558
obtained separately for pelagic and benthic environments, were used for the modifiers in Eqs. 32.3 and 32.4, respectively. Parameters for slope (α = 0.2885 and
γ = 0.0531) and PAH concentration threshold (β = 907.4306 and δ = 28.422 in ppb)
were fit by maximum likelihood estimation (Dornberger et al. 2016; Ainsworth
et al. 2018).
32.2.6 Estimation of Ixtoc 1 Spill Distribution and Oil
Concentration
We estimated the amount of oil spilled during each calendar month, from June 1979
to March 1980, from the flow rates reported by the Coordinated Program of
Ecological Studies in Campeche Sound (PCEESC 1980): 4770 tons per day (t·d
−1
)
during the first 41 days of the spill, 3180 t·d
−1
for the next 32 days, then 1590 t·d
−1
for 104 days, 318 t·d
−1
for 35 days, and 229 t·d
−1
for 83 days. The PCEESC (1980)
report also indicates that approximately 58.1% of the spilled oil was burned or evaporated, 8.9% was recovered either at the surface or near the well head, and only 33%
of the oil remained in the environment. Therefore, for estimation of oil effects, we
only included 33% of the monthly estimates of spilled oil.
The oil was spatially allocated to the model area based on the results from Sun
et al. (2015) who identified Ixtoc 1 oil slick in satellite images. A total of 82 satellite
images processed by Sun et al. (2015) were grouped by calendar month, and the
areas of Ixtoc 1 oil slick represented by polygon features were overlaid in ArcGIS
(Fig. 32.2). The number of oil slick features overlaid in any particular area each
month served as coefficient to determine relative oil concentration: an area with one
oil slick feature had a coefficient of 1, while areas with two overlaying slick features
in a month received a coefficient of 2. The estimated oil spilled each month was then
divided over the entire area covered by the images and adjusted by the coefficients,
resulting in estimates of oil volume by squared meter. These values were multiplied
by a factor or 0.0001 to estimate the concentration of bioavailable oil (ppb) per
feature. The oil concentration was used in Eqs. 32.5 and 32.6 to estimate mortality
and growth effects in the feature layers. The total monthly mortality and growth
effects in each Atlantis polygon were estimated by prorating the effect in any oil
slick features present, as a proportion of the total area of the Atlantis polygon.
The highest oil concentration was estimated at 44183 ppb for sediments for a
small area (2.8 hectares) of polygon 13, in the immediate vicinity of the Ixtoc 1 oil
well, in December 1979. Oil concentrations greater than 8400 ppb were only estimated for small portions of polygon 13. Concentrations greater than the 907.4 ppb
mortality effect threshold were estimated for portions of 36 Atlantis polygons during at least 1 month. Concentrations greater than the 28.4 in ppb growth effects
threshold were estimated over a larger area in the same 36 polygons. The only polygons without estimated oil concentrations greater than the mortality or growth effect
thresholds were six estuaries (polygons number 33, 35, 36, 37, 38 and 41) where oil
J. G. Ortega-Ortiz et al.
obtained separately for pelagic and benthic environments, were used for the modifiers in Eqs. 32.3 and 32.4, respectively. Parameters for slope (α = 0.2885 and
γ = 0.0531) and PAH concentration threshold (β = 907.4306 and δ = 28.422 in ppb)
were fit by maximum likelihood estimation (Dornberger et al. 2016; Ainsworth
et al. 2018).
32.2.6 Estimation of Ixtoc 1 Spill Distribution and Oil
Concentration
We estimated the amount of oil spilled during each calendar month, from June 1979
to March 1980, from the flow rates reported by the Coordinated Program of
Ecological Studies in Campeche Sound (PCEESC 1980): 4770 tons per day (t·d
−1
)
during the first 41 days of the spill, 3180 t·d
−1
for the next 32 days, then 1590 t·d
−1
for 104 days, 318 t·d
−1
for 35 days, and 229 t·d
−1
for 83 days. The PCEESC (1980)
report also indicates that approximately 58.1% of the spilled oil was burned or evaporated, 8.9% was recovered either at the surface or near the well head, and only 33%
of the oil remained in the environment. Therefore, for estimation of oil effects, we
only included 33% of the monthly estimates of spilled oil.
The oil was spatially allocated to the model area based on the results from Sun
et al. (2015) who identified Ixtoc 1 oil slick in satellite images. A total of 82 satellite
images processed by Sun et al. (2015) were grouped by calendar month, and the
areas of Ixtoc 1 oil slick represented by polygon features were overlaid in ArcGIS
(Fig. 32.2). The number of oil slick features overlaid in any particular area each
month served as coefficient to determine relative oil concentration: an area with one
oil slick feature had a coefficient of 1, while areas with two overlaying slick features
in a month received a coefficient of 2. The estimated oil spilled each month was then
divided over the entire area covered by the images and adjusted by the coefficients,
resulting in estimates of oil volume by squared meter. These values were multiplied
by a factor or 0.0001 to estimate the concentration of bioavailable oil (ppb) per
feature. The oil concentration was used in Eqs. 32.5 and 32.6 to estimate mortality
and growth effects in the feature layers. The total monthly mortality and growth
effects in each Atlantis polygon were estimated by prorating the effect in any oil
slick features present, as a proportion of the total area of the Atlantis polygon.
The highest oil concentration was estimated at 44183 ppb for sediments for a
small area (2.8 hectares) of polygon 13, in the immediate vicinity of the Ixtoc 1 oil
well, in December 1979. Oil concentrations greater than 8400 ppb were only estimated for small portions of polygon 13. Concentrations greater than the 907.4 ppb
mortality effect threshold were estimated for portions of 36 Atlantis polygons during at least 1 month. Concentrations greater than the 28.4 in ppb growth effects
threshold were estimated over a larger area in the same 36 polygons. The only polygons without estimated oil concentrations greater than the mortality or growth effect
thresholds were six estuaries (polygons number 33, 35, 36, 37, 38 and 41) where oil
J. G. Ortega-Ortiz et al.