Environmental Pollution 252 (2019) 1367e1376

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Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol

Oiling of the continental shelf and coastal marshes over eight years
after the 2010 Deepwater Horizon oil spill+
R. Eugene Turner a, *, Nancy N. Rabalais a, Edward B. Overton b, Buffy M. Meyer b,
Giovanna McClenachan a, c, Erick M. Swenson a, Mark Besonen d, Michael L. Parsons e,
Jeffrey Zingre e
a
Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA, 70803, USA
b
Department of Environmental Sciences, Louisiana State University, Baton Rouge, LA, 70803, USA
c
Presently, Department of Biology, University of Central Florida, Orlando, FL, 32816, USA
d
Harte Research Institute for Gulf of Mexico Studies, Texas A&M University - Corpus Christi, Corpus Christi, TX, 78412, USA
e
Coastal Watershed Institute, Florida Gulf Coast University, Fort Myers, FL, 33965, USA

a r t i c l e i n f o a b s t r a c t

Article history: We measured the temporal and spatial trajectory of oiling from the April, 2010, Deepwater Horizon oil
Received 24 January 2019 spill in water from Louisiana's continental shelf, the estuarine waters of Barataria Bay, and in coastal
Received in revised form marsh sediments. The concentrations of 28 target alkanes and 43 target polycyclic aromatic hydrocar-
18 May 2019
bons were determined in water samples collected on 10 offshore cruises, in 19 water samples collected
Accepted 26 May 2019
Available online 12 June 2019
monthly one km offshore at 13 inshore stations in 2010 and 2013, and in 16e60 surficial marsh sediment
samples collected on each of 26 trips. The concentration of total aromatics in offshore waters peaked in
late summer, 2010, at 100 times above the May, 2010 values, which were already slightly contaminated.
Keywords:
Oil residues
There were no differences in surface or bottom water samples. The concentration of total aromatics
Alkanes declined at a rate of 73% y
1 to 1/1000th of the May 2010 values by summer 2016. The concentrations
Aromatics inside the estuary were proportional to those one km offshore, but were 10e30% lower. The oil con-
Louisiana continental shelf centrations in sediments were initially different at 1 and 10 m distance into the marsh, but became equal
Estuaries after 2 years. Thus, the distinction between oiled and unoiled sites became blurred, if not non-existent
Salt marsh then, and oiling had spread over an area wider than was visible initially. The concentrations of oil in
sediments were 100e1000 times above the May 2010 values, and dropped to 10 times higher after 8
years, thereafter, demonstrating a long-term contamination by oil or oil residues that will remain for
decades. The chemical signature of the oil residues offshore compared to in the marsh reflects the more
aerobic offshore conditions and water-soluble tendencies of the dissolved components, whereas the
anaerobic marsh sediments will retain the heavier molecular components for a long time, and have a
consequential effect on the ecosystems.
© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction 2010 (McNutt et al., 2012a, b). This marine oil spill was the largest in
the world history, equal to 11 times the previous largest marine oil
The Deepwater Horizon (DWH) drilling platform collapsed in the spill in the US until then from the grounding of the oil tanker Exxon
northern Gulf of Mexico (GOM) April 20, 2010, killing 11 workers Valdez in Alaska 21 years earlier (0.48  106 bbl; Paine et al., 1996),
and injuring 17 other crew members. It subsequently released 25 times the oil discharges during Hurricane Katrina (Pine, 2006),
4.9  106 barrels (bbl) of oil from 4000 m below the ocean floor in but two-thirds of the land-based 1908 Dos Bocas spill in Veracruz,
1500 m water depth until completely closed off on September 19, Mexico (Santiago, 2006). It followed the first large-scale subsurface
spill in the Gulf of Mexico, the IXTOC I spill (3 million barrels), into
the Bay of Campeche, Mexico, from June 1979 to March 1980. The
DWH accident created a singularly massive oil spill delivered
+
This paper has been recommended for acceptance by Maria Cristina Fossi.
within the background of oil contamination from the Mississippi
* Corresponding author.
E-mail address: euturne@lsu.edu (R.E. Turner).
River, atmosphere, releases from ships, historic and recent oil

https://doi.org/10.1016/j.envpol.2019.05.134
0269-7491/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1368 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376

production platforms and infrastructure, and natural seeps in the ensued (Michaels and Howard, 2012; United States Coast Guard,
GOM (National Research Council, 2002). The daily release of DWH 2011) to contain, remove and mitigate the damage estimated at
oil over 87 days was about 34e128 times higher than the daily $17 billion for natural resources (Bishop et al., 2017) and $37 billion
hydrocarbon release from natural seeps over the entire GOM, for the combined environmental and economic losses (Smith et al.,
which disperses and evaporates quickly (0.16e0.6  106 bbl yr
1; 2011). Tens of billion dollars ($US) in fines and restitution were
MacDonald et al., 2015). made to Federal, State and regional governing bodies, non-
The oil that entered the Gulf from the reservoir changed governmental organizations, and private citizens. Multiple
chemical composition and physical properties as a result of disso- research efforts were funded (Cornwall, 2015; Murphy et al., 2016).
lution, photo-oxidation, incorporation of other elements, etc., to Part of the general framework for the research was to document
become what we call ‘oil residues’ herein. Burning, oil skimmers damages to the ecosystem, potential long-term threats and con-
and 6965 m3 of the dispersants COREXIT 9500A and COREXIT sequences, mitigation and restoration opportunities, and to prevent
9527A were applied in an attempt to disperse oil, hasten decom- future spills. These damage valuations were partially dependent on
position, and disrupt slicks (Kujawinski et al., 2011). Twenty-nine knowing: 1) how much of the original oil and oil residue reached
percent of the 4.9 mb DWH oil dispersed within the water col- the continental shelf, then entered the estuaries, and were depos-
umn, another, 23% evaporated or dissolved, and 23% was unac- ited in marshes, and, 2) if the amounts persisted or changed.
counted for (McNutt et al., 2012a). The oil and its residues were not We document the petroleum contamination after the DWH oil
rapidly flushed out of the GOM because the residence time of the spill in water on the Louisiana continental shelf, in estuarine wa-
deep water is 250 years (Rivas et al., 2015; Chapman, 2017). Oil ters, and in marsh sediments from May 2010 to June 2018. We
residues spread away from the surface near the well head and were compare the changes in total amounts of alkanes and aromatics
found on 1773 km of the 7058 km beaches and coastal marshes that over time, the coherence among concentrations in nearshore shelf
were surveyed from Florida to Texas (Fig. 1; Michel et al., 2013). The and inshore estuarine waters, the initial accumulation in marsh
oiled shoreline was predominantly in Louisiana (60.6%), and 38% of sediments, and estimate the legacy of oil remaining within the
that remained visually evident after two years (Michel et al., 2013). marshes as of June 2018.
The hydrocarbons interfacing with beaches and marshes were
immediately toxic or subsequently generated chronic effects on the
2. Methods
flora, fauna and human population as documented in an extensive
Federal damage assessment (NRDA, 2016). The media produced
2.1. Study area description
disturbing visuals of oiled pelicans, dolphins, turtles, beaches and
marshes (Fig. 1) (Morse, 2012). Much of commercial fishing was
The DWH spill occurred at 1500 m depth in the northern Gulf of
closed causing a >$1 billion deficit to the industry (NMFS, 2010;
Mexico (GOM), a marginal sea of the Atlantic Ocean bordered by
Carroll et al., 2016).
five U.S. states on the northern and eastern border, five Mexican
A massive cleanup effort by 47,489 people and over 6000 vessels
states on its western and southern border, and Cuba to the

Fig. 1. Oil on the continental shelf and marsh. (A) Oil coating marsh vegetation at the shoreline in Bay Batiste, LA. (B) Oil residues, mostly black asphaltenes, at the base of salt marsh
plants; note the overhang with exposed roots beneath. (C) The surface oil and mousse on surface waters in May 2010. D. A line of mousse and oil sheens off the Mississippi River
delta. E. An oiled pelican (Pelecanus occidental - the Louisiana State Bird) covered in oil. A, B and C are photos by the authors. D is from NOAA, and E is from an open source (https://
www.flickr.com/photos/49937499@N08/4667375844)
 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376 1369

southeast. The average depth of the GOM is 1615 m and the glass bottles, refrigerated at 4  C, and extracted within 14 days, as
maximum is 4400 m. There are broad continental shelves off Flor- recommended by the US EPA (2007).
ida, the Yucata
n Peninsula and the northwestern GOM. The volume Water samples for oil analyses were also collected from a small
flux of warm and salty Caribbean water entering through the boat in Barataria Bay (Fig. 2B) and 1 km offshore of the bay entrance
Yucata
n and Florida straits (Mooers and Maul, 1998) is equivalent to on monthly transects during two periods: 1) from May to
a 2.5 year refill rate for the GOM, compared to a 1/50,000th lower December 2010, and 2) for each month in 2013 as part of routine
refill rate (weeks to months) for the shelf volume where coastal monthly sampling (Turner et al., 2019). Sample water (0.5 L and 1 L)
currents mix a smaller water volume (Barron and Vastano, 1994). was collected at 13 inshore stations (station numbers 1e14 in
There is, therefore, a relatively slower water turnover rate (years) in Turner et al., 2019) using a bucket rinsed with surface water, re-
the center of the GOM, compared to turnover rates of weeks-to- filled and immediately put into a commercially-prepared amber
months on the periphery that contains a smaller water volume of glass bottle.
shallower depths. Marsh sediment samples were collected from fixed locations
The surface temperature of the warm and calm winds in sum- with vegetated cover dominated by Spartina alterniflora, Juncus spp.
mer GOM fuels hurricane winds, which have come ashore every- or Schneoplectus americanus. There were three sampling schemes:
where in the GOM. The transfer of water between continental shelf 1) five trips were made to sample 18e30 different sites in St. Ber-
and deeper GOM waters is partially documented, or implied by nard, Barataria and Terrebonne estuaries (n ¼ 183). The May 2010
strong inference (Fry et al., 2015; Turner and Rabalais, 2019). Ninety sampling occurred before oil was observed on beaches or behind
% of the water and 62% and 88% of the terrestrial riverine-derived N barrier islands; 2) there were 14 biannual trips to east and west
and P sources into the GOM, respectively, are from the Mississippi Barataria Bay and Terrebonne Bay from 2011 to 2018 (n ¼ 326); and
and Atchafalaya rivers on the northern boundary. The resulting 3) there were seven trips to the north shore of Bay Batiste (these are
stratification and enhanced nutrients drive the formation and the same sites used by McClenachan et al., 2013) from 2011 to 2018
maintenance of a low oxygen zone off the Louisiana and northern (n ¼ 210). The oiling of these marsh sites from 2010 to 2012 was
Texas coast, which is the second largest human-caused coastal reported in Turner et al. (2014a, b). Only samples collected at 1 and
hypoxic area in the global ocean (Rabalais et al., 2007). It extends 10 m from the shoreline are reported here. All sediment samples
from near shore to as much as 125 km offshore, and in water depths were collected from the upper 5 cm of the sediment surface, stored
up to 60 m (Rabalais et al., 2007. Hydrocarbons from the DWH oil in pre-cleaned amber jars on ice until delivery to the laboratory,
spill moved directly onto this shelf, and some continued into the and either immediately extracted or refrigerated at 4  C for no more
estuaries of the northern GOM into the 10,000 km2 þ of wetlands than 14 days until extraction, as recommended by the US EPA
(Turner and Rabalais, 2019). (2007).
The salinity in the northern GOM estuaries is influenced by
water exchange between the estuarine entrance and the coastal 2.3. Oil analyses
zone, and local forcing (tidal advection, river discharge, precipita-
tion) occurring within the estuary proper. The monthly tidal range We targeted 28 alkanes and 43 polycyclic aromatic hydrocar-
in these estuaries averages 30 cm with a low in winter and high in bons (PAHs) and their respective alkyl homologs (18 parent PAHs,
late summer. Barataria Bay is northwest of the DWH spill site and is and 25 alkyl homolog groups) (herein, “alkanes” and “aromatics”)
adjacent to the west bank of the Mississippi River in southeastern for quantitative analysis using GC/MS-SIM (gas chromatography/
Louisiana, USA, and empties into the Gulf of Mexico and Terrebonne mass spectrometry in selective ion monitoring mode), as well as
Bay is to the west (Fig. 2B). Some of the Mississippi River discharge isoprenoid hydrocarbons pristane and phytane, and cyclic forensic
enters the Barataria estuary from offshore to alter the salinity of the petroleum biomarker compounds like the hopanes, steranes and
lower Barataria Bay and also brings nutrients into the estuary triaromatic steroids (SIM ions 191, 217, 218 and 231 eluting be-
(Wiseman et al., 1990; Wissel et al., 2005; Turner et al., 2019). The tween C23 and C31). The more complete methods description is
majority of all oiled shorelines and most marsh oiling were in found in the Supplemental Materials. Briefly, we used accepted
Louisiana (>60% and 95%, respectively; Michel et al., 2013). standard operating and QA/QC procedures to prevent contamina-
The sample collections described below were used to determine tion and avoid sample degradation. The sediments were speed
the quantity and quality of oil in continental shelf water and one extracted with dichloromethane (DCM) and spiked with surrogate
estuary near the DWH spill site, and in marsh sediments from the recovery standards to achieve a final standard concentration of
spring of 2010e2018. 20 mg mL
1 in extracts. Water samples were liquid-liquid extracted
with DCM. All extracts were concentrated by rotary evaporation
2.2. Sample collection and concentrated under a stream of nitrogen to a final volume of 1-
ml and then 0.5 ml portions were used for injection. The
Water samples without visible signs of oil slicks, sheens, tarballs target alkane and PAH concentrations were determined by an in-
or mousse were collected on the continental shelf from 17 to 19 ternal standard method and response factors calculated from a 5-
May 2010 through July 2018 (Fig. 2). The 375 offshore water sam- point calibration curve using a commercially-available standard
ples were sub-sampled from Niskin water bottles tripped within containing the normal alkanes from n-C10 through n-C35 and the
0.5e1.0 m above the bottom or within 0.5 m of the surface from ten parent PAH analytes of interest. The MS detector was tuned to
cruises across the hypoxia mapping area (Rabalais et al., 2018) PFTBA (perfluorotributylamine) before each set of analyses. The
(Fig. 2A). Six of these cruises were the summer shelf-wide hypoxia instrument was routinely checked for errors and a daily calibration
cruises with transects from shallow water to 50-m water depth that standard and blank were analyzed with each sample batch to verify
occurred in July 2010, 2013, 2014, 2015, 2017 and 2018 (Rabalais proper instrument performance. The identities of all analytes were
et al., 2018). Four cruises were conducted in May, June, August confirmed using retention time comparisons from the ion chro-
and October 2010 and covered the C and F transects with 155 matogram using full scanning mass spectral analysis of the riser
samples (Fig. 2A). The average sample number for shelf-wide source oil sample. The spectral data were processed by Chem-
cruises was 40 per cruise (range n ¼ 12 to 71). The sample distri- station Software (Agilent Technologies). The data are archived at
bution for these offshore samples is in Supplemental Table S2. https://data.gulfresearchinitiative.org/data/R1.x139.142:0004/.
Water samples were placed immediately into pre-cleaned amber Contamination from the oil spill was detected using two
1370 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376

Fig. 2. Sample locations from offshore waters (A) and from marsh sediments (B). The DWH oil spill site is indicated with an X in 2A. The offshore station transects are identified with
letters whose sampling frequency is in Supplemental Table S2.

methods. First, the concentrations of target aromatics and alkanes 3. Results and discussion
from petroleum has a specific composition, even after weathering,
that is distinct from background target aromatic and alkane dis- 3.1. Offshore
tributions within marsh samples. Second, samples containing oil
residues within the marsh have detectable levels of petroleum The concentration of the total aromatics was not different in
biomarker compounds, such as the hopanes and steranes. In surface and bottom water samples within each sampling trip
addition to the target alkanes and aromatic compounds measured (p > 0.05), but was different among dates. The average concentra-
in all chemical analyses, C30 hopane was quantified from the m/z tion of target aromatics in all stations was 3e9 mg L
1 on the first
191 mass chromatogram as well as the chromatographic profile for three trips in 2010 (May, June, July), and then rose to 153 mg L
1 in
the other hopanoid biomarkers. Additionally, the sterane August and 323 mg L
1 in September, after which it declined until it
biomarker profiles at m/z 217 and 218 mass chromatograms were was below 0.01 mg L
1 by 2015, 2017 and 2018 (Fig. 3A). These
also recorded. The chromatographic profiles of these biomarker values are a plausible consequences of the spill on this shelf. For
compounds were visually paired with the same profiles in MC252 example, if all of the DWH oil (density 0.839) were equally
source oil for all samples to establish a link between the oil residues distributed throughout the entire water mass of the GOM after one-
in samples with the spilled oil. Thus, marsh sediments were half was volatilized, then the concentration of all target aromatics
examined using these two analytical metrics to determine would equal 1453 mg L
1, equal to nine times the concentration of
contamination from the DWH oil spill. dissolved target aromatics measured on the Louisiana shelf in July
2010.
The decline in total target aromatics in the July cruises from
2010 to 2018 was 73% y
1 (Y ¼
1.329*X þ 2676; p ¼ 0.014;
2.4. Statistics F ¼ 17.1; R2 ¼ 0.81), which is a first order decay rate, meaning that
the concentration was declining without regard to the decay rate
The percent of the average analyte composition for each sam- itself or dilution (Fig. 3B). There was no significant change in the
pling trip was computed for comparison purposes. The mean and total target alkane concentrations over the same interval (p > 0.45).
standard error of the mean (m ± 1 SE) for the total alkane and total The average ratio of target aromatics to total alkanes (by measured
aromatics were computed for each sampling trip collecting water weight; mg L
1/ng L
1) was about 7 in May 2010, quickly climbed
or marsh sediments, and a linear regression calculated for the log10 100-fold by September, and fell below 0.1 from 2015 to 2018
transformation of concentration vs year. A linear regression was (Fig. 3C). Several water samples collected in August and September
derived between the total aromatics in the Barataria Bay inshore of 2010 contained trace levels of petroleum biomarker compounds,
stations and one offshore station for 2010 and 2013. A t-test was run indicating that these water samples contained entrained small
using Prism software to test if there was a difference between the residues of oil, not just the more water soluble components from
concentrations of aromatics in surface and bottom water samples the oil.
collected on each offshore cruise. A Welch's correction for unpaired The average concentration of total target aromatics in May 2010
samples was applied. The statistical significance was p < 0.05 for all was three orders of magnitude higher than in July 2018 (Fig. 3A),
linear regression slopes and comparisons. and it is necessary to ask if the July 2018 concentration could be a
 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376 1371

collected in May 2010 were sourced or indicative of dissolved oil

contamination from the DWH oil spill; 2) there were three years of
offshore samples in 2015, 2017 and 2018 that were below the
detection limit for target aromatics and alkanes, and these were
considered to be unlikely ‘oiled’ conditions; 3) the ratio of target
aromatics/alkanes was lower in the same last three sampling years
compared to in May 2010; and 4) a minimum sustained current
speed of 0.4 cm s
1 for 30 days would have been sufficient to
transport the oil from the DWH spill site to this continental shelf
location, assuming it took the most direct path. This speed was
observed in Alabama by Mulabagal et al. (2013) who determined
the chromatographic signatures of petroleum biomarkers to iden-
tify source material of tar balls found on Alabama beaches after the
DWH oil spill. While there were no tar balls observed about 2 weeks
after the spill and 3 weeks before beaches were oiled, mousse was
found in Alabama beaches on June 11, 2010, 8 weeks after the spill
began. Beazley et al. (2012) found oil residues in Alabama surface
sediments in marshes on June 8, 2010 (50 mg g
1). A current speed
of 0.4 cm s
1 over 30 days would be sufficient to take oil to the
Alabama coast from the well head, assuming it went the most direct
route.
Algae can produce alkanes (Schirmer et al., 2010; Gelin et al.,
1999) which add to the pool of target alkanes. Bottom waters in
the inner and mid Gulf of Mexico continental shelf off southeastern
Louisiana are hypoxic because of the excess production of phyto-
plankton in surface waters that eventually sinks to the seabed and
through decomposition by bacteria and the waters become devoid
of dissolved oxygen (Rabalais et al., 2007). The algal populations in
surface waters vary among years and seasons (Walker and Rabalais,
2006; Parsons et al., 2015). It is not surprising, therefore, that the
total target alkane concentration in surface waters among years
demonstrates no trends in summer. However, the distribution of
target alkanes in May 2010 water samples is distinct from the
alkane distribution seen from typical biogenic sources, having no
evidence of odd carbon preference in the (Cn, n ¼ 20e30) range
(see Fig. S1).

3.2. DWH oil exchange through tidal passes

The concentration of target aromatic hydrocarbons in water

samples taken within Barataria Bay changed in concert with the
concentrations in nearshore waters (Fig. 4). The concentration of
total target aromatics within Barataria Bay, for example, rose
sharply in September 2010, compared to the previous three months
(Fig. 4A) e a rise similar to that in offshore waters (Fig. 3A). The
concentration of total aromatics within the estuary each month
was proportional to the amount found 1 km off of the estuary
Fig. 3. The concentration of total aromatics and total alkanes in offshore water sam-
ples taken on the continental shelf from May 2010 through July 2018. A. Average
entrance, but 15e35% lower (Fig. 4B). Concurrent fluctuations be-
amounts of aromatics in surface and bottom water samples (m ± 1 SE). B. The natural tween the salinity offshore and in Barataria Bay are well-
log of the average concentration of total aromatics and total alkanes for the July documented (Wiseman et al., 1990; Turner et al., 2019). We
samples taken on the annual hypoxia mapping cruises from the mouth of the Mis- conclude that what is in the water column offshore can be assumed
sissippi River to the Texas coast. C. The ratio of total aromatics/total alkanes for each
to enter the estuary e including dissolved petroleum hydrocarbons
sampling trip (by measured weight; mg L
1/ng L
1). The vertical dotted line is the
beginning of the oil spill offshore. and oil residues.

3.3. Oil residues in the marsh

baseline value to use for conditions before the oil spill residues
reached the inner Louisiana continental shelf. There are four sup- Our first sampling in the southeastern Louisiana marshes was
portive logic points to support the view that the May samples were 10 m from the shoreline in May 2010, one month after the begin-
already contaminated with oil and do not represent baseline con- ning of the DWH oil spill. At this time, only occasional samples with
centrations: 1) tar balls and sheens were found on the Grand Isle detectable concentrations of target aromatics in May 2010 (before
beach on May 21, 2010 (Rioux, 2010). A dive team (N.N. Rabalais, oiling of the marshes) were found (Fig. 5). The peak concentration
personal observation) at the 20-m isobath near the entrance to was in February 2011, when there was a thousand-fold higher
Barataria Bay in mid-May 2010 encountered oil residues in and just concentration of total target aromatic concentrations in marsh
below the water surface (Fig. 1D). The shipboard water samples surficial sediments, compared to May 2010 (Fig. 5). Thereafter, the
petroleum hydrocarbon concentrations of both target aromatics
1372 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376

Fig. 4. The concentration of aromatics in 2010 (dark circles) and 2013 (open circles) at the offshore station 1 km outside of Barataria Bay and five stations (m ± 1 SE) within lower
Barataria Bay measured on a single day. A. Concentration versus date. The dotted line is the start of the Deepwater Horizon oil spill. B. The relationship between the concentrations
within 1 km offshore and inshore for each month. The linear regression lines for each year are shown separately and the dotted line is the 1:1 correspondence line.

distance (Turner et al., 2014a) (also Fig. 6A and B).

The only comparable study of oil residue concentrations along
transects leading into the marsh was by Hester et al. (2016). Hester
et al.’s first sample was in fall 2011, 18 months after the spill started
and after considerable oiling of the marsh. The sites were divided
into five classes of oiling, which were based on a visual estimation,
not analytically. They estimated that there were lower amounts
100 m into the marsh than at the edge. There was a one order of
magnitude reduction after four years, which is similar to the
average oiling change demonstrated by our results (Fig. 6). Our
conclusion is that sites that were unoiled became contaminated
with petroleum hydrocarbons and from oil residues within two
years. Further, defining sites using a binary choice, such as ‘oiled’ or
‘not oiled,’ at the beginning of the intrusion of oil residues into the
marsh is not valid after 18 months without an analytical
Fig. 5. The concentration of total aromatics (filled symbols, ng g
1 ±1 SE) and total confirmation.
alkanes (open symbols, mg g
1 ±1 SE) in marsh sediments for each sampling trip from
May 2010 to May 2018. Proportional amounts of aromatics are in Supplemental
Table S3. The red symbols are the samples taken before oil reached the marsh. The 3.4. Differing compositions offshore and inshore in the first year
vertical dotted line is the beginning of the DWH oil spill offshore. The sample number
averaged for individual trips are below each symbol (along the x-axis) and are the
Lehr et al. (2010) estimated that the sum of collected, direct
same for alkanes and aromatics.
recovery, burned and skimmed oil from offshore represented 23%
of the total oil spilled. Their estimate leaves the remaining 77%
and alkanes dropped 100 fold (i.e., to about 10 times higher than potentially in the GOM, where another 25% either evaporated or
the May 2010 sediment sample concentrations) and remained dissolved, and 10% was dispersed. The quality of oil buried in res-
higher through spring 2018. The target alkane concentrations ervoirs was transformed after dissolution, evaporative losses, nat-
increased approximately 100 times the May 2010 values in the ural and chemical dispersion, particle attachment, photo- and bio-
marsh sediments compared to a 1000efold increase observed for degradation, and re-complexations (Fig. 1). A portion went to the
the offshore water samples. The standard error of the mean (SE) seabed as mostly fossilized carbon from the spill, which Chanton
was greater in the first few years after the oil spill than in later years et al. (2015) estimated was 3e5%. The term ‘fossilized carbon’ re-
(2013e2018), indicating the spotty nature of hydrocarbon fers to “dead carbon” from the spilled DWH oil that was converted
contamination in the immediate aftermath of oiling, but was so into biomass residues that ultimately ended up on the seafloor.
small in later years, when oil residues were more dispersed and Some of this biomass came from mineralization of oil on the sea-
evenly distributed, that the SE is obscured by the data point (Fig. 5). floor, but much came from mineralization of oil hydrocarbons in
The concentration of target aromatics and alkanes in surficial the water column and on the surface that ended up being trans-
sediments was at least one order of magnitude higher 1 m versus ported to the seafloor associated with sinking marine particles
10 m into the marsh during the first 12 months after the oil spill (marine snow). Thus, a considerable portion of oil residue remained
(Fig. 6A and B). After the spring of 2011, however, the concentration in and on the water surface.
at 1 and 10 m and their ratios were about equal (Fig. 6C). The oil The proportional amount of naphthalene and alkyl naphtha-
distribution within sites and among estuarine basins became lene(s) declined from the peak of 58% of all target aromatics at the
similar within two years. A June and September (2012) sampling well head source oil (measured using the analyte molecular
along a 100-m transect into the marsh could not document an weight) to shelf and inshore waters and then on the marsh sedi-
attenuation of concentrations within marsh sediments with ments (Fig. 7) (Supplemental Table S1). The percent dropped to 22%
in the May 2010 water samples from offshore, and 1% at the highest
 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376 1373

Fig. 6. The concentration for the marsh sediment samples taken 1 m and 10 m into the
marsh by year. The vertical dotted line is the start of the oil spill offshore. A. Total
alkanes. B. Total aromatics. C. The ratio between the concentration of alkanes and
aromatics in the 1- and 10-m samples. The average (m ± 1 SE) is shown for each
sampling trip.

concentration of total target aromatics in September 2010, before

rising to 27% in July 2014 (Supplemental Table S1). In contrast, the
average of the naphthlalene and alkyl naphthalenes for each of the
marsh sediment sampling trips was 8% of the total aromatics,
including <2% at the peak concentration in February 2011
(Supplemental Table S3). Sixty percent of the petroleum hydro-
carbons in oil and oil residues contaminating water samples on the
coast at the peak concentration were comprised of three rings (mol. Fig. 7. The percent of the total aromatics by molecular weight for (A) the DWH source
wt. < 234) (Table S1), whereas it was 28% in the marsh sediment oil at the well head (Table S3); (B) during the peak concentration of oil residues in
continental shelf waters in September 2010; and, (C) during the peak concentration of
samples. The low proportional amounts of naphhthalene and alkyl oil residues in marsh sediments in February 2011. Naphthalene and naphthalenes have
naphthalenes indicates significant volatilization and biodegrada- a molecular weight less than 130 g mol
1.
tion, which is higher for smaller target aromatic hydrocarbons. The
low amounts of naphthalenes in marsh sediments is consistent
with the highly depleted concentrations in oil residues that reached weathered and indicative of the composition found in the source oil
the marsh shoreline, and left residues containing mostly the from the well head (Meyer et al., 2018).
heavily-weathered oil residues, such as alkyl chrysenes isomers,
asphaltenes, and resins which are the heavier by-products of oiling
that accumulated on the marsh surface. Further, these oil residues 3.5. Degradation rates
also contain petroleum biomarker compounds, some of which were
The degradation rate of target alkanes and aromatics is affected
1374 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376

by a variety of factors, especially the molecular weight or carbon (2003) provided several striking outcomes arising from a 10-year
number (Cn), volatility, molecular solubility, eH (oxidation poten- analysis of the 1989 Exxon Valdez oil spill. One of the strongest
tial) and pH, sediment organic content, sediment type, oxygen observations was that the impacts were more than from the im-
availability and temperature. The target alkanes have a lower mo- mediate effect of toxins, but also from the unexpected persistence
lecular weight and are more easily volatilized than alkanes with of toxins creating chronic stressors that continued to affect wildlife
higher molecular weights, whereas the mid-length (Cn ¼ 14 to 20) through indirect cascades of food web relationships that postponed
alkanes are generally non-polar liquids with minimal water solu- recovery. The foundational species for many organisms is the
bilities (Overton et al., 2016). The target aromatics are, in general, emergent marsh vegetation that is used in many metrics of marsh
100 times more soluble in water than alkanes of the same carbon restoration (Fleeger et al., 2018; Zengel et al., 2018). Michel and
number (McAuliffe, 1966), which means that target aromatics are Rutherford (2014), for example, conducted a review of marsh oil-
more prone to evaporative losses than are the alkanes. The ing and concluded that six of 32 marshes on five continents had not
oxygenated water plume near the well-site was enriched with completely recovered from an oil spill after 10 years. Lin et al.
water soluble components indicating some decomposition of DWH (2016) reported that in their heavily-oiled sites in Louisiana,
oil (Reddy et al., 2012). The evaporative losses of target aromatics which experienced near complete plant mortality in 2010, were
moving from the well head to estuary were about 64% of the 10- somewhat recovered 42 months after the DWH spill with live
and 12-carbon naphthalenes (Liu et al., 2012). In general, the higher aboveground biomass only 50% of reference values and below-
the molecular weight of PAHs indicates a higher hydrophobicity ground biomass in the upper 12 cm only 24% of reference values.
and toxicity, and a slower degradation rate in coastal marshes. The Further, shoreline erosion accelerated as a result of oiling
effect of these and other physiochemical properties means that (McClenachan et al., 2013; Hester et al., 2016; Turner et al., 2016;
mid-length target aromatic hydrocarbons (Cn ¼ 14 to 20) that are Beland et al., 2017).
not readily volatilized or leached from sediments can remain bio- Some hydrocarbons from the spilled oil do make their way into
logically active unless degraded. Further, the decomposition of or- food webs (Bonisoli-Alquati et al., 2016; Wilson et al., 2016). The
ganics in anaerobic sediments, in general, is considerably slower indirect rather than toxic effects may be substantial and diverse,
than in aerobic sediments (Hambrick et al., 1980; Delaune et al., although not visually obvious. The concentrations of target PAHs in
1981). Laboratory studies by Bauer and Capone (1985) revealed June 2013 marsh sediments remained at levels that affected the
no re-mineralization of either anthracene or naphthalene in reproduction and growth of resident fish in laboratory experiments
intertidal sediments without oxygen and a temperature de- (Whitehead et al., 2012). The phytoplankton community shifted in
pendency. The results of other laboratory studies by Boyd et al. 2010 along with a loss of abundance in some groups compared to a
(2005) showed that the oxygen supply was the determining fac- 19-y baseline (Parsons et al., 2014, 2015); fledged nests of the
tor controlling PAH degradation. The result is that target alkanes resident seaside sparrows were less frequent in marsh plots
and aromatics will only very slowly degrade once accumulated in determined to be oiled versus those determined as unoiled
anaerobic areas of marsh sediments, and much faster in aerobic (Bergeon Burns et al., 2014); and marsh periwinkle population size
waters or aerated surface sediments. Reddy et al. (2002), for in heavily-oiled marshes remained reduced for at least five years
example, found that pristane, phytane and other branched alkanes after the spill (Zengel et al., 2017). Insect communities were
remained 30 years after the West Falmouth, MA oil spill in 1969 and depressed after this oiling (McCall and Pennings, 2012; Pennings
predicted that “hydrocarbon contamination will persist indefinitely et al., 2014; Bam et al., 2018). Fleeger et al. (2018) found that the
in the sedimentary record.” This is in sharp contrast to the rapid meiofauna, nematodes, copepods, and juvenile annelids, excluding
degradation of the DWH oil's target alkanes and aromatics found in the polychaete Manayunkia aestuarina, recovered in about three
lab weathering studies under ideal mixing and nutrient levels using years in near synchrony with Spartina alterniflora stem density, but
waters collected from the Louisiana marsh shoreline (Olson et al., other common taxa either recovered more slowly (i.e., juvenile
2017). bivalves and amphipods) or not at all. The microbes have not re-
established to pre-spill compositions as of 2016 (Summers Engel
Oil legacies et al., 2017). However, Rabalais et al. (2018) could find no differ-
ence in the low oxygen concentrations or mid-summer hypoxic
The results of multiple studies suggest that some of the spilled area of offshore waters in 2010 compared to a 27-y history.
oil and its residues may persist for decades and continue to nega-
tively affect coastal ecosystems (Teal et al., 1992; National Research 4. Conclusions
Council, 2002; Reddy et al., 2002; Culbertson et al., 2007a, b, 2008).
Peacock et al. (2007) documented the persistence of oil 25 years The oil released at 1500 m during the DWH oil spill created a
after the Bouchard 65 oil barge spill, and oil persisted from the spike in oil concentrations on the continental shelf which did not
Arrow oil spill for more than 20 years (Vandermeulenn and Singh, return to pre-spill concentrations for more than 5 years. The pe-
1994). Linden et al. (2004) reported that salt marshes oiled in the troleum constituents found offshore were temporally synchronous
2001 Gulf War had not recovered after 10 years because of the with the residual oil in the Barataria Bay estuary, implying trans-
“absence of physical energy, wave action, and the anaerobic envi- port and mixing. The oil residues in the water fraction, having lost
ronment,” which are the same conditions in the northern GOM salt much of the lighter fractions, were not identical to those deposited
marshes. We observed fresh oil in crab burrows on our sampling on marsh vegetation and sediments. The initial oiling was wide-
trips. D’Sa (2016) examined the fluorescent components of dis- spread throughout Louisiana coastal marshes and eastwards, but
solved organic matter offshore before, during and after the spill and not in all areas. The smearing of oil residues into the marsh after
found that the variable fluorescence amounts and intensities in two years suggests that the total area oiled was larger than the
2013 were consistent with the long-term persistence of the oil initial oil distribution. These oil residues in sediments were in the
residues in the dissolved carbon pool until that time. form of a viscous fluid emulsion with heavier petroleum constitu-
An oiling legacy can affect organisms through a complex suite of ents that were further transformed under anaerobic conditions.
subtle interactions, perhaps with threshold effects, or de- The total target alkanes and total target aromatics remaining in
pendencies from one species in both offshore (Joye et al., 2016) and marsh sediments in the eight years after oiling are each at least one
inshore ecosystems (Rabalais and Turner, 2016). Peterson et al. order of magnitude greater than the background levels in
 R.E. Turner et al. / Environmental Pollution 252 (2019) 1367e1376 1375

sediments immediately before the oiling. Some of these DWH oil Boyd, T.J., Montgomery, M.T., Steele, J.K., Pohlman, J.W., Reatherford, S.R.,
Spargo, B.J., Smith, D.C., 2005. Dissolved oxygen saturation controls PAH
residues will be re-suspended and distributed in marsh sediments
biodegradation in freshwater estuary sediment. Microb. Ecol. 49, 226e235.
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