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 micro bial 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 ori gins of metabolic end products. However, in zones of methanogenesis an d/or where the delta(13)C values of endogenous plant matter overlap th ose of hydrocarbons, delta(13)C measurements can produce ambiguous res ults. In such cases, simultaneous measurement of the radiocarbon (C-14 ) contents of metabolic end products can be used to determine their so urces. 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 car bon in groundwater enabled quantification of carbon inputs. Furthermor e, low C-14 contents of high delta(13)C CO2 were crucial in establishi ng that the soil gas CO2 was derived from methanogenesis of hydrocarbo ns and not shell dissolution. In addition, low C-14 content coupled wi th 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.