To control the impact of nitrate and its sustainable mitigation in groundwater systems used for drinking water production, it is crucial to understand and quantify sources as well as biochemical processes which (permanently) remove nitrate.
In an alluvial aquifer in Germany (Hessian Ried) that serves as major drinking water recourse for the Frankfurt metropolitan area, water quality is challenged by nitrate contamination from intensive agricultural land use locally by far exceeding the drinking water limits of 50 mg/L.
In order to evaluate the capability of the aquifer for natural mitigation of the nitrate contamination, we investigated the denitrification potential with respect to the availability of electron donors and the predominant reduction pathways in different sections of the aquifer. The content of sedimentary sulfide and organic carbon was quantified by solid-phase analyses of drill core samples from aquifer sediments. Water samples from vertical profiles gave access to information on the isotope-hydrochemical composition of the groundwater (multi-parameter profiles, major ions, nitrate isotope signature, sulfate isotope signature). Using this hydrochemical and isotope information in concert with the results of a groundwater flow model allowed determining the nitrate input and the average nitrate reduction kinetics along the flow path upstream of selected groundwater monitoring wells. Batch and column experiments provided detailed information on prevailing reaction pathways and the associated isotope fractionation pattern enabling the recognition and quantification of processes on field scale. Our results suggest that litho-autotrophic denitrification using sedimentary sulfide as an electron donor is preferably responsible for the nitrate degradation in the aquifer. However, due to the low sulfide content (max. 123 mg-S/kg), the potential for autotrophic denitrification is very limited. Consequently, if no active measures reducing the input of fertilizer-derived nitrate will be implemented in the near future, the limited potential for autotrophic denitrification will ultimately exhaust and a severe deterioration of the groundwater quality can be expected.