93
resistance against its relatively large surface area. The result of these forces is that
the droplets rise more slowly; are advected laterally; undergo dissolution, dilution,
and biodegradation (i.e., partitioning and weathering); and may never reach the
ocean surface. For droplets and aggregations of droplets that do eventually surface,
the resulting surface slick will be more diffuse as the droplets are transported laterally during their rise. Droplets that never reach the surface can form a subsurface
anomaly or diffuse “plume” of oil, which travels within the water column at a density boundary.
In contrast to shallower or surface releases, gasses released at depth are likely to
dissolve into the water column and not be available to evaporate from a surface
slick. For example, Reddy et al. (2012) summarize multiple studies which confirm
that during the DWH oil spill, nearly all methane present in the released material
was dissolved and remained present in the water column at approximately 1100 m
depth and use that information to examine the retention of ethane and propane,
which were also nearly completely dissolved at depth (Reddy et al. 2012). The longer contact time with the water column and higher surface area also result in the
increased dissolution of volatile and soluble components of the liquid oil, resulting
in higher dissolved concentrations and less release to the atmosphere. Larger droplets of oil rose rapidly to the surface, while smaller neutrally buoyant oil microdroplets (approximately 10–60 microns in diameter) remained in the 1000–1200 m
depth range advecting with the water currents (Camilli et al. 2010). The enhanced
formation of these small droplets resulting from dispersant application promoted
the dissolution of the most soluble components of the oil (Reddy et al. 2012), and
microbial action further degraded the petroleum hydrocarbons in the water column
(Atlas and Hazen 2011; Hazen et al. 2010).
Spills that have been studied extensively resulting in large data collections, such
as the DWH, Ixtoc, and Exxon Valdez spills, have shown that chemicals associated
with oil spills, such as polycyclic aromatic hydrocarbons (PAHs), reach maximum
water concentrations early after release and decrease rapidly with time and distance
from the release point (Boehm and Fiest 1982; Boehm et al. 2007; Boehm et al.
2016). Within the water column, aggregated chemical parameters such as the “total
concentration of PAHs” or sums of other oil components neither completely nor
adequately characterize the important chemical exposure to organisms. As components dissolve into the aqueous fraction, the resulting aqueous profile is controlled
by the relative solubilities in addition to the composition of the original oil. This
phenomenon results in different profiles based on the relative amounts of dissolved
and particulate oil, even from a single source. For example, filtration of two water
samples collected during the DWH oil spill contained approximately 10 parts per
billion (ppb) total PAH (TPAH) had very different distributions in the aqueous and
particulate (filtered) phases (Fig. 6.1).
As discussed above, the vast majority of crude oil compounds have a specific
density less than the surrounding water, meaning they will naturally float on the
surface. However, weathering processes lead to the removal or alteration of the
lighter/smaller compounds leaving the relatively heavier, more resistant, compounds behind. Although these compounds may still be less dense than the
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…
resistance against its relatively large surface area. The result of these forces is that
the droplets rise more slowly; are advected laterally; undergo dissolution, dilution,
and biodegradation (i.e., partitioning and weathering); and may never reach the
ocean surface. For droplets and aggregations of droplets that do eventually surface,
the resulting surface slick will be more diffuse as the droplets are transported laterally during their rise. Droplets that never reach the surface can form a subsurface
anomaly or diffuse “plume” of oil, which travels within the water column at a density boundary.
In contrast to shallower or surface releases, gasses released at depth are likely to
dissolve into the water column and not be available to evaporate from a surface
slick. For example, Reddy et al. (2012) summarize multiple studies which confirm
that during the DWH oil spill, nearly all methane present in the released material
was dissolved and remained present in the water column at approximately 1100 m
depth and use that information to examine the retention of ethane and propane,
which were also nearly completely dissolved at depth (Reddy et al. 2012). The longer contact time with the water column and higher surface area also result in the
increased dissolution of volatile and soluble components of the liquid oil, resulting
in higher dissolved concentrations and less release to the atmosphere. Larger droplets of oil rose rapidly to the surface, while smaller neutrally buoyant oil microdroplets (approximately 10–60 microns in diameter) remained in the 1000–1200 m
depth range advecting with the water currents (Camilli et al. 2010). The enhanced
formation of these small droplets resulting from dispersant application promoted
the dissolution of the most soluble components of the oil (Reddy et al. 2012), and
microbial action further degraded the petroleum hydrocarbons in the water column
(Atlas and Hazen 2011; Hazen et al. 2010).
Spills that have been studied extensively resulting in large data collections, such
as the DWH, Ixtoc, and Exxon Valdez spills, have shown that chemicals associated
with oil spills, such as polycyclic aromatic hydrocarbons (PAHs), reach maximum
water concentrations early after release and decrease rapidly with time and distance
from the release point (Boehm and Fiest 1982; Boehm et al. 2007; Boehm et al.
2016). Within the water column, aggregated chemical parameters such as the “total
concentration of PAHs” or sums of other oil components neither completely nor
adequately characterize the important chemical exposure to organisms. As components dissolve into the aqueous fraction, the resulting aqueous profile is controlled
by the relative solubilities in addition to the composition of the original oil. This
phenomenon results in different profiles based on the relative amounts of dissolved
and particulate oil, even from a single source. For example, filtration of two water
samples collected during the DWH oil spill contained approximately 10 parts per
billion (ppb) total PAH (TPAH) had very different distributions in the aqueous and
particulate (filtered) phases (Fig. 6.1).
As discussed above, the vast majority of crude oil compounds have a specific
density less than the surrounding water, meaning they will naturally float on the
surface. However, weathering processes lead to the removal or alteration of the
lighter/smaller compounds leaving the relatively heavier, more resistant, compounds behind. Although these compounds may still be less dense than the
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…