Biogeochemical controls on biodegradation of MC252 crude oil in coastal wetland systems

The persistence and fate of MC252 oil as well as the presence of nutrients and their effect on biodegradation were studied in both an oil-contaminated marsh and a nearby oil-contaminated mangrove site near Port Fourchon, Louisiana as well as a second marsh site in Barataria Bay adjacent to Bay Jimmy. High-resolution nutrient profiles were obtained at different locations at each site during winter and summer using dialysis samplers. Areas of sulfate depletion were observed in the top 20 cm of dialysis samplers in the Fourchon marsh and mangrove sites. When sulfate depletion was observed, it was accompanied by an increase in the ammonia and/or phosphorous concentration signifying an anoxic zone in the sediment. This anoxic zone was found more frequently in summer sampling compared to winter sampling. Flux chamber were installed at each site to capture evolved CO2 to determine if carbon from the MC252 oil was being mineralized at each site. Carbon derived from oil and carbon derived from indigenous organic matter was differentiated using the stable carbon isotope ratio. d13C signatures of the bulk soil, root matter, and soil with roots removed were measured at depth to compare contaminated sites to nearby inland sites with little to no oil contamination. A d13C signature of −24.8 ± 0.8 ‰ was observed in the bulk soil of the contaminated shoreline site in the Port Fourchon Marsh while an adjacent uncontaminated inland bulk soil sample had a d13C signature of −17.3 ± 0.8 ‰. d13C signatures were measured for sample before and after being washed with a hexane acetone solvent which removed oil. The signature of the samples became less negative after being washed with the solvent. To supplement the field findings, a laboratory microcosm study was performed to observe the effects of adding nutrients to a wetland soil/water/oil mixture. The study focuses on 5 different treatments: natural attenuation, sulfate amended, ammonia amended, sulfate and ammonia amended, and a killed control. A second set of microcosm experiments were conducted in triplicate under aerobic conditions along with a kill control sample. There was no observed degradation in the anaerobic microcosm experiments for any of the treatments during the time of the study. Aerobic microcosm results are still pending. Of particular interest are petrogenic PAHs (i.e, C1-phenanthrenes, C2-dibenzothiophenes, and C3-dibenzothiophenes) since there is little information in the peer-reviewed literature on the effectiveness of biostimulation in marsh systems.