Ground-water contamination by petroleum hydrocarbons: Natural biodegradation in a dynamic hydrologic and geochemical system

A surficial aquifer contaminated with gasoline was studied as part of the US Geological Survey Toxic Substances Hydrology Program to determine how ground-water chemistry is controlled by microbial degradation processes. This paper presents a study of the temporal changes in hydrologic and geochemical conditions in an aquifer in the Atlantic coastal plain that was affected by hydrocarbon contaminants. The biogeochemical reactions and the resulting chemical composition of shallow ground water were controlled by (1) microbially-mediated reactions that occurred at a small spatial scale, and by (2) changes in these reactions over time due to mixing with infiltrating water, and rising and falling water levels. The concentration gradients observed in contaminated ground water were complicated by seasonal changes in the local hydrology that caused variability in the availability of electron acceptors. Shifts in the biogeochemical reactions occurred rapidly in response to these changes in the hydrogeochemical conditions