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

Ground-water chemistry and the stable C isotope composition (delta(13)C(DIC )) Of dissolved inorganic C (DIC) were measured in a sand aquifer contamina ted with JP-4 fuel hydrocarbons. Results show that ground water in the upgr adient zone was characterized by DIC content of 14-20 mg C/L and delta(13)C (DIC) values of -11.3 parts per thousand to -13.0 parts per thousand. The c ontaminant source zone was characterized by an increase in DIC content (12. 5 mg C/L to 54 mg CL), Ca, and alkalinity, with a significant depletion of C-13 in delta(13)C(DIC) (-11.9 parts per thousand to -19.2 parts per thousa nd). The source zone of the contaminant plume was also characterized by ele vated levels of aromatic hydrocarbons (0 mu g/L to 1490 mu g/L) and microbi al metabolites (aromatic acids, 0 mu g/L to 2277 mu g/L), non-detectable di ssolved O-2, NO3 and SO4. Phospholipid ester-linked fatty acid analyses sug gest the presence of viable SO4-reducing bacteria in ground water at the ti me of sampling. The ground-water chemistry and stable C isotope composition of ground-water DIC are interpreted using a chemical reaction model involv ing rainwater recharge, contributions of CO2 from soil gas and biodegradati on of hydrocarbons, and carbonate dissolution. The major-ion chemistry and delta(13)C(DIC) were reconciled, and the model predictions were in good agr eement with field measurements. It was concluded that stable C isotope meas urements, combined with other biogeochemical measures can be a useful tool to monitor the dominant terminal electron-accepting processes in contaminat ed aquifers and to identify mineralogical, hydrological, and microbiologica l factors that affect delta(13)C Of dissolved inorganic C. (C) 1999 Elsevie r Science Ltd. All rights reserved.