Study Ranks Parameters in Oil Biodegradation Models and Quantifies Dispersant Effects
– JANUARY 9, 2020
Scientists compared oil biodegradation model parameters and ran simulations to understand the relative importance of variables that affect predictions for oil fate from a deep-water release. Simulations, with and without sub-surface dispersant injection, showed that the most important parameter affecting oil surfacing was initial droplet diameter, with biodegradation contributing substantially to the fate of droplets ≤0.5 mm. The parameters of biodegradation rates and lag time prior to the onset of biodegradation were of secondary importance, and oil type had limited influence on oil fate.
Simulations with dispersant treatment (2% volume ratio) showed more than an order of magnitude decrease in oil droplet volume median diameter (from 5.69 mm to 0.29 mm), with ~30% of the released oil reaching the ocean surface. The rest of the oil remained in the subsurface where degradation occurred on time scales of days to weeks, with some compounds able to persist well beyond the 2-week model simulations. Simulations without dispersant treatment showed droplets rising to the surface almost immediately, with ~90% of the released oil reaching the ocean surface. The researchers offered recommendations to improve oil fate and transport models related to systematic assessment and comparison of biodegradation rates, degradation pathways, and biodegradation characterization.
The authors published their findings in Marine Pollution Bulletin: The treatment of biodegradation in models of sub-surface oil spills: A review and sensitivity study.
Currently-available oil spill models adopt simplified algorithms to accout for biodegradation, a key and complex process for natural remediation of oil spills. However, the algorithms have challenges: measured biodegradation rates vary widely in the literature, there are multiple approaches for describing and parameterizing the biodegradation process in operational oil spill models, and biodegradation rates influence the predicted far-field fate of sub-surface oil droplets.
Study author Scott Socolofsky explained, “Studies that can predict droplet sizes, both at the release of an oil spill and due to wind/wave action at the surface, are critically needed to improve the veracity of numerical model predictions. There is also a need to develop a standard method to measure biodegradation rates of crude oil in water. Most studies use different methods, and few studies are able to distinguish between biodegradation of the liquid crude oil and biodegradation of dissolved components in the water.”
To address this need, the team conducted a literature review and convened two workshops in 2016 with experts (representatives of various modeling groups and academia) to assess the current state-of-the-art of modeling oil degradation. They also held individual discussions with modelers to clarify formulation of oil fate processes, including the description of dissolution and biodegradation kinetics. These efforts resulted in a summary of modeling degradation, including algorithms, rate constants, and model formulations, which are detailed in this study.
Then, guided by their earlier work (the inter-comparison of oil spill prediction models), the team used the Texas A&M Oil spill Calculator (TAMOC) to identify how differences in biodegradation formulations affected the predicted fate of oil droplets from a deep-sea release (2000 m depth). Thirty-two simulations of 14-day scenarios included: 1) a steady, uniform flow field; 2) Louisiana Sweet Crude and Hoops Oil, which have similar properties and a large range of hydrocarbon compositions (116 were mapped); and 3) a relatively high dispersant-to-oil ratio (2%) when sub-surface dispersant injection was considered.
The authors acknowledged that a smaller dispersant-to-oil ratio would reduce the preponderance of the model sensitivity to droplet size, but it would likely still be the dominant parameter given its strong influence on predicted oil fate and transport.
Recommendations that the authors offered to improve oil biodegradation and transport models include:
- Agree on protocols to measure biodegradation rate in the laboratory that can facilitate inter-comparison of rates measured in multiple laboratories;
- Distinguish biodegradation of liquid and dissolved hydrocarbon separately in laboratory and field studies;
- Develop an updated common database of biodegradation half-lives, for individual hydrocarbons, that could be mapped to pseudo-components to better support multiple oil spill models;
- Develop improved algorithms to account for factors that modulate half-lives (e.g., temperature, dissolved oxygen, nutrients);
- Incorporate generation and decay of abiotic and biotic transformation products through mapped decay chains in oil fate assessment;
- Develop and calibrate models that treat biodegradation of liquid oil as a function of the area of the oil/water interface; and
- Better understand the lag time associated with the onset of biodegradation in laboratory experiments and whether/how it relates to microbial growth on dissolved and droplet oil in the field.
Socolofsky put their results in an oil spill response context, “During an oil spill, responders basically have two options: collect the oil or disperse it. Especially during the early stages of a spill, collection is often impossible for much of the oil. Biodegradation is an important, natural process that will remove spilled oil that has been dispersed over time. This study helps to understand how far, and for how long, the dispersed oil could be transported in the subsurface before it surfaces or is degraded.”
Data for this study are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at DOI 10.7266/n7-5v1h-1t66.
By Nilde Maggie Dannreuther. Contact firstname.lastname@example.org with questions or comments.
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