Biodegradation of aromatic hydrocarbons under anoxic conditions in a shallow sand and gravel aquifer of the Lower Rhine Valley, Germany

The groundwater in the area of the former gasworks at Düsseldorf in the Lower Rhine Valley, Germany, is contaminated with up to 80 mg l−1 of aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylenes (BTEX), other alkylated benzenes and low molecular weight PAH. About 100 m downgradient from the contamination source a rapid decrease of these compounds to less than 0.5 mg l−1 is observed due to biodegradation under anoxic conditions. Abundant organic acids are generated as metabolic products of the aromatic hydrocarbons in the contaminant plume. Degradation of the aromatic hydrocarbons is related to nitrate reduction and particularly sulfate reduction. Two possible explanations for the enhanced concentrations of dissolved iron can be given: (1) Biogenic hydrogen sulfide originating from degradation of aromatic hydrocarbons by sulfate reducing bacteria is used for the abiotic ferric iron reduction; (2) Organic acids generated from microbial degradation of aromatic hydrocarbons act as ligands for complexing insoluble Fe(III) oxides in the aquifer. Thereby Fe(III) is made available for iron-reducing bacteria intensifying the degradation process of the aromatic hydrocarbons. Downgradient from the contaminant plume the groundwater is reoxygenated and the dissolved iron is reprecipitated.