Field and in vitro evidence for in-situ bioremediation using compound-specific ¹³C/¹²C ratio monitoring

This work describes the use of δ¹³C values of residual hydrocarbons as a method for demonstrating in-situ biodegradation. Microbial growth, hydrocarbon loss and increase in δ¹³C values were demonstrated in vitro using benzene and styrene as carbon substrates. Isotope evidence of biodegradation were subsequently sought in four field sites contaminated with a wide variety of hydrocarbons. Analysis of residual hydrocarbons in the field indicated that an overall increase in the δ¹³C generally accompanied loss of hydrocarbons, an observation consistent with in-situ biodegradation. The field samples were analyzed using vapor or soil extracts, and the increases in δ¹³C were observed using both types of samples. Vapor sampling is of practical interest because stable isotope ratio monitoring of soil vapor could dramatically reduce the number of wells required for monitoring of ongoing remediation efforts. Our preliminary studies of contaminated field sites allude to the potential of compound-specific isotopic monitoring techniques as a cost-effective measure of in-situ biodegradation.