Interaction of dissolved and sedimentary sulfur compounds in contaminated aquifers

A sulfate rich Quaternary urban aquifer is contaminated with a variety of different petroleum hydrocarbons. To reveal the details of the sulfur cycling and to prove the occurrence of bacterial sulfate reduction (BSR), as one major process of anaerobic biodegradation, geochemical and isotope investigations of different dissolved (sulfate and sulfide) and reduced inorganic sedimentary sulfur compounds (monosulfide, disulfide, and elemental sulfur) were conducted. In order to recognize possible biodegradation, the carbon isotopic composition of dissolved inorganic carbon was examined as well. The local significant depletion of 13C in DIC is a first indicator for the occurrence of biodegradation at the site. The enrichment of both heavy sulfur and heavy oxygen isotopes in dissolved sulfate and the simultaneous decrease of sulfate concentrations at several sampling locations provide clear evidence for the occurrence of BSR in the aquifer. The isotope distribution pattern of sulfate suggests a straightforward bacterial reduction process. However, the isotopic composition of dissolved sulfide indicates the occurrence of secondary processes superimposed on BSR. Primary or secondary pyrite is the dominant sedimentary sulfur species in the aquifer matrix. The ubiquitous occurrence of sedimentary monosulfide is further evidence for the important role of BSR. However, the presence of elemental sulfur and the isotope distribution pattern of sedimentary sulfur species point to a temporal variability of the hydrochemical conditions at the site with the occurrence of secondary sulfur transformations such as partial re-oxidation. Nevertheless, isotope and hydrochemical data indicate an overall high natural attenuation potential of the investigated aquifer. ►Sulfur isotope ratios indicate bacterial sulfate reduction in contaminated aquifer. ►Organic contaminants are the dominant electron donor for sulfate reduction. ►Amount of reduced sulfur species indicates high biodegradation potential. ►Secondary sulfide oxidation is locally superimposed on sulfate reduction. ►Sulfide oxidation competes with biodegradation for electron acceptors.