Contrasting release of redox-sensitive arsenic and uranium induced by heterotrophic denitrification in alluvial sediments

Arsenic (As) and uranium (U) are toxic trace elements and can contaminate groundwater. Elevated As and U concentrations exceeding the WHO drinking water guideline values were found within the capillary fringe and shallow groundwater at an agricultural site in the Hessian Ried, Germany. The elevated U concentrations can be attributed to the nitrate-triggered roll-front release of geogenic U in the redoxcline, while the elevated As concentrations are associated with the reductive dissolution of Fe-Mn oxyhydroxides at greater depth. To investigate the release of these elements and specifically the role of nitrate, we conducted a 90-day incubation experiment of the alluvial sediments in an inert atmosphere (N₂) glovebox. Mesocosms amended with nitrate (220 mg L^-1) and without nitrate were incubated in triplicates. In the control mesocosms, we observed a progressive decline in Eh, accompanied by the release of Mn and Fe due to reductive dissolution of (oxyhydr)oxides, which led to a release of As. In contrast, in the nitrate-amended mesocosms, heterotrophic denitrification caused (sub)oxic conditions, which inhibited the reductive dissolution of (oxyhydr)oxides, but led to the oxidative release of U. Sequential and single extractions showed that the geochemical fractions of U did not change and that about 93 % occurred as reduced U(IV) before and after the experiment. Our findings suggest that agricultural inputs of nitrate can cause the release of geogenic U, while simultaneously reducing the reductive release of As.