Stable carbon isotope biogeochemistry of a shallow sand aquifer contaminated with fuel hydrocarbons

Ground-water chemistry and the stable C isotope composition (δ13CDIC) of dissolved inorganic C (DIC) were measured in a sand aquifer contaminated with JP–4 fuel hydrocarbons. Results show that ground water in the upgradient zone was characterized by DIC content of 14–20 mg C/L and δ13CDIC values of −11.3‰ to −13.0‰. The contaminant source zone was characterized by an increase in DIC content (12.5 mg C/L to 54 mg C/L), Ca, and alkalinity, with a significant depletion of 13C in δ13CDIC (−11.9‰ to −19.2‰). The source zone of the contaminant plume was also characterized by elevated levels of aromatic hydrocarbons (0 μg/L to 1490 μg/L) and microbial metabolites (aromatic acids, 0 μg/L to 2277 μg/L), non-detectable dissolved O2, NO3 and SO4. Phospholipid ester-linked fatty acid analyses suggest the presence of viable SO4-reducing bacteria in ground water at the time of sampling. The ground-water chemistry and stable C isotope composition of ground-water DIC are interpreted using a chemical reaction model involving rainwater recharge, contributions of CO2 from soil gas and biodegradation of hydrocarbons, and carbonate dissolution. The major-ion chemistry and δ13CDIC were reconciled, and the model predictions were in good agreement with field measurements. It was concluded that stable C isotope measurements, combined with other biogeochemical measures can be a useful tool to monitor the dominant terminal electron-accepting processes in contaminated aquifers and to identify mineralogical, hydrological, and microbiological factors that affect δ13C of dissolved inorganic C.