Radiocarbon-depleted CO2 evidence for fuel biodegradation at the Naval Air Station North Island (USA) fuel farm site

摘要

Dissolved CO2 radiocarbon and stable carbon isotope ratios were measured in groundwater from a fuel contaminated site at the North Island Naval Air Station in San Diego, CA (USA). A background groundwater sampling well and 16 wells in the underground fuel contamination zone were evaluated. For each sample, a two end-member isotopic mixing model was used to determine the fraction of CO2 derived from fossil fuel. The CO2 fraction from fossil sources ranged from 8 to 93% at the fuel contaminated site, while stable carbon isotope values ranged from -14 to +5‰_(VPDB). Wells associated with highest historical and contemporary fuel contamination showed the highest fraction of CO2 derived from petroleum (fossil) sources. Stable carbon isotope ratios indicated sub-regions on-site with recycled CO2 (δ~(13)CO2 as high as +5‰_(VPDB)) - most likely resulting from methanogenesis. Ancillary measurements (pH and cations) were used to determine that no fossil CaCO3, for instance limestone, biased the analytical conclusions. Radiocarbon analysis is verified as a viable and definitive technique for confirming fossil hydrocarbon conversion to CO2 (complete oxidation) at hydrocarbon-contaminated groundwater sites. The technique should also be very useful for assessing the efficacy of engineered remediation efforts and by using CO2 production rates, contaminant mass conversion over time and per unit volume.