Pathways of sulfate and hydrogen sulfide transformations in a BTEX- contaminated groundwater system

The gradually increasing anthropogenic impact on the environment over the last 60 years has altered the groundwater quality in many areas around the world. Groundwater represents the main drinking water resource in nearly all European countries. Therefore, the task of ensuring good status of groundwater demands an improved understanding of the efficiency and limitations of microbially driven reactions on the degradation of pollutants in contaminated groundwater systems. In this context stable isotope approaches represent a powerful tool to evaluate and elucidate biogeochemical processes in complex environmental systems. As a result, compound specific stable isotope signatures in various sulfur species were determined in a tar-oil contaminated site and were linked to the microbial community distribution in the aquifer. The goal of the study was to reach an integrated understanding of sulfur cycling processes in contaminated aquifers relative to their importance for the biodegradation. The results show that sulfur cycling is an important process driving microbial degradation of contaminants in porous groundwater systems. For the anoxic plume core it appears that elemental sulfur is a key intermediate during abiotic oxidation of hydrogen sulphide, with the latter formed during bacterial sulfate reduction. The formed elemental sulfur may be used by the specific microbial community found in this aquifer for the oxidation of organic contaminants such as toluene. In contrast, reoxidation of hydrogen sulfide to sulfate by molecular oxygen may affect sulfur cycling within the transition between the unsaturated and the saturated zones and therefore attenuate concentrations of contaminants in groundwater as well.