Biogeochemical controls on fate of subsurface oiled sands on a coastal headland beach

Abstract

The fate of subsurface oiled sands collected from Fourchon Beach in Louisiana were determined while modifying biogeochemical controls on the degradation of PAHs. Groundwater on the beach has intrinsically low dissolved oxygen concentrations, which may limit natural biodegradation of the crude oil components. The intent of this research was to characterize the biogeochemical properties and degradability of oiled sands (with >10% of pore filled with MC252 oil) using a combination of laboratory flow-through reactor studies, field measurements and time-series microelectrode profiles of down-flow and cross flow geometries. Reactor experiments indicate that optimal conditions for substantial oil degradation are aerobic under advective-dispersive transport processes, and when amended with N and P (DO > 5 mg/L, > 6 mg-N/L, and > 0.6 mg-P/L). Cross-flow reactor studies, which mimic the presence of oiled sands over impermeable clayey deposits, showed no significant (P < 0.05) degradation of phenanthrenes or benzenothiophenes. Time series O2. microelectrode profiles showed that down-flow reactors had greater O2 penetration (>14 mm) than cross-flow geometries, which remained largely oxygen deficient at depths greater than 7 mm. Aerobic degradation of MC252 oil deposits on Fourchon Beach will be controlled by field transport mechanisms of available nutrients and oxygen. Due to low oxygen concentrations in natural groundwater at Fourchon Beach, in conjunction with low oxygen penetration depths and availability of nutrients, the rate of oil biodegradation in these environments is likely to be severely impeded.