Coupled biogeochemical dynamics of nitrogen and sulfur in aquifer systems and implications for groundwater quality threatened by agricultural nitrogen pollution

Nitrate contamination due to agricultural activity represents a worldwide threat to groundwater resources. Denitrification is the major pathway of removal of nitrate from groundwater systems. This anaerobic process may be coupled to the oxidation of organic matter or to that of pyrite. Coupled denitrification - pyrite oxidation may lead to mobilization of trace metals and sulfate production, and may, therefore, cause degradation of groundwater and surface water quality. Relatively little is known about the quantitative role of pyrite oxidation for nitrate removal and factors controlling this process in groundwater systems. We propose an integrated combination of experimental, field and local/regional reactive transport modeling to (1) elucidate the key factors controlling the rate of denitrification plus the relative contribution of organic matter and pyrite as electron donors at two study sites (these factors include pH, temperature, nitrate concentration, the surface chemistry of pyrite and reactivity of the organic matter), (2) determine the impact of denitrification coupled to pyrite oxidation on local and regional groundwater quality, and (3) assess how the natural capacity of the groundwater systems for denitrification will evolve with time at the two locations. We will focus on two sites in the Netherlands (Oostrum in Limburg and the Kempen region in Noord Brabant) where preliminary field results indicate substantial agricultural nitrate pollution, nitrate removal through denitrification coupled to pyrite oxidation, and trace metal mobilization. Particular innovative aspects of this study are (1) the combined use of S and N isotopes to assess the sources and transformation of nitrate and sulfate, and (2) the reconstruction of present-day and historical groundwater quality and the prediction of its future evolution on a regional scale. The results of this project will provide much needed data on crucial biogeochemical processes that control groundwater quality, as well as new modeling tools to assess the impact of nitrate contamination on groundwater resources.