International Journal of Geosciences, 2013, 4, 39-45
http://dx.doi.org/10.4236/ijg.2013.45B007 Published Online September 2013 (http://www.scirp.org/journal/ijg)
Legal Issues and Scientific Constraints in the
Environmental Assessment of the Deepw ater Horizon Oi l
Spill in Mexico Exclusive Economic Zone (EEZ)
in the Gulf of Mexico
Luis A. Soto, Alfonso Vázquez-Botello
UA-Oceanic and Coastal Processes-Instituto de Ciencias del Mar y Limnología,
Universidad Nacional Autónoma de México, México, D.F., México
Received July 2013
ABSTRACT
The largest accidental marine oil spill (4.9 million barrels) in the Gulf of Mexico (GoM) seabed (1600 m) caused by the
sinking of the Deepwater Horizon oil rig in 2010, put to the test once again the resilient capacity of the pelagic and
benthic realms of this Large Marine Ecosystem. Many are the ecological services provided by its waters (fisheries,
tourism, aquaculture and fossil fuel reserves) to neighboring countries (US, Mexico and Cuba). However, the unprece-
dented volumes of hydrocarbons, gas and chemical dispersants (Corexit) introduced in the system, represent ecological
stressors whose deleterious effects are still the subject of civil claims and scientific controversy. Presumably, the short
scale effects were confined to the Gulf’s northeastern shallow waters, and the combined actions of weathering, biode-
gradation, and oil recovery left the system almost under pre-spill conditions. Unfortunately, surface and subsurface oil
plumes were detected in the spill aftermath, and their dispersion trajectories threatened Mexico EEZ. Surface o il slicks
were detected in the pristine waters of northern Yucatán, while subsurface oil plumes from the Macondo’s well blow out
were dangerously advancing southwest towards key fishing grounds in the northwestern GoM. This disaster prompted
the Mexican government to implement an ambitious ocean monitoring program adopting a bottom-up approach focused
on building a base line for more than 42 physicochemical and biological variables for water, sediment and biota from
the continental shelf-slope region of the NW GoM. Technological constraints have precluded systematic observations in
the vast Mexican EEZ that could discriminate natural variability and oil seep emissions from antropic disturbances.
Therefore, preliminary risk analyses relied on seasonal and historical records. Two years of field observations revealed
subtle environmental changes in the studied area attributed to antropic disturbances. Waters maintained oligotrophic
conditions and zooplankton and benthic infaunal biomass were also poor. Biomarkers in sediments and biota did not
exceed EPA’s benchmarks, and sediment’s fingerprinting (δ13C) indicated marine carbon sources. Geomarkers revealed
an active transport fro m the Mississippi towa rds the NW GoM of phyllosilicates bearing a weathered oil coating. Con-
sequently, shelf and slope sediment toxicity begins to show an increasing trend in the region. The complexity of hydro-
carbons bioaccumulation and biodegradation processes in deep waters of the GoM seems to indicate that meso- and
large-scale observations may prove to be essential in understanding the capacity of the GoM to recover its ecological
stability.
Keywords: Deepwa t er Horizon; Macondo’s Oil Spill Accident; Mexic o ’s Exclusive Economic Zone; G ul f of Mexico
1. Introduction
The oil spill caused by the sinking of the Deepwater Ho-
rizon Platform (DWH) in the northern Gulf of Mexico
(GoM) is unprecedented in the history of catastrophic
events in the marine environment in both coastal and
deep-waters. The assessment of the environmental con-
sequences caused by this spill has represented a major
challenge for the scientific community of the neighboring
countries sharing the waters and resources of this Large
Marine Ecosystem.
The precise volume of oil spilled and the trajectory of
surface and subsurface oil slicks are still controversial
issues. Similarly, the persistence in time and space of
weathered oil, its by-products, and their environmental
effects, constitute major research uncertainties for most
of the current studies hitherto accomplished. In the af-
termath of the oil spill accident, the reports released by

L. A. SOTO, A. VÁZQUEZ-BOTELLO
40
U.S. agencies NOAA and EPA [1,2] offered to the gen-
eral public a rather optimistic assessment on the percen-
tages of recovered oil, the volume burned and evaporated
from the Macondo’s well blowout [3,4]. Nonetheless,
there was a tacit admission about the severe environ-
mental damages caused by the accumulation of oil on
shorelines of the coastal zone of the northeastern GoM.
In spite of this environmental emergency, that lasted for
more than four months, there was a consensus among
experts [5-7], that while the expected damage in the
coastal zone was not as severe as anticipated, it was rec-
ognized that the Corexit dispersant injected near the base
of the oil well, promoted the formation of subsurface
hydrocarbon plumes, whose dispersion and degradation
in the deep sea was enigmatic given our limited know-
ledge on deep water circulation of the GOM.
Under this appalling scenario, the Mexican govern-
ment implemented in the summer of 2010, an ambitious
ocean monitoring program covering its own Exclusive
Economic Zone in the GoM. Initially a bottom-up ap-
proach was adopted in a multidisciplinary research pro-
gram focused on building a base line for more than 42
physicochemical and biological variables for water, se-
diment and biota obtained from the continental shelf-
slope region of the NW GoM. The present study high-
lights the most relevant environmental issues related to
the Macondo’s oil spill from a Mexican perspective .
Background
The GoM is one of the 61 Large Marine Ecosystems
(LME) recognized on the Planet. These ecosystems are
relatively large oceanic units characterized by environ-
mental properties such as bathymetric conditions, hydro-
graphic, productivity and trophically dependent popula-
tions [8]. In the particular case of the Go M, its waters
are shared by three countries, the U.S., Mexico and Cuba,
and much of the economy of these nations revolves
around energy and fishery resources from the coastal
zone and the ocean’s seabed.
The DWH well blow out located just off the Missis-
sippi River Delta spilled for nearly 84 days, 4.3 million
barrels of crude oil and an equivalent amount of methane
gas. As part of the first mitigation procedures it was de-
cided to employ 2.1 million gallons of chemical disper-
sants both on the surface, and at the base of the well [7].
This last action provoked the formation of hydrocarbon
subsurface oil plumes [9,10], approximately 100 m wide,
with a southwest trajectory and an estimated speed of 4
NM/ day [11,12 ].
According to our own projections [13,14], based on
the surface circulation models of the waters of the GoM,
the regions most likely to receive in the mid-term, the
impact of the oil slick from the Macondo’s blowout,
were the NE and SW coasts of Mexico. Our own re-
search efforts in the first two years of monitoring were
concentrated in the NW sector of the GoM (Figure 1).
This sector of the GoM is strongly influenced by the
cyclone and/or anticyclone gyres derived from the Loop
Current. When these processes are absent in the NW
Gulf, the surface oceanic circulation is predominantly
towards the north. Gyres in the NW Gulf are known for
their significant time/space variability. The salinity and
density vertical profiles obtained in winter indicated the
intrusion of cold and more diluted water originated from
the Louisiana-Texas continental shelf.
Figure 1. A. Location of the area of study in the NW. Gulf of Mexico. The extension of Mexico’s Exclusive Economic Zone
and the approximate position of the Deepwater Horizon Platform (DWH) are indicated. B. Position of the oceanographic
stations on the continental shelf and slope of the NW Gulf of Mexico (depth scale in meters).
Exclusive
Economic Zone
DWH
Area
of
Study
AB

L. A. SOTO, A. VÁZQUEZ-BOTELLO
41
It is worth noting to ind icate at this point the technical
difficulties involved in the identification of Macondo’s
crude oil once it was over dispersed by surface and bot-
tom currents, and exposed to weathering and biodegrada-
tion processes. This situation seems augmented by the
facts that in the GoM there are natural emission of hy-
drocarbons and gas [15]. Sometimes the geochemical
record of GoM sediments reflects a composition of veg-
etable waxes, biofilms, and weathered hydrocarbons
from subsurface natural filtrations beneath the seabed. In
the particular case of the Macondo’s blowout, the envi-
ronmental damage assessment is also magnified by mix-
ing processes and fractionating of the oil components
[16-18].
For tu nately, there is valuable oceanographic informa-
tion [13,14,19-22] available from the regions under study
(SW marine ecosystem—Campeche Sound , and th e NW
sector—continental shelf of Tamaulipas and Veracruz)
that documents the pre-spill environmental conditions of
the GoM.
2. Main Oceanographic Features in Mexico’s
EEZ: NW Gulf of Mexico
The Gulf of Mexico is one of the most productive water
bodies on earth. Its area is about 1.5 million km2, with a
maximum depth of 3,800 m in the area known as Sigsbee
Abyssal Plain. It is relevant to note, that the valuable
natural resource of the GoM are also exposed to the ac-
tion stressful natural chemical and biological factors
whose synergy causes environmental changes with a
certain degree of predictability (seasonality). There are
also catastrophic weather events such as storms and hur-
ricanes, which modify the coastline, and can alter the
patterns of sediment transport, thus promoting erosion.
Its water chemical balance can also be altered by the ex-
traction of fossil fuels and gas beneath the seabed.
In recent years, excessive discharge of organic mate-
rials (fertilizers) in areas near the Mississippi River [23]
and Coatzacoalcos have generated the formation of hy-
poxic areas in which the O2 concentration level drops
alarmingly, threatening the survival of communities oc-
cupying the marine seabed. In reference to the Tamauli-
pas and northern Veracruz coasts (Figure 1), these are
highly influenced by river discharges (Bravo, Soto la
Marina, Pánuco) that enrich with nutrients and organic
matter the waters of the adjacent continental shelf. The
continental shelf in this region is narrow with a maxi-
mum length of 50 km and with steep and rugged slopes,
particularly off the coast of northern Veracruz. The se-
dimentary environment in the coastal strip is dominated
by terrigenous input from rivers and lagoons; sand and
biogenic muds predominate in deeper areas (Figure 2).
Other major physiographic features in the GoM are the
presence of extensive coastal lagoons namely Laguna
Madre and Tamiahua; both function as natural nursery
grounds for many estuarine-dependent species (penaeid
shrimp and oysters). The waters of the continental shelf
in this region of the GoM exhibit stratification and mix-
ing conditions from April to September and November to
March, respectively. During the rainy season (June-Sep-
tember), the rivers have a significant dilution and cooling
effects in the salinity (30 psu) and temperature of the
river plume (23˚C). Due to the intrusion of ocean water
toward the coast, these parameters are maintained at
23˚C and 36 psu, and shallow seasonal thermocline (18
m). Winter northern fronts promote the mixing of the
water column causing low salinity and temperature (34
psu and 15˚C) and the deepening of the thermocline (80
to 160 m). The waters of this region are considered well
ventilated to 100 m, with oxygen concentrations of 4 - 5
ml·L−1. The reference values of nutrients (PO4, NO3, NO2,
and SiO2) tend to be more enriched during the mixing
phase of the water column, reducing its concentration
values when stratified conditions are established. There
is a positive correlation between significant nutrient val-
ues and sites under the influence of river and lagoon dis-
charges. However, the rapid consumption of nutrients by
neritic phytoplankton maintains oligotrophic conditions.
The surface concentration values of chlorophyll a (>0.10
mg·m−3) correlate well with sites receiving low fluvial
influence, while oceanic environments are characterized
by poor chlorophyll concentrations (<0.05 mg·m−3).
A vital information in this study are the reference
concentrations of heavy metals and polycyclic aromatic
hydrocarbons (PAH’s) in surface sediments previously
recorded in the study area. Heavy metals such as Cr, Ni,
and Cd reach significant concentrations along the Ta-
maulipas’ shelf [24]. There are also high levels (26.9
μg·g−1) of PAH’s in sediments off Tamaulipas and Vera-
cruz attributable to chronic oil spills in the area.
3. Economic Damage Assessment
The estimation of natural resource damages caused b y oil
spills is extremely controversial. The main difficulty re-
sides in the complexity involved in assessing the ecolog-
ical services offered by the different biotic components
of the marine ecosystem. An event such as the Macon-
do’s oil spill is subject of legal claims from physical
damages, cleanup and preventative actions to the loss of
fishing grou nds and landscape prop erty value. Acco rding
to international agreements (International Oil Pollution
Fund) and US federal legislatio n (Oil Po llution Act, 1990)
addressing oil spill accidents, the responsible party, Brit-
ish Petroleum in this case, can be held accountable for
the caused environmental damages, while the affected
parties must provide sufficient evidence of the source of

L. A. SOTO, A. VÁZQUEZ-BOTELLO
42
98.0 97.0 96.0
22.0
23.0
24.0
25.0
26.0
12345
6 7
8
9 10
11 12 13 14 15
16
17
18 19 20
21 22
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26
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32
33 3435
0.3
0.8
1.3
1.8
2.3
Bravo River
Madre
Lagoon
Soto La
Marina
River
Pánuco
River
Tamiahua
Lagoon
%
Longitude O
Latitude N
(a)
98.097.096.0
22.0
23.0
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12345
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41
Bravo River
Madre
Lagoon
Soto La
Marina
River
Pánuco
River
Tamiahua
Lagoon
%
Longitude W
Latitude N
(c
98.0 97.0 96.0
22.0
23.0
24.0
25.0
26.0 12345
6
7
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78
Bravo River
Madre
Lagoon
Soto La
Marina
River
Pánuco
River
Tamiahua
Lagoon
%
Longitude W
Latitude N
(b)
Figure 2. Dis tr ibution and composition of surface sediments in the NW Gulf of Mexico. (a) Sand. (b) Silt. (c) Clay.
the spilled oil, and an economic quantification of the
alleged damages. Sumaila et al. (2012) have briefly ex-
plained some of the various attempts made in assessing
economic environmental damages due to oil spills. Ap-
parently, accidents caused by oil tankers are highly vari-
able in nature. They depend on the type of oil, the vo-
lume spilled and the area where the accident occurs.
While oil rig accidents like the Deepwater Horizon plat-
form, located in a “sensitive ecoregions” have substantial
ecological and economic damages [25].

L. A. SOTO, A. VÁZQUEZ-BOTELLO
43
4. General Discu ssi on
The Large Marine Ecosystem Gulf of Mexico is a
semi-enclosed basin whose hydrodynamic and topo-
graphic features may contribute to the confinement or the
dispersion of crude oil. Thus for instance, it was assumed
that the circulation system prevailing in the summer in
the northern gulf was responsible for the hydrocarbon
concentration towards the northeast [1], where oil slick
remains were then trapped into the Eddy Franklin, a ma-
jor gyre system of the Loop Current. Satellite images
obtained in June, 2010 revealed oil residues at 200 km
north of Yucatan [26] possibly transported by this circu-
lation system.
Perhaps, during the summer the surface transport of
Macondo’s crude oil was not as intense towards the
northwestern GoM. However, in the mid-term, such
transportation could have taken place through the sub-
surface layers below the mixed layer (~500 m). We were
able to detect in the winter of 2011, the transport and
deposition of oil mineral aggregates (OMA) in the EEZ
of Mexico on the slope (>1000 m) in front of Tamaulipas
(Figure 3). Presumably, the intrusion of water from
Texas-Louisiana shelf observed in winter, favored these
processes. Interestingly enough, at this time of year, an
increasing gradient in the levels of sediment toxicity was
also recorded in that region of the GoM [27].
The Mexican waters and coastal environments in the
GOM have been internationally recognized for its mega-
biodiversity. These habitats are home to over 15,000
species of flora and fauna and many of them have consi-
derable commercial value. That is the case of the penaeid
shrimp populations exploited in the offshore waters of
Tamaulipas, Veracruz, Campeche and Quintana Roo.
Other important fishing resources of high economic val-
ue are the pelagic stocks of red snapper, grouper and tuna.
The blue crab, oysters, and clams, which are normally
exploited in the complex lagoon systems around the gulf
(Laguna Madre-Tamiahua-Alvarado), constitute vital
artisanal fisheries. All these marine living resources are
highly vulnerable to toxic compounds incorporated into
Figure 3. Microphotograph of oil mineral aggregates ob-
tained from surface sediments from the continental slope of
the NW Gulf of Mexico.
the crude oil molecules, such as naphthalene, phenanth-
rene, benzenes, and also the trace elements such as vana-
dium, cadmium, chromium, zinc, lead, and nickel.
From an ecological point of view, it is important to
recognize that the vast majority of organisms inhabiting
the GoM conduct, as part of their life cycle, circadian
and seasonal migratory movements at different spatial
scales that could seriously be disturbed by the presence
in the water column of spilled crude oil mixed with
chemical dispersants (Corexit). The direct exposure, the
disruption of breeding and spawning areas, or the transfer
of contaminants through the food web, are just some of
the deleterious effects one may expect to occur in the
GoM due to the Macondo’s blow out [28,29]. Consider-
ing the existing ecological connectivity within the GoM,
it is doubtful to accept the argument that the conditions
of environmental damage caused by the accidental oil
spill in the northern gulf were spatially and temporally
confined to that region.
The Ixtoc-1 (1979), Exxon Valdez (1989) and Prestige
(2002) oils spill accidents are painful remainders of the
lasting harmful effects to the marine ecosystem. Micro-
biologists were the first to report the unusual oil microb i-
al degradation that contributed significantly to minimize
the seriousness of the Macondo’s oil spill in surface wa-
ters [30,31]. However, it was difficult to reconcile the
bacterial degradation rate in deep-water with an average
temperature of 4˚C and possibly anaerobic conditions in
sediments. We presume that many of the toxic elements
of crude oil still remain bio-available in the sediments of
the GoM at sublethal levels mainly for benthic dwellers.
Our current toxicity analyses in tissues of shelf-fauna
inhabitants (mollusks, decapods crustaceans, and demer-
sal fish) from the northwestern GoM reveal concentra-
tions of HA P’s and heavy metals not exceeding the levels
to cause mortality [32,33]. However, the bioaccumula-
tion effects and possible magnification in the troph ic w eb
cannot be rul ed out.
On the other hand, prior to the accidental Macondo’s
oil spill, Mexican fisheries experts [34,35] have already
sent warning signs about the overexploitation levels
reached by demersal and finfish stocks traditionally ex-
ploited in Mexico’s EEZ waters. An additional stress
factor such as the presence of chemical pollutants in the
GoM may seriously undermine the immune capacity of
individuals and their bioenergetics balance (respiratory
metabolism-growth-rate-reproduction).
Bearing in mind the legacy left by accidental oil spills
occurred at different latitudes (IXTOC-1, 1979, SW Gulf
of Mexico; EXXON Valdez, 1989, Alaska; PRESTIGE,
2002, northern coast of Spain), we could expect that the
MACONDO massive oil spill effects will be present in
the GoM for several decades. The restoration of the eco-
logical stability of the GoM would depend on its resi-

L. A. SOTO, A. VÁZQUEZ-BOTELLO
44
lience capacity. Indeed, the existing natural oil and gas
seeps and the chronic contamination due to the current
extraction of fossil fuels in the north and south of the
GoM, increase the uncertainty in detecting the chemical
fingerprint from MACONDO. However, the seriousness
of the environmental damage of this unprecedented phe-
nomenon in the annals of contemporary science cannot
be met with hesitation or skepticism. An answer to this
challenging issue can be found in the implementation of
a modern observational system of oceanographic condi-
tions of the GoM and the periodic determinations of oil
biomarkers in sediments and biota (δ13 C, PAHs, hopanes,
steranes, and trace metals).
5. Conclusions
In the aftermath of the unfortunate Macondo’s blowout
in which nearly 5 million barrels of crude oil were spilled
both in deep and surface waters of the northern GoM, a
heated debate emerged among experts on the fate and
persistence of the hydrocarbons in coastal and deep en-
vironments. The severe disruption of the ecological bal-
ance absorbed by the GoM due to this massive spill
tended to be officially underrated by the contention and
emergency procedures implemented during the acute
phase of the accident. Natural processes such as evapora-
tion, dispersion, dilution, and a significant microbial
biodegradation were largely credited for the substantial
loss of the released oil (>74%). The remaining percen-
tage still poses potential damages specially for the deep
sea environment since we ignore the degradation rates of
the different oil components, the areas of deposition, and
the dispersion trajectories of subsurface oil plumes. Un-
fortunately, our current knowledge on the deep circul a-
tion patterns and the deep sea biodiversity of the GoM is
not sufficient to foresee the degree and duration of the
environmental disturbance that an input of crude oil may
have in the Mexican EEZ. This is a challenging research
issue that requires the best available science and multila-
teral cooperation from neighboring countries in the GoM.
From a Mexican perspective, the answer to this riddle
can be found in the rescue of historical data, and the im-
plementation of a Monitoring Pr ogram of our coastal a nd
oceanic waters in order to ensure the sustaining devel-
opment of this LME.
6. Acknowledgements
We are indebted to the research group integrated in the
project “Marco Ambiental de las Condiciones Ocea-
nográficas en el Sector NW de la ZEE de México en el
Golfo de México (MARZEE)” for their invaluable sup-
port and tireless enthusiasm. The authors would like to
acknowledge the financial support of the Instituto Na-
cional de Ecología y Cambio Climático (INECC).
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