Geochemical heterogeneity of a gasoline-contaminated aquifer

The scale of biogeochemical reactions was studied in a physically and chemi cally heterogeneous surficial Coastal Plain aquifer contaminated by a gasol ine spill. The physical heterogeneity of the aquifer is manifested in two h ydrologic units, a shallow local aquifer of perched water and a regional sa ndy aquifer. Over the studied vertical interval of 21.3 ft (6.5 m), concent rations of reactive sixties varied by orders of magnitude, and the impact o f biodegradation,was expressed to widely varying degrees. A thin (3 ft thic k) section of the perched-water zone was the most contaminated; total aroma tic hydrocarbons were as high as 19.4 mg/l, Hydrocarbons were degraded by m icrobially mediated reactions that varied over short vertical distances and time. Anaerobic processes dominated within the low-permeability clay unit, whereas in the more permeable sandy layers nitrate reduction and aerobic d egradation occurred. Hydrocarbons were more persistent over time in the low -permeability layer due to the limited availability of electron accepters f or degradation. The microbial degradation of hydrocarbons was-linked to sul fate and iron reduction in the clay unit and led to alterations in the aqui fer solids; electron microscopy revealed the presence of FeS minerals encru sting primary aquifer grains. High concentrations of Fe2+ in groundwater, u p to 34.5 mg/l, persist in kinetic disequilibrium in the presence of elevat ed H2S levels of 1.0 mg/l. Assessment of aquifer heterogeneities and ground water contamination was possible due to sample discrimination at a scale of approximately 2 ft (similar to 0.6 m), a much finer resolution than is att empted in many remedial investigations of polluted aquifers. The informatio n obtained in this type of study is essential to the development of models capable of estimating the fate of hydrocarbons at a site scale. (C) 1999 El sevier Science B.V. All rights reserved.