97
due to the limited surface area of more concentrated oil. The key determinants are
the oil concentration (Prince et al. 2017), which was exceptionally dilute in the
subsurface—almost all samples were lower than 250 ppb total petroleum hydrocarbon (TPH), 1 ppb TPAH (the sum of all the two- to six-ring aromatic hydrocarbons
including their alkylated forms detectable by gas chromatography-mass spectrometry) (Wade et al. 2016; Boehm et al. 2016). Surface area and surface area (S)/
volume(V) played an important role in the biodegradation process in that small oil
droplets afford a large surface area (high S/V) for biodegradation to take place,
while, by contrast, the surface oil slick’s S/V was much smaller. For dilute dispersed
oil with high S/V, seawater provides the necessary biologically available nutrients
that oil does not provide—nitrogen, phosphorus, iron, etc.—as well as the requisite
oxygen for aerobic biodegradation. It was no surprise that biodegradation was only
marginally slower at great depths and low temperatures than nearer the ocean surface (Prince et al. 2016; Nguyen et al. 2018; Marietou et al. 2018). Biodegradation
of oil at the surface was probably negligible until the S/V increased after the oil
reached the shoreline (Atlas et al. 2015).
Determining the extent of dissolution of aromatic hydrocarbons from oil droplets, and distinguishing this from biodegradation in field samples, remains an area
of active research and investigation. It is well known that the small aromatics benzene, toluene, ethylbenzene, and the xylenes dissolve out of oils and gasolines
(Poulsen et al. 1992), and the oil/water partition coefficient increases significantly
at high pressure in the presence of methane, so-called live oil (Jaggi et al. 2017).
This presumably slows dissolution at depth, although of course the equilibrium partition coefficient does not reveal much about the kinetics of dissolution or about the
effect of the essentially infinite dilution available as hydrocarbons leave an oil droplet for the aqueous phase. Modeling (Gros et al. 2016) suggests that high pressures
dramatically favor the aqueous dissolution of C 1 − C 4 hydrocarbons and also influence the buoyancies of bubbles and droplets. These authors believe solubility was a
far more important process than dispersion (Gros et al. 2017), although clearly both
depend on the generation of small droplets, which can be facilitated by dispersant
injection at source. What is clear is that the vast majority of hydrocarbons in dispersed crude oils are biodegraded within a few weeks when oil concentrations are a
few parts per million (Prince et al. 2017), irrespective of their initial presence in
solution or in droplets. This is the great benefit of using chemical dispersants (Prince
2015)—not only do dispersants protect birds and shorelines from oiling by surface
slicks, they also deliver oil to the microbes that provide the only process that will
remove the spilled oil from the biosphere. It is now clear that dispersants have no
significant influence on hydrocarbon biodegradation rates once the oil is dispersed
(Brakstad et al. 2018b) and that they themselves are degraded at similar rates to the
hydrocarbons (Brakstad et al. 2018c; McFarlin et al. 2018).
It is well known that the GoM has thousands of oil seeps (MacDonald et al.
2015), so it is no surprise that a large “standing crop” of oil-degrading microbes was
available to utilize the dispersed oil. A succession of genera proliferated as the spill
progressed (Redmond and Valentine 2011; Dubinsky et al. 2013; Hu et al. 2017;
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…
due to the limited surface area of more concentrated oil. The key determinants are
the oil concentration (Prince et al. 2017), which was exceptionally dilute in the
subsurface—almost all samples were lower than 250 ppb total petroleum hydrocarbon (TPH), 1 ppb TPAH (the sum of all the two- to six-ring aromatic hydrocarbons
including their alkylated forms detectable by gas chromatography-mass spectrometry) (Wade et al. 2016; Boehm et al. 2016). Surface area and surface area (S)/
volume(V) played an important role in the biodegradation process in that small oil
droplets afford a large surface area (high S/V) for biodegradation to take place,
while, by contrast, the surface oil slick’s S/V was much smaller. For dilute dispersed
oil with high S/V, seawater provides the necessary biologically available nutrients
that oil does not provide—nitrogen, phosphorus, iron, etc.—as well as the requisite
oxygen for aerobic biodegradation. It was no surprise that biodegradation was only
marginally slower at great depths and low temperatures than nearer the ocean surface (Prince et al. 2016; Nguyen et al. 2018; Marietou et al. 2018). Biodegradation
of oil at the surface was probably negligible until the S/V increased after the oil
reached the shoreline (Atlas et al. 2015).
Determining the extent of dissolution of aromatic hydrocarbons from oil droplets, and distinguishing this from biodegradation in field samples, remains an area
of active research and investigation. It is well known that the small aromatics benzene, toluene, ethylbenzene, and the xylenes dissolve out of oils and gasolines
(Poulsen et al. 1992), and the oil/water partition coefficient increases significantly
at high pressure in the presence of methane, so-called live oil (Jaggi et al. 2017).
This presumably slows dissolution at depth, although of course the equilibrium partition coefficient does not reveal much about the kinetics of dissolution or about the
effect of the essentially infinite dilution available as hydrocarbons leave an oil droplet for the aqueous phase. Modeling (Gros et al. 2016) suggests that high pressures
dramatically favor the aqueous dissolution of C 1 − C 4 hydrocarbons and also influence the buoyancies of bubbles and droplets. These authors believe solubility was a
far more important process than dispersion (Gros et al. 2017), although clearly both
depend on the generation of small droplets, which can be facilitated by dispersant
injection at source. What is clear is that the vast majority of hydrocarbons in dispersed crude oils are biodegraded within a few weeks when oil concentrations are a
few parts per million (Prince et al. 2017), irrespective of their initial presence in
solution or in droplets. This is the great benefit of using chemical dispersants (Prince
2015)—not only do dispersants protect birds and shorelines from oiling by surface
slicks, they also deliver oil to the microbes that provide the only process that will
remove the spilled oil from the biosphere. It is now clear that dispersants have no
significant influence on hydrocarbon biodegradation rates once the oil is dispersed
(Brakstad et al. 2018b) and that they themselves are degraded at similar rates to the
hydrocarbons (Brakstad et al. 2018c; McFarlin et al. 2018).
It is well known that the GoM has thousands of oil seeps (MacDonald et al.
2015), so it is no surprise that a large “standing crop” of oil-degrading microbes was
available to utilize the dispersed oil. A succession of genera proliferated as the spill
progressed (Redmond and Valentine 2011; Dubinsky et al. 2013; Hu et al. 2017;
6 The Importance of Understanding Transport and Degradation of Oil and Gasses…