Combining Uranium, Boron, and Strontium Isotope Ratios (234U/238U, δ11B, 87Sr/86Sr) to Trace and Quantify Salinity Contributions to Rio Grande River in Southwestern United States

In semi-arid to arid regions, both anthropogenic sources (urban and agriculture) deeper Critical Zone (groundwater with long flow paths water residence times) may play an important role in controlling chemical exports rivers. Here, we combined two isotope tracers: uranium ratios ( 234 U/ 238 U) boron (δ 11 B), the 87 Sr/ 86 Sr identify quantify multiple solute (salinity) Rio Grande river southern New Mexico western Texas. The is a major source of freshwater for irrigation municipal uses southwestern United States. There has been large disagreement about dominant salinity particularly significant are (agriculture practices shallow groundwater flows, pumping, urban developments) and/or geological (natural upwelling) origins. Between 2014 2016, collected monthly samples at 15 locations along 200-km stretch from Elephant Butte Reservoir, El Paso, Texas, as well agricultural canals drains, effluents wells. Our study shows that due presence localized inputs, total dissolved solids (TDS) U, B, show high spatial temporal variability. Several agricultural, urban, watershed have characteristic distinguishable Sr, B signatures. However, common issue overlapping signatures identified by previous tracer studies (such δ 18 O, δD, 34 S), no single or isotopes was powerful enough distinguish sources. combining elemental signatures, applied multi-tracer mass balance approach relative contributions various end members during different seasons. results (high flow) seasons, had uniform isotopic compositions, similar reservoir where stored well-mixed, reflecting contribution headwater regions temperate Colorado northern Mexico. non-irrigation (low seasons when reservoir, many downstream showed heterogeneous variable inputs upwelling (deeper CZ), displacement groundwater, return effluents. highlights needs using investigate solutes rivers complex geology human impacts.