FATE OF MICROBIAL METABOLITES OF HYDROCARBONS IN A COASTAL-PLAIN AQUIFER - THE ROLE OF ELECTRON ACCEPTERS

A combined field and laboratory study was undertaken to understand the distribution and geochemical conditions that influence the prevalence of low molecular weight organic acids in groundwater of a shallow aqu ifer contaminated with gasoline. Aromatic hydrocarbons from gasoline w ere degraded by microbially mediated oxidation-reduction reactions, in cluding reduction of nitrate, sulfate, and Fe(III). The biogeochemical reactions changed overtime in response to changes in the hydrogeochem ical conditions in the aquifer. Aliphatic and aromatic organic acids w ere associated with hydrocarbon degradation in anoxic zones of the aqu ifer. Laboratory microcosms demonstrated that the biogeochemical fate of specific organic acids observed in groundwater varied with the stru cture of the acid and the availability of electron accepters. Benzoic and phenylacetic acid were degraded by indigenous aquifer microorganis ms when nitrate was supplied as an electron acceptor. Aromatic acids w ith two or more methyl substituents on the benzene ring persisted unde r nitrate-reducing conditions. Although iron reduction and sulfate red uction were important processes in situ and occurred in the microcosms , these reactions were not coupled to the biological oxidation of arom atic organic acids that were added to the microcosms as electron donor s.