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

Abstract

Redox conditions are seen as the key to controlling aqueous uranium concentrations (cU). Groundwater data collected by a state-wide groundwater quality monitoring study in Mecklenburg-Western Pomerania (Germany) reveal peak cU up to 75 μg L but low background uranium concentrations (median cU < 0.5 μg L). To characterize the hydrogeochemical processes causing such groundwater contamination by peak cU, 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 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 within the redoxcline. Moreover, the modeling results indicate that peak cU occurring at this redox front increase along with the temporal progress of such redox conversion within the aquifer.

Model-Based Analysis of Nitrate Concentration in the Leachate—The North Rhine-Westfalia Case Study, Germany

“…In reduced aquifers, such a relationship is missing due to denitrification processes [20][21][22][23], and thus the related groundwater bodies are in good status with regard to nitrate in spite of high nitrogen inputs. Consequently, no measures to reduce N input have to be implemented in such cases.…”

Metal Mobilization As An Effect of Anthropogenic Contamination in Groundwater Aquifers in Tutuila, American Samoa

“…In this study, we found minimal levels of NO 2 − , so its role in U oxidation and correlation with U could not be evaluated. Finally, numerical model simulations conducted by van Berk et al [32] confirmed their hypothesis that the addition of NO 3 − (derived from fertilizers) shifted groundwater redox conditions to an oxidizing environment, thus mobilizing U.…”

“…The mobility of U is primarily dependent on its oxidation state, where oxidized U(VI) is generally more soluble than reduced U(IV) [10,31]. With an increasing amount of oxidants present in the groundwater system, uraninite (UO2) will dissolve from minerals to aqueous forms of U [32]. For example, NO3 − can oxidize UO2 following the reaction: UO2 + NO3 -+ 2H + → UO2 2+ + NO2 -+ H2O,…”

Hydrochemistry of the Tuxertal, NW Tauern Window, Austria: water use and drinking water supply in an alpine environment

“…The first four correlations are explainable by the paragenesis discussed above. A correlation between uranium and nitrate can be a result of uranium mobilization caused by nitrate as oxidizing agent (Berk & Fu, 2017;Bonotto et al, 2019). Spearman's rank correlation matrices become more meaningful, if single rock types are evaluated.…”