Water Chemistry

GEO 309
Outline of Presentation
1. Introduction

2. Concentration Scales and Unit of analysis and

3. Natural water quality (Groundwater Composition)

4. Types of Chemical Reactions in Water

5. Geochemical Processes affecting Groundwater Chemistry

6. Water quality parameters

7. Major Ion Chemistry

8. Presentation of Chemical Analyses

1. Introduction
• Water chemistry analyses are carried out to identify and quantify the chemical
components and properties of water samples. The type and sensitivity of the
analysis depends on the purpose of the analysis and the anticipated use of the
water.
• Chemical water analysis is carried out on water to be used for a variety of uses
• In all cases the results of the analysis provides information that can be used to
make decisions or to provide re-assurance that conditions are as expected.
• The analytical parameters selected are chosen to be appropriate for the
decision making process or to establish acceptable normality.
• Analytical methods routinely used can detect and measure all the natural
elements and their inorganic compounds and a very wide range of organic
chemical species using methods such as gas chromatography and mass
spectrometry.
• Water quality
• Expresses the suitability of water to sustain various uses.
• Important component of integrated water resource management.
• Water composition
• Results from both natural and anthropogenic sources.
• May vary within the lateral or vertical limits.
• Difficult to determine. Determination is systematic and statistically controlled.
Hydrogeochemistry
• Study of the chemical composition of natural waters
• Both natural (geogenic) and anthropogenic (human)
• Rain and snow purest forms of natural water
• Chemical composition of natural water varies widely depending upon
climate,geology and hydrology in a specific drainage area (seasonality)
• Chemical composition of water can be affected by human intervention,
withdrawal of water which can alter temperature
• Matter can exist in the subsurface in several ways
- Separate gas or solid phases e.g CO2 gas in the soil zone or minerals that
form the porous medium
- As a separate component of the liquid phase e.g crude oil and liquid
organic contaminants
- Matter dissolved in the water itself (solutes) e.g Na+ and Cl-
• More focus on the latter
 2. Concentration Scales
• An element that dissolves into water (or some other liquid) is called a
solute
• Frequently the individual elements or molecules that are dissolved will
be called dissolved species or constituents
• The liquid it dissolves into is called a solvent
• Molar concentration (M) defines the number of moles of a species per
liter of solution in (mol/L)
• A mole is the formula weight of a substance expressed in grams E.g. A 1
liter solution containing 1.42g of Na2SO4 has a (Na2SO4) molarity of 1.42/
(2*22.99 +32.06+4x16.00) =0.01M
Note: This uses atomic weights for the different elements
• Na2SO4 dissociates completely in water: Na2SO4 = 2Na+ + SO42-
• The molar concentrations of Na+ and SO42- are 0.03 and 0.01 respectively
• Reaction states that one mole of Na2SO4 dissolves to produce two moles
of Na+ and one mole of SO42-
 2. Concentration Scales
• Molal concentration (m) defines the number of moles of a species per
kilogram of solvent (mol/kg)
• This scale for concentrations in dilute solutions is almost the same as molar
concentration (water) but becomes increasingly different for concentrated
solutions
• Equivalent charge concentration is the number of equivalent charges of an
ion per liter of solution with units such as eq/L or meq/L
• The equivalent charge for an ion is equal to the number of moles of an ion
multiplied by the absolute value of the charge. E.g. with a singly charged
species such as Na+, 1M Na+ =1eq/L and with a doubly charged species such
as Ca2+, 1M Ca2+ =2eq/L
• Equivalent charges can also be represented as equivalent charges per unit
mass of solution with units such as eq/Kg
• Mass per unit mass concentrations define a scale in terms of a species or
element per total mass of the system. Many older analyses have been
reported using this scale with concentrations in parts per million (ppm) or
parts per billion).
• More recently, these units of concentration have given way to corresponding
concentrations in mg/kg or µg/kg.
• Again, there is a close correspondence between these last two scales of
concentration. For dilute solutions, 1ppm=1mg/kg=1mg/L
2. Concentration Scales
 2. Concentration Scales
• Recalculation of the results to meq/l is done by multiplying the values in
mmol/l by the charge z of the ion or molecule. Figure below presents
equations for the conversion between different units.
2. Concentration Scales
Unit of analysis
• mg/l milligrams per liter of sample;
• ppm parts per million by weight of the sample;
• ppb parts per billion by weight of the sample;
• mmol/l millimoles per liter of sample (= millimolarity);
• meq/l milliequivalents per liter of sample;
• epm equivalents per million, by weight of sample;
• M molality, moles per kg of H2O;
• mM millimoles per kg of H2O;
• mi molality of i in this text;
• N normality, equivalents per liter;
• EC Electrical Conductivity, in μS/cm (= μmho/cm),
• pH -log[H+], the log of H+ activity.
3.Natural Water Chemistry
Pure water (H2O) does not occur in nature it contains:
• Dissolved solids:
• major cations and anions, minor and trace constituents.
• ranges <10 ppm in rain and snow to > 300,000 ppm in brine
• Dissolved gasses:
• Major gasses: O2, CO2, N2 ; Minor gases of concern: H2S
• ranges from 1 to 100 ppm
• Suspended matter:
• Organic compounds: carbon containing compound
• Concentration ranges from 0.1 to 10 mg/l
• Bacteria and viruses
 3.Natural Water Chemistry
Groundwater Composition
• The chemistry of groundwater depends on the composition of the recharge water, the interactions
between the water and the soil, soil-gas and rocks with which it comes into contact in the unsaturated
zone, and the residence time and reactions that take place within the aquifer
• Naturally occurring, dissolved organic compounds in groundwater could number in the hundreds. They are
typically present in minor or trace quantities
• The important groundwater gases include oxygen, carbon dioxide, hydrogen sulfide and methane
• Dissolved Inorganic Constituents
A classification of the inorganic species that occur in groundwater is shown in Table XX. The concentration
categories are only a general guide. In some groundwaters, the concentration ranges are exceeded
Major constituents commonly referred to as the major ions (Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, SO42-)
The total concentration of these seven major ions normally comprises more than 90% of the total dissolved
solids in the water
• Organic Constituents
Organic compounds are those that have carbon and usually hydrogen and oxygen as the main elemental
components in their structural framework
The species H2CO3, CO2, HCO3-, and CO32-, which are important constituents in all groundwater, however, are
not classified as organic compounds
• Dissolved Gases
The most abundant dissolved gases in groundwater are N2, O2, CO2, CH4 (methane), H2S, and N2O. The first
three make up the earth’s atmosphere and it is, therefore, not surprising that they occur in subsurface water
CH4, H2S and N2O can often exist in groundwater in significant concentrations because they are the product of
biogeochemical processes that occur in non-aerated subsurface zones. The concentrations of these gases can
serve as indicators of geochemical conditions in groundwater
 3.Natural Water Chemistry
Factors that influence natural water chemistry
• Physical, chemical, and biological processes control the chemistry
and chemical evolution of water in natural and contaminated
systems
• Climate – controls rock weathering, vegetation growth hence determine the
type and amount of solutes entering water body
• Geology – the type of rock, degree of weathering, geological structure,
temperature of rocks.
• Biochemical process – life forms and chemical processes
• Hydrologic Cycle – Determines the type and total ionic concentration
• Solutes in atmosphere – reach water via precipitation
• Influence of humans – Solutes directly added to water body, or changing
land use and land cover.
• Mixing and stratification –lake's water quality and ability to support life
are affected by the extent to which the water mixes.
 3.Natural Water Chemistry
Reaction Models
• The state of a chemical equilibrium for a closed system describes a position of maximum
thermodynamic stability
• At equilibrium, there is no chemical energy available to alter the relative distribution of
mass between the reactants and products in a reaction
• Away from equilibrium, energy is available to spontaneously drive a system toward
equilibrium by allowing the reaction to progress.
• Theoretical approaches are available for modelling the chemical composition of a
solution at equilibrium. However, these calculations provide no information about the
time required to reach equilibrium or the reaction pathways involved
• A different approach is required, based on kinetic models of reaction
• Chemical kinetics is the study and discussion of chemical reactions with respect to
reaction rates, effect of various variables, re-arrangement of atoms, formation of
intermediates e.t.c. There are many topics to be discussed, and each of these topics is a
tool for the study of chemical reactions
• Chemical kinetics provide a useful framework for studying reactions in relation to time
and reaction pathways
• A kinetic description is applicable to any reaction, but is necessary for irreversible
reactions or reversible reactions that are slow in relation to mass transport
 3.Natural Water Chemistry
Reaction Models
• Reactions can be categorized into reversible and irreversible
• Most of the reactions that are affecting groundwater chemistry involve reversible reactions, like precipitation and
dissolution of mineral phases and certain oxidation-reduction reactions. Reversible reactions are reactions in which
the system reaches a dynamic equilibrium. For example, lets consider a glass of water with a piece of calcite in it. The
water will dissolve the calcite until it is saturated and can hold no more water. At that point, the system is at
equilibrium. However, the reactions don’t just stop; what happens is that calcite is precipitating and dissolving at the
same rate, so the concentration in the water doesn’t change
• This is the dynamic part of the equilibrium – the reactions still occur, but are proceeding in both directions at an equal
rate. The characteristics of this equilibrium (e.g., how much is in solution, how much is in the mineral, etc.) are based
on the thermodynamics of the system – in other words, these reactions are all about changes in energy in the system,
and they are all about the system trying to reach a state of minimum energy
• A lot of the chemical models that we use to understand these systems are based on the thermodynamics of
equilibrium. These models and approaches to groundwater chemistry ignore reaction rates – in other words, if we
assume that a system is at equilibrium, we know nothing about how long it took to get there or how fast the reactions
are proceeding
• When we study and try to understand the dynamics of geochemical systems, we often make the assumption that the
system is at equilibrium. This is just like a lot of other assumptions that we make – it simplifies things for us and turns
a complicated system into something that we can understand, but in reality it is wrong
• Natural aqueous systems are rarely at equilibrium; instead, they are in a state of moving towards equilibrium at a rate
that is related to the reaction rates. In this case, it is more appropriate to use a kinetic approach to modeling the
chemistry. Kinetic models are appropriate for reactions that proceed slowly
• Some reactions in groundwater systems are irreversible; they just go in one direction. Examples include radioactive
decay, some oxidation-reduction reactions, and degradation of organic contaminants. When dealing with irreversible
reactions, the concept of equilibrium does not apply and we are required to use a kinetic approach
 3.Natural Water Chemistry
Groundwater Composition
• Considerable variation can be found, even in the same general
area, especially where rocks of different compositions and
solubility occur.
• The principal processes influencing water chemistry in aquifers are
- physical (dispersion/dilution, filtration and gas movement),
- geochemical (complexation, acid-base reactions, oxidation-
reduction, precipitation-solution, and adsorption-desorption)
- biochemical (microbial respiration and decay, cell synthesis)
- Groundwater chemistry is also influenced by the effects of human
activities which cause pollution at the land surface because most
groundwater originates by recharge of rainwater infiltrating from
the surface
- Chebotarev Sequence
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Precipitation/Dissolution Reactions
- Dissolution of neutral salts
Dissolution of a neutral salt is a reversible reaction in which the acid/base
equilibria of water are not affected. An example is the dissolution of gypsum:
CaSO4 · 2H2O ↔ Ca2+ + SO4 2–
A typical example of dissolution of neutral salts is the dissolution of evaporites.
These salts are very soluble and their dissolution strongly increases the salinity of
the groundwater. When saline groundwater exfiltrates in closed semi-arid or arid
basins precipitation is likely
- Weathering reactions
In these reactions primary minerals become dissolved or altered and secondary
minerals may be produced. The secondary minerals are closer to chemical
equilibrium with the earth surface conditions than the primary ones. Weathering
reactions are associated with shifts in the acid/base equilibria
Carbonate reactions are reversible. Note that dissolved CO2 is hydrated in
groundwater and exchange with the gas phase may be limited or impossible. CO2
in groundwater is therefore indicated as H2CO3
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Redox Reactions
In these reactions ions electrons are transferred from one species to
another. Redox half-reactions are used to describe the change of an
element from the reduced state to the oxidised state or vice versa.
These reactions may co-occur with precipitation /dissolution
reactions, yielding redox-controlled dissolution/precipitation
reactions.
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Sorption Reactions
- Cation exchange reactions
• Cation-exchange reactions are the result of charge compensation of
negatively charged clay minerals and organic matter. The occupation of
the exchange complex is determined by the aqueous composition
together with the affinity of the exchange complex for the cations
present

- Surface complexation
• Surface complexation reactions are reactions in which species are
transferred from the solid surface to the aqueous phase and vice versa.
They typically occurs for oxides, humic and fulvic acids. Sites at the
surface of the solids are protonated or deprotonated
• Sorption of metals and oxyanions is strongly pH-dependent. Sorption of
oxyanions is strongest at acid to neutral pH, and sorption of metals is
strongest at neutral to alkaline pH. This behaviour is related to
increasing deprotonation of the surface sites with increasing pH, which
gives rise to a gradual change in charge from positive to negative
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Aqueaous Complexing
Aqueous complexing does not involve solid minerals. However, it
indirectly influences reactions with the porous medium. It gives rise
to an increased concentration of species, since the free species
react together to form complexes. The solubility of Ca-carbonate is
increased by the forming of the aqueous Ca-bicarbonate complex:

Ca2+ + HCO3- ↔ CaHCO3+

In groundwater systems aqueous complexing reactions are

instantaneous
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Gas Transfer
Some of the gases dissolved in groundwater are important reactants with the
solid matrix. Consider for example the role of CO2 in dissolving carbonates and
that of O2 as an oxidant of reduced compounds. Gases become dissolved in pore
water proportionally to the partial gas pressure in the associated gas phase:

CO2g ↔ CO2(aq)

where subscripts g and aq refer to the gaseous and aqueous phases.

Gases remain dissolved as long as the sum of partial gas pressures does not
exceed the hydrostatic pressure.
Groundwater can often be considered as closed to gas transfer, i.e., gases that
react are not replenished and gas that is produced remains in the dissolved state
during groundwater flow.
Exceptions to this rule are groundwater near the water table and groundwater in
hydrothermal or volcanic systems.
 3.Natural Water Chemistry
Geochemical Processes affecting Groundwater Chemistry
• Ion Filtration and Osmosis
These reactions occur in clays and other fine-grained sediments in which
the unchanged water molecule can pass freely through the semi-
permeable membrane but the anions are repelled by the negative charge
on the clay layers and the cations remain with the anions to maintain
electrical balance
It is important in large, thick sedimentary basins, in areas with a long
residence time and at interfaces between seawater and fresh water in
clayey deposits

• Radioactive Decay
Radioactive decay and fractionation of isotopes produce radioactivity in
the groundwater and alter the isotopic composition of the water
4. Quality Evaluation of Groundwater
• In specifying the quality characteristics of groundwater,
physical, chemical and biological analyses are normally
required
• Physical parameters: Temperature, Color, Turbidity, Tastes and
Odors
• Chemical parameters:Total Dissolved Solids (TDS), Total
Suspended Solids (TSS), Total Solids, Electrical Conductance (or Specific
Conductance), pH, Dissolved Oxygen (OD) (including Biochemical
Oxygen Demand (BOD)), Hardness, Alkalinity, Dissolved Inorganic
Constituents, Organic Constituents, Dissolved Gases
• Bological parameters: Coliform bacteria
• Analytical Methods
 4.Quality Evaluation of Groundwater
Physical Analysis
• Temperature
• Impinging solar radiation and atmospheric temperature brings about spatial and
temporal changes in temperature, setting up convection currents and thermal
stratification.
• With respect to water quality, one important aspect of water temperature is the
influence it has on dissolved oxygen, the weight or volume of oxygen dissolved in water.
The solubility of a gas in water decreases as the water temperature increases, so warmer
water simply holds less oxygen.
• Temperature is also affecting the various parameters such as alkalinity, salinity, electrical
conductivity. Temperature is also important in other chemical reactions, such as those
involving water pH. In an aquatic system, these parameters affect the chemical and
biological reactions such as solubility of oxygen, carbon dioxide-carbonate-bicarbonate
equilibrium, increase in metabolic rate and physiological reactions of organisms, etc.
Water temperature is important in relation to fish life. The temperature of drinking
water has an influence on its taste.

• Procedure: Temperature measurement is made by taking a portion of the water sample

(about 1litre) and immersing the thermometer into it for a sufficient period of time (till
the reading stabilizes) and the reading is taken, expressed as °C (normally taken as a
field measurement immediately after collecting sample and later when being analyzed
in the lab)
 4.Quality Evaluation of Groundwater
Physical Analysis
• Color
• In natural water, color is due to the presence of humic acids, fulvic
acids, metallic ions, suspended matter, plankton, weeds and
industrial effluents
• Colour in water may be due to the presence of coloured organic
matter, e.g. humic substances, metals such as iron and
manganese, or highly coloured industrial wastes
• Drinking-water should be colourless
• Color is removed to make water suitable for general and industrial
applications and is determined by visual comparison of the sample
with distilled water (Todd, 1980)
• Color in groundwater may be due to mineral or organic matter in
solution and is reported in mg/l by comparison with standard
solutions
• Changes in the colour of water and the appearance of new colours
serve as indicators that further investigation is needed.
 4.Quality Evaluation of Groundwater
Physical Analysis
• Turbidity
• a measure of the cloudiness of water. It is measured by
passing a beam of light through the water and
measuring photometrically the light scattered at right
angles to the beam. Results are expressed in
nephelometric turbidity units (NTU)
• Suspended and colloidal matter such as clay, silt, finely
divided organic and inorganic matter; plankton and
other microscopic organisms cause water cloudiness
• Therefore, turbidity is an indirect measure of total
suspended solids (TSS), even if the correlation will hold
only for the particular sample from which it was derived
 4.Quality Evaluation of Groundwater
Physical Analysis
• Tastes and Odors
Taste and odor can originate from natural inorganic and organic chemical
contaminants and biological sources or processes (e.g., aquatic
microorganisms), from contamination by synthetic chemicals, from
corrosion or as a result of water treatment (e.g., chlorination)
Taste and odor may also develop during storage and distribution due to
microbial activity
These characteristics are subjective sensations that can be defined only in
terms of the experience of a human being.
Quantitative determinations of taste and odor have been developed based
on the maximum degree of dilution that can be distinguished from taste
free and odor free water.
 4.Quality Evaluation of Groundwater
Chemical Analysis
• A complete chemical analysis of a groundwater sample includes the
determination of the concentrations of the dissolved inorganic constituents,
dissolved organic constituents, and dissolved gases.
• The analysis also includes measurement of pH, TDS and specific electrical
conductance.
• Depending on the purpose of a water quality investigation, partial analyses of
only particular constituents will sometimes suffice (be adequate)
• Basic Chemical Parameters
When analyzing groundwater, it is common practice to measure temperature, pH,
alkalinity, total dissolved solids (TDS), and specific conductance (conductivity)
• Total Solids
Total solids is the term applied to the material residue left in the vessel after
evaporation of the sample and its subsequent drying in an oven at a temperature
of 103-105oC. Total solids include Total Dissolved Solids (TDS) and Total
Suspended Solids (TSS).
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Total Dissolved Solids (TDS)
• The total dissolved solids represent the total concentration of dissolved constituents in the water; usually
measured by evaporating a liter of the sample and measuring the weight of the remaining solids
• TDS is measured in mg/L. Shallow groundwater (within 200 meters of the surface) generally ranges from
100 mg/L to 10,000 mg/L TDS (seawater is 35,000 mg/L). The standard limit for drinking water is generally
considered to be less than 1000 mg/L, with less than 500 preferable (although people can tolerate up to
2000 mg/L)
• "Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes
anything present in water other than the pure water (H2O) molecule and suspended solids
• Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as
wood pulp. Waters with high dissolved solids generally are of inferior palatability and may induce an
unfavorable physiological reaction in the transient consumer
• The concentration of total dissolved solids (TDS) in groundwater is determined by weighing the solid
residue detained by evaporating a measured volume of filtered sample to dryness. The solid residue
almost invariably consists of inorganic constituents and very small amounts of organic matter. The TDS
concentrations in groundwater vary over many orders of magnitude. A simple but widely used scheme for
categorizing groundwater based on TDS is presented in Table below.
Class TDS (mg/l)

Fresh Water 0 – 1,000

Brackish Water 1000 – 10,000

Saline Water 10,000 – 100,000

Brine Water More than 100,000

 4.Quality Evaluation of Groundwater
Chemical Analysis
• Total Dissolved Solids (TSS)
Suspended solids are the portions of solids that are retained on a filter of standard specified size (generally
2.0 µ) under specific conditions. Water with high-suspended solids is unsatisfactory for bathing, industrial
and other purposes

• Electrical Conductance (or Specific Conductance)

Specific conductance is a measure of the ability of the sample to conduct electricity. This acts as an
approximation of the TDS, since the electrical conductivity of water is a function of the amount of dissolved
material in the water
It is a measure of the ability of water to conduct an electrical current. This ability depends on the presence of
ions; on their total concentration, mobility, and valance; and on the temperature of measurement
The conductance of groundwater ranges from several tens of microsiemens for water nearly as nonsaline as
rainwater to hundreds of thousands microsiemens for brine water
A rapid determination of total dissolved solids can be made by measuring the electrical conductance of a
groundwater sample. Conductance is preferred rather than its reciprocal, resistance, because it increases
with salt content
Because natural water contains a variety of ionic and undissociated species, conductance cannot be simply
related to total dissolved solids. However, conductance is easily measured and gives results that are
convenient as a general indication of dissolved solids
An approximate relation for most natural water in the range of 100 to 5000 μS/cm leads to the equivalencies
1meq/l of cations = 100 μS/cm and 1mg/l = 1.56 μS/cm (Todd, 1980)
 4.Quality Evaluation of Groundwater
Chemical Analysis
• pH
The balance of positive hydrogen ions (H+) and negative hydroxide ions (OH-) in water
determines how acidic or basic the water is
When analysts measure pH, they are determining the relative concentration (expressed in
exponential or "power" form) of hydrogen ions; the term "pH" comes from the power of
Hydrogen
The pH scale ranges from 0 (high concentration of positive hydrogen ions, strongly acidic)
to 14 (high concentration of negative hydroxide ions, strongly basic)
In pure water, the concentration of positive hydrogen ions is in equilibrium with the
concentration of negative hydroxide ions, and the pH measures exactly 7
pH is defined as the inverse log of hydrogen ion activity in the water (activity is basically
like concentration). For example, a pH of 7 means that the activity (or concentration) of
hydrogen in the water is equal to 1 x 10-7 mol/L. A pH of 5.6 means that hydrogen activity
is equal to 1 x 10-5.6 mol/L
• Dissolved Oxygen
Dissolved oxygen (DO, pronounced) is oxygen that is dissolved in water. It gets there by
diffusion from the surrounding air; aeration of water that has tumbled over falls and
rapids; and as a waste product of photosynthesis
Dissolved oxygen analysis measures the amount of gaseous oxygen (O2) dissolved in an
aqueous solution
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Hardness
It is defined as its content of metallic ions which reacts with sodium soaps to produce solid soaps or scummy residue and
which react with negative ions, when the water is evaporated in boilers, to produce solid boiler scale (Camp, 1963).
It is predominantly caused by divalent cations such as calcium, magnesium, alkaline earth metal such as iron, manganese,
strontium, etc. It is a water quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium.
Hardness is normally expressed as the total concentration of Ca2+ and Mg2+ as milligrams per liter equivalent CaCO3. The total
hardness is defined as the sum of calcium and magnesium concentrations, both expressed as CaCO3 in mg/l. It can be
determined by substituting the concentration of Ca2+ and Mg2+, expressed in milligrams per liter, in the expression
Total Hardness = 2.5 (Ca2+) + 4.1 (Mg2+)
Each concentration is multiplied by the ratio of the formula weight of CaCO3 to the atomic weight of the ion; hence the
factors 2.5 and 4.1 are included in the hardness relation.
There are two types of water hardness, temporary and permanent.
• Temporary Hardness
This hardness in water is due to the bicarbonate ion, HCO3-, and carbonate ion, CO32-, being present in the water. Carbonates
and bicarbonates of calcium and magnesium cause temporary hardness. This type of hardness can be removed by boiling the
water to expel the CO2, as indicated by the following equation:
HCO3- H2O + CO2
Carbonate and bicarbonate hardness is classified as temporary hardness
• Permanent Hardness
It is due to the presence of the ions Ca2+, Mg+2, Fe3+, Cl- and SO4-. Sulphate and chloride cause permanent hardness. This type
of hardness cannot be eliminated by boiling. The water with this type of hardness is said to be permanently hard
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Why be Concerned about Hard Water?
The determination of water hardness is a useful test that provides a measure of quality of water for households and industrial
uses. Originally, water hardness was defined as the measure of the capacity of the water to precipitate soap. Hard water is
not a health hazard. People regularly take calcium supplements. Drinking hard water contributes a small amount of calcium
and magnesium toward the total human dietary needs of calcium and magnesium
Hard water does cause soap scum, clog pipes and clog boilers. Soap scum is formed when the calcium ion binds with the
soap. This causes an insoluble compound that precipitates to form the scum you see. Soap actually softens hard water by
removing the Ca2+ ions from the water. When hard water is heated, CaCO3 precipitates out; this then clogs pipes and
industrial boilers. This leads to malfunction or damage and is expensive to remove
• Identifying Hard Water
The best way to determine whether or not your water is hard is to have it tested. However, you can usually detect hard water
by the evidence in your home, including:
Increased water heating costs due to scale buildup and mineral deposits, and more frequent replacement of hot water
heating elements; Soap scum on bathtubs, shower tiles, and basins; Film left on the body resulting in dry skin and dull, limp
hair; Decreased sudsing and cleaning capabilities of soaps and detergents, resulting in dingy laundry and reduced life of;
Increased buildup of scale on plumbing fixtures and cooking utensils such as a tea kettle, coffee maker, pasta pot, and dish;
and, Clogged pipes or appliances resulting in reduced water flow and increased repairs.
• Treatment
The most common method to treat hard water is through ion exchange water softening. Ion exchange water softening is a
process in which the hardness ions, magnesium and calcium, are exchanged with either sodium or occasionally, potassium
ions.
• Water Softeners
A water softener may use to remove the hardness from hard water. Salt is mixed with water. The Na+ ion from the salt
replaces the Ca2+ ion, but this causes the water to be too salty for drinking. Water that has been softened should be used only
for laundry and bathing
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Alkalinity
Alkalinity refers to the ability of the water to neutralize an acid
This is directly related to specific dissolved species in the water; in most groundwater systems, the
predominant acid-neutralizing species is the bicarbonate ion. Therefore, alkalinity is usually used as a direct
measurement of the concentration of bicarbonate in groundwater
Alkalinity is usually measured in the field, when the samples are taken, because exposure to air can cause
some of the bicarbonate in solution to turn into carbon dioxide and leave the water
It is a total measure of the substances in water that have "acid-neutralizing" ability. pH measures the strength
of an acid or base whereas alkalinity indicates a solution’s power to react with acid and "buffer" its pH — that
is, the power to keep its pH from changing
Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks
(limestone) which are mostly CaCO3
If CaCO3 actually accounts for most of the alkalinity, hardness in CaCO3 is equal to alkalinity
Since hard water contains metal carbonates (mostly CaCO3) it is high in alkalinity
Conversely, unless carbonate is associated with sodium or potassium which don't contribute to hardness, soft
water usually has low alkalinity.
The main sources of natural alkalinity are rocks, which contain carbonate, bicarbonate, and hydroxide
compounds
Borates, silicates, and phosphates may also contribute to alkalinity
Limestone is rich in carbonates, so waters flowing through limestone regions are generally high alkalinity —
hence its good buffering capacity. Conversely, granite does not have minerals that contribute to alkalinity.
Therefore, areas rich in granite have low alkalinity and poor buffering capacity
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Accuracy of Chemical Analyses
In general two types of errors are discerned in chemical analyses: precision or statistical errors and accuracy or
systematic errors. Precision or statistical errors reflect random fluctuations in the analytical procedure
whereas accuracy or systematic errors display systematic deviations due to faulty procedures or interferences
during analysis
The precision can be calculated by repeated analysis of the same sample. It is always a good idea to collect a
number of duplicate samples in the field as a check on the overall procedure
Systematic errors can be tested only by analyzing reference samples and by interlaboratory comparison of
the results. At low concentrations, duplicate analyses may show large variations when the sensitivity of the
method is insufficient
The accuracy of the analysis for major ions can be estimated from the Electro Neutrality (E.N.) condition since
the sum of positive and negative charges in the water must balance:

Where cations and anions are expressed as meq/l.

The sums are taken over the cations Na+, K+, Mg2+, and Ca2+, and anions Cl-, HCO3-, SO42- and NO3-
Differences in E.N. of up to 2 % are inevitable in almost all laboratories. Sometimes an even larger error must
be accepted, but at deviations of more than 5 % the sampling and analytical procedures should be examined
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
- Tables showing results of analyses of chemical quality of
groundwater may be difficult to interpret, particularly
where more than a few analyses are involved
- To overcome this, graphic representations are useful for
display purposes, for comparing analyses, and for
emphasizing similarities and differences
- Graphs can also aid in detecting the mixing of water of
different compositions and in identifying chemical
processes occurring as groundwater moves
- A variety of graphic techniques have been developed for
showing the major chemical constituents; some of the
more useful graphs are described and illustrated
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Stiff diagrams
The Stiff diagrams are plotted for
individual samples as a method of
graphically comparing the
concentration of selected anions
and cations for several individual
samples
The shape formed by the Stiff
diagrams will quickly identify
samples that have similar
compositions and are particularly
useful when used as map symbols to Stiff diagram for Ca-Mg-SO4-HCO3 water (Hem, 1985)

show the geographic location of

different water facies
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Radial Diagrams
The radial diagrams are plotted for
individual samples as a method of
graphically comparing the
concentrations of measured
parameters for several individual
samples
The shape formed by the radial
diagrams will quickly identify samples
that have similar compositions and
are particularly useful when used as
map symbols to show the geographic Radial diagram for Mg-Ca-Na-Cl-HCO3 water (Hem, 1985)

location of different water facies

 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Pie Charts
The pie charts are used to plot the
concentrations ratio of the major
ions (or any combination of
parameters) for individual samples
As with the Stiff and radial diagrams,
the pie chart is used to graphically
compare the concentration ratios of
several measured parameters for
several different samples
The color and patterns used to Pie Chart for Na-Mg-Ca-HCO3 water (Hem, 1985)
identify each parameter are
customizable
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Piper diagrams
The Piper diagram plots the major ions as
percentages of milli-equivalents in two base
triangles
The total cations and the total anions are set
equal to 100% and the data points in the two
triangles are projected onto an adjacent
diamond grid
The diamond shaped grid is used to represent
the composition of water with respect to both
cations and anions
This plot reveals useful properties and
relationships for large sample groups
The main purpose of the Piper diagram is to
show clustering of data points to indicate
samples that have similar compositions
The term hydrochemical facies is used to
describe the bodies of groundwater, in an
aquifer, that differ in their chemical
composition.
The facies are a function of the lithology,
solution kinetics, and flow patterns of the
aquifer Piper diagram for the three different samples (Hem, 1985)
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Piper diagrams
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Durov diagram
The Durov diagram is an alternative to
the Piper diagram
The Durov diagram plots the major
ions as percentages of milli-
equivalents in two base triangles
The total cations and the total anions
are set equal to 100% and the data
points in the two triangles are
projected onto a square grid that lies
perpendicular to the third axis in each
triangle
This plot reveals useful properties and
relationships for large sample groups.
The main purpose of the Durov
diagram is to show clustering of data Durov plot for 3different samples (Petalas and Diamantis, 1999)
points to indicate samples that have
similar compositions
 4.Quality Evaluation of Groundwater
Chemical Analysis
• Presentation of Results
• Scatter Plots
The X-Y scatter plots are the most
simple initial approach to the
interpretation of geochemical data.
Single plots of ion relationship and
parameters that show significant data
can be easily created and patterns are
quickly identified and easily
understood
Both normal scale and log scales are
supported for the x and y axes and
multiplication factors can be applied
to either the x or y element
Element ratios and sums may also be
included for either axes
The scatter plot can be used to plot all
samples in the open database or
selected sample groups Scatter plot for the three different samples

They are calculated and plotted on the

basis of analytical data.
 4.Quality Evaluation of Groundwater
Biological Analysis
• Bacteriological analysis is important for detecting biological
pollution of groundwater
• Biological analysis includes tests to detect the presence of
Coliform bacteria, which indicate the sanitary quality of water for
human consumption
• Because certain coliform organisms are normally found in
intestines of humans and animals, the presence of these in
groundwater is tantamount to its contact with sewage sources
• Because bacteria of the coliform group are relatively easy to
isolate and identify, standard tests to determine their presence or
absence in a water sample are taken as a direct indication of the
safety of the water for drinking purposes
• Coliform test results are reported as the most probable number
(MPN) of coliform group organisms in a given volume of water
• By analysis of a number of separate portions of a water sample,
the MPN is computed from probability tables for this purpose
Water Quality
GEO 309
Introduction
• Quality of water we ingest  quality of life
• Water quality determined by the solutes and gases
dissolved in the water, as well as the matter suspended
in and floating on the water
• Consequence of the natural physical and chemical state
of the water as any alterations that may have occurred
as a result of anthropogenic activity
• Usefulness of water for a particular purpose is
determined by its quality
• Water quality is a term used to describe the chemical,
physical, and biological characteristics of water, usually
in respect to its suitability for a particular purpose
Quality Criteria for Various Uses: Groundwater
• Groundwater is mainly used for drinking, irrigation and industrial
purposes
• Therefore, quality criteria depend on the use of water for a particular
purpose, and quality standards have to be maintained in water supply
for different uses to avoid deleterious effects
• In other words, whether a groundwater of a given quality is suitable for
a particular purpose depends on the criteria or standards of acceptable
quality for that use

• Domestic Use
Groundwater forms an important source of water for drinking and other
domestic purposes. Therefore, groundwater, in general, is safer for use
than surface water especially from the point of view of bacterial pollution;
but the chemical composition of water is also important
Certain chemical constituents become toxic beyond at a particular
concentration, although they may be beneficial in lower amounts
Prescribed standards for drinking-water vary from country to country,
depending upon economic conditions, climate, food habits and geographic
location. There is also conflicting evidence with respect to safe limits for
certain constituents
 Drinking water standards as prescribed by WHO (1984) et al
given

Take into considerations such as social,

economic, environmental, political,
and financial and have to balance a
number of criteria.

Different countries may have different

standards but within the limit of WHO
guideline values.
BOTSWANA STANDARDS
Pollution
• Definition: alterations in the physical, chemical, or
biological characteristic of water to make it harmful to
public health or to the health for other biotic life or to
become unsuitable for use for a purpose it was
previously used for
• Groundwater pollution is a change in the properties of
groundwater due to contamination by microbes,
chemicals, hazardous substances and other foreign
particles
• It is a major type of water pollution
• The sources of groundwater pollutants are either
natural (mineral deposits in rocks) or man-made
• Water pollution occurs when pollutants are discharged
directly or indirectly into water bodies without
adequate treatment to remove harmful compounds
Sources and types of groundwater Pollution
• Based on their origin sources of groundwater pollution can be categorized into point source and
non-point source
• Point source water pollution refers to the types of water contamination which enter the water
system via one certain, identifiable source, such as a pipe or a ditch. This type of contamination
source includes municipal sewage systems and industrial, construction sites, and discharges
from a sewage treatment plant, a factory, or a city storm drain. On-site septic systems; Leaky
tanks or pipelines containing petroleum products
• Leaks or spills of industrial chemicals at manufacturing facilities; Underground injection wells
(industrial waste); Municipal landfills; Livestock wastes; Leaky sewer lines; Chemicals used at
wood preservation facilities; Mill tailings in mining areas; Fly ash from coal-fired power plants;
Sludge disposal areas at petroleum refineries; Land spreading of sewage or sewage sludge;
Graveyards; Road salt storage areas; Wells for disposal of liquid wastes; Runoff of salt and other
chemicals from roads and highways; Spills related to highway or railway accidents; Coal tar at old
coal gasification sites; and, Asphalt production and equipment cleaning sites.
• Non-point (distributed) source contamination refers to the types of water contamination which
are diffuse and not just from a single source. Small amounts of contaminants spread over a large
area, finding their way into the water system, would be considered a non-point source of
contamination. NPS pollution is often the cumulative effect of small amounts of contaminants
gathered from a large area. The leaching out of nitrogen compounds from agricultural land
which has been fertilized is a typical example. Nutrient runoff in storm water from “sheet flow”
over an agricultural field or a forest are also cited as examples of NPS pollution.
• Fertilizers on agricultural land
• Pesticides on agricultural land and forests
• Contaminants in rain, snow, and dry atmospheric fallout
Factors associated to the accelerating freshwater pollution
• – urbanization
• – population increase
• – intensification of agricultural and industrial activities
• – inadequacy of waste control or waste treatments
• – deforestation
• – damming of rivers
• – destruction of wetlands
• – mining
• – increase in primary energy consumption (leading to acidification)
• – burst pipes and tanks
• – major leaks
• – fires and oil spills, and industrial accidents

Pollutant types (from point or non-point sources)

• – pathogens
• – Biodegradable oxygen-depleting organic matter (i.e. biodegradable organic wastes)
• – non-biodegradable organic chemicals
• – suspended sediments
• – inorganic nutrients
• – toxic chemical compound of heavy metals
• – water soluble radioactive isotopes
• – hot water releases causing thermal
• – volatile Synthetic Organic Chemicals
Groundwater Pollution by Organic Contaminants
Organic Water Pollutants Include
• Detergents
• Disinfection by-products found in chemically disinfected
drinking water, such as chloroform
• Food processing waste, which can include oxygen-
demanding substances, fats and grease
• Insecticides and herbicides, a huge range of organohalides
and other chemical compounds
• Petroleum hydrocarbons, including fuels (gasoline, diesel
fuel, jet fuels, and fuel oil) and lubricants (motor oil), and
fuel combustion by-products, from storm water runoff
• Tree and bush debris from logging operations
• Volatile organic compounds (VOCs), such as industrial
solvents
• Various chemical compounds found in personal hygiene and
cosmetic products
Groundwater Pollution by Inorganic Contaminants
Inorganic water pollutants include
• Pre-production industrial raw resin pellets
• Heavy metals including acid mine drainage,
chemical waste as industrial by-products
• Acidity due to industrial discharges like sulphur
dioxide
• Silt in surface runoff due to logging, slash and burn
practices, construction sites or land clearing sites
• Fertilizers in runoff from agriculture including
nitrates and phosphates
• Water chemistry therefore quality alteration
caused-by human activities possesses full potential
to destroy the complete aquatic ecosystems,
making their supplies totally unfit for use even
after treatment
Therefore;
• To avoid such situations environmental awareness
in the entire society is most urgently required.
Examples of contaminants and possible health effects

Contaminant Sources to groundwater Potential health and other effects

Enters environment from natural Causes acute and chronic toxicity,
processes, industrial activities, liver and kidney damage;
Arsenic
pesticides, and industrial waste, decreases blood hemoglobin. A
smelting of copper, lead, and zinc ore. carcinogen.
Decreases incidence of tooth
decay but high levels can stain or
mottle teeth. Causes crippling
Fluoride
Occurs naturally or as an additive to bone disorder (calcification of the
municipal water supplies; widely used bones and joints) at very high
in industry. levels.
Occurs naturally in mineral deposits,
soils, seawater, freshwater systems, the
atmosphere, and biota. More stable
form of combined nitrogen in
Toxicity results from the body's
Nitrate (as oxygenated water. Found in the highest natural breakdown of nitrate to
nitrogen) levels in groundwater under extensively nitrite. Causes "bluebaby
developed areas. Enters the
disease," or methemoglobinemia,
environment from fertilizer, feedlots, which threatens oxygen-carrying
and sewage. capacity of the blood.
Imparts a bitter astringent taste
Occurs naturally as a mineral from to water and a brownish color to
Iron
sediment and rocks or from mining, laundered clothing and plumbing
industrial waste, and corroding metal. fixtures.
Mottle or teeth staining, Dental
Fluorosis
Tables X & X Classification of dissolved inorganic constituents in groundwater.
How Hard is the Water?

The degree of hardness of the water is classified in terms of its calcium carbonate
concentration as follows:

Table 2.3. Hardness classification of water (after Sawyer and McCarty, 1967).

Concentration of Calcium
Hardness rating Carbonate (mg/l)

Soft 0 to < 75

Medium hard 75 to < 150

Hard 150 to < 300

Very hard 300 and greater

Analytical Methods
Evolution of Groundwater chemical composition
Sources of Constituents
Piper Plot
Important processes affecting water quality
• Nutrient
cycling:
movement of
nutrients
(nitrogen,
phosphorus,
carbon, etc.)
within the
environment
• For example-
• Figure 1.
Nitrogen
movement
Table 5. US national drinking water regulation (cont…)

Constituent Recommended level

Chloride 250mg/l
Color 15 color units
Copper 1 mg/l
Corrosively Non corrosive
Foaming agents 0.5 mg/l
Iron 0.3 mg/l
Manganse 0.05 mg/l
Odour 3 threshold odour
number
pH 6.5-8.5
Sulphate 250 mg/l
Total dissolved solids 500 mg/l
Zinc 5.0 mg/l