Combined ¹⁴C and δ¹³C Monitoring of <i>in Situ</i> Biodegradation of Petroleum Hydrocarbons

Abstract

Measurements of the stable carbon isotope ratios (δ13C) of microbial metabolic end products presents a promising method for monitoring in situ bioremediation of petroleum hydrocarbons. Differences between the δ13C 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 δ13C values of endogenous plant matter overlap those of hydrocarbons, δ13C measurements can produce ambiguous results. In such cases, simultaneous measurement of the radiocarbon (14C) 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 14C-free carbon. Combined δ13C and 14C measurements of soil gas CO2 and dissolved inorganic carbon in groundwater enabled quantification of carbon inputs. Furthermore, low 14C contents of high δ13C CO2 were crucial in establishing that the soil gas CO2 was derived from methanogenesis of hydrocarbons and not shell dissolution. In addition, low 14C content coupled with a 16‰ drop in the δ13C values of CO2 across a semipermeable layer beneath the gas station building confirmed that microbial oxidation of methane was occurring within this zone.