Redox Roll-Front Mobilization of Geogenic Uranium by Nitrate Input into Aquifers: Risks for Groundwater Resources

Redox conditions are seen as the key to controlling aqueous uranium concentrations (cU_(aq)). Groundwater data collected by a state-wide groundwater quality monitoring study in Mecklenburg-Western Pomerania (Germany) reveal peak cU_(aq) up to 75 μg L^-1 but low background uranium concentrations (median cU_(aq) < 0.5 μg L^-1). To characterize the hydrogeochemical processes causing such groundwater contamination by peak cU_(aq), we reanalyzed measured redox potentials and total concentrations of aqueous uranium, nitrate, and sulfate species in groundwater together with their distribution across the aquifer depth and performed semigeneric 2D reactive mass transport modeling which is based on chemical thermodynamics. The combined interpretation of modeling results and measured data reveals that high cU_(aq) and its depth-specific distribution depending on redox conditions is a result of a nitrate-triggered roll-front mobilization of geogenic uranium in the studied aquifers which are unaffected by nuclear activities. The modeling results show that groundwater recharge containing (fertilizer-derived) nitrate drives the redox shift from originally reducing toward oxidizing environments, when nitrate input has consumed the reducing capacity of the aquifers, which is present as pyrite, degradable organic carbon, and geogenic U(IV) minerals. This redox shift controls the uranium roll-front mobilization and results in high cU_(aq) within the redoxcline. Moreover, the modeling results indicate that peak cU_(aq) occurring at this redox front increase along with the temporal progress of such redox conversion within the aquifer.