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

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.

Mobility of trace elements underneath irrigated agricultural fields: implication of dryland soil and water quality along the Rio Grande Valley

“…However, studies are still needed for water quality from perspectives of toxic metals and nutrients. Irrigation, especially, intensive irrigation like ooding, hydrologically link the vadose zone soils and underlying groundwaters and also the surface rivers, and thus impacts the regional water quality (Szynkiewicz et al, 2015ab;Garcia et al, 2021). This study shows that most of seven trace elements loaded by agricultural practices are sequestered in stable forms of pedogenic carbonates.…”

“…3). Eventually B and Zn can be mobilized and return to the river (e.g., Garcia et al, 2021). Phosphorus concentration however increases with depth in the soil waters, and then drops signi cantly in shallow groundwater (Fig.…”

“…The irrigated water in agricultural elds in ltrates to shallow groundwaters and discharges to the Rio Grande through return ow (e.g., Fuchs et al, 2019). Such connectivity of regional hydrological systems also implies that the chemical composition of the river water is altered due to soil amendments and fertilizers (Szynkiewicz et al, 2011(Szynkiewicz et al, , 2015Sanchez et al, 2017;Cox et al, 2018;Garcia et al, 2021). Indeed, Miyamoto et al (1995b) observed an increase in salinity, sodicity, and sulfate concentrations during the irrigation season where the main source of water to the Rio Grande is irrigation return ow and sewage water.…”

A critical review on the occurrence and distribution of the uranium- and thorium-decay nuclides and their effect on the quality of groundwater

“…Measurements of 234 U/ 238 U in groundwater have been used as tracers of distinct water masses, as indicators of certain chemical processes, and as indicators of groundwater age. The 234 U/ 238 U ratios of groundwater masses often differ from each other and from surface water (e.g., Snow and Spaulding, 1994;Durand et al, 2005;Garcia, 2021) and can be used to trace groundwater migration directions, recharge from rivers, and groundwater discharge into surface water. The excess 234 U that occurs where aquifers transition from oxic to anoxic has been used…”

Climate-driven mid- to late Holocene hydrologic evolution of arid wetlands documented by strontium, uranium, and oxygen isotopes from Lower Pahranagat Lake, southern Nevada, USA

“…Thus, Sr- and U-isotope measurements have been used together and effectively applied to identify multiple sources and mixing relationships among end members in waters from a variety of settings (Drexler et al, 2014; Paces and Wurster, 2014; Neymark et al, 2018; Miller et al, 2021; Garcia et al, 2021). In arid settings, Sr and U isotopes have the advantage of retaining their distinct signatures when exposed to evaporation, transpiration, and other near-surface processes that are especially significant for other chemical tracers (Drexler et al, 2014; Paces and Wurster, 2014).…”