H146-07
Combining Uranium, Boron, and Strontium isotope ratios (234U/238U, 11B, 87Sr/86Sr) to trace and quantify salinity contributions to Rio Grande river in Southwestern United States

Monday, 14 December 2020: 05:54
Virtual
Lin Ma, University of Texas at El Paso, Geological Sciences, El Paso, TX, United States, Sandra Garcia, SOLARIS-EPCC, EL PASO, TX, United States, Pascale Louvat, Institute de Physique d Globe Paris, Paris, France, Jerome Gaillardet, Institut de Physique du Globe de Paris, Paris, France and Syprose K Nyachoti, University of Texas at El Paso, El Paso, TX, United States
Recorded Presentation
Abstract:
In semi-arid to aird regions, deeper Critical Zone (with long flow paths and travel times) and anthropogenic sources may play a role in controlling chemical exports to rivers. Here, we combined two novel anthropogenic isotope tracers: Uranium (234U/238U) and Boron (11B) isotope ratios, with the conventional 87Sr/86Sr ratios to better identify and quantify solute (salinity) sources in the Rio Grande River in southern New Mexico and western Texas. The Rio Grande River is a major freshwater resource for irrigation and municipal uses in southwestern United States. There has been a large disagreement about the major salinity sources to the Rio Grande with anthropogenic (agriculture practices, groundwater pumping, and urban developments) and geological (natural groundwater upwelling and shallow groundwater flows) origins.

Between 2014-2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande, as well as agricultural canals and drains, urban effluents and drains, and groundwater wells, from Elephant Butte Reservoir, New Mexico to El Paso, Texas. Our study shows that agricultural, urban, and geological sources of salinity in the Rio Grande watershed have characteristic and distinguishable U, Sr, and B isotope signatures. Due to the presence of localized and multiple salinity inputs, total dissolved solids (TDS) and U-B-Sr isotope ratios in the Rio Grande river show high spatial and temporal variability. We applied a multi-tracer (U, Sr, and B isotopes) mass balance approach to quantify the relative contributions from the identified various salinity end members along the 200-km stretch of the Rio Grande during different river flow seasons. During irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the upstream Elephant Butte reservoir water where water is stored and well mixed, reflecting the dominant contribution from shallow Critical Zone in headwater regions in temperate southern Colorado and northern New Mexico. 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 upwelling of groundwater, displacement of shallow groundwater, agricultural return flows, and urban effluents.

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