Combined C-14 and delta C-13 monitoring of in situ biodegradation of petroleum hydrocarbons

Measurements of the stable carbon isotope ratios (delta(13)C) of microbial metabolic end products presents a promising method for monitoring in situ bioremediation of petroleum hydrocarbons. Differences between the delta(13)C values of hydrocarbons and indigenous carbon sources (e.g., plant matter, soil carbonates) can be exploited to trace the origins of metabolic end products. However, in zones of methanogenesis and/or where the delta(13)C values of endogenous plant matter overlap those of hydrocarbons, delta(13)C measurements can produce ambiguous results. In such cases, simultaneous measurement of the radiocarbon (C-14) contents of metabolic end products can be used to determine their sources. This method was applied at a gasoline station spill site where hydrocarbons were the only source of C-14-free carbon. Combined delta(13)C and C-14 measurements of soil gas CO2 and dissolved inorganic carbon in groundwater enabled quantification of carbon inputs. Furthermore, low C-14 contents of high delta(13)C CO2 were crucial in establishing that the soil gas CO2 was derived from methanogenesis of hydrocarbons and not shell dissolution. In addition, low C-14 content coupled with a 16 parts per thousand drop in the delta(13)C values of CO2 across a semipermeable layer beneath the gas station building confirmed that microbial oxidation of methane was occurring within this zone.