Formation of alkylated aromatic acids in groundwater by anaerobic degradation of alkylbenzenes

Abstract

Evidence for the presence of previously unreported alkylated aromatic acids in anaerobic groundwater samples from a jet fuel-contaminated site is presented. The redox conditions are dominated by sulfate reduction. A gas chromatography/mass spectrometry screening showed a composition of aromatic hydrocarbons ranging from BTEX (C 0–C 2-substituted benzenes) up to C 7-benzenes. Known metabolites like methylbenzoates (C 1-BA), C 2-BA, C 3-BA as well as methylbenzylsuccinates (C 1-BSA) were detected. Additionally, previously unreported metabolites of higher alkylated benzenes have been identified in the extracts: those are C 2-BSA to C 5-BSA as well as C 4-BA to C 6-BA, which were detected in the μg/l-concentration range. The formation of C 1- and C 2-BSA as well as the C 1- and C 2-BA can be linked to structurally corresponding parent contaminants like the xylene isomers and the trimethylbenzene isomers. The C 3-BSA and C 3-BA represent metabolites of C 4-benzenes. The occurrence of C 4–C 5-BSA and C 4–C 6-BA is attributed to C 5–C 7-benzenes acting as parent compounds. The concentration of total aromatic contaminants decreased along the plume centre line. In contrast, benzoates and benzylsuccinates (BA and BSA) showed constant concentrations over a distance of 84 m. BA concentrations were up to 162 times higher compared to BSA, as indicated by the ratio f BA/BSA. A pronounced transient behaviour of BSA or an overall persistent behaviour of BA can explain this. Hence, along the plume centre line, f BA/BSA was constant over a distance of 128 m. The degradation products detected in a monitoring well are not necessarily tied to contaminants detected in that well. Therefore, the metabolite plume is partly located in front of the contaminant plume, but does not significantly migrate downstream the plume toe.