Decadal variations in groundwater quality: A legacy from nitrate leaching and denitrification by pyrite in a sandy aquifer

Twenty‐five years of groundwater quality monitoring in a sandy aquifer beneath agricultural fields showed large temporal and spatial variations in major ion groundwater chemistry, which were linked closely to the nitrate (NO3) content of agricultural recharge. Between 1988 and 2013, the NO3 content of water in the oxidized zone of the aquifer nearly halved, following implementation of action plans to reduce N leaching from agriculture. However, due to denitrification by pyrite oxidation in the aquifer, a plume of sulfate‐rich water migrates through the aquifer as a legacy of the historical NO3 loading. Agriculture thus is an important determinant of major ion groundwater chemistry. Temporal and spatial variations in the groundwater quality were simulated using a 2D reactive transport model, which combined effects of the historical NO3 leaching and denitrification, with dispersive mixing into the pristine groundwater residing deeper in the aquifer. Reactant‐to‐product ratios across reaction fronts are altered by dispersive mixing and transience in reactant input functions. Modelling therefore allowed a direct comparison of observed and simulated ratios of concentrations of NO3 (reactant) in the oxidized zone to those of SO4 (product) in the reduced zone, which aided a stoichiometric assessment of the mechanisms of denitrification. Denitrification by pyrite in the Rabis Creek aquifer results in oxidation of S−1 and Fe2+ in pyrite to S6+ in dissolved SO4 and Fe3+ in Fe‐oxide. Neither precipitation of elemental sulfur (S0), nor of jarosite, was supported by observations, and adsorption of sulfate was also dismissed. Key Points Land‐use controls major ion groundwater chemistry: Every farmer makes his own groundwater quality! Reactant‐to‐product ratios across reaction fronts become altered by dispersion and transience in reactant input functions The nitrate legacy causes decadal fluctuations in groundwater quality as NO3 and SO4 derived from denitrification sweep through aquifers Plain Language Summary Agricultural fertilization has a major impact on groundwater chemistry in underlying aquifers. Popularly stated, every farmer makes his own groundwater quality. The study shows that this is the case for the major ions dissolved in groundwater. Hence, agricultural activities also may alter groundwater quality when it comes to trace compounds (e.g., pesticides and trace metals) and nitrate in itself. In the study, up to three quarters of the total dissolved solutes