90
prompted investigations to characterize the environmental distributions, the degradation, and the fate of the released petroleum. More than 100 separate cruises were
involved in collecting more than 10,000 water samples in the offshore waters of the
GoM as part of the collaboration between the US government and BP during the
response to the spill and as part of Natural Resource Damage Assessment (NRDA)
phases of the DWH investigation.
6.2 How Deep Subsea Spills Differ from Surface Releases
The fate of the DWH oil differed from the Amoco Cadiz, the Exxon Valdez, and most
other well-studied spills because of the depth of the release and the transit of the oil
through the water column before surfacing. Nevertheless, several subsea spills
occurred before the DWH oil spill, and in each case, studies provided insight into
the specific oil properties and environmental conditions that controlled the ultimate
fate of the oil. On January 28, 1969, a leak occurred during the drilling of well A-21
on Platform A, approximately 6 miles off the coast of Santa Barbara, California, and
oil and gas were released from approximately 190 ft. of water depth (Swift et al.
1969). Oil reached the surface of the ocean within 14 minutes, forming a slick, and
despite occurring in relatively deep water, the oil from the release reached nearby
shorelines a week later. The well was capped on February 7, but oil continued to
seep from the sediments at increased rates in the general area for months. Because
of the extended and variable nature of the release, the exact spill volume is not well
understood but has been estimated to be between 4,000 and 9,600 tonnes (NOAA
2018b). Evaluations of the impact of the oil were complicated by the presence of
naturally occurring hydrocarbon seeps in the area, which have been estimated to
contribute more than 12 tonnes of oil per day to the local ecosystem (Hornafius et al.
1999). Additionally, local weather conditions at the time of the spill resulted in
higher than average coastal runoff, which may have contributed to increased oil
sedimentation through the presence of denser-than-water particulate matter
(Straughan and Kolpack 1971).
The 1979 Ixtoc 1 well blowout in the Gulf of Campeche, Mexico, released more
than 3 million barrels (360,000 tonnes) of a light crude oil (API gravity 34.5) at high
pressure at a depth of approximately 50 m. The released material formed a water/
gas/oil emulsion that reached the ocean surface where the lighter components were
lost via burning and evaporation. The resulting oil/water mixture interacted with
particles, and ultimately, a fraction became heavier than the surrounding water and
was removed from the water column through sedimentation (Boehm and Fiest
1980). There was debate over the amount of oil which reached the sediments, which
was estimated by difference (i.e., the unaccounted for oil) at up to 25% of the initial
oil (Jernelöv and Lindén 1981). However, direct investigation using sediments and
sediment trap results suggests that much less of the oil reached the seafloor via sedimentation (ca. 1–3%), and analysis of the alkane composition of material in the
sediment traps found that sinking material was actually scouring previously
K. J. Murray et al.
prompted investigations to characterize the environmental distributions, the degradation, and the fate of the released petroleum. More than 100 separate cruises were
involved in collecting more than 10,000 water samples in the offshore waters of the
GoM as part of the collaboration between the US government and BP during the
response to the spill and as part of Natural Resource Damage Assessment (NRDA)
phases of the DWH investigation.
6.2 How Deep Subsea Spills Differ from Surface Releases
The fate of the DWH oil differed from the Amoco Cadiz, the Exxon Valdez, and most
other well-studied spills because of the depth of the release and the transit of the oil
through the water column before surfacing. Nevertheless, several subsea spills
occurred before the DWH oil spill, and in each case, studies provided insight into
the specific oil properties and environmental conditions that controlled the ultimate
fate of the oil. On January 28, 1969, a leak occurred during the drilling of well A-21
on Platform A, approximately 6 miles off the coast of Santa Barbara, California, and
oil and gas were released from approximately 190 ft. of water depth (Swift et al.
1969). Oil reached the surface of the ocean within 14 minutes, forming a slick, and
despite occurring in relatively deep water, the oil from the release reached nearby
shorelines a week later. The well was capped on February 7, but oil continued to
seep from the sediments at increased rates in the general area for months. Because
of the extended and variable nature of the release, the exact spill volume is not well
understood but has been estimated to be between 4,000 and 9,600 tonnes (NOAA
2018b). Evaluations of the impact of the oil were complicated by the presence of
naturally occurring hydrocarbon seeps in the area, which have been estimated to
contribute more than 12 tonnes of oil per day to the local ecosystem (Hornafius et al.
1999). Additionally, local weather conditions at the time of the spill resulted in
higher than average coastal runoff, which may have contributed to increased oil
sedimentation through the presence of denser-than-water particulate matter
(Straughan and Kolpack 1971).
The 1979 Ixtoc 1 well blowout in the Gulf of Campeche, Mexico, released more
than 3 million barrels (360,000 tonnes) of a light crude oil (API gravity 34.5) at high
pressure at a depth of approximately 50 m. The released material formed a water/
gas/oil emulsion that reached the ocean surface where the lighter components were
lost via burning and evaporation. The resulting oil/water mixture interacted with
particles, and ultimately, a fraction became heavier than the surrounding water and
was removed from the water column through sedimentation (Boehm and Fiest
1980). There was debate over the amount of oil which reached the sediments, which
was estimated by difference (i.e., the unaccounted for oil) at up to 25% of the initial
oil (Jernelöv and Lindén 1981). However, direct investigation using sediments and
sediment trap results suggests that much less of the oil reached the seafloor via sedimentation (ca. 1–3%), and analysis of the alkane composition of material in the
sediment traps found that sinking material was actually scouring previously
K. J. Murray et al.