Water-column Measurements and Observations from the Deepwater Horizon Oil Spill Natural Resource Damage Assessment

Abstract

ABSTRACT NO. 2017-167 As part of the Natural Resource Damage Assessment (NRDA) effort following the Deepwater Horizon (MC252) blowout and oil spill in 2010, over 5,300 water samples were forensically evaluated both as evidence of exposure and to validate oil fate and transport modelling. In addition to whole water-sample grabs, particulate-oil and dissolved-phase samples from the subsurface release were separated (filtered) in the field to provide detailed information on the partitioning behavior of oil droplets in a deepwater plume (1,000–1,400m) extending to the southwest (SW) of the wellhead. Offshore, the subsurface plume was visually observed and photographed using remotely operated vehicles (ROVs), and tracked in conductivity, temperature, and depth (CTD), dissolved oxygen (DO), and fluorometry profiles. The farthest reach of the plume was 412 km (250 mi) SW of the wellhead as confirmed by multiple lines of evidence (i.e., depth, fluorometry spikes, DO sags, and dispersant indicators) and out to 267 km as weathered, phase-discriminated, confirmed hydrocarbon profiles. With increasing time and distance from the wellhead, the plume’s polycyclic aromatic hydrocarbon (PAH) signal became diluted and eventually no longer detectible using selected-ion-monitoring (SIM) gas chromatography/mass spectrometry (GC/MS), although the plume was still discernible in the corroborating data. We hypothesize that microbial degradation at depth converted the PAH and aliphatics into oxygenated and polar products not detectible using SIM GC/MS methods. Near-surface oil samples showed evidence of substantial dissolution weathering as the oil droplets rose through the water column, and further evaporative losses of lower-molecular-weight n-alkanes and aromatic hydrocarbons occurred after the oil reached the surface. Surface oil also showed evidence of photo-oxidation of alkylated chrysenes and triaromatic steranes. Typical of surface oil dynamics, increases in dissolved and particulate-oil fractions were observed in the shallow sub-surface as a result of both dispersant effects and wave reentrainment of surface films. Dispersant treatment effects, both as surface applications and injected at the wellhead, showed uniquely enhanced-dissolution weathering patterns in PAH profiles with limited or delayed microbial degradation of saturated hydrocarbons (SHC) close to the wellhead. From an oil-fate-and-transport standpoint, these data document that the dispersant applications at depth were functionally effective in breaking up the oil droplets and thereby preventing some portion of the oil from reaching the surface.