Assessing selenium fate and transport in a semi-arid river basin with and without human influence.

Selenium (Se) is an essential micro-nutrient for humans and animals but can be toxic at high levels of intake. Quantifying the transport of Se in environmental systems is essential for understanding and mitigating Se contamination in soils, groundwater, and surface waters. In this study, we investigate the transport, storage, and contamination of Se in highly managed, irrigated watershed systems to explore historical conditions, the dominant environmental controls on Se contamination and transport, and the impact of anthropogenic influences such as irrigation and urbanization. We use a portion (23,600 km) of the Arkansas River Basin, Colorado as an example river basin system, and the SWAT+ watershed model as the numerical tool, amended in this study with a Se reactive transport module. The module accounts for concentrations and mass loads of selenate (SeO) and selenite (SeO) for landscapes, soils, aquifers, channels, and lakes and reservoirs. The model is applied to the 2000-2020 period and tested against streamflow, groundwater head, in-stream Se loads, and in-stream Se concentration within an extensive sample network, and then applied to the 1981-2020 period to examine anthropogenic influences on the Se transport cycle. Canal diversions are the major control on streamflow in the Arkansas River, with up to one-third of the diverted volume replaced by groundwater discharge. Canal seepage to the unconfined aquifer drives much of the groundwater discharge. Similarly, Se mass loading in the Arkansas River, approximately 23 kg/day in a downstream irrigated watershed, is controlled by canal diversions (14.4 kg/day) and groundwater loading (8.2 kg/day), with Se in groundwater controlled by oxidation of Cretaceous shale present as outcrops or bedrock. If anthropogenic influences are removed for a 40-year period, there is more flow and Se load in the Arkansas River. While groundwater discharge and Se loading decrease under this scenario, canal diversions also are eliminated, leading to a net increase in average flow (13 m/sec → 21 m/sec; 60 %), Se load (14.5 kg/day → 26.4 kg/day; 80 %), and concentration (13 µg/L → 15 µg/L; 15 %). These results can be used to formulate management plans for Se contamination. While methods are applied to the Arkansas River Basin, we expect that general patterns and relationships between shale, canal diversions, canals, and irrigated fields found in this study are applicable to other semi-arid river basins where Cretaceous shale is present as bedrock.