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

Abstract 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 (NO 3 ) content of agricultural recharge. Between 1988 and 2013, the NO 3 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 NO 3 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 NO 3 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 NO 3 (reactant) in the oxidized zone to those of SO 4 (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 Fe 2+ in pyrite to S 6+ in dissolved SO 4 and Fe 3+ in Fe‐oxide. Neither precipitation of elemental sulfur (S 0 ), nor of jarosite, was supported by observations, and adsorption of sulfate was also dismissed.