Surface irrigation induced uranium mobility in the vadose zone – groundwater system

Title : Surface irrigation induced uranium mobility in the vadose zone – groundwater system

Abstract:

Irrigation is known to enhance nitrate loss, which can mobilize uranium in the saturated system. High nitrate concentrations occur within the unsaturated or vadose zone, but the cascading effect of irrigation and nitrate on uranium mobility within the vadose zone is unknown. The current study evaluated the impact of surface sprinkler irrigation practice and linked it to the impact of inorganic nitrogen species, nitrate, on uranium mobilization in the vadose zone. The study replicated the vadose zone-groundwater system using columns representing different redox-active biogeochemical zones. The columns (150 cm length) simulated a sprinkler-irrigated system (n=3) and a rainfed (n=3) system, where biweekly pore water samples were collected at 10 cm (rootzone), 140 cm (capillary fringe), and groundwater. The irrigated capillary fringe pore water contained significantly higher uranium and nitrate levels (U: 41.6 ± 3.0 μg L^-1, NO₃^--N: 11.1±1.4 mg L^-1) than rainfed (U:21.5±3.3 μg L^-1, NO₃^--N: 7.1±1.6 mg L^-1). The porewater ammonium concentration was significantly higher in rainfed (NH₄⁺-N: 2.1±0.5 mg L^-1) than in irrigated (NH₄⁺-N: 0.8±0.1 mg L^-1) columns. Though groundwater nitrate-N concentrations were similar, uranium concentrations were significantly higher in irrigated (33.8±3.7 μg L^-1) than rainfed (26.0±2.3 μg L^-1). The study’s findings indicate that surface irrigation application, such as sprinkler that covers more than 72% of irrigated acreage in the US, could be crucial in uranium mobilization at the capillary fringe, leading to groundwater contamination.

Keywords: Uranium, nitrate, ammonium, root zone, capillary fringe, groundwater