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Water Resources
Edward D. Schroeder
University of California, Davis

I.The Role of Water

II.Movement of Water in the Hydrosphere
III.Water Quality
IV. Modification of Water Quality
V. Requirements for Water Quantity and Water
Quality
VI. Water Resources Management

GLOSSARY Turbidity Cloudiness in water resulting from the pres-

ence of colloidal particles.
Biochemical oxygen demand The quantity of oxygen Watershed An area that drains to a specific point.
required by bacteria to metabolize organic matter in
water.
Groundwater Water that accumulates in the soil. Where WATER is fundamental to life, and human societies can-
accumulation is great enough to saturate the soil pores not survive without adequate supplies of water. Living
in quantities great enough to be extracted economically, organisms are over 70% water by mass; the large ma-
the region is termed a groundwater aquifer. jority of organisms live in water and obtain their suste-
Hydrograph A graph of flow in a stream versus time. nance from water; and terrestrial organisms can survive
Hydrologic cycle The pathway through which global wa- for only brief periods without water. Our natural environ-
ter cycles from the oceans through the atmosphere, ment has been shaped to a great extent by the occurrence,
precipitation, runoff, infiltration, and back to the the availability, and the physical, chemical, and biologi-
oceans. cal characteristics of water. Water has shaped the physical
Hydrosphere The total quantity and distribution of characteristics of our environment through glacial action,
global water. floods, and erosion. Reservoirs and canals have been used
Suspended solids Particulate matter carried in water and since ancient times to control floods, improve transporta-
measured as the dry residue on a filter. tion, and change the availability of water. Today, approxi-
Transpiration Loss of water vapor from plants. Evapo- mately 19% of the global supply of electricity is generated
transpiration is the combined evaporation from surface with water, and approximately 12% of global food supply
puddles and transpiration over an area. is grown using stored water.

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722 Water Resources

I. THE ROLE OF WATER TABLE I Worldwide Distribution of Water by Type

Volume,
The importance of water availability is illustrated by com- Type 1000 km3 %
paring the ecology of the Florida Everglades and Death
World oceans 1,370,323 94.2
Valley in California, and the importance of the chemical
Groundwater (fresh and saline) 60,000 4.1
and physical characteristics of water can be illustrated by
Glaciers 24.000 1.65
considering Lake Superior, the Dead Sea, and Antarctica.
Lakes and reservoirs 280 .019
We tend to set relative values on particular types of en-
Soil moisture 85 0.006
vironments; for example, most of us would say that Lake
Atmospheric water 14 0.001
Superior is “superior” to the Dead Sea. However, each en-
River water 1.2 0.001
vironment provides competitive advantages for particular
types of life. The diversity in life forms resulting from the Total 1,454,700
range in availability and characteristics of water on our [From L’vovich, M. I. (1979). “World Water Resources and
planet appears to be a significant factor in stability of the Their Future” (R. L. Nase, translator), American Geophysical Union,
worldwide ecosystem. Washington, DC.]
Usefulness is strongly related to the chemical charac-
teristics of water. For example, rainwater usually con- inaccessible or economically unavailable. Surface waters,
tains minimal concentrations of dissolved ions, is poorly rivers, lakes, and reservoirs comprise approximately
buffered, and has a pH of about 5.6. As a result, rainwater 155,000 km3 , less than 1% of the total freshwater volume.
tends to be slightly corrosive, generally good for irrigat- Extraction of freshwater for municipal, agricultural, and
ing plants, and is generally thought to be good tasting. industrial use places considerable stress on the available
Subsurface water (more commonly called groundwater) water resources. About 1300 km3 /day of freshwater is
is often relatively high in dissolved matter and may have used in the United States alone. Clearly, the worldwide
concentrations of ions that make it unsuitable for drinking demand for freshwater would rapidly exhaust the avail-
or irrigation of some crops. The high salt concentrations able volumes if the system were not dynamic. Although
of seawater are required by some species of organisms but the total amount of global water is constant, there is a
are unsuitable for consumption by terrestrial animals and continuous movement from the oceans to atmospheric
for irrigation of most crops. water, through evaporation, to surface water, through
Most organisms that live in water are microscopic bac- precipitation, and to groundwater, through infiltration.
teria, algae, protozoans, and invertebrates that serve as Thus extracted water is replaced in a process known
bottom rungs in the food chain and as primary degraders as the hydrologic cycle. Moreover, extracted water is
of organic material. However, water also serves as a con- returned to the system through wastewater discharges,
duit for the transmission of many diseases caused by mi- agricultural drainage, and evaporation.
croorganisms. In most cases, the pathogenic organisms are
added to water and are not natural members of the aquatic B. The Role of Water Quality
community. This is particularly true of human viruses Water quality is as important as water availability in sup-
and pathogenic bacteria. However, some diseases such as porting ecosystems and human society. The two most
schistosomiasis, filariasis, and guinea worm are caused by
organisms whose life cycle includes periods in water. TABLE II Worldwide Distribution of Freshwater by Type
Volume, % of total % of total
Type 1000 km3 freshwater type of water
A. Distribution and Availability of Water
Worldwide distribution of water by type is shown in Glaciers 24,000 85 100
Table I. Of the total global volume of water, about 1.5 Groundwater 4,000 14 6.7
trillion km3 , only 2% (28,000,000 km3 ) is freshwater, that Lakes and reservoirs 155 0.6 55.4
is, water usable for consumption and for agriculture. The Soil moisture 83 0.3 97.7
definition of freshwater is imprecise but can be nominally Atmospheric water 14 0.05 100
defined as water having total dissolved matter concentra- River water 1.2 0.004 100
tions of less than 1500 mg/L. As indicated in Table II, 85% Totals 28,253.2 100
of global freshwater is frozen. Groundwater, that is, water
[From L’vovich, M. I. (1979). “World Water Resources and Their Fu-
completely filling pores in soil, gravel, and sand, accounts ture” (R. L. Nase, translator), American Geophysical Union, Washing-
for 4000 km3 , or 14% of the global freshwater. Much ton, DC; Heath, R. C. (1982). “Basic Groundwater Hydrology,” USGS,
of the total groundwater resources are either physically Water Supply Paper No. 2220.]
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Water Resources 723

important water quality parameters are the total dissolved River, the Ethiopian plateau, receives heavy rains between
solids (TDS) concentration and temperature. Dissolved April and October. Despite the fact that precipitation is
matter in water is principally composed of four cations badly located both spatially and temporally, large-scale
(calcium, magnesium, potassium, and sodium) and for an- agricultural development in California has been possible,
ions (bicarbonate, chloride, nitrate, and sulfate), although through the development of storage and distribution fa-
smaller quantities of virtually every element and com- cilities. British agriculture depends very little on seasonal
pound can be found in water at some location on the earth. water storage and irrigation because rainfall is generally
The relative concentrations of the major ions can be impor- plentiful throughout the growing season.
tant. For example, water having high ratios of sodium to Historically, floods have been one of the principal fac-
calcium and magnesium cause clay soils to swell and be- tors controlling the development of communities and soci-
come impermeable, and low permeability soils are unsuit- eties. The flooding Nile eroded nutrient-rich upland soils
able for agriculture. However, the total salt concentration and deposited them in the delta, allowing Egyptian agri-
is the principal determinant of the type of life that can be culture to flourish. However, more common effects of
supported. Organisms have adapted to specific ranges of floods have been to limit use of land on flood plains and
TDS, either physiologically or with respect to food and nu- to wreak calamitous damage during unusually large flood
trient sources. Thus we find different plants, animals, and events. During the 20th century major efforts were made
microorganisms living in fresh, brackish, and saline wa- to develop flood control systems in major river basins
ters. Some organisms such as the brine shrimp (Artemia) throughout the world. These efforts combined dams, reser-
and halophilic algae and bacteria grow in the Dead Sea voirs and levees to temporarily store flood waters, protect
and Great Salt Lake where salt concentrations are up to land along river banks, and move flood waters rapidly
25%. to the ocean. One result has been increased flood plain
The ability of organisms to grow, compete, and resist development, faster storm runoff, and higher peak flows
disease is strongly impacted by water temperature. Species during floods. A second result has been loss of habitat
of plants, animals, and microorganisms tend to do best in for many aquatic species. For example, floods clean out
rather specific temperature bands or seasonal temperature spawning gravels and deposit new gravels eroded from up-
patterns. Fish are perhaps the most familiar examples of stream hills. When dams are constructed, flood peaks are
organisms that are distributed according to temperature decreased, which decreases cleaning, and newly eroded
patterns. Trout and pike are typical coldwater gamefish gravels are deposited in the reservoir formed by the dam.
while bass do better in warmer waters of shallow lakes and Such unintended results of flood control measures have
slow moving streams. Anadromous fish, such as salmon, increased understanding of system complexity and raised
wait for temperatures to reach specific values before pro- questions about the sustainability of modern society.
ceeding upstream to spawn.
Physical, chemical, and biological water quality char-
acteristics are important in determining the suitability of II. MOVEMENT OF WATER IN THE
water for municipal, agricultural, and industrial use. Mu- HYDROSPHERE
nicipal water supplies must be essentially free of sus-
pended matter, have low concentrations of specific ions The distribution of water shown in Table II can be con-
and compounds, and be free of pathogenic organisms. As sidered a snapshot in time. Small changes in the distribu-
noted above, TDS concentrations and the ratio of sodium tion occur on a seasonal basis, and major changes occur
to calcium and magnesium are limiting factors in agricul- over periods of centuries and millennia. During the Pleis-
tural water use. Additionally, certain contaminants, such tocene Epoch (the most recent ice age ending approxi-
as boron, place constraints on the type of crop that can be mately 10,000 years ago), the volume of water stored as
grown. The principal use of water in industry is for cool- ice was several times greater than at present, the earth
ing, where scaling and corrosion are the principal water was more arid, with regions such as Florida being sandy
quality issues. deserts, and the mean sea level elevation was about 130 m
lower than at present. The global climate, and hence the
distribution of water in space and time, will continue to
C. Spatial and Temporal Distribution
change on several time scales and the distribution of water
of Water Resources
will vary correspondingly. Currently, there is a general re-
Spatial and temporal distribution of water resources has treat of global ice that is thought to be at least partially due
been a definitive factor in the development of human so- to anthropogenic causes, such as the production of green-
cieties. Although the land was extremely arid, ancient house gases. While the effects of the warming trend are
Egypt was able to develop because the source of the Nile not completely clear, most predictions include increased
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724 Water Resources

variability in weather and changes in annual precipitation B. The Hydrologic Cycle

patterns as well as a rise in mean sea level of 0.35 to 0.55 m
The hydrologic cycle, depicted in Fig. 1, involves
by 2100. Efforts to predict impacts of increased mean sea
five fundamental steps: evapotranspiration, transport of
levels is the subject of considerable research at the present
water vapor, condensation/precipitation, overland flow/
time.
percolation, and channel runoff. Evapotransporation is the
vaporization of water through the combined processes of
A. Short-Term Water Resource Dynamics evaporation from surface water and transporation from
plants. Sublimation of frozen water in glaciers is relatively
Human society must necessarily focus on much shorter
small but is effectively included in the general category.
time scales than those that result in global climate change.
The amount of water vapor that can be held in the atmo-
Two principal areas are of importance in water resources:
sphere is a function of the ambient temperature and can
provision of water for human activities or environmental
be approximated using Eqs. (1) and (2)
management and management of runoff or flood control.
eS
The time scale associated with water supply is defined ρ S = 0.622 (1)
by annual rainfall patterns, particularly the length of dry RT
 
periods. Flood management requires consideration of the 4278.6
intensity and volumes of precipitation, and frequency of e S = 2.7489 × 10 exp −
7
(2)
T − 30.37
particular storms. Thus flood management facilities must
be designed for the maximum storm that can reasonably where ρS = saturation density of water vapor, kg/m3 ,
be expected. eS = saturation vapor pressure, kN/m2 ,
Consideration of the movement of water on a shorter
R = dry air gas constant, 0.287 kN · m/kg · K,
time scale is built around the hydrologic cycle, which pro-
T = temperature, K.
vides a conceptual picture of the short-term dynamics of
the system. Time scale is an important factor. Surface wa- Thus, at 20◦ C (293.16 K) the saturation vapor pressure is
ter dynamics have time scales of hours and days, while approximately 2.33 kN/m2 and the amount of water vapor
subsurface (groundwater) dynamics have time scales of that can be held in the air is 0.017 kg/m3 . At 30◦ C the
months and years. For example, flood waves move down saturation vapor density is 0.030 kg/m3 and at 10◦ C the
rivers at velocities of several km/hr while groundwater saturation vapor density is 0.009 kg/m3 . Thus warm air
surface elevations may change by a few meters during the (e.g., 30◦ C air) containing 0.017 kg/m3 of water vapor
course of an irrigation season and tens of meters during will be unsaturated. Cooling the air to 20◦ C will result in
periods of continued overdraft. the air being saturated. Further cooling to 10◦ C will result

FIGURE 1 The hydrologic cycle.

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Water Resources 725

in super saturation by approximately 0.008 kg/m3 . Water Although evapotranspiration is continuous, both evapora-
vapor in supersaturated air condenses around microscopic tion and transpiration are functions of temperature, and
particles that precipitate as rainfall. Thus cooling of moist hence are seasonal. The largest source of atmospheric
air will result in precipitation if the temperature falls below moisture is evaporation from the oceans. Evaporation from
the saturation temperature for a particular water density surface waters is a very small fraction of the total, with
(20◦ C in the above example). The saturation temperature transpiration from plants being somewhat more signifi-
is called the dew point. cant. Of course, transpiration requires growing plants and
Pressure gradients in the atmosphere resulting from un- stops altogether when annual plants die, or when decid-
even heating and cooling result in winds that move water uous trees lose their leaves. However, some transpira-
vapor with other gases. Air expands and cools as it rises tion occurs all year from perennials and nondeciduous
(e.g., due to the orographic lift when winds move air over trees.
mountains). Air is also cooled by passing over cold sur- Movement of water vapor in the atmosphere is a func-
faces (e.g., snow fields) and due to thermal convection. tion of weather patterns that result in generally repeat-
The density of wet air is less than that of dry air. Wet air able annual precipitation cycles. However, precipitation
may be warmed by radiation from warm surfaces, causing is highly stochastic on a short-term basis, and annual total
it to decrease in density and rise through cooler overlying precipitation can be highly variable, as indicated in Fig. 2.
air. Expansion occurs as the pressure decreases and the The form of precipitation, rain, ice, or snow, is important
temperature drops to the dew point. Condensation of the because rain begins moving through the ground or over the
water vapor releases the latent heat of vaporization, re- surface on impact while frozen precipitation is stored on
sulting in decreased air density, further rising, and further the surface for a period of time. Regions receiving large
condensation. Thermal convection is responsible for the amounts of snow have spring floods as melting occurs.
formation of cumulus clouds which have a clearly defined Many arid regions, such as the southwestern United States,
base and fluffy, cottony tops. depend on snow melt from mountains for water during the
Depending on the temperature and other meteorologic long dry summers.
conditions, the form of the precipitation is rain, snow,
sleet, or hail. Precipitation reaching the surface infiltrates
into the soil, becomes surface runoff, is temporarily stored
as snow, or falls directly onto surface waters such as the
oceans, lakes, or rivers. Most river systems discharge to
the world oceans, but a few discharge to lakes such as the
Great Salt Lake and the Dead, Caspian, and Aral Seas that
have no surface outlets. Water that infiltrates into the soil
may be assimilated by plants, percolate into groundwater
aquifers, and/or resurface as springs and become part of
the surface runoff. Estimates of residence times in each
phase of the hydrologic cycle are given in Table III.

TABLE III Residence Times of Water Molecules in the

Hydrologic Cycle
Location Residence time

Atmosphere 10 days
Rivers (speed 1 m/sec) 2 weeks
Soil moisture 2 weeks to 1 year
Largest lakes 10 to 1,000 years
Shallow groundwater (speed 1–10 m/sec) 10 to 100 years
Mixed layer of oceans (150 m depth) 120 years
World ocean 3,000 years
Deep groundwater up to 10,000 years FIGURE 2 Precipitation patterns at the headquarters weather
Antarctic ice cap 10,000 years station of University of California Agricultural Experiment Station,
Hopland, CA. Elevation is 244 m. Annual total precipitation values
[Source: Heath, R. C. (1982). “Basic Groundwater Hydrology,” are shown in (a) with mean and standard deviation lines. Monthly
USGS, Water Supply Paper No. 2220.] averages shown in (b) are for the period 1953 through 1999.
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726 Water Resources

Rate of infiltration into soil is dependent on the recent return period, the largest storm has occurred once and
storm history and soil characteristics. When storms are hence the return period is n years. The second largest storm
frequent, sorption sites on soil particles remain saturated has been equaled or exceeded once and the return period
and infiltration is solely due to flow of water downward is n/2 years. Return periods of smaller storms are defined
through the soil. Additionally, smaller pores remain full in a similar manner. When precipitation records are ad-
due to capillary forces, and the available drainage cross equate, more sophisticated estimates of storm frequency
section is reduced. can be applied using appropriate statistical distributions.
Usually normal or log-normal distributions provide ade-
quate descriptions of storm frequency. However, distribu-
C. Return Period tions that include skewness, such as the Pearson Type III
Considerable effort is made to evaluate the return period and Gumbel, may be used as storm magnitudes have a
of storms, that is, the probability that a storm of a partic- lower bound. Application of the distributions allows esti-
ular magnitude will occur within a given period of time. mating the probability of a storm of a given magnitude oc-
The magnitude of storms varies considerably, and even curring within a selected time increment. It is important to
the maximum storm that might occur annually is highly remember that in frequency analysis storms are assumed to
variable. An example of the variation in storm magnitudes be random events. Thus the occurrence of a storm of a par-
is given by the annual maximum storm flows in the Don ticular magnitude in a given time period does not change
River, shown in Fig. 3. The simplest and most common the probability that a storm of equal magnitude will occur
method of characterizing storms is by estimation of the in the next time period. Thus a 100-year storm has the same
volume of water precipitated and the time over which the probability (0.01) of occurring in any given year. Corre-
precipitation took place. An additional factor is the area spondingly, it is not surprising to have several successive
over which the precipitation occurred. The importance of years in which the total rainfall is less than or greater than
the quantity of water precipitated is quite obvious. Im- average. Additionally, return periods and other predictive
portance of storm duration results from the fact that the analyses are based on the available records. Each year
rate of runoff is directly related to the storm intensity. For the record increases and the estimates of average rainfall,
example, flood damage resulting from a 25-mm storm oc- 100-year storm magnitude, and other values change.
curring during a 1-hr period may be considerably greater Relationships between storm intensity and duration are
than the damage resulting from a 100-mm storm occurring developed using return period as the reference. By plot-
during a 24-hr period. The area over which precipitation ting (or sorting numerically) storms according to intensity
occurs is a complicating factor in defining the return pe- (in mm/hr) and duration (in minutes), the relationship be-
riod. Storms are rarely uniform in intensity and duration tween storms of a given magnitude or return period and
over an entire drainage basin. Hence, judgments must be the intensity or duration can be determined. This informa-
made in defining areas of impact and methods of aver- tion can be used as the basis for design capacity of flood
aging. In summary, storm return period has at least two control facilities.
dimensions, volume and intensity. A third factor, area of
impact, increases the uncertainty of the calculations.
D. Overland Flow
Return period is estimated by classifying all recorded
storms in a given period of time and ranking them ac- Precipitation reaching the ground surface is sorbed onto
cording to frequency. In the simplest approach to defining soil surfaces and infiltrates into the soil. When the pre-
cipitation rate is greater than the rates of sorption and in-
filtration, water accumulates and begins to move as free-
surface flow. Conceptually, the flow is no different than
free-surface flow in engineered facilities. Gravity is the
principal driving force, and inertial and pressure forces
can be neglected. Gravitational force is nearly balanced
by bed friction. Flow rates can be estimated using the
kinematic wave equation
Q = αy m (3)

where Q = volumetric flow rate, m3 /sec,

FIGURE 3 Maximum annual flows in the Don River at the y = depth of flow, m, and
Razderskaya gauging station for the years 1911 to 1951. α, m = kinematic wave parameters.
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Water Resources 727

The Manning equation for open channel flow is of this E. Infiltration and Groundwater Flow
form:
1 Water below the ground surface may be sorbed onto the
q = s̄ 1/2 ȳ 5/3 (4) surface of soil particles, flow downward as a thin liquid
n
film through unsaturated pores, flow in locally saturated
where q = volumetric flow rate per unit width, pores, be held in place by capillary forces, or be stored in
m3 /m · sec, a saturated zone called an aquifer, as indicated in Fig. 4.
n = Manning roughness coefficient, Water in aquifers is often moving as the result of pressure
s̄ = mean slope, m/m, and gradients generated by springs or by pumping.
ȳ = mean flow depth, m. Soil above the water table is generally unsaturated; the
pores between soil particles are at most locally full, and
Values of the roughness coefficient, n, range from about a significant fraction of the pore volume is filled with air.
0.01 for smooth pavement to 0.4 for dense shrubbery and Some moisture is bound to soil by osmotic forces and a
grass. certain amount is held above the water table in small pores
Boundary conditions for overland flow are typically dif- by capillary forces. However, most of the water entering
ficult to describe, and the flow is inherently nonuniform the subsurface migrates toward the saturated zone under
and non-steady-state. Ground surfaces are uneven, rivulets the dominant action of gravitational forces.
and channels develop, plants and debris retard the flow Subsurface flow is nearly always laminar, and thus there
and make friction factor estimates highly variable, lateral are similarities between flow in saturated and unsaturated
boundaries are not constant, and slopes change. Each of zones. Because of the heterogeneous nature of the subsur-
these factors can be incorporated in overland flow mod- face, flows are extremely difficult to describe and nearly
els, but the coefficients are averaged over discrete areas all models are based on Darcy’s law
of the drainage area or watershed. The finer the grid, the
more accurate a predictive overland flow model can be. h
ν S = −K C (5)
However, obtaining coefficient values for drainage basins L
is difficult and costly. The values are functions of past
precipitation history and are affected by changes in land where νS = superficial velocity, m/sec,
use, accumulation of debris, and many other factors. Con- KC = hydraulic conductivity, m/sec,
sequently, there is a great deal of empiricism involved in h = head loss, m,
applying overland flow relationships. L = length of flow path, m.

FIGURE 4 Movement of water below the ground surface.

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728 Water Resources

F. Storm Hydrographs
Storm hydrographs are used to describe the response of
streams and rivers to overland flow discharges. Water flow-
ing across the land surface eventually comes together in
channels to form creeks and rivers. At any point selected,
the flow versus time response to a storm will show a rela-
tively rapid rise, a peak, and a gradually declining reces-
sion back to the base flow fed by springs, as indicated in
Fig. 6. The shape of the hydrograph is a function of the
size, slope, and physical characteristics of the drainage
area. For example, flow from parking lots characteristi-
cally peaks rapidly because of the relatively short flow dis-
tances, smooth surfaces, and high slopes (required to min-
imize accumulation of water on the surface). Flow rates
FIGURE 5 Apparatus for measuring head loss in soil sample. in rivers change much more slowly because overland flow
The gravel layer has much less resistance to flow than the soil, velocities are generally slower and distances traversed are
and head loss through the gravel can be considered insignificant. larger than in urban settings. An example is provided in
Fig. 6b for the Portage River in northern Ohio. Note that
the response time to the two storms is fairly rapid but the
Superficial velocity is determined by dividing the volu- hydrographs are of the order of days in length.
metric flow rate by the total cross-sectional area. Pore area
is approximated by the porosity, and average pore veloc-
ity is usually estimated by dividing the superficial velocity
by the porosity. Pore velocity is important in estimating
transport time of chemical contaminants.
Application of Darcy’s law is illustrated for saturated
flow in the test system sketch shown in Fig. 5. Note that
head loss in the gravel support layer is assumed to be
negligible.
Flow in the subsurface is always three-dimensional and
the hydraulic conductivity, K C , is a function of posi-
tion and local soil characteristics. Because flow is lam-
inar, the hydraulic conductivity should be a function of
pore size and viscosity, and a typical expression is given
by
ρg
K C = Cd 2 (6)
µ

where C = proportionality coefficient,

d = soil grain size, m,
ρ = density of water, kg/m3 , and
µ = viscosity of water, N · s/m2 .

Values of K C range from 10−8 m/sec for fine clays to

10−3 m/sec for coarse sands.
Flow in the unsaturated vadose zone is best de-
scribed using values of hydraulic conductivity obtained
through experimental observation. Not surprisingly, pre-
diction of unsaturated flow rates is considerably less FIGURE 6 Characteristic storm hydrograph for a drainage basin
satisfactory than prediction of rates in the saturated and actual flow records for the Portage River at Woodville, San-
zone. dusky County, OH for the month of April, 1997.
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Water Resources 729

Storm hydrograph characteristics are reasonably con- buildings decreases erosion considerably. Paved surfaces,
sistent at particular locations, and this fact allows the de- roofs, concrete pipes, and channels are much less resistant
velopment of unit hydrographs that are characteristic of to flow than grass, forests, and fields. Consequently, both
one unit of rainfall on the upstream drainage area in a the time to peak flow and the overall watershed drainage
unit time period (usually 1 hr). Unit hydrographs can be time decrease with urbanization.
combined to construct hydrographs for storms of any vol- Each of these changes can be considered a benefit or a
ume and any duration. Unit hydrograph theory is based detriment, depending on one’s viewpoint. Increasing the
on the assumption that discharge is a linear function of fraction of precipitation that becomes surface runoff can
precipitation. Although the assumption is not valid, the be useful if storage facilities are available and the water can
unit hydrograph concept is useful and can often provide be used in some manner. However, in arid areas that receive
satisfactory estimates of response. the most precipitation during the winter, the result is a loss
Hydrographs for locations downstream of a gauging of groundwater recharge. Rapid drainage of urban areas
station can be constructed by a process known as flood is highly desirable. For example, accumulation of water
routing which incorporates a stream flow model and the in streets is a traffic hazard. However, downstream flood
equation of continuity to predict flow rates. The stream flow peaks increase with urbanization and in some cases
flow model must incorporate physical characteristics of the damage to downstream facilities and ecosystems may
the channel. A number of routing models are available as be high. Erosion control is generally considered to be a
computer software packages. benefit. However, in southern California where beach sand
is supplied by erosion of sedimentary material in the sur-
rounding mountains, urbanization has resulted in a need
G. Watershed Models
to buy sand to replace that washed away by tidal action.
Watershed models integrate overland flow, infiltration,
streamflow, and routing to provide estimates of stream
flow as a function of storm characteristics. The most
widely used watershed models are distributed by the Hy- III. WATER QUALITY
drologic Engineering Center of the U.S. Army Corps of
Engineers. Models require calibration for each applica- Water quality is defined by physical, chemical, and biolog-
tion. Such models have become essential tools for predic- ical characteristics and the intended use. Pure liquid wa-
tion of flows and river depths resulting from storms. ter, that is, pure H2 O without dissolved ions, compounds,
General-use watershed models include features such as particles, and gases, does not exist in nature. From the
reservoir storage, variable land characteristics, and soil moment that liquid droplets condense in the atmosphere,
moisture as a function of time. Inclusion of such features materials begin to accumulate (see Table IV). Raindrops
allows application of watershed models for reservoir oper- quickly reach equilibrium with atmospheric gases, most
ation and evaluation of alternative strategies for respond- notably oxygen, nitrogen, and carbon dioxide. Minerals
ing to a range of storm scenarios. For example, watershed present in the atmosphere as a result of evaporation of
models are used to predict the impact of large storms, the spray over the ocean and entrainment of dust by winds
potential for local or regional flooding, and areas that will dissolve in the drops as they fall. Combustion processes
be most severely impacted. At present, construction is dis- emit sulfur dioxide and oxides of nitrogen which dissolve
couraged in areas that will be inundated by floods having a in raindrops and are converted to mineral acids. Local ap-
100-year return period. Thus flood control facilities, such plication of pesticide and herbicide sprays may result in
as levees and reservoirs, are constructed to contain such temporary presence of these compounds. Surprisingly, the
a flood, and significant damage can be expected if a flood urban application of pesticides and herbicides is often a
exceeding the 100-year flood occurs. larger contributor than that from agriculture. Concentra-
tions of selected contaminants found in rainfall at various
locations in California are given in Table IV. Note that the
H. Impacts of Land Development
pesticides chlorpyrifos and diazinon are much higher in
Urban development results in significant changes in wa- the Lodi samples because of their use as dormant sprays
tershed characteristics. The most significant features are in orchards.
covering of soil by pavement and buildings and installation After reaching the ground surface, water comes in con-
of stormwater collection systems. Pavement and buildings tact with an array of organic and inorganic materials,
prevent infiltration and for this reason the fraction of pre- particles are carried by overland flow, and materials dis-
cipitation that becomes surface runoff is much greater in solve into the water. The characteristics of water change
urban than in rural areas. Covering soil with pavement and continually during transport through the hydrologic cycle.
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730 Water Resources

TABLE IV Concentration of Selected Constituents in Rainfall at Five Locations in California

during February 1999
Los Orange
Constituent Bakersfield Angeles Riverside Lodi County

Copper, total (µg/L) 3.5 5.8 2.8 4.0 2.4

Lead, total (µg/L) 3 6.6 0.9 3.3 2.7
Zinc, total (µg/L) 34.0 51.9 15.5 34.0 100.7
Total Kejldahl (mg/L) 1.03 0.76 1.00 1.08 0.85
nitrogen
Nitrate (mg/L) 0.56 0.02 0.33 0.87 1.98
Phosphorus (mg/L) 0.03 0.04 0.03 0.11 0.05
Chlorpyrifos (µg/L) 0.07 0.02 0.01 0.87 0.04
Diazinon (µg/L) 0.09 0.09 0.03 0.65 0.15

[Source, California Department of Transportation.]

A. Water Quality as Defined by Water Use The nutrients were necessary to maintain benthic organ-
ism and algal populations that were food sources for the
Because of the huge range of materials that may be present
sardines, anchovies, and shrimp.
in natural water, any definition of quality must be related
When we speak of water quality, our first thought is the
to intended use. Thus a water that is quite acceptable for
water we drink. In the United States, Congress has enacted
drinking may not be acceptable for irrigation of citrus trees
regulations on water quality under the Safe Drinking Water
because of the presence of boron. Water quality for ship-
Act of 1986 and its later amendments. The requirements
ping is defined by viscosity and perhaps corrosiveness.
are summarized as the National Primary Drinking Water
When water temperature drops below freezing, the phys-
Standards. These standards are based on safety for human
ical state of water becomes a problem for ships. Survival
consumption and do not include factors related to corro-
of bacteria living in water requires that carbon and en-
sive or scaling properties of water. Thus it is possible for
ergy sources as well as inorganic nutrients be present. The
water that is “acceptable” to cause major problems in the
materials that serve as nutrients for the bacteria are gen-
distribution system by corroding pipes or plugging pipes
erally very unwelcome in public drinking water supplies.
with scale. The water quality requirements of the distri-
Boron is not regulated in drinking water supplies but is
bution system are different from those of the consumer.
a problem at concentrations of 1 mg/L in water used to
irrigate citrus trees. We need to think of maintenance of
environmental quality, particularly the aquatic habitat, as
B. Physical Characteristics of Water
a water quality issue and therefore environmental man-
agement is a type of use. Unfortunately, the aquatic or- For most people, physical characteristics such as temper-
ganisms do not have uniform water quality requirements. ature, turbidity, solids, color, and odor form the first im-
Many live in ecological niches provided by local water pression of the water quality. Streams that are warm, tur-
quality characteristics, and changes in water quality can bid, dark-colored, and odorous would often be assumed
impact the aquatic community is surprising ways. Con- to be polluted and perhaps unhealthful. Correspondingly,
struction of the Aswan High Dam on the Nile River in streams that are cool, clear, and odorless are generally as-
Egypt, completed in 1964, has provided one of the best sumed to be of good quality and pollution free. In fact,
examples of the complexity of aquatic ecosystems. The most natural waters are somewhat cloudy at best and
dam and its reservoir, Lake Nassar, were conceived of as may be colored by tannins and humic matter from de-
a solution for water resources management, flood control, caying plants. Sloughs, particularly in tropical regions,
and electric power requirements for Egypt. Unexpected have characteristic odors associated with decaying veg-
results have included increases in the incidence of schis- etation. However, the ecological health of such systems
tosomiasis, a northward movement of malaria, and sharp may be very good. Mountain streams receiving acid mine
decreases in the sardine, anchovy, and shrimp population drainage may be virtually sterile and contain toxic concen-
in the Mediterranean Sea. Increases in schistosomiasis and trations of metals but may also be extremely clear, cold,
malaria resulted from the decreases in flooding and conse- and odorless. Thus the physical characteristics of water
quent stability of water levels. Loss of the fisheries resulted must be interpreted in terms of specific situations, and
from trapping of nutrient-laden sediments behind the dam. other characteristics may be more important.
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Water Resources 731

1. Temperature are more likely to survive disinfection. Thus drinking wa-

ter treatment is focused on removal of colloidal particles
Temperature is a major water quality characteristic be-
and the production of extremely clear water—which is
cause of the relationship of temperature and the type of or-
also aesthetically pleasing.
ganisms that can compete, because the solubility of solids
In natural waters, a Secchi disk is used to assess turbid-
and gases in water is a function of temperature, and be-
ity. The black and white Secchi disk is lowered into the
cause the properties of water are functions of temperature.
water until the markings become indistinguishable and
Organisms at all levels of complexity are adapted to cer-
the depth is recorded as the Secchi depth. Extremely clear
tain ranges of temperature. Growth and respiration rates
lakes may have Secchi depths of 30 to 40 m while ex-
of aquatic organisms increase with temperature. Fish pro-
tremely turbid lakes will have Secchi depths of less than
vide a familiar example in that cold-water species, such
1 m. Turbidity in drinking water is measured by determin-
as trout, are found in mountain streams and lakes of the
ing light transmission using standard light sources and
temperate zone, and warm-water species, such as bass, are
reported in nephelometric turbidity units (NTUs). Drink-
found in warmer regions. Cold- and warm-water fish may
ing water standards require turbidities of less than 1 NTU.
also be found in the same streams or lakes, but occupy-
While only a qualitative relationship exists between tur-
ing different zones. For example, cold-water fish will be
bidity units and Secchi depth, ultraclear natural waters are
found in the deeper portions of a lake while warm-water
nearly always below 5 NTU. The United States National
fish will be found along the edges in shallow water were
Primary Drinking Water Standard for turbidity is 1.0 NTU.
temperatures are higher.

2. Turbidity 3. Solids
Turbidity in water results from the presence of colloidal Solids in water include all contaminants other than gases
particles that scatter light. As a result, objects in wa- and are classified according to state, size, size distribu-
ter become indistinct. Lake Tahoe on the California– tion, and chemical characteristics. The most widely used
Nevada border, Crater Lake in Oregon, and the reefs in the measurements of solids in water and their significance are
Caribbean Sea are famous for the clarity, or lack of turbid- summarized in Table V, and the size distribution of solids
ity, of their water. Shoreline development and wastewater is summarized in Fig. 7.
discharges increase particle density and support growth Three classifications, suspended, dissolved, and volatile
of phytoplankton that increases turbidity. Decreased light solids, are of particular importance in defining water qual-
penetration and energy absorption in the near surface lay- ity. Suspended solids and dissolved solids are measures
ers affect both the aquatic ecology and the heating and of size, while volatile solids is based on a procedure—
cooling of the water. burning. Filtration through a standard 1.2-µm pore size
Turbidity is an important characteristic in drinking wa- filter is used to define suspended and dissolved solids. The
ter supplies because microorganisms attached to particles practical definition of suspended solids is particles larger

FIGURE 7 Size distribution of solids in water.

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732 Water Resources

TABLE V Standard Measures of Solids Content of Water

Test Description and use

Settleable solids (mg/L) A measure of the volume of sludge that can be produced by gravity settling.
The test is used to predict sludge accumulation in treatment facilities.
Total solids (TS, mg/L) The residue remaining after drying at 103◦ C. Total solids measurements are
used in conjunction with other measurements.
Total volatile solids (TVS, mg/L) The difference between TS and the residue remaining after burning at 550◦ C.
TVS is a measure of the organic solids present in a water.
Suspended solids (SS, mg/L) The residue retained on a 1.2-µm pore size filter dried at 103◦ C. Suspended solids
are regulated in wastewater discharges, urban stormwater discharges,
and runoff from construction sites.
Volatile suspended solids (VSS, mg/L) The difference between SS and the residue remaining after burning at 550◦ C.
VSS are used to estimate the organic suspended matter concentration and as a
measure of microbial concentration in wastewater treatment.
Total dissolved solids (TDS, mg/L) The solids passing through the standard 1.2-µm pore size filter. TDS includes
colloidal particles and thus might be better defined as filterable solids.
TDS can be measured by drying filtrate at 103◦ C or as the difference between
total solids and suspended solids (TS–SS).

Note. Detailed descriptions can be found in Water Environment Federation (1998). “Standard Methods for the Examination of Water and
Wastewater,” 20th edition, Alexandria, VA.

than 1 µm because the standard filter retains most of the Groundwaters tend to have higher dissolved solids concen-
particles down to that size. Included are most, but not all, trations than surface waters because of the time of con-
bacteria, algae, and protozoa. Organic particles larger than tact with solutes sources (see Table III). Concentrations
100 µm and dense particles (e.g., clay, silt, and sand) larger of dissolved matter found in groundwaters varies from
than 10 µm settle relatively rapidly and are removed in below 100 mg/L to values more concentrated than seawa-
moderately quiescent conditions. Thus suspended solids ter. Waters having dissolved solids concentrations greater
are closely related to, but not synonymous with, settleable than 1000 mg/L are considered unacceptable as municipal
solids. Settleable material forms shoals and sludge banks water supplies.
in natural waters, fills reservoirs, and clogs wastewater Volatile solids, the material that combusts at a tempera-
collection systems. Removal of settleable material is a ture of 550◦ C, is used as a measure of the organic content.
major task in both water and wastewater treatment. In ad- Like suspended and dissolved solids, volatile solids is a
dition to being less aesthetically pleasing, water contain- lumped parameter and no information is provided about
ing suspended matter is less suitable for drinking because the chemical characteristics of material. However, know-
pathogenic organisms are shielded from disinfectants. ing the volatile solids content of a water provides infor-
Dissolved solids, as measured by filtration, include col- mation about the source and history of the water. High
loidal matter, as noted above. In most natural waters nearly volatile solids concentrations are associated with decay-
all of the material making up the TDS is inorganic. The ing organic matter such as might be found in swamps and
principal organic component is complex, high molecular in streams receiving poorly treated wastewaters.
weight humic material that is present at concentrations
up to a few mg/L. Concentrations of dissolved solids in
4. Color
surface waters range from about 10 mg/L in lakes and
streams at the upper end of watersheds to several hundred Color in water is imparted by dissolved material such as
mg/L at the lower end of watersheds. As streams progress tannins from decaying plants. As a result, colored water
through watersheds, agricultural runoff, municipal and in- is most often brownish in tint. Colloidal organic matter
dustrial waste discharges, and erosion add dissolved mat- found in wastewater will give receiving waters a gray color
ter. For example, municipal use of water adds 200 to 300 if concentrations are high, and swamp waters are often
mg/L of TDS to extracted waters. Total dissolved solids black due to the presence of ferrous sulfide precipitates.
concentrations in the Mississippi River increases from True colors are due to dissolved materials but in practice,
about 170 mg/L at Grand Rapids, Minnesota, to about colors associated with colloidal material are included as a
250 mg/L at St. Francisville, Louisiana. Dissolved solid characteristic of water.
concentrations may vary seasonally due to factors such Color is usually associated with poor water quality by
as snowmelt, seasonal agricultural practices, and weather. the public. In general this assessment is correct, although
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Water Resources 733

many natural waters that are suitable as municipal sources drink two liters of water per day. MCLs in surface waters
are slightly colored. are often set on the basis of toxicity to the most sensitive
species present. The water flea Cerodaphnia is commonly
C. Chemical Characteristics of Water used as a test organism for toxicity in water because of
its place in the food chain and ease of cultivation in the
The chemical characteristics of water are most commonly laboratory. Sensitivity of Cerodaphnia to the organophos-
described by the concentrations of a limited number of phate pesticide diazinon is based on the concentration at
dissolved inorganic ions, lumped parameters such as alka- which 50% of Cerodaphnia die in 48 hr (LC50 ) which is
linity, acidity, hardness, conductivity, the aqueous carbon approximately 360 to 600 ng/L. Note that precipitation
dioxide concentration, the radioactivity, and lumped mea- concentrations of diazinon shown in Table IV are in this
sures of the organic content such as biochemical oxygen range. Stream concentrations in areas where diazinon is
demand, as indicated in Table VI. Concentrations of other used are often several times this value.
inorganic ions and specific organic compounds are im-
portant in relation to particular uses. The most common
1. Chemical Characteristics
issues with respect to concentrations of particular ions and
of Water—Inorganic Matter
compounds are related to toxicity. For example, the max-
imum concentration limit (MCL) for arsenic in drinking The most common parameters used to define the inorganic
water of 50 µg/L is based on the average ability to excrete chemical characteristics of water are given in Table VI. As
approximately 900 µg of arsenic per day, the probable in- noted above, four cations (calcium, magnesium, sodium,
take of arsenic from other sources (principally food), and and potassium) and four anions (bicarbonate, chloride,
the assumption that we will follow recommendations to sulfate, and nitrate) comprise most of the inorganic matter

TABLE VI General Measures of Chemical Characteristics of Water Quality

Abbreviation/
Measure definition Description

Cations
Calcium Ca+2
Magnesium Mg+2
The four major cations and anions comprise over 99%
Sodium Na+ of the inorganic chemical mass of most freshwaters.
Potassium K+ The principal chemical characteristics associated with
Anions water are most agricultural and domestic uses of
Bicarbonate HCO3 − defined by these ions.
Chloride Cl−
Nitrate NO3 −
Sulfate SO4 −2
pH − log10 [H+ ] Living organisms survive within fairly narrow pH ranges.
The pH value is important in determining the tendency of
a water to corrode or scale in distribution systems.
Alkalinity [HCO3 ] + 2[CO3 −2 ] Alkalinity measures the ability of a water to neutralize acid.
+[OH− ] − [H+ ]
Hardness 2([Ca+2 ] + [Mg+2 ]) A measure of the tendency of a water to form scale and
consume soap.
Gas
Carbon dioxide CO2 A measure of the corrosiveness of a water and used in
determination of chemical requirements in water
treatment.
Conductivity µS/cm Approximately proportional to the TDS of a water.
Organics
Biochemical BOD A measure of the concentration of biodegradable organic
oxygen demand material present using units of oxygen consumed.
Chemical oxygen COD A measure of the concentration of total organic material
demand present using units of oxygen consumed.
Total organic carbon TOC A measure of the total organic carbon present.
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734 Water Resources

found in natural waters. Typical water quality analyses The pH of most natural waters falls between 6 and 8.5,
are focused on these ions and three other parameters, a region in which the equilibrium between bicarbonate
pH, alkalinity, and hardness. Approximately 20 minor and carbonate is strongly in favor of bicarbonate. In most
ionic species are significant in defining water quality. Sev- cases the bicarbonate concentration is far greater than the
eral species are important because of toxicity. Examples hydroxyl or hydrogen ion concentrations. Thus for most
include arsenic (As− ) and copper (Cu+2 ). Other minor situations the alkalinity is approximately equal to the bi-
ionic species, such as ammonium (NH+ −
4 ), nitrate (NO3 ), carbonate concentration.
−3
and phosphate (PO4 ), are nutrients for plants and al-
gae. Growth of aquatic plants and blooms of algae re- c. Hardness. As noted in Table VI, hardness is a
sult from the presence of nutrients. As bodies of water measure of the tendency of a water to form scale and to
age, nutrients accumulate and the water becomes more eu- consume soap. Hardness is defined as the sum of the Ca+2
trophic, that is, capable of supporting larger quantities of and Mg+2 concentrations measured in equivalents per liter.
organisms. Because both ions have a charge of +2, the relationship
Some inorganic ionic species found in water are pri- can be expressed algebraically as
marily from anthropogenic sources. The majority of these
H = 2([Ca+2 ] + [Mg+2 ]) (8)
ions are heavy metals such as cadmium (Ca+2 ), chromium
(Cr+3 , Cr+6 ), lead (Pb+2 ), and mercury (Hg+2 ) that tend where H = hardness, eq/L.
to accumulate in living organisms until toxic levels are Scale formation results from the precipitation of CaCO3
attained. and Mg(OH)2 . Precipitation of CaCO3 is a particular prob-
lem in heated water systems such as steam power gener-
a. pH. Liquid water is a weak acid and disassoci- ation systems, industrial cooling systems, and home hot
ates into the hydrogen (H+ ) and hydroxide (OH) ions. water systems because the equilibrium between HCO− 3
The equilibrium is strongly toward water within the envi- and CO−2 −2
3 increasingly favors CO3 as the temperature in-
ronmental temperature spectrum. However, the presence creases. Although the solubility of CaCO3 increases with
of other chemicals can alter the equilibrium significantly. increasing temperature, the increase in CO−2 3 availability
In most cases the equilibrium state is characterized by often results in precipitation and scaling. Both Ca+2 and
the negative log of the molar hydrogen ion concentration, Mg+2 react with soap to form a precipitate. The result is
which is defined as the pH of the water. Pure water at that hard waters require more soap for cleaning purposes.
25◦ C has a pH of 7, which means that the hydrogen ion Substitution of synthetic detergents has greatly reduced
concentration is 10−7 mol/L, a very small number. The the problem.
molar concentration of undisassociated water will be ap-
proximately 55.5 mol/L. d. Major cations. The major cations, Ca+2 , Mg+2 ,
The activity of the hydrogen ion makes the pH value Na+ , and K+ are virtually always the dominant chemical
very important. Waters below pH 7 are increasingly cor- species in natural waters. The balance between the divalent
rosive to pipes. The majority of aquatic organisms require cations and sodium is particularly important in agriculture
pH values between 6.5 and 9. Precipitation reactions are because sodium has a very large radius of hydration. Clays
characteristically strong functions of pH. in soils have a net negative surface charge which attracts
cations. When the hydrated sodium ion sorbs onto the clay
b. Alkalinity. Alkalinity has been defined in Table VI surface the clay swells, soil pores become smaller, and soil
and mentioned in connection with other water quality pa- permeability decreases. Waters with a high ratio of diva-
rameters. The quantitative definition of alkalinity is lent cations (i.e., Ca+2 and Mg+2 ) to Na+ ions are highly
   −2   −   +  desirable for irrigation. All four major cations are essen-
A = HCO− 3 + 2 CO3 + OH − H (7)
tial nutrients for life. The United States Food and Drug
where A = alkalinity, eq/L, and [i] = molar concentration Administration has reported that dietary intake of sodium
of chemical species i. should be between 500 and 2400 mg per day. However,
Note that the equivalent weights of the bicarbonate, sodium has been implicated in hypertension and thus peo-
hydroxide, and hydrogen ions are the same as the gram ple with high blood pressure, heart disease, or a family
molecular weights and that the equivalent weight of car- history of stroke are often advised to drink low sodium
bonate is one-half the gram molecular weight. Hence the water as one method of minimizing their sodium intake.
units of alkalinity are in equivalents per liter. Because
constituent concentrations are generally in the millimolar e. Major anions. As described above, HCO− 3 is a key
range, alkalinity is usually reported in milliequivelents per constituent in regulation of the pH of natural waters, and
liter (meq/L). because of the equilibrium relationship with CO−2
3 , HCO3
−
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Water Resources 735

moglobinemia. Nitrate has been shown to be involved in

the microbial production of carcinogenic nitrosamines and
an apparent relationship between high nitrate levels and
gastric cancer has been shown in epidemiological studies.

f. Minor ionic species in water. Virtually every

mineral will be found in water at some location in some
concentration. Ionic species found at lower concentration
and with less frequency than the major ionic species are
termed minor ionic species. In some cases, where solubil-
ity is very low, the term trace species is used. However,
many of the minor ionic species are extremely important
in defining water quality. Arsenic was used above as an ex-
ample in the setting of drinking water standards. A number
of minor ions found in water have human activities as the
FIGURE 8 Reactions and transport processes involved in crown principal sources. Examples include phosphorus and the
rot in wastewater collection systems. Note that the wastewater heavy metals cadmium, chromium, copper, lead, mercury,
must be anaerobic for the sulfate reduction reactions to occur. and zinc.

g. Water quality of rivers and groundwaters.

is a key factor determining the scaling properties of a wa- Comparison of the winter and summer chemical charac-
ter. Sulfate in water is derived chiefly from mineral sources teristics of the Grand River, a small stream in Northeastern
but undergoes microbial reduction to sulfide (S−2 ) in the Ohio, provides some insights into the importance of inor-
absence of oxygen. A number of bacterial species use sul- ganic ionic species and the relationship between chemical
fide as an energy source, oxidizing it to sulfate. The oxi- and biological characteristics of water. Overall, the Grand
dation process also releases two hydrogen ions per mole River is low in dissolved constituents and would be con-
of sulfide oxidized, resulting in the formation of sulfuric sidered of high chemical quality. The data presented in
acid. The sulfur cycle is a particular problem in wastewater Table VII are not completely consistent. For example, no
collection systems where it is responsible for crown rot. information is given on the calcium concentrations, which
In the crown rot process (see Fig. 8) anaerobic wastewa- are certainly significant, and the sum of the dissolved ion
ter partially fills a pipe. Sulfate present in the wastewater concentrations differs considerably from the TDS concen-
serves as an electron acceptor for the oxidation of organic trations on January 16th.
material and is reduced to hydrogen sulfide gas (H2 S). Note that the pH and alkalinity of the Grand River are
The H2 S partitions into the air and then into the aerobic lower in January when the temperature is low. In natural
condensate film on the pipe crown where it serves as an waters, carbonate chemistry is the dominating factor in
energy source for the sulfide oxidizing bacteria. The con- determining pH and alkalinity. When water temperature is
densate is poorly buffered, and the pH will often drop to low the equilibrium between carbon dioxide in the air and
values between 1 and 2 as a result of the sulfide oxida- water shifts toward higher concentrations in the water. In
tion. Corrosion of the pipe crown results from the acidic water there is an equilibrium reaction between four forms
conditions. Crown rot is a serious problem in collection of the carbonate species, dissolved carbon dioxide gas
systems, causing tens of millions of dollars per year in (CO2aq ), carbonic acid (H2 CO3 ), bicarbonate ion (HCO− 3)
damage. and carbonate ion (CO−2 3 ), as indicated by
Nitrate is an important plant nutrient, as noted above, → H2 CO3 ←
→ HCO3− ←
→ CO3−2
CO2aq ← (9)
and is also the cause of methemoglobinemia. Normally the
pH of the human stomach is low enough to prevent growth The CO2 concentration is fixed by the gas–liquid equi-
of nitrate reducing bacteria. However, in infants under six librium relationship. Thus the increased CO2 concentra-
months old and adults with depressed gastric acid produc- tion at lower temperatures results in greater quantities of
tion the stomach pH is higher. Nitrite produced through carbonic acid and a drop in both pH and alkalinity.
denitrification reacts with the iron in hemoglobin to form The total Kjeldahl nitrogen is the sum of the organic
methemoglobin, a form that does not carry oxygen. Accu- and ammonia nitrogen present. In January, TKN is low
mulation of methemoglobin results in the disease methe- and most of the nitrogen present is in the form of nitrate.
moglobinemia, which can be fatal. Nitrate concentrations In July, the TKN concentration has increased nearly six-
as nitrogen above 10 mg/L have been implicated in methe- fold and nearly all of it is organic. The organic nitrogen is
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736 Water Resources

TABLE VII Characteristics of the Grand River near tralize acid, is made up almost exclusively of bicarbonate
Harperville, Ohio, on January 16 and July 31, 1997 in natural waters, and waters of low alkalinity tend to be
Day acidic, as indicated by the pH of 5.4 for the Tampa water.
Constituent Units 16 Jan. 31 Jul. Such waters are corrosive in pipes, and consequently the
pH and alkalinity must be raised prior to discharge into
Flow m3 /sec 7.6 0.4 distribution systems.
Total dissolved solids (TDS) mg/L 128 228 Hardness of the three waters of Table VIII is calculated
Conductivity µS/cm 291 384 in Table IX using Eq. (8). Waters are classified as soft,
Temperature (T) ◦C 0 24 moderately hard, hard, and very hard for hardness values
pH — 7.5 8 of <1, between 1 and 3, between 3 and 6, and greater
Alkalinity (A) meq/L 1.56 2.34 than 6 meq/L, respectively. Thus the Ocala water is hard,
Magnesium, Mg+2 mg/L 4 11 the Tampa water is soft, and the Virginia Beach water is
Sodium, Na+ mg/L 8.4 19 moderately hard.
Potassium, K+ mg/L 1.5 2.9
Bicarbonate, HCO−
3 mg/L 95 143
Sulfate, SO−2 mg/L 22 27 2. Chemical Characteristics
4
Chloride, Cl− mg/L 19 28 of Water—Organic Matter
Fluoride, Fl− mg/L 0.2 0.2 Organic matter in water is derived from the decay of plant
Ammonia, as N mg/L 0.05 <0.02 and animal matter, wastewater discharges, and storm and
Total Kjeldahl nitrogen (TKN) mg/L 0.07 0.4 irrigation runoff containing agricultural chemicals. Nat-
Nitrate and nitrite, as N mg/L 0.41 0.06 ural organic matter (NOM) is composed largely of very
Phosphorus, as P mg/L 0.07 0.04 complex, high molecular weight materials that fall under
Iron, Fe+3 mg/L 0.18 0.024 the general category of humic substances. Most natural
Manganese, Mn+2 mg/L 0.033 0.073 waters contain between 1 and 10 mg/L of NOM. In some
Silica, as SiO2 mg/L 3.1 1.7 cases, the NOM gives water a brownish tint. Of greater im-
Total organic carbon (TOC) mg/L 5.7 7.7 portance are the reactions between NOM and the widely
Dissolved oxygen (DO) mg/L 12 8.4 used disinfectant chlorine (Cl2 ). Disinfection byproducts
produced in these reactions include the carcinogenic tri-
[Source: Water Resources Data, Water Year 1997, Vol. 2, United
States Geological Survey.] halomethanes and haloacetic acids.
Organic matter in wastewater discharges can be clas-
sified as natural or synthetic. Natural organic compounds
probably associated with increased phytoplankton con- in wastewaters might be considered precursors of NOM
centrations, and this conclusion is supported by the in- in that they are composed of proteins, carbohydrates, and
crease in total organic carbon. Note also that both phos- lipids derived from living organisms. Synthetic organic
phorus and nitrate have decreased from January to July. compounds include surfactants, pesticides, herbicides,
Both results would be expected with increased phyto- solvents, and a variety of chemicals related to manu-
plankton growth. facturing of petrochemicals. Importance of organics in
Inorganic chemical characteristics of groundwaters dif- wastewaters is derived from their use as food sources
fer significantly from surface waters, in part because of the by various organisms in the aquatic ecosystem and the
longer time of contact with soil minerals (see Table III) toxicity or mutagenicity of many of the compounds.
and in part because of groundwater being isolated from Wastewater discharges typically increase the supply of
the atmosphere. Characteristics of three shallow ground- organics in receiving waters, with a resulting increase in
waters from the southeastern coastal plain of the United microbial activity. The increase in activity at the lowest
States are given in Table VIII. trophic level results in increased growth and activity at
The three groundwaters described in Table VIII are higher levels. Often species diversity decreases and chemi-
typical of high-quality groundwaters with respect to low cal characteristics such as dissolved oxygen concentration
TDS concentrations and relatively low concentrations of that are important in overall water quality are modified.
the hardness ions, calcium and magnesium. The Ocala, Toxic and mutagenic compounds that are discharged con-
FL, water would be classified as moderately hard and the tinuously often have impacts at extremely low concentra-
Tampa, FL, and Virginia Beach, VA, waters would be clas- tions. The MCLs placed on such compounds are often less
sified as soft. Note the extremely low bicarbonate concen- than 5 µg/L and in some cases are less than 100 ng/L.
tration in the Tampa, FL, water. Such low concentrations Organic molecules in wastewater discharges range
are very unusual. Alkalinity, the ability of a water to neu- widely in size with molecular weights from less than 20
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Water Resources 737

TABLE VIII Characteristics of Three Groundwaters from the Southeastern Coastal Plain of the
United States
Virginia
Constituent Units Ocala, FL Tampa, FL Beach, VA

Temperature (T) ◦C 23.9 23.5 19.5

Conductivity µS/cm 380 195 321
Total dissolved solids (TDS) mg/L 214 121 216
pH 6.9 5.4 7.1
Calcium, Ca+2 mg/L 57.5 8.1 33
Magnesium, Mg+2 mg/L 3.4 1.75 7.2
Sodium, Na+ mg/L 8.1 10.1 21
Potassium, K+ mg/L 1.15 2.80 2.3
Iron, Fe mg/L 0.014 0.155 3.8
Manganese, Mn+2 mg/L 0.0045 0.0065 0.110
Bicarbonate, HCO−
3 mg/L 126 10 154
Sulfate, SO−2
4 mg/L 10.3 17.5 10
Chloride, Cl− mg/L 4.15 18.5 22
Nitrate, NO− 3 as N mg/L 1.2 0.06 0.05
Total phosphorus mg/L 0.28 0.015 0.06
Bromide, Br− mg/L 0.075 0.030 0.14
Silica, as SiO2 mg/L 7.5 5.0 42
Dissolved oxygen (DO) mg/L 2.9 0.35 0.15
Total organic carbon (TOC) mg/L 0.65 3.55 1.8

[Source: Berndt, M. P., et al. (1997). Water Resources Investigation Report 97-423m, USGS.]

to several million. Solubility of organic compounds varies variety of compounds that have been inadvertently or
greatly, as well. Hydrophilic compounds such as ethanol illegally applied to soil. Examples include waste oil and
and acetic acid are completely miscible in water while solvents from home shops, agricultural chemicals washed
polycyclic aromatic hydrocarbons (PAHs) and polychlo- off soil and plants during irrigation or rains, and animal
rinated biphenyls (PCBs) generally have solubilities of wastes from pets and livestock.
less than 1 mg/L. Many of the organic compounds that
impact water quality are volatile, and this fact leads to the a. Measurement of organic concentration. Be-
possibility of the air being a source as well as a sink for cause of the wide range organic compounds present in
these materials. For example, partial combustion of gaso- most natural waters, two approaches to measurement are
line results in the production of a number of compounds taken. Specific compounds may be monitored if they pose
that are soluble in water. Thus surface waters near urban a particular hazard to the environment or in connection
areas contain trace quantities of many compounds through with a use such as drinking. Compounds on the USEPA
exchange with the atmosphere. list of priority pollutants are examples of specific com-
Stormwater and agricultural runoff is generally low in pounds that may require monitoring. Measurement of in-
organic matter concentration relative to municipal and dividual compounds in the complex mixture of organics
industrial wastewater discharges. The organic materials found in most natural waters is usually quite difficult and
present include NOM, pesticides, herbicides, and a wide expensive. Often the compounds of interest are present at

TABLE IX Hardness of Groundwater Supplies in Ocala, FL, Tampa, FL, and

Virginia Beach, VA
Ca+2 concentration Mg+2 concentration
Hardness
Water mg/L mmol/L mg/L mmol/L meq/L

Ocala 57.5 1.44 3.7 0.15 3.18

Tampa 8.1 0.20 1.75 0.07 0.54
Virginia Beach 33 0.83 7.2 0.30 2.26
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738 Water Resources

   −3 
extremely low concentration, and sophisticated methods η1 [Organics] + η2 [O2 ] + η3 NH+
4 + η4 PO4
of extraction, concentration, identification, and quantifica-
bacteria
tion must be applied. An example is provided by gasoline −−−−→η5 [NewCells] + η6 [CO2 ] + η7 [H2 O] (11)
which is often present in both groundwater and surface wa-
ter at trace levels. Gasoline in groundwater is derived from The amount of oxygen required in biological reactions
leaking storage tanks while gasoline in surface waters is is less than in abiotic reactions such as Eq. (10) because
derived from boat engines and the atmosphere. Gasoline is a portion of the organic material is used for produc-
composed of approximately 60 hydrocarbon compounds tion of new cells. Additionally, nutrients such as nitrogen
ranging in size from 5 to 12 carbons. Monitoring the indi- and phosphorus are required as for any process involving
vidual compounds making up gasoline is difficult because growth of living organisms. A BOD test has been devel-
of the generally low concentrations and the difficulty of oped to measure the oxygen consumed through biological
separating out individual organic species among the wide oxidation. The BOD test environment is dilute and the re-
range of compounds normally present. action rates are relatively slow. The standard measurement
In most cases, organic matter in water is measured us- of BOD is based on the amount of oxygen consumed in
ing lumped parameters such as oxygen demand, total or- 5 days at 20◦ C. Thus the organic strength of a water is
ganic carbon, or total petroleum hydrocarbons. The most often given in mg BOD5 /L.
widely used measures are two forms of oxygen demand,
the chemical oxygen demand (COD) and the biochemical b. Recalcitrant organics. Many organic materials
oxygen demand (BOD) tests. Oxygen demand provides are either nonbiodegradable or very difficult to degrade
two types of information, a direct lumped measure of the biologically. Recalcitrance is caused by factors such
transformable organic content based on stoichiometry and as low solubility, branching of the molecular structure,
a measure of the quantity of oxygen that will be required characteristic bond structures such as ethers, and substi-
in treatment or from the environment. Both types of in- tutions such as chloride and nitro radicals. Solvents such
formation can be visualized by examination of Eq. (10) as trichloroethylene (TCE) and methylene chloride have
where the general conceptual form is given first and an become major water quality problems because of their
example of sugar oxidation follows. resistance to biodegradation. TCE is a carcinogen and has
a density of 1462 kg/m3 and water solubility of 1100 mg/L.
η1 [Organics] + η2 [O2 ] → η3 [CO2 ] + η4 [H2 O] (10a)
Methylene chloride is a suspected carcinogen with a dens-
C6 H12 O6 + 6O2 → 6CO2 + 6H2 O (10b) ity of 1326 kg/m3 and a water solubility of 1320 mg/L. The
From Eq. (10), we can see that a stoichiometric relation- compounds are found in groundwater as a result of im-
ship exists between the amount of organic material initially proper handling of and leaking storage tanks. Both comp-
present and the amount of oxygen consumed. In the ex- ounds tend to migrate to the water table as pure liquids,
ample of Eq. (10b), 6 mol of oxygen (192 g) are required settle to the bottom of the aquifer because of their density,
to oxidize 1 mol of sugar (180 g). Thus 1.07 g of oxygen is and gradually dissolve into the water column. Sorption
equivalent to 1.0 g of organic material in this example. The onto soil organics retards the movement and results in
amount of oxygen consumed in a reaction such as given residual soil contamination. TCE can be biologically
in Eq. (10) provides a lumped measure of the amount of degraded through a process known as co-metabolism
organic initially present. Because dissolved oxygen is the which requires another organic compound to serve as both
source of the oxygen used in aquatic reactions, the amount the energy and the carbon source for growth. Relatively
of oxygen consumed is of importance in estimating im- few microorganisms have been found that will degrade
pacts on the aquatic ecosystem. Equation (10) is a model TCE, as the compound is very stable in aquifers and soil.
for abiotic oxidation of organics. Because we rarely know Methylene chloride does serve as a sole source of carbon
what compounds are present in water, a chemical proce- and energy for a few bacteria. The fact that the compound
dure in which the amount of oxygen required to carry out has only one carbon is a limiting factor and the substitu-
the reaction of Eq. (10a) is determined. The oxygen con- tion of a chloride atom increases the recalcitrance. Thus
sumed is called the COD and is reported in milligrams of methylene chloride is also quite stable in aquifers and soil.
O2 required per liter.
Most organic materials found in water are stable. For
3. Chemical Characteristics of Water—Gases
example, sugar will not oxidize spontaneously when dis-
solved in water. However, many organic materials are me- The principal gases of interest in defining water quality are
tabolized by microorganisms, thus we are more often in- oxygen (O2 ), carbon dioxide (CO2 ), and hydrogen sulfide
terested in the amount of oxygen consumed in biological (H2 S). The importance of oxygen relates to its low sol-
oxidation of these materials, as depicted by ubility and its requirement for the respiration of aerobic
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Water Resources 739

organisms from bacteria to fish. Carbon dioxide is im- The Climate Monitoring and Diagnostics Laboratory
portant because the pH and alkalinity of natural waters (CMDL) of the National Oceanic and Atmospheric Ad-
is controlled by carbonate chemistry as noted above. Hy- ministration (NOAA) has reported that the mean atmo-
drogen sulfide results from biological processes in which spheric concentrations of CO2 increased from 330 to
sulfate is reduced to H2 S as discussed above. In addition to 365 ppm between 1975 and 2000. Using Henry’s law,
wastewater, swamps and wetlands often have considerable and assuming that raindrops come to equilibrium with the
sulfate reduction. atmosphere (approximately correct), and an atmospheric
Estimates of dissolved gas concentrations can be made temperature of 10◦ C, we would predict that the CO2aq con-
using Henry’s law (Eq. 12) which provides an accurate centration of raindrops increased from 0.79 to 0.87 mg/L.
estimate of equilibrium conditions for dilute conditions. The pH of rain is usually controlled by the equilibrium
mCi = Pi (12) with atmospheric carbon dioxide. The corresponding pH
values based on Eq. (9) are 5.60 and 5.58. Acid rain re-
where m = Henry’s law coefficient, L · atm/mg,
sulting from absorption of oxides of sulfur and nitrogen
Ci = concentration of gas species i, mg/L, and
may have substantially lower pH values. In soils, acid rain
Pi = partial pressure of gas species i, atm.
may increase the solubility of minerals with the result that
The Henry’s law constant is a function of temperature, toxic ions are introduced into waterways or that mineral
as indicated in Table X. nutrients such as phosphorus are stripped from the soil.
Because oxygen is consumed during the biodegrada- Lakes are often poorly buffered, and continued influx of
tion of organics and the oxidation of inorganics such as acidic water results in pH decreases.
ammonia and sulfide, natural waters are generally not in
equilibrium with the atmosphere. The rate of oxygen trans-
D. Biological Characteristics of Water
fer is usually predicted using Eq. (12), which is based on
the two-film model. Biological characteristics of water are related to the
 
M O2 = K L a C S − C O2 (13) resident population of aquatic organisms. Most of the
properties of concern result from the presence of mi-
where MO2 = mass transfer rate, mgO2 /L · sec, croorganisms. Larger organisms, such as fish, rooted
K L = mass transfer rate coefficient, m/sec, plants, and amphibians such as frogs and alligators re-
a = gas–liquid interface area per unit quire certain types of water quality but do little to change
volume, m−1 , water quality themselves. Transmission of disease by
C S = equilibrium concentration of oxygen pathogenic organisms is the most important biological
with the atmosphere as predicted by
impact of water quality, at least from a short-term hu-
Henry’s law, mg/L, and
CO2 = concentration of oxygen in the water, man perspective. Microoroganisms are important factors
mg/L. in defining water quality and in modifying both physical
and chemical characteristics of water. Daily pH and dis-
Determination of the mass transfer rate coefficient in nat- solved oxygen cycles, seasonal heating and cooling, and
ural water systems is very difficult. Reported values range long-term geological changes are impacted by microbial
from about 0.1 d−1 for small ponds and backwaters to growth.
about 1.1 d−1 for rapids and waterfalls. Equation (12) is
generally used for estimating mass transfer rates between
1. Classifications of Microorganisms in Water
gas and water or volatilization rates from water to gas. The
equation is not specific to oxygen. The principal groups of microorganisms found in water
are bacteria, algae, protozoa, and invertebrate animals.
Bacteria are single-celled organisms, characterized by
TABLE X Henry’s Law Coefficients of Several Gases
(atm/L · mg1 )
the procaryotic cell structure, which is simpler than that
of the other microorganisms, and are characteristically
T, ◦ C O2 CO2 H2 S N2 Air a smaller, ranging in size from about 0.2 to about 3 µm.
0 0.0144 0.00030 0.00015 0.0340 0.0271
Bacteria are often referred to as primary degraders
10 0.0171 0.00042 0.00021 0.0429 0.0344 because they carry out degradation of organic detritus in
20 0.0231 0.00058 0.00027 0.0519 0.0414 water and serve as food sources for larger organisms such
30 0.0267 0.00076 0.00034 0.0596 0.0590 as the protozoa and invertebrates. Algae, like bacteria,
40 0.0300 0.00099 0.00042 0.0669 0.0632 are single celled but they have eucaryotic cell structure
[Sources: Butler (1964), and Perry and Green (1984).] similar to plants and animals. Most importantly, algae
a Based on molecular weight of 28.8 g/mol. are photosynthetic and autotrophic, that is, they obtain
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740 Water Resources

TABLE XI Common Water-Related Diseases

Category and method of

contamination Disease Causative agent Symptoms

Waterborne: ingesting Amebiasis Protozoan Prolonged diarrhea with bleeding, abscesses

contaminated water (Endameba histolytica) of the liver and small intestine.
Shigellosis (dysentery) Bacteria (Shigella, several species) Severe diarrhea
Cholera Bacteria (Vibrio cholerae) Extremely heavy diarrhea, dehydration
Cryptosporidiosis Protozoan
Gasteroenteritis Virus (enteroviruses, parvovirus, Mild to severe diarrhea
rotovirus)
Giardiasis Protozoan (Giardia lamblia) Mild to severe diarrhea, nausea, indigestion,
flatulence
Hepatitis Virus (hepatitis A virus) Jaundice, fever
Leptospirosis Bacteria (Leptospira interrogans) Jaundice, fever, renal failure
Salmonellosis Bacteria (all species of Fever, nausea, diarrhea
Salmonella)
Typhoid fever Bacteria (Salmonella typhosa) High fever, diarrhea, ulceration of
the small intestine
Water-washed: washing Shigellosis (dysentery) Bacteria (Shigella, several species) Severe diarrhea
with contaminated Scabies Mite Skin ulcers
water Trachoma Virus Eye inflammation, blindness
Water-based diseases Filariasis Worms Blocking of lymphnodes, swelling and
(Wuchereria bancrofti, Brugia malayi) permanent tissue damage
Guinea worm Worm (Dracunculus medinensis) Larvae are ingested when water flea
host is swallowed. Worm grows for
one year and then exits through skin
lesion.
Onchocerciasis Worm (Onchocerca volvulus) Disease is transmitted by blackflies.
Human hosts have chronic eye and
skin conditions, blindness.
Schistosomiasis Worm (Schistosoma mansoni, Intestinal and venus damage,
japonicum, haematobium) blood loss in bladder.

[Sources: Hawkes (1971); Salvatto (1982). Madigan, et al. (2000).]

their energy from light and use CO2 for a carbon source. 2. Pathogenic Organisms in Water
Protozoa and invertebrate animals, such as worms,
Pathogenic organisms in water include bacteria, proto-
rotifers, and crustaceans, graze on the bacteria and algae
zoans, worms, and viruses. A list of common diseases as-
and in turn serve as food for larger organisms.
sociated with water is given in Table XI. Many of the dis-
Viruses are nonliving particles composed of a strand of
eases listed in Table XI are transmitted in other ways, such
either deoxyribonucleic acid (DNA) or ribonucleic acid
as by food preparation or through person to person contact.
(RNA) and a protective protein coat and are also found
in water. Viruses are too small and too simple to contain
the molecular machinery necessary to replicate on their
3. Impacts of Microbial Growth on Water Quality
own, which is the essential definition of life, but can infect
an organism and force it to carry out replication of the Environmentally, the largest impact of microorganisms is
viral particles. (Perhaps viruses were the inspiration for on the development of phytoplankton, the microscopic
Jack Finney, author of Invasion of the Body Snachers.) communities of bacteria, algae, and plants that grow near
Viruses are extremely host specific, often infecting only the water surface. Phytoplankton form the lowest level
one strain in a species. Thus human viruses, which all of the food chain and the largest mass of aquatic organ-
cause a disease, will only be found in water as the result isms. The initial state of most natural water systems is one
of a discharge from people. A few viruses cause diseases of minimal mineral and organic content. Very little life
in more than one species. Examples are Hantavirus and can be supported in such waters. Heterotrophic organisms,
Ebola virus, neither of which is a waterborne infection. such as bacteria, have no organic matter to supply energy
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Water Resources 741

and carbon, and autotrophic organisms have little in the safe from accelerated eutrophication due to societal activ-
way of energy sources other than sunlight. Thus photo- ities and near-shore waters are also at risk. For example,
synthetic organisms, the Cyanobacteria and the algae, are the habitat around coral reefs throughout the world has
generally the first organisms to grow in these oligotrophic been damaged in ways that are not entirely understood,
waters. Over long periods of time the photosynthetic or- but the damage is certainly related to discharges from ur-
ganisms accumulate, many die and provide food sources ban and agricultural areas. Approximately 10% of coral
for heterotrophic organisms, and the mineral components reefs throughout the world were seriously damaged in the
are recycled. Nutrients and organic matter are washed into last half of the 20th century.
the water, and the overall accumulation becomes a richer Microorganisms are also factors in the development of
habitat. Diversity of life increases and species present taste, odors, and colors in water, and the biofouling of heat
may change due to competition. This process, called eu- transfer surfaces in cooling systems. Both Cyanobacte-
trophication, will eventually lead to heavy phytoplankton ria and algae release organic compounds that cause tastes
growth, floating algal blooms, and increased deposition and odors in water. Problems usually occur in late sum-
of organic detritus. Species diversity will pass through a mer when drinking water supplies are warmer. Microbial
maximum. When extreme eutrophic conditions develop, fouling of heat transfer surfaces is a very serious problem
oxygen cycling occurs due to net oxygen production dur- industrially. The principal related water quality problem
ing daylight hours and net oxygen consumption during results from the chemicals used for controlling fouling.
nighttime hours. Consumption of CO2 by photosynthetic
organisms during the day is greater than production by IV. MODIFICATION OF WATER QUALITY
heterotrophic organisms, and the pH tends to rise. Dur-
ing the night, when photosynthesis shuts down, a net CO2 Water is pure only at the moment of condensation. From
production occurs that results in pH decreases. The com- that time forward materials in the environment interact
bined oxygen and pH cycling, illustrated for an extreme with each drop of water. As shown in Table IV, rural and
case in Fig. 9, results in a habitat supporting some types of urban rainfall contain a surprising array of chemicals. The
macroscopic organisms and excluding others. Under ex- admonition don’t eat yellow snow might be considered
treme conditions, the dissolved oxygen concentration may conservative in that water quality changes continuously
fall to near zero. Usually such extreme conditions are lo- as each droplet moves through the hydrologic cycle, and
cal, that is, near the bottom or in a stagnant backwater we never really know what may be present. Not all changes
area with little mixing. Only a few types of macroscopic in water quality are deleterious, but the general result of
organisms will survive in such a habitat. The high pH val- contact with other components of the environment results
ues that may occur in highly eutrophic environments limit in water becoming less useful. Municipal, industrial, and
the species diversity and also change the water chemistry agricultural use of water results in severe deterioration of
considerably. Over millennia, naturally eutrophic waters water quality that requires treatment to remove contami-
gradually fill up with highly organic soils and become nants before discharge.
meadows. Human activities have caused rapid increases
in eutrophication of most natural aquatic habitats on the
A. Deterioration of Water Quality
earth. Well advertised cases such as Lake Tahoe on the
California–Nevada border have received a great deal of The deterioration of water quality during passage through
public attention. However, few lakes can be considered the hydrologic cycle can be classified as natural or anthro-
pogenic. Natural deterioration results from contact with
mineral deposits, vegetation and animal wastes, atmo-
spheric deposition, heating and cooling, and growth or
addition of microorganisms. Anthropogenic deterioration
results from the wide range of human activities that result
in discharges to water courses and groundwater. Obviously
there is some ambiguity and overlap in the definitions.
However, two distinct differences between natural and an-
thropogenic deterioration should be noted: the ability or
desirability of control and, in the case of anthropogenic
deterioration, the involvement of synthetic or xenobiotic
FIGURE 9 Dissolved oxygen and pH cycling in highly eutrophic compounds.
waters. Note that the oxygen concentration may fall to zero under Atmospheric deposition resulting from forest fires is
extreme conditions. natural, while deposition from agricultural burning is
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742 Water Resources

anthropogenic. The results are essentially the same, but The importance of anthropogenic deterioration of water
the ability to control natural processes is far less than our quality has been recognized for over 150 years. However,
ability to control anthropogenic processes. Natural dete- the accelerated industrial development of the past 50 years
rioration of water quality would occur even if humans together with an increasing willingness to investigate and
did not exist. We can modify natural deterioration in a an increased ability to measure impacts of discharges has
number of ways, but the long-term impact is unclear. For greatly changed society’s views on water quality manage-
example, managing forests to prevent fires may result in ment. We find synthetic chemicals in significant concen-
increasing the period between fires but larger conflagra- trations throughout the natural water system. Additionally,
tions when fires do occur. Considering longer time scales, we have learned that very low concentrations of chemi-
the resulting erosion and atmospheric deposition may re- cals may cause serious damage to the ecosystem. In the
sult in greater rates of water quality deterioration than if late 1980s, selenium was found to be the source of de-
fire prevention practices were minimal. formed birds and decreased birthrates of resident water-
In evaluating natural water quality deterioration we suf- fowl in Kesterson Reservoir in the San Joaquin Valley
fer from the lack of a clear baseline. There are few pristine of California. The source of the selenium was agricul-
environments that can be used as a reference. For exam- tural drainage from the Panoche Fan, a region on the west
ple, hikers must contend with Giardia lamblia in even the side of the San Joaquin Valley in which the soils had been
most remote places. How long this has been the case is un- washed from a high selenium region of the Coastal Range.
known, but there is little reason to believe that the organism At present, concentrations as low as 2 µg/L are believed to
has not been historically resident in the wild animal popu- be a problem for some bird species. Analogously to sele-
lation. Suspended solids in streams are a natural result of nium, heavy metals, such as copper, are accumulated in the
erosion caused by floods. Beaches are formed by sand car- food chain and very low concentrations can result in seri-
ried downstream in floods. Flood control practices have ous damage to organisms depending on the aquatic habi-
both decreased erosion and flooding and trapped solids tat. Sources of copper in streams include discharges from
in reservoirs, and sand losses from beaches becomes a mines, automobile break linings, and pipes. The maxi-
problem in places such as southern California. mum contaminant limit goal for copper set by the USEPA
Water quality of streams changes during passage for drinking water is 1.3 mg/L, while the recommended
from the source to the final discharge point, as indicated receiving water quality criterion for dissolved copper in
in Table XII. Each of the water quality parameters freshwater is 13 µg/L because of the toxicity to certain
listed in Table XII changes considerably on a seasonal aquatic species. The result is that many drinking waters
basis as well as on a spatial basis. Some knowledge of may not meet discharge standards to sensitive streams.
river geography is helpful in interpreting the data. For More recently, there has been the discovery that many
example, following the temperature of the Colorado River natural and synthetic compounds mimic the actions of
downstream from Cisco, Utah, we see a temperature of hormones and can disrupt reproductive patterns. The most
22.5◦ C which is typical of a large, unshaded stream in late widely noted phenomenon is the feminization of male fish,
summer. The much lower temperature of 11◦ C at Lees but certainly the problem extends throughout the food
Ferry, Arizona, would suggest that water releases from chain. Initially, most endocrine disrupters were believed
Lake Powell were from the lower depths. Similarly, the to be compounds with some structural similarity to natu-
temperature of 13.5◦ C downstream of Lake Mead would ral hormones. More recent work has indicated that a wide
have been from some depth. Finally, the temperature range of materials, including heavy metals such as arsenic
near the international border at Imperial Dam, Arizona, and mercury, may be involved.
represents temperatures in the desert. The dissolved oxy-
gen concentrations should be considered both in absolute
B. Water Quality Modification
terms and in terms of relative saturation. At Cisco, Utah,
the river is 81% saturated with oxygen, while at Lees Water quality is modified to meet the requirements of spe-
Ferry the water is at 67% saturation, which would be cific uses. In most cases, we think of water quality mod-
expected of water from some depth in a reservoir. We note ification in terms of treatment of surface or groundwa-
that the water coming from Armistad Reservoir on the ters for drinking or industrial purposes or of treatment of
Rio Grande River is substantially depleted of oxygen and wastewaters to meet discharge requirements; and discus-
only 19% saturated. Total dissolved solids are expected to sion of these topics is the principal purpose of this sec-
increase as water flows downstream, and this is generally tion. However, water quality can be selectively changed
the case for all for rivers in Table XII. Decreases shown on a large scale through use of reservoirs and other con-
result from the contributions of large tributaries or structed systems, as noted above. Temperature control
averaging of TDS values over time in reservoirs. is one of the most desirable potential benefits of large
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Water Resources 743

TABLE XII Water Quality of Four Major U.S. Rivers in Late Summer 1999
Colorado River Rio Grande River
Below
Below Armistad
Lees Hoover Imperial El Paso, Presidio, Reservoir, Brownsville,
Cisco, UT Ferry, AZ Dam, AZ Dam, AZ TX TX TX TX

Date 8/30/99 8/17/99 8/25/99 8/25/99 8/24/99 8/31/99 9/10/99 8/24/99

Q, m3 /sec 170 560 462 250 34 6 21 6
T, ◦ C 22.5 11 13.5 30 24.5 23 22.5 27
DO, mg/L 7 7.4 7.5 7.3 6.4 5.6 1.6 4.8
pH 8.3 8 8.1 8.2 8.3 7.9 7.2 7.7
SS 3303 2 8 146 724 582 622 39
TKN, mg/L ND 0.2 0.2 0.3 0.8 0.7 0.2 0.4
P, mg/L 1.86 ND 0 0.02 0.37 0.08 0.01 0.15
DOC, mg/L 3.4 3.1 2.7 2.7 3.8 4.8 2.3 3.9
Turb, NTU 910 0.3 0.4 1.6 90 150 1.8 22
TDS, mg/L 636 438 563 688 547 1512 622 628

Missouri River Mississippi River

Culbertson, Pierre, Omaha, Hermann, Jordan, Clinton, Thebes, St. Francisville,
MT SD NE MS MN IO IL LA

Date 9/7/99 8/17/99 9/14/99 9/14/99 9/3/99 9/9/99 9/1/99 9/14/99

Q, m3 /sec 287 939 1492 2003 65 1149 4511 5050
T, ◦ C 17 21 23.5 23.1 21.6 22 26.9 28.2
DO, mg/L 8.1 8 7.8 8.1 8.2 7.5 8.2 7.5
pH 8.4 8.4 8.4 7.8 8.2 8 8 7.8
SS 269 1 267 144 218 42 102 70
TKN, mg/L 0.3 0.3 0.5 0.5 1.3 0.7 0.8 0.5
P, mg/L 0.14 ND 0.19 0.19 0.23 0.16 0.24 0.18
DOC, mg/L 2.9 3.3 3.7 3.9 6.2 7.8 5.6 3.7
Turb, NTU 42 0.3 15 27 — 10 16 32
TDS, mg/L 352 509 545 514 23 221 383 311

[Source: US Geological Survey, National Stream Quality Accounting Network, www.water.usgs.gov/nasqan.]

impoundments. Drawing water off at selected depths al- The most important contaminants in most surface waters
lows improved support of fish migration, germination of are particulate matter and microorganisms. Other contam-
seeds, and protection of ecosystems. Reservoirs also act inants of concern include metal ions, surfactants, solvents,
as sediment traps. Downstream waters generally have low petroleum products, and agricultural chemicals.
turbidity. A negative impact is that gravels trapped in reser-
voirs are required for breeding areas of many species and,
a. Removal of suspended solids. Particulate mat-
as noted, loss of sand necessary for beach maintenance.
ter in surface waters ranges in size from colloidal to large
rocks. The rocks are removed quite easily, but colloid re-
moval requires four distinct steps: coagulation to destabi-
1. Water Treatment
lize the particles, flocculation to promote particle growth,
Treatment of water to modify water quality nearly always and sedimentation and filtration for the actual particle re-
requires a treatment train, that is, a sequence of steps moval. Colloids are kept in suspension by Brownian mo-
in which various constituents are removed to produce a tion. Their small size makes filtration extremely expen-
product water that meets specific requirements. Perhaps sive. Coagulation is a process of particle destabilization by
the simplest case is treatment of surface waters for mu- adding positive counterions to neutralize the net negative
nicipal use. Municipal use requirements are dominated by surface charge of the mostly clay colloids. A second ap-
the aesthetic and health requirements for drinking water. proach is to form a precipitate in which the colloidal
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744 Water Resources

particles are enmeshed. The most common coagulant used is very effective for a wide range of organisms and that
is alum (Al2 SO4 · 18H2 O). Other coagulants in use include a residual concentration of chlorine can be maintained in
ferric chloride (FeCl3 ), lime (CaOH2 ), and organic poly- the distribution system. The contact time necessary varies
mers. In flocculation, the destabilized colloids are sub- with the organism of concern and the chlorine concentra-
jected to gentle mixing that results in particle collisions. tion. Fecal coliforms are relatively sensitive to chlorine,
On collision, the particles are bound by van der Waals while the protozoan Cryptosporidium parvum is quite re-
forces and gradually grow to a size where they will settle sistant. Thus the use of fecal coliforms as indicator organ-
from the water under quiescent conditions. Similar pro- isms has its limits.
cesses occur in natural waters and account for the accu- New approaches to monitoring biological quality of
mulation of muds in many estuaries. drinking water will be developed using techniques of
In sedimentation, particles settle to the bottom of qui- molecular biology. At present, the methods require too
escent tanks under gravitational forces, and treated water much time and are too expensive for regular monitoring.
is removed at the surface. Typical hydraulic loading rates, Additionally, there is not a generally accepted measure of
based on tank surface area, are 35 to 45 m3 /m2 · d. Particles probable presence of pathogens at the present time.
reaching the bottom are scraped to a sump and the result-
ing slurry, or sludge, is pumped to a thickening and drying
c. Metal ion removal. Two types of metal ion prob-
facility. The product water from sedimentation usually has
lems exist in water: the presence of ions that cause aes-
a turbidity of a few NTU. Granular media filtration is used
thetic problems and the presence of ions that are toxic.
to remove remaining particles down to a size of approx-
Aesthetic problems include hardness and staining. Hard-
imately 1 µm. Following filtration the turbidity should
ness is due to the presence of multivalent cations, princi-
be less than 1 NTU and the bacterial counts are typically
pally calcium and magnesium. These ions react with soap
reduced by 99% (commonly referred to as two logs of
to form a precipitate and cause scaling in hot-water pipes.
removal, based on base 10 logarithms).
Removal of hardness can be accomplished by precipita-
tion of the metals as calcium carbonate and magnesium
b. Disinfection. Microorganisms in water are killed
hydroxide or by ion exchange. Both methods are rela-
using a variety of disinfection processes. Because mon-
tively inexpensive. The principal cause of staining is the
itoring each human pathogen is logistically impossible,
presence of reduced iron and manganese in groundwa-
indicators of the possible presence of pathogens are mon-
ter. Concentrations of iron and manganese are rarely more
itored. The most commonly used indicators are fecal co-
than a few milligrams per liter and both ions are rapidly
liforms, bacteria known to make up a large fraction of
oxidized and precipitated by contact with molecular
the flora in the gut of warm-blooded animals. Presence of
oxygen.
these organisms is taken as evidence of recent contamina-
Toxic metals in water include arsenic, barium, cad-
tion by feces from warm-blooded animals and the possible
mium, hexavalent chromium (Cr+6 ), lead, mercury,
presence of human pathogens. Fecal coliforms are com-
selenium, and silver. The most common sources are natu-
monly present in surface waters with typical most proba-
ral mineral formations, and consequently, toxic metals are
ble numbers, or MPNs, of 1000 to 10,000 per 100 mL of
most commonly associated with groundwater. Removal
sample. Following disinfection, the MPN values should
using ion exchange processes or by reverse osmosis is
average less than one per 100 mL. One of the reasons that
most common.
turbidity is an important parameter in drinking water qual-
ity is that bacteria attach to particles and are shielded from
disinfection. d. Organic matter removal. Organic material is
Chlorine gas is the most widely used disinfectant. Other rarely present in large quantities in natural waters, either
forms of chlorine, such as chlorine dioxide, are also used. surface or ground. The principal problems associated with
Monochloramine (NH2 Cl) and dichloramine (NHCl2 ) are organic matter in drinking water sources is the presence of
used to a limited extent. Ozone is used extensively in surfactants, solvents, petroleum products, and agricultural
Europe but less so in the United States. Ultraviolet ra- chemicals. Allowable concentrations of these materials in
diation is becoming the disinfection method of choice for finished water is generally less than 100 µg/L, and in many
filtered wastewaters because fewer problematic disinfec- cases, the maximum concentration limits are in the ng/L
tion by-products are formed. range. Sorption onto activated carbon is the most widely
A disadvantage of chlorine is that reactions with or- applied treatment method. Some contaminants, such as the
ganic matter, particularly humic materials, produce car- components of gasoline, are volatile and can be removed
cinogenic trihalomethanes such as chloroform and bro- using air stripping. Reverse osmosis is also used to remove
moform. Advantages of chlorine are that the compound organic matter from water in some cases.
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2. Wastewater Treatment clay are normally present. Biodegradable particulate mat-

ter that is removed by physical processes undergoes anaer-
Two principal objectives form the basis for wastewater
obic biological treatment in a separate process stream.
treatment: protection of public health and protection of
The purposes of biological treatment of the solids are to
the aquatic environment. Wastewater treatment systems
reduce the volume of sludge and to stabilize the organic
are similar to water treatment systems in many respects.
solids into nonputrescible materials. A benefit of biologi-
In most cases, a treatment train must be used, and re-
cal solids treatment is the production of methane gas that
moval of suspended matter is very important. Toxic ma-
can be used for heating or power generation.
terials, including carcinogens, are sometimes present, and
maximum concentration limits in the discharge for these b. Biological wastewater treatment. In biological
materials are often very low. Two major differences be- wastewater treatment organic material is oxidized by mi-
tween water and wastewater treatment are that biodegrad- crobial communities maintained in either a suspended
able organic material in municipal and many industrial growth or an attached growth reactor. Both types of re-
wastewaters is often present in relatively high concentra- actors make use of mixed cultures, that is, cultures in-
tions and that nutrients such as nitrogen and phosphorus cluding a number of microbial species. Such systems are
that may cause eutrophication of receiving waters may be self-optimizing in that the most competitive organisms
present. A simplified schematic of a typical wastewater for a particular set of environmental conditions domi-
treatment system is shown in Fig. 10. nate the culture. If the environmental conditions change
Because of the diverse nature of the organic mat- (e.g., if temperature rises or falls) the population make-up
ter in wastewaters, a lumped, surrogate parameter must will shift in species dominance. Bacteria are the dominant
be used to monitor the concentration. The parameter of group of microorganisms involved in biological wastewa-
choice, as noted above, is oxygen demand because oxy- ter treatment. Higher organisms, such as protozoa, fungi,
gen is consumed when biodegradation takes place, and the and invertebrates are present as components of the com-
relationship between organic matter initially present and munity but have mostly indirect impacts on the process
oxygen consumed on degradation should be stoichiomet- performance.
ric. Additionally, dissolved oxygen is very important in The general concept of biological treatment processes
maintaining aquatic ecosystems. Using oxygen demand can be understood by examination of Eq. (11), repeated
to measure organic concentration allows estimation of the below:
damage that might be caused by discharge of a particular    −3 
wastewater. η1 [Organics] + η2 [O2 ] + η3 NH+ 4 + η4 PO4
bacteria
−−−−→η5 [NewCells] + η6 [CO2 ] + η7 [H2 O] (11)
a. Particulate matter removal. Particulate matter in
wastewater is removed by sedimentation, and, if necessary, Biodegradation of organic material in wastewater has
filtration. Because turbidity standards are rarely placed on two principal products, new cells and CO2 . The new
discharges, coagulation and flocculation are usually not cells are a waste product and must be removed from the
required. In municipal wastewaters the particulate mat- treated water because they are a form of reduced carbon
ter is principally organic matter, although some sand and compound and will impose a load on the receiving water.

FIGURE 10 Schematic diagram of typical wastewater treatment system. The secondary system schematic diagram
is for an activated sludge process, a type of suspended growth process. Attached growth systems recycle the effluent
stream rather than the settled solids stream.
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746 Water Resources

Carbon dioxide is the most oxidized form of carbon and concentrations from a well operating plant should be be-
imposes no load on receiving waters. Note that oxygen, low 20 mg/L and are often below 10 mg/L.
nitrogen, and phosphorus are required by the bacteria car- Biological processes are subject to upset due to toxic
rying out the reaction. Oxygen is the terminal electron ac- contaminants, overloading, or limiting nutrients. Upset
ceptor for the reaction process, and nitrogen and phospho- conditions are defined by a poor effluent quality and a
rous are components of the bacterial cells produced. Other change in the microbial culture that results in poor set-
trace elements are required also, but these are normally tling and high effluent suspended solids. Often the changes
present in wastewater in satisfactory quantities. Municipal in the microbial culture are difficult to correct, and re-
wastewaters usually have excess concentrations of nitro- turning the process to good operation may take several
gen and phosphorus and thus these materials need not be weeks.
added during treatment. Many industrial wastes are nitro-
gen or phosphorus limited, and one or both must be added.
d. Nutrient removal. Un-ionized ammonia nitrogen
In suspended growth, or activated sludge, processes the
is very toxic to fish and other forms of aquatic life. For this
microbial culture is grown as flocculent particles, or flocs,
reason nitrification, the oxidation of ammonia nitrogen
and kept in suspension by the action of the aeration system.
to nitrate, is a common requirement placed on wastew-
Two types of aeration systems are in common use: diffused
ater discharges to surface waters. Excess nitrogen and
aeration and surface aeration. In diffused aeration, air, or
phosphorus in wastewaters is available to organisms in
in some cases pure oxygen, is injected at the bottom of the
receiving waters and often is the source of algal blooms in
reactor as small bubbles. Oxygen is transferred from the
lakes, reservoirs, and rivers. Additionally, nitrate in water
bubbles into the liquid phase where it is used by the mi-
presents a health problem for infants, as described above.
croorganisms. The rising bubbles mix the liquid and keep
Thus, in some cases removal of nitrogen from wastewater
the organisms in suspension. The mixed liquor composed
discharges is required.
of wastewater and microbial community leaves the reactor
Nitrification can be achieved using a conventional bio-
and flows to a secondary sedimentation tank where the mi-
logical treatment process operated at a low loading rate.
crobial flocs and the treated wastewater are separated. A
Nitrate, the product of nitrification, serves as an electron
portion of the settled floc equal to that grown in the cycle
acceptor, similar to the role of oxygen, but the reduced
is discarded as waste sludge to the solids treatment stream,
product is molecular nitrogen. By arranging the biological
and the rest is returned to the aerated reactor for reuse.
treatment sequences appropriately, nitrate nitrogen pro-
In attached growth processes, typified by trickling fil-
duced through nitrification can be reduced to molecular
ters, the microbial community is grown on inert pack-
nitrogen through denitrification. This process sequence is
ing such as rock or shaped polystyrene. Wastewater is
in extensive use worldwide.
sprinkled over the top of the packing and flows down
Phosphorus can be removed by precipitation with lime
and through the reactor as a thin liquid film. Air flows
or iron. However, biological removal of phosphorus is in-
in the interstices of the packing, and oxygen is trans-
creasingly utilized. In biological removal of phosphorus,
ferred from the air into the liquid film and then into the
the process sequence is set up to encourage the growth
biofilm where the degradation reactions occur. Microbial
of polyphosphate accumulating organisms, bacteria that
growth, as indicated in Eq. (11), results in increasing
store polyphosphate in very high concentrations. These
biofilm depth. Eventually, the weight of the biofilm re-
organisms are harvested, which effectively removes ex-
sults in sloughing of small portions of the biofilm. The
cess phosphorus from the treated wastewater.
sloughed biofilm particles are removed in the secondary
sedimentation tank and treated in the anaerobic sludge
digester. e. Tertiary wastewater treatment. Primary and
secondary treatments are focused on the removal of par-
c. Performance of biological treatment systems. ticles and biodegradable organic matter. Metal ions and
Biological treatment is expected to remove 85% of the many synthetic organic compounds pass through these
entering organic matter measured as BOD and suspended processes virtually intact. Generally the concentrations of
solids. Most biological processes exceed this level of per- these recalcitrant materials are quite low. However, many
formance because virtually all of the biodegradable ma- of the materials are major environmental hazards even at
terial entering is at least partially degraded. Additionally, extremely low concentrations, either directly or because
most of the BOD leaving the secondary sedimentation tank they accumulate in the food chain. Appropriate methods of
consists of particulate material that has not settled. Influent treatment to remove these materials include ion exchange,
organic concentrations are usually in the 200 to 350 mg precipitation, adsorption on activated carbon, microfiltra-
BOD5 /L range while effluent BOD5 and suspended solids tion, nanofiltration, and reverse osmosis.
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Water Resources 747

C. Stormwater Treatment range of other uses. However, cooking and drinking are the
dominant factors in setting standards for municipal water
Stormwater presents a difficult treatment problem because
quality. Protection of human health will continue to be
of the high volume flows generated over short time peri-
the dominating factor in selection of municipal water sup-
ods. In the Southwestern United States, a high fraction of
plies and in the management of treatment and distribution
the annual rainfall may occur in 10 to 15 storm events.
systems. Water quantities required for municipal use are
Contaminants that collect on soil and paved surfaces over
highly variable but average a little under 400 L/person · d
a periods of weeks and months are washed off in a short
in the United States. We are advised to drink about 2 L/d,
time frame, and concentrations in the initial flow can be
and therefore the quality requirements for a tiny fraction of
quite high. Much of the contaminant load is soil related.
the municipal water supply set the standard for the entire
However, animal droppings, litter, dust from break lin-
volume.
ings, herbicides, pesticides, solvents, spilled petroleum
products, sanitary sewer overflows, and similar materials
are also present. Dust from break linings includes cop-
per which is hazardous to aquatic communities at very 1. Drinking Water Quality
low concentrations. Animal droppings contain microor-
Drinking water quality requirements have been focused on
ganisms and viruses that may cause human diseases. All
elimination of pathogenic organisms for over 100 years.
of the spills and accidents that occur in urban life leave
Introduction of filtration in 1906 and chlorine disinfection
residues of materials that are potentially dangerous to hu-
in 1913 resulted in a very rapid drop in the incidence of wa-
mans or the aquatic environment. Much of the stormwater
terborne diseases in the United States. The use of coliforms
runoff is classified as non-point source flow. That is, the
as an indicator of the possible presence of human contam-
point of entry into water channels is distributed over the
ination has clearly been successful in reducing the inci-
channel which makes treatment extremely difficult. Man-
dence of waterborne disease. At present, the standard for
agement of stormwater has until recently been focused on
treated drinking water is that less than 1% of the samples
flood control and the focus has been on moving the runoff
will be positive for coliform organisms. In most years there
through urban areas as rapidly as possible. For these rea-
are 20 to 30 incidents in which at least two people become
sons, management of stormwater quality is a problem that
ill from the same water source, and the total number of ill-
has not been solved. Construction of facilities to treat large
nesses is annually 2000 to 3000. An exception occurred in
volumes of water that occur periodically to a very high
March 1993 in Milwaukee, Wisconsin, when 400,000 peo-
standard will not be easy. At present the favored proce-
ple were infected with Cryptosporidium, an organism with
dure in arid regions is to pond the water and percolate it to
considerably more resistance to chlorine than coliforms.
groundwater aquifers. However, this approach will deprive
Cryptosporidium was virtually unknown prior to 1990 and
near-shore ecosystems of needed freshwater and poten-
is completely absent from textbooks on water quality prior
tially contaminate aquifers. Solutions to the stormwater
to that date. Thus the utility of the coliform standard must
quality management problem will need to be developed
be questioned, although an adequate substitute is yet to be
but the appropriate direction is not clear at this time.
found.
Drinking water supplies must also be safe in terms of
chemical constituents. Of particular concern are heavy
V. REQUIREMENTS FOR WATER metals, chlorinated hydrocarbons, and agricultural chem-
QUANTITY AND WATER QUALITY icals. Periodic monitoring of public water supplies is re-
quired to determine if contaminants determined by the
Water use can be classified as municipal, environmental, USEPA to be priority pollutants are present above their
agricultural, and industrial. Each use of water has both respective MCLs.
quantity and quality requirements. These requirements Aesthetic quality of drinking water is extremely impor-
provide a framework for selection of municipal water sup- tant also. Turbidity requirements for public health result
plies, setting recreational water, aquatic environment stan- in a clear, attractive water in most cases. However, high
dards and wastewater discharge, operation of reservoirs, dissolved solids concentrations, while safe, result in wa-
and selection of crops and other plantings. ter with a slightly alkaline taste. Water containing several
milligrams per liter of humic material often has a brown-
ish tint. Some groundwaters contain small concentrations
A. Municipal Water Supplies
of sulfides that result in objectionable odors. Such aes-
Municipal water is used for irrigation, car washing, dish- thetic characteristics usually result in large bottled water
washing, toilet flushing, bathing, cooking, drinking, and a sales.
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748 Water Resources

2. Quality Desired for Domestic Use environment. The aquatic environment provides a base-
line reference for water quality standards. Many aquatic
Desired characteristics of water used for nondrinking do-
organisms are very sensitive to heavy metals, pesticides,
mestic use include low hardness, a slightly alkaline pH
and other materials washed from the urban environment by
to prevent corrosion in the distribution system, and an
storms or found in wastewater discharges. Insect larvae are
absence of ions that are toxic to plants. An example of a
often characterized by the need for high dissolved oxygen
toxic ion is boron which is toxic to citrus at concentrations
concentrations, and most organisms can thrive over a lim-
below 1 mg/L.
ited pH range. Additionally, many species have adapted to
very specific environmental conditions and can be found in
3. Municipal Demand and Water Conservation only a few habitats. Contaminants, such as the heavy metal
ions arsenic, barium, cadmium, chromium, copper, lead,
Municipal water consumption can be reduced consider- mercury, and zinc, bioaccumulate in the food chain. For
ably by introduction of low water use appliances and care- example, bacteria or aquatic plants assimilate the metals
ful use of water for irrigation. Low-flow shower heads, and store them in higher concentration than in the water.
low-flush volume toilets, low water use dish and clothes Organisms that graze on the bacteria or plants are ex-
washers are extremely effective and have minimal if any posed to larger quantities because they feed over a period
detectable differences from conventional units for the user. of time. The concentrations increase in each food chain
Water consumption can be decreased by approximately step until toxic levels are attained. Bioaccumulation is a
50% with little difficulty. Where irrigation is required for partial explanation for the fact that many aquatic ecosys-
lawns, gardens, and plantings, there is a strong tendency to tem standards are much more stringent than drinking water
overwater, and equally large savings can often be achieved. standards.
Unfortunately water conservation efforts do not have a Changes in the aquatic habitat result from changes in
linear impact on water cost. A major factor in the cost of water availability and flow patterns. Aquatic plants grow
water to the consumer is the capital cost of the distribu- in waters of specific depth ranges. These plants provide
tion system and maintenance costs over time. Pipe size is habitat for a wide range of organisms forming a major part
set by fire protection requirements and thus the distribu- of the ecosystem. Increases or decreases in flow or reser-
tion system costs are not a direct function of consumption voir level resulting from flood control, power generation,
rates. However, because of the cost of developing new or irrigation requirements can destroy an aquatic ecosys-
water supplies and the limited supplies available in many tem very quickly, such as the example of the Nile river
locations, there is a need to practice conservation virtually described above.
everywhere. Urban drainage is now recognized as a serious ecolog-
ical hazard. A complete understanding of the problems
B. Water Quality Standards for Recreation associated with urban drainage does not exist. However,
impacts on the diversity of life in urban waters have been
Water quality standards for recreation are focused on pre- observed, and comparative measurements of the toxicity
vention of waterborne infections. The standards in use are of urban and nonurban drainage indicate that there is rea-
directly derived from drinking water standards. However, son to be concerned.
the levels of contamination are considerably less strict.
In 1986 the USEPA recommended that Escherichia coli
and enterococci be substituted for fecal coliform and fecal D. Wastewater Discharge Standards
streptococci counts as measures of biological water qual- Wastewater discharge standards are structured to protect
ity for contact recreation such as swimming and water public and ecological health and to maintain aesthetic
skiing. Current recommendations are that the steady-state qualities of the nation’s waters. As for municipal water
geometric mean of freshwater samples not exceed 126 per supplies and recreational waters, public health require-
100 mL for Escherichia coli and not exceed 33 per 100 mL ments are based on the bacterial quality, and coliforms,
for enterococci. Marine samples, such as are taken in the fecal coliforms, or Escherichia coli and enterococci counts
surf, should not exceed 35 enterococci per 100 mL. are used as the principal parameters. Toxic ions and com-
pounds are of concern for public health reasons but in
most cases the sensitivity of the aquatic environment is
C. Water Quality of the Aquatic Environment
considerably greater than that of humans, as noted above.
In the United States a water quality plan must exist for In most cases discharge requirements for lumped pa-
every drainage basin. The primary focus of basin plans is rameters are based on running averages of daily compos-
to set standards of water quality that protect the aquatic ite values. For example, the 30-day running average of the
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Water Resources 749

effluent BOD5 value may not exceed 30 mg/L. Maximum protection and recreation is similar to determining the
values for any sample of about three times the 30-day av- value of municipal water. The value of flood protection is
erage are typical. Average value requirements are based based on the probability of floods of particular magnitudes
on the assumption that some variability in treatment pro- and the estimated damage that would result from flooding.
cess operations is inevitable. Discharge standards for spe- Generally, other alternatives such as preventing construc-
cific pollutants tend to be based on single measurements. tion in the flood plain and moving communities currently
For example, copper concentration requirements as low in the flood plain are not investigated. The value of water
as 2 µg that may not be exceeded in any sample are be- for recreation is calculated by the amount of money spent
ing imposed on many treatment facilities and on some on specific recreational activities: for example, the num-
stormwater discharges. ber of visitors to a facility, the prorated cost of their boats,
fishing gear, and water skis, camping, hotel, transporta-
tion, and meal costs and everything else that goes into
VI. WATER RESOURCES MANAGEMENT using a recreational site. Hydroelectric power generation
has a clear market value and is perhaps the easiest of the
The acceptance that water resources management and wa- classical water uses to evaluate.
ter quality management are tightly connected began in the Placing a value on water used for environmental pur-
last half of the 20th century. Prior to that time the disposal poses is perhaps the most difficult task in economic eval-
of urban runoff and wastewater discharges in rivers, lakes, uation of water resources. Obtaining general agreement
and near-shore waters was considered to be a reasonable on the value of maintaining a marsh or wetland area is
use of the resources. As evidence of the importance of virtually impossible. Releasing water from a reservoir to
aquatic ecosystems accumulated, pressure mounted both protect an aquatic ecosystem has a clear cost in terms of al-
to stop pollution and to rehabilitate waters that had been ternative uses of the water. However, these costs should not
damaged. Fortunately aquatic ecosystems have proven to be used to define the value of the ecosystem. Similarly the
be resilient and rivers such as the Ohio and the Willamette value of a mountain valley suitable for reservoir construc-
that were little more than open sewers in the 1950s were tion cannot be assessed simply by the recreational dollars
nearly recovered by 1990. It is now generally accepted spent by visitors or the number of cattle it will support.
that the water quality component is an essential part of any Some value must be placed on maintaining the ecosys-
water resources management plan. Municipal, industrial, tem and some value must be placed on the availability of
and agricultural water supply, flood control, recreation, wild lands to the general public even if they are not used.
and hydroelectric power generation are even more signifi- Clearly, the value placed on environmental water use is
cant issues at the beginning of the 21st century than at the subjective, but that is the case for most other uses as well.
beginning of the 20th century. Adding the constraints im- The principal problems in setting the value of water
posed by water quality requirements to the increasing de- occur in arid regions such as the Southwestern United
mand for water makes water resources management ever States. Some help in developing management strategies
more challenging. is provided by regulatory agencies and legislation that re-
Water resources management programs are nearly al- quire maintenance of water quality and aquatic ecosys-
ways based on economic evaluation of alternative uses. tems. Laws and regulations form a set of constraints on
Assigning value to specific water uses is a difficult pro- water use and set priorities among water uses and indi-
cess. People are willing to pay over one dollar per liter rectly define the value of water for environmental uses.
for bottled drinking water that has no more intrinsic value
than tap water costing one cent. As municipal water prices
A. Conservation as a Management Strategy
increase, more lawns will go brown in the summer and a
value becomes somewhat defined for that component of The most severe problems with the volume of munici-
municipal water. The value of having water available for pal water use are in arid regions. In the United States the
fire protection can be set using risk factors, insurance costs, Southwestern states fall into this category. West Texas,
and building values. None of these values will be precise New Mexico, Arizona, Utah, Nevada, and California have
and none will be constant in time. Estimation of the value annual droughts of four to eight months duration, and the
of water to agriculture and industry is somewhat easier in entire area is faced with serious long-term water supply
that the cost of water is part of the overall balance sheet. deficits. Other parts of the United States experience pe-
Farmers pay for seed, fertilizer, equipment, fuel, and water riodic droughts that impact municipal water supplies, but
and thus the cost of water is a factor in the price they must these problems can be addressed by increasing storage or
receive for their crops. The value for water is set by the modest amounts of conservation. However, the situation in
ability to make a profit. Determining the value of flood the Southwestern states has reached a point where water
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750 Water Resources

conservation has become a major priority. Competition sider that the Mississippi River receives most of the mu-
exists between municipal, agricultural, and environmen- nicipal and industrial wastewater generated between the
tal uses of water. The value of water for municipal use is Allegheny and Rocky Mountains and the fact that New
much greater than for agricultural use, and one solution Orleans, Louisiana, produces potable water from the mix-
is for municipalities to purchase water from agricultural ture. Planned reuse is focused on a particular application
users or those holding rights to agricultural water. There is and the provision of water that would otherwise not be
no clear place for maintaining environmental uses except available. For example, the groundwater aquifers of the
through legislation and regulation. Approximately 80 to Los Angeles basin in California have been extensively
90% of water use in the Southwestern states is for irriga- overpumped. Treated wastewater from the Los Angeles
tion. Thus competition for water may result in decreases County Sanitation Districts and the Orange County Sani-
in agricultural water use and corresponding decreases in tation Districts is discharged to spreading basins and used
agricultural production. for recharge of the aquifers. Treated wastewater is directly
Considerable opportunities to conserve water exist in reused extensively for landscape irrigation in many areas.
both municipal and agricultural applications. As noted Direct reuse of reclaimed wastewater for irrigation of non-
above, introduction of water-saving appliances can de- food crops is used throughout the world, and in many areas
crease household water consumption by 50% or more. In reclaimed wastewater is used for food crop irrigation. The
arid regions, lawn and garden irrigation often accounts technology is available to produce water of any desired
for the largest fraction of water use. Changing plantings quality, and reclamation of wastewater for potable use is
to drought tolerant plants, decreasing or eliminating lawn feasible.
area, and careful irrigation practice greatly decrease water
consumption. Evaporation losses from swimming pools,
ornamental lakes, and open reservoirs can also be signifi- SEE ALSO THE FOLLOWING ARTICLES
cant sources of water consumption.
A significant reason for conservation in both arid DRINKING WATER QUALITY AND TREATMENT • EN-
and nonarid regions is the cost of wastewater disposal. VIRONMENTAL GEOCHEMISTRY • GREENHOUSE EFFECT
Decreasing domestic use of water results in decreased AND CLIMATE DATA • HYDROGEOLOGY • OCEAN-
hydraulic loading on wastewater treatment plants. The ATMOSPHERIC EXCHANGE • POLLUTION, AIR • SOIL AND
cost of wastewater treatment and disposal is very closely GROUNDWATER POLLUTION • STREAMFLOW • TRANS-
linked to the flow being treated. Thus communities with PORT AND FATE OF CHEMICALS IN THE ENVIRONMENT
an abundant supply of water can also benefit from water • WASTEWATER TREATMENT AND WATER RECLAMA-
conservation. TION • WATER CONDITIONING, INDUSTRIAL • WATER
POLLUTION
B. Wastewater Reclamation and Reuse
In arid regions there is increasing use of wastewater recla-
mation and reuse as part of the overall water resources BIBLIOGRAPHY
management strategy. Wastewater reclamation involves
the treatment or processing of wastewater to a quality level Asano, T., and Levine, A. D. (1998). “Wastewater Reclamation, Recy-
acceptable for reuse. Wastewater reuse may be direct or cling and Reuse: An Introduction,” In Wastewater Reclamation and
indirect. Direct wastewater reuse involves a direct link Reuse (T. Asano, ed.), Technomic Publishing Co., Lancaster, PA.
Bedient, P. B., and Huber, W. C. (1992). Hydrology and Flood Plain
between reclamation and processing. Examples of direct Analysis, 2nd ed., Addison-Wesley, Reading, MA.
reuse include application of reclaimed water for irriga- Berndt, M. P., Galeone, D. R., Spruill, T. B., and Crandall, C. A. (1997).
tion of crops or landscaping, industrial cooling water, and Groundwater Quality in Three Urban Areas in The South Costal Plain
cleaning. Indirect reuse occurs when reclaimed water is of the Southeastern United States, 1992, Water Resources Investiga-
discharged to a stream, impoundment, or aquifer where tion Report 97-423m, U.S. Geological Survey.
Bouchard, D. C., Williams, M. K., and Surampalli, R. Y. (1992). “Ni-
it is diluted by and mixed with freshwater prior to reuse. trate Contamination of Groundwater: Sources and Potential Health
Indirect reuse may be planned or unplanned. At this time, Effects,” J. Am. Water Works Assoc. 84(9), 85–90.
most wastewater reuse is indirect and unplanned. Most Butler, J. N. (1964). Solubility and pH Calculations, Addison-Wesley,
rivers are used as receiving waters for wastewater and Reading, MA.
downstream users of all types are involved in indirect, Davis, S. N., and DeWiest, R. J. M. (1966). Hydrogeology, John Wiley
and Sons, New York.
unplanned wastewater reuse. Department of Health Services (1999). Primary Drinking Water Stan-
It is very important to understand that indirect, un- dards, California State Department of Health Services, Sacramento,
planned wastewater reuse is in widespread practice. Con- CA.
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Encyclopedia of Physical Science and Technology EN017G-821 August 3, 2001 16:50

Water Resources 751

Diazinon Review (2000). National Registration Authority for Agri- Nazaroff, W. W., and Alvarez-Cohen, L. (2000). Environmental
cultural Chemicals, Minister of Fisheries and Wildlife, Canberra, Engineering Science, John Wiley & Sons, New York.
Australia. Pan, N. L., and Schroeder, E. D. (1997). Impact of Storm Water
Engman, E. T. (1986). “Roughness Coefficients for Routing Surface Loading on Santa Monica Bay, Report to the California Department
Runoff,” J. Irrigation Drainage Engineering, ASCE, 112(1), 39–53. of Transportation, Center for Environmental and Water Resources
Ergas, S. J., Kinney, K., Fuller, M. E., and Scow, K. M. (1994). Engineering, University of California, Davis, CA.
“Characterization of a Compost Biofiltration System Degrading Perry, R. H., and Green, D. W., eds. (1984). Chemical Engineers
Dichloromethane,” Biotechnol. Bioengineering 44, 1048–1054. Handbook, 6th ed., McGraw-Hill Book Company, New York.
Eweis, J. B., Ergas, S. J., Chang, D. P. Y., and Schroeder, E. D. (1998). Public Health Service (2001). 9th Report on Carcinogens, Revised
Bioremediation Principles, WCB McGraw-Hill, New York. January 2001, National Toxicology Program, U.S. Department of
Freeze, R. A., and Cherry, J. A. (1979). Groundwater, Prentice-Hall, Health and Human Services, Washington, DC.
Englewood Cliffs, NJ. Tchobanoglous, G., and Schroeder, E. D. (1985). Water Quality,
Heath, R. C. (1982). Basic Groundwater Hydrology, U.S. Geological Addison-Wesley, Reading, MA.
Survey, Water Supply Paper 2220. U.S. Environmental Protection Agency (2000). Draft Implementation
Kaltreider, R. C., Davis, A. M., Laniviere, J. P., and Hamilton, J. W. Guidance for Ambient Water Quality Criteria for Bacteria-1986,
(2001). “Arsenic Alters the Function of the Glucocorticoid Receptor EPA 823-D-00-001, Office of Water, USEPA, Washington, DC.
as a Transcription Factor,” Environmental Health Perspect. 109(3). U.S. Environmental Protection Agency (1999). Enhanced Coagulation
Logan, B. E. (1999). Environmental Transport Processes, John Wiley and Enhanced Precipitative Softening Guidance Manual, EPA
& Sons, New York. 815-R-99-012, Washington, DC.
L’vovich, M. I. (1979). World Water Resources and Their Future Warrick, R. A., Le Provost, C., Meier, M. F., Oerlemans, J., and
(translated by R. L. Nase), American Geophysical Union. Woodworth, P. L. (1996). “Changes in Sea level,” In Contribution of
Madigan, M. T., Martinko, J. M., and Parker, J. (2000). Brock Biology Working Group I to the Second Assessment Report of the Intergov-
of Microorganisms, 9th ed., Prentice-Hall, Upper Saddle River, NJ. ernmental Panel on Climate Change (J. T. Houghton, et al., eds.),
Mansell, B. O. (2000). Biological Denitrification of Nitrate Con- pp. 362–405.
taminated Groundwater in a Microporous Membrane Bioreactor, Water Environment Federation (1998). Standard Methods for the
Doctoral dissertation, Department of Civil and Environmental Examination of Water and Wastewater, 20th edition, Alexandria,
Engineering, University of California, Davis. VA.
Metcalf and Eddy, Inc. (2001). Wastewater Engineering, 4th ed., World Commission on Dams (2000). Dams and Development,
McGraw-Hill Book Company, New York. London.