COMBINING URANIUM, BORON, AND STRONTIUM ISOTOPE RATIOS (234U/238U, δ11B, 87SR/86SR) TO TRACE SALINITY SOURCES IN THE SEMI-ARID RIO GRANDE RIVER OF SOUTHWESTERN UNITED STATES

The Rio Grande River is a major source of surface water for irrigation and municipal uses in the arid to semi-arid regions of southwestern United States. However, elevated salinity present in the Rio Grande water significantly impacts soil quality and crop productivity. There has been a large disagreement about the main salinity sources to the Rio Grande that is believed to originate from multiple anthropogenic (agriculture practices, groundwater pumping, and urban developments) and geological (natural groundwater upwelling) processes. In this study, we combined two novel anthropogenic isotope tracers: uranium (²³⁴U/²³⁸U) and boron (d¹¹B) isotope ratios, with the conventional ⁸⁷Sr/⁸⁶Sr ratios and major element concentrations to better identify and quantify main salinity sources in the Rio Grande in southern New Mexico and western Texas. Between 2014 and 2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande from Elephant Butte Reservoir, New Mexico to El Paso, Texas. Additionally, agricultural canals and drains, urban effluents and drains, and groundwater wells were sampled. Our results show that agricultural, urban, and geological sources of salinity have respective and characteristic isotope signatures in the Rio Grande watershed. Total dissolved solids (TDS) and U-B-Sr isotope ratios in the Rio Grande river showed high spatial and temporal variability, revealing the presence of localized and multiple salinity inputs. During irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the upstream Elephant Butte reservoir water. In non-irrigation (low flow) seasons, the Rio Grande at many downstream locations showed signatures of heterogeneous chemical and isotopic compositions, reflecting variable inputs from agricultural return flows, local groundwater used for irrigation, and urban effluents. Generally, natural upwelling of deep groundwater appears to be insignificant contributor of salinity in the Rio Grande, expect near the hot spring areas of Elephant Butte Reservoir and in local areas in Mesilla basin. Surface processes related to agricultural practices (flood irrigation, return irrigation flows, displacement of shallow groundwater, and extraction of groundwater) and urban effluent discharges are main factor significantly increasing water salinity, especially in agricultural and urban areas.