The Davis Strait - DCE - Nationalt Center for Miljø og Energi

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m deep. Dispersants were applied at the wellhead and huge subsea plumes
of dispersed oil were formed in different depths and they moved long distances
with the water currents (Diercks et al. 2010, Thibodeaux et al. 2011).
Oil also settled on the ocean floor far from the spill site (Schrope 2011). The
Deepwater Horizon oil spill has been estimated at 840,000 oil tonnes, making
it the largest recorded peacetime spill. The oil dispersed at the wellhead and
had a very slow buoyant migration towards the surface, which allowed volatile
hydrocarbon to be dissolved in the water column. Adding of dispersants
at the wellhead contributed to the formation of huge plumes of dispersed
oil at different depths ranging between 800 and 1,200 m (Hazen 2010,
Valentine 2010). It is estimated that 50% of the oil ‘remains’ dispersed, has
sunk to the seabed or has degraded in the water column (Kerr 2010).
Studies of deepwater blowout events have predicted that a substantial fraction
of the released oil and gas would become suspended in pelagic plumes,
and that this may take place even in the absence of added dispersant agents
(Johansen et al. 2001). The fate of oil in deep water is likely to be very different
from that of surface oil because processes such as evaporative loss and
photooxidation do not take place (Joye & MacDonald 2010). Microbial oxidation
and perhaps sedimentation on the seabed are the primary fates expected
of the oil suspended in the deep sea (Joye & MacDonald 2010). In the
Gulf of Mexico, natural oil seeps contribute to the marine environment with
an estimated 140,000 tonnes of oil annually (Kvenvolden & Cooper 2003),
which means there should be intrinsic potential for microbial degradation
(i.e. presence of the responsible organisms) (Hazen 2010). This was confirmed
by bio-degradation rates faster than expected in the deep plumes at
5° C.
However, microbial degradation of oil may have derived effects such as oxygen
depletion, which in the deep water may persist for long periods of
time, because deep water oxygen is not replenished in situ by photosynthesis
as it is in surface waters (Joye & MacDonald 2010).
There are indications of unexpected and severe deep-sea impacts (Schrope
2011). However, at the time of writing the environmental impacts are not really
understood or described (Graham et al. 2011), (Schrope 2011) and therefore
it has not been possible to include clear conclusions in this SEIA. But a
natural resource damage assessment is under preparation (Graham et al.
2011) and the consequences of the Deepwater Horizon subsea blowout will
be discussed in more detail in a later version of this assessment.
11.1.3 Dissolution of oil and toxicity
Total oil concentration in water is a combination of the concentration of
small dispersed oil droplets and oil components dissolved from these and
the surface slick. The process of dissolution is of particular interest as it increases
the bioavailability of the oil components. The toxic components can
increase the potential for acute toxicity to marine organisms. The rate and
extent to which oil components dissolve in seawater depends mainly on the
amount of water-soluble fractions (WSF) in the oil. The degree of natural
dispersion is also important for the rate of dissolution; although surface
spreading and water temperature may also have some influence.
PAHs are among the toxic components of crude oil. The highest PAH concentration
found in the water column in Prince William Sound within a six-