RATE OF PETROLEUM HYDROCARBON BIODEGRADATION IN ANOXIC WETLAND SEDIMENT WITH THE ADDITION OF COMMON CO-CONTAMINANTS

Biodegradation rates of soluble components of crude oil in the subsurface are controlled by the availability of electron acceptors as well as the presence of competing electron donors. Given the widespread use of nitrogen as fertilizer and ethanol as a fuel additive, it is important to understand the effects of combined ethanol and nitrate releases on the biodegradation rates of crude oil. In-situ experiments conducted in 2012 at the National Crude Oil Spill Research site near Bemidji, MN indicated that BTEX (benzene, toluene, ethylene, and xylene) mobility was enhanced in the presence of ethanol, and geochemical observations indicated that nitrate may have stimulated the microbial populations, but the experiment was not conducted long enough to fully assess these effects. Follow-up laboratory microcosm studies were designed to evaluate whether the biodegradation of crude oil in wetland sediments is enhanced or reduced by the addition of ethanol and/or nitrate. Deep and shallow wetland sediments collected from the site were homogenized to reflect dominant sediment types observed in wetland cores. Sediments were then placed in an anaerobic atmosphere and placed into ten sealed 100ml glass vessels attached to a closed-loop respirometer. Water taken from the contaminated aquifer was augmented with BTEX (500µg/L), ethanol (1000 µg/L) and/or nitrate (100mg/L) in various combinations. Changes in the rates of CO₂ and CH₄ generation were measured hourly for six months to identify activity patterns between tests. Analysis showed that the presence of both ethanol and nitrate resulted in an increase in respiration rates, but the dominant control was sediment depth. In experiments with shallow sediment, CO₂ generation rates were consistently 40-60% higher than those with deep sediments, likely due to the shallow sediments having approximately 50% more organic matter. The combined presence of ethanol and nitrate enhanced BTEX attenuation rates most effectively, producing increased respiration rates of 0.25µL CO₂/hour in shallow and 0.15µL CO₂/hour in deep sediments. These results illustrate the importance of competing electron acceptors and donors as well as the presence of naturally occurring organic matter on overall rates of biodegradation.