Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone

The vadose zone is a reservoir for geogenic and anthropogenic contaminants. Nitrogen and water infiltration can affect biogeochemical processes in this zone, ultimately affecting groundwater quality. In this large-scale field study, we evaluated the input and occurrence of water and nitrogen species in the vadose zone of a public water supply wellhead protection (WHP) area (defined by a 50-year travel time to groundwater for public supply wells) and potential transport of nitrate, ammonium, arsenic, and uranium. Thirty-two deep cores were collected and grouped by irrigation practices: pivot (n = 20), gravity (n = 4) irrigated using groundwater, and non-irrigated (n = 8) sites. Beneath pivot-irrigated sites, sediment nitrate concentrations were significantly (p < 0.05) lower, while ammonium concentrations were significantly (p < 0.05) higher than under gravity sites. The spatial distribution of sediment arsenic and uranium was evaluated against estimated nitrogen and water loading beneath cropland. Irrigation practices were randomly distributed throughout the WHP area and presented a contrasting pattern of sediment arsenic and uranium occurrence. Sediment arsenic correlated with iron (r = 0.32, p < 0.05), uranium negatively correlated to sediment nitrate (r = −0.23, p < 0.05), and ammonium (r = −0.19 p < 0.05). This study reveals that irrigation water and nitrogen influx influence vadose zone geochemistry and mobilization of geogenic contaminants affecting groundwater quality beneath intensive agricultural systems.