Hydrogen and carbon isotope fractionation during anaerobic biodegradation of aromatic hydrocarbons--a field study

The aquifer of a former manufactured gas plant site, highly contaminated by dissolved monocyclic, heterocyclic, and polycyclic aromatic hydrocarbons, was studied to evaluate the applicability of carbon and hydrogen isotope fractionation to prove ongoing biodegradation of these compounds even in complex aquifer settings. The loss of toluene, o-xylene, p,m-xylene, and 2-methylnaphthalene was accompanied by a considerable carbon isotope fractionation. Additionally, a strong 2H enrichment in residual o-xylene was detected. All isotope fractionations observed could be related to established biochemical degradation mechanisms, each involving a C-H bond cleavage in the rate-determining step. In contrast, other compounds such as 1-methylnaphthalene, methylbenzofuran, and acenaphthene exhibited a uniform stable carbon isotope composition. However, a decrease in concentration for these compounds was observed in the flowpath of the aquifer. High threshold concentrations of acenaphthene downgradient indicate that this contaminant is, if at all, only marginally biodegraded. Detailed analyses of xylenes provided support that compound specific isotope analyses and subsequent application of the Rayleigh model may provide a valuable basis to distinguish between different biodegradation mechanisms as well as dissolution processes in heterogeneous aquifers.