Abstract
In semi-arid to arid regions, both anthropogenic sources (urban and agriculture) and deeper Critical Zone (groundwater with long flow paths and water residence times) may play an important role in controlling chemical exports to rivers. Here, we combined two anthropogenic isotope tracers: uranium isotope ratios (234U/238U) and boron isotope ratios (δ11B), with the 87Sr/86Sr ratios to identify and quantify multiple solute (salinity) sources in the Rio Grande river in southern New Mexico and western Texas. The Rio Grande river is a major source of freshwater for irrigation and municipal uses in southwestern United States. There has been a large disagreement about the dominant salinity sources to the Rio Grande and particularly significant sources are of anthropogenic (agriculture practices and shallow groundwater flows, groundwater pumping, and urban developments) and/or geological (natural groundwater upwelling) origins. Between 2014 and 2016, we collected monthly river samples at 15 locations along a 200-km stretch of the Rio Grande river from Elephant Butte Reservoir, New Mexico to El Paso, Texas, as well as water samples from agricultural canals and drains, urban effluents and drains, and groundwater wells. Our study shows that due to the presence of localized and multiple salinity inputs, total dissolved solids (TDS) and isotope ratios of U, B, and Sr in the Rio Grande river show high spatial and temporal variability. Several agricultural, urban, and geological sources of salinity in the Rio Grande watershed have characteristic and distinguishable U, Sr, and B isotope signatures. However, due to the common issue of overlapping signatures as identified by previous tracer studies (such as δ18O, δD, δ34S), no single isotope tracer of U, Sr, or B isotopes was powerful enough to distinguish multiple salinity sources. Here, combining the multiple U, Sr, and B isotope and elemental signatures, we applied a multi-tracer mass balance approach to quantify the relative contributions of water mass from the identified various salinity end members along the 200-km stretch of the Rio Grande during different river flow seasons. Our results show that during irrigation (high river flow) seasons, the Rio Grande had uniform chemical and isotopic compositions, similar to the Elephant Butte reservoir 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 when the river water is stored at Elephant Butte reservoir, the Rio Grande river at many downstream locations showed heterogeneous chemical and isotopic compositions, reflecting variable inputs from upwelling of groundwater (deeper CZ), displacement of shallow groundwater, agricultural return flows, and urban effluents. Our study highlights the needs of using multi-tracer approach to investigate multiple solutes and salinity sources in rivers with complex geology and human impacts.
Highlights
The Critical Zone, extending from surface vegetation and shallow soils to deeper permeable bedrock, provides chemical constituents to rivers and controls water quality (Brantley et al, 2006)
We explored the potential of combining uranium, boron and strontium isotopic tracers (234U/238U, δ11B, 87Sr/86Sr) to distinguish and quantify contributions from anthropogenic and natural processes that lead to salt loads in a semi-arid portion of the Rio Grande as well as in shallow groundwater aquifers
Our study focuses on the salinity (TDS) and chemical and isotopic compositions of U, Sr, and B observed in Rio Grande river samples for both irrigation and non-irrigation seasons as well as in different possible salinity end members
Summary
The Critical Zone, extending from surface vegetation and shallow soils to deeper permeable bedrock, provides chemical constituents to rivers and controls water quality (Brantley et al, 2006). Natural processes from the deeper Critical Zone such as upwelling of saline groundwater and dissolution of evaporites can significantly increase the salinity of arid rivers (e.g., Allison et al, 1990; Meybeck, 2003; Phillips et al, 2003; Hogan et al, 2007; Szynkiewicz et al, 2015a) Human activities, such as dam construction, agricultural practices (e.g., irrigation, fertilizer/pesticide applications), groundwater pumping, and urbanization, have significantly modified the hydrologic cycle with respect to both water quantity and quality (e.g., Chetelat and Gaillardet, 2005; Lyons et al, 2012; National Research Council, 2012; Chen et al, 2014). Previous salinity tracers (e.g., major elemental concentrations and ratios, light stable isotope ratios such as δ34S, δ18O, and δD) have shown overlapping signatures for multiple salinity end members (e.g., Szynkiewicz et al, 2011, 2015b)
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