Sulfur Cycling and Biodegradation in Contaminated Aquifers: Insights from Stable Isotope Investigations

We applied the dual isotope System (delta S-34-delta O-18-SO42-) to investigate the relevance of bacterial sulfate reduction (BSR) for natural biodegradation in an anaerobic, sulfate rich aquifer contaminated with petroleum hydrocarbons. Isotope fractionation parameters were determined in column experiments operated under near in situ conditions at the site of the contaminated aquifer. Using those fractionation parameters as a reference, we showed that differences between field derived and experimental fractionation parameters provide essential information on the determination of secondary sulfur transformation processes superimposing BSR and competing with the actual biodegradation reactions. Most important of those processes is the reoxidation of reduced sulfur species consuming electron acceptors that would be relevant for contaminant oxidation. Furthermore,the detailed, flow path related analysis of the sulfate isotope distribution pattern revealed that BSR and consequently biodegradation is predominately occurring in hot spots. It also showed the occurrence of sharp hydrochemical gradients that change the natural attenuation potential of the aquifer over a very short distance. Generally, this hydrochemical heterogeneity limits the applicability of isotope investigation for quantifying bacterial sulfate reduction. Nevertheless, the identification of sulfate reducing hot spots and hydrochemical gradients as well as the recognition of geochemical processes competing for electron acceptors are essential to understand natural attenuation of contaminants in aquifers.