Spatial Heterogeneity of Microbial and Geochemical Parameters in Gasoline Contaminated Aquifers

Abstract

Spatial heterogeneity of microbial populations (total heterotrophs or total bacteria (AODC), iron reducers, sulfate reducers, methanogens, aerobic, and anaerobic protozoa) and geochemical parameters (hydrocarbon concentrations, inorganics, grain size, sediment Fe) were studied at gasoline contaminated aquifers located at Rocky Point, N.C. and Pope Air Force Base (AFB), N.C. At Rocky Point, heterogeneity was studied on samples collected on vertical scale (2–3 cm and 0.3 m intervals) and horizontal scale (>40 m). At Pope (AFB) heterogeneity was studied in samples collected on vertical scale (7.5 cm intervals) and horizontal scale (>20 m). Distribution of microbial populations at Rocky Point showed that there is little variability in 2–3 cm spaced samples, while there is substantial variability over 0.3 m intervals. Elevated numbers of iron reducers and anaerobic protozoa were measured at specific depth intervals that corresponded with reduced contaminant concentrations. On the horizontal scale little variability was observed in microbial and geochemical parameters. At Pope AFB, distribution of microbial populations varied on horizontal and vertical scales. Shifts in the dominance of trophic groups were observed on vertical scale and dominant microbial processes varied on the horizontal scale. In addition aqueous inorganic analysis and sediment Fe analysis showed that geochemistry at the two aquifer varied. At Rocky Point extractable Fe(III) was greater relative to Pope AFB and significantly greater fraction of total Fe(II) was on the solid phase (>98%) relative to Pope AFB (60–72%). The measured heterogeneity suggests that techniques utilizing blended sediment for the measurement of in situ biodegradation rates and vertically averaged measurement of hydrocarbons in the field will provide misleading results.