Intrinsic Aromatic Hydrocarbon Biodegradation for Groundwater Remediation

Abstract

Intrinsic biodegradation, representing the key process in natural attenuation, is increasingly considered for the remediation of contaminated sites as an alternative to more active measures. In this paper, intrinsic biodegradation is discussed with respect to BTEX and PAH. In the first part, an overview is given summarizing the current understanding of microbial aromatic hydrocarbon degradation and the methods available for the assessment of intrinsic bioremediation. In the second part, the concept and selected results of a case study are presented. Both aerobic and anaerobic biodegradation of aromatic hydrocarbons contribute to pollutant elimination at contaminated sites such as former manufactured gas plants and tar-oil polluted disposal sites. Intrinsic biodegradation processes usually result in a sequence of redox zones (methanogenic, sulfate-reducing, Fe(III)-reducing, denitrifying, aerobic) in the groundwater plume downgradient the source of contamination. Methods to assess redox zonation include hydro- and geochemical analysis, measurement of the redox potential, and determination of hydrogen. Biodegradation of target pollutants can be demonstrated by alterations in the pollutant profiles, isotopic fractionation, specific metabolic products, and by microcosm studies with authentic field samples. Microcosm studies in particular are a useful tool to identify degradation mechanisms and to understand the role of specific electron acceptors and redox conditions. In a case study, intrinsic biodegradation was examined at a tar-oil polluted disposal site. Due to the low sorption capacity of the aquifer, decreasing pollutant concentrations with increasing plume length were attributed predominantly to biodegradation. Sulfate reduction and Fe(III) reduction were the most important redox processes in the anaerobic core of the groundwater plume. Changing pollutant profiles with increasing plume length indicated active biodegradation processes, e.g. biodegradation of toluene and naphthalene in the anaerobic zones. In microcosms amended with model pollutants, biodegradation of toluene and ethylbenzene was observed under sulfate-reducing conditions. Degradation of toluene, ethylbenzene, benzene and naphthalene occurred in the presence of Fe(III). Under aerobic conditions, all BTEX and PAH were rapidly degraded.