The influence of droplet size and biodegradation on the transport of subsurface oil droplets during the Deepwater Horizon spill: a model sensitivity study

Abstract

A better understanding of oil droplet formation, degradation, and dispersal in deep waters is needed to enhance prediction of the fate and transport of subsurface oil spills. This research evaluates the influence of initial droplet size and rates of biodegradation on the subsurface transport of oil droplets, specifically those from the Deepwater Horizon oil spill. A three-dimensional coupled model was employed with components that included analytical multiphase plume, hydrodynamic and Lagrangian models. Oil droplet biodegradation was simulated based on first order decay rates of alkanes. The initial diameter of droplets (10-300 μm) spanned a range of sizes expected from dispersant-treated oil. Results indicate that model predictions are sensitive to biodegradation processes, with depth distributions deepening by hundreds of meters, horizontal distributions decreasing by hundreds to thousands of kilometers, and mass decreasing by 92-99% when biodegradation is applied compared to simulations without biodegradation. In addition, there are two-to four-fold changes in the area of the seafloor contacted by oil droplets among scenarios with different biodegradation rates. The spatial distributions of hydrocarbons predicted by the model with biodegradation are similar to those observed in the sediment and water column, although the model predicts hydrocarbons to the northeast and east of the well where no observations were made. This study indicates that improvement in knowledge of droplet sizes and biodegradation processes is important for accurate prediction of subsurface oil spills.

How Do Oil, Gas, and Water Interact Near a Subsea Blowout?

“…Finally, droplets with a diameter of 0.02-0.2 mm, typical of those expected at DWH for dispersant-treated oil experiencing significant latent breakup (Nagamine, 2014) could theoretically be transported 3-20 km before rising out of the intrusion layer. These transport distances are in general agreement with predictions for similar droplet sizes in farfield transport models (Paris et al, 2012;North et al, 2015).…”

“…simulate subsequent fate and transport of multi-fraction Lagrangian elements (Paris et al, 2012;North et al, 2015).…”

“…For an LSM, oil is typically represented by Lagrangian elements representing dissolved hydrocarbons or oil droplets with appropriate droplet size and density (Paris et al, 2012;North et al, 2015). These Lagrangian elements are advected by mean ocean currents and droplet rise velocity that varies based on the temperature and salinity of the ambient water, diffused by ambient turbulence, and transformed by a host of physical, chemical, and biological processes, including dissolution, biodegradation (Lindo-Atichati et al, 2014), high pressure , and sediment particle interactions (Paris et al, 2012;North et al, 2015). While oil transformations are critical for determining oil fate, we focus here on transport and mixing.…”

Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon

“…Superimposed on these changes in physical distribution, hydrocarbons trapped in the deep ocean were subject to biologically mediated loss processes (i.e., biodegradation) (12,16,18,(29)(30)(31). For sparingly soluble hydrocarbons, biodegradation is expected to serve as the primary cause of weathering in the deep ocean because other key weathering processes (e.g., evaporation, photooxidation) depend on atmospheric and solar exposure.…”

“…Hydrocarbons that remained undissolved became trapped in the deep ocean in a suspension of small (less than ∼100 μm) droplets of liquid oil that lacked the buoyant force to rise through the water column. These droplets remained concentrated close to the well's coordinates (2,11,12,15), but modeling suggests that droplet size drove further vertical partitioning, with droplets >50 μm mixing upwards by August 2010 and smaller droplets remaining suspended in the deep ocean (7,17,18). Some suspended oil was eventually deposited to the seafloor, likely via oilmineral aggregates or microbial flocs (8,19,20), with intense contamination within ∼5 km of the well (21)(22)(23)(24)(25)(26).…”