Mobilization of Naturally Occurring Uranium in Groundwater Under Intensely Managed Farmland

Groundwater has been demonstrated to bear elevated uranium (U) concentrations in excess of regulatory limits in many regions throughout the world. Within some of these regions, direct anthropogenic U contaminant sources do not exist indicating a naturally occurring U source that results in elevated groundwater U concentrations. Naturally occurring U is ubiquitous and is deposited in evaporative sequences and/or organic carbon-rich soils and sediments, which includes the sedimentary matrix within aquifers. Thus one potential source of U in groundwater is dissolution or release from sedimentary deposits. To date two primary mechanisms have been described that can lead to U mobilization in these aquifers: (i) desorption and binding of U by complexing ligands such as carbonate or (ii) oxidative dissolution of reduced U (U(IV)) minerals. While the former has been studied and described, the latter presents a newly described mechanism that should be considered in shallow alluvial aquifers. Uranium is commonly deposited as reduced U(IV) phases that are stable in reduced environments; however, the influx of molecular oxygen (O2) or nitrate (an agricultural contaminant) can stimulate oxidative dissolution of U(IV) minerals. An influx of O2 or nitrate as well as other oxidants during groundwater recharge have been linked to groundwater with elevated U concentrations in major US aquifers. These results highlight the instability of naturally deposited subsurface U and highlight a potential risk to water quality from mobilization. Controlling influxes of oxidants such as nitrate, however, may help mitigate U contamination of groundwater and drinking water supplies at risk from geogenic uranium.