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

Abstract

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 (δ¹³CO₂ 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.