Data_Sheet_1_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.xlsx

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.

在半干旱至干旱地区，人为来源（城市和农业）以及更深层次的临界带（具有长径流路径和较长的水停留时间的地下水）可能在控制化学物质输送到河流方面发挥着重要作用。本研究中，我们结合了两种人为同位素示踪剂：铀同位素比（234U/238U）和硼同位素比（δ11B），以及87Sr/86Sr比，以识别和量化新墨西哥州南部和德克萨斯州西部格兰德河中的多种溶质（盐度）来源。格兰德河是美国西南部灌溉和市政用水的主要淡水来源。关于格兰德河主导盐度来源的问题存在较大分歧，尤其是重要的来源包括人为来源（农业耕作和浅层地下水流动、地下水抽取和城市发展）和/或地质来源（自然地下水上升）。在2014年至2016年期间，我们在新墨西哥州的象峰水库至德克萨斯州的埃尔帕索的200公里格兰德河段上的15个地点收集了月度河流样品，以及来自农业灌溉渠道和排水系统、城市废水和排水系统以及地下水井的水样。我们的研究表明，由于存在局部和多种盐度输入，格兰德河中的总溶解固体（TDS）以及铀、硼和锶同位素比表现出高空间和时间变异性。格兰德河流域内农业、城市和地质来源的盐度具有特征性的和可区分的铀、锶和硼同位素特征。然而，由于与前人同位素示踪研究（如δ18O、δD、δ34S）中识别出的重叠特征问题，没有任何单一的同位素示踪剂（铀、锶或硼同位素）足以区分多种盐度来源。在本研究中，通过结合多种铀、锶和硼同位素和元素特征，我们采用多示踪剂质量平衡方法，量化了在不同河流流量季节中沿200公里格兰德河段上识别出的各种盐度端元的水团相对贡献。我们的结果表明，在灌溉（高流量）季节，格兰德河具有统一的化学和同位素组成，类似于储存有水和充分混合的象峰水库，反映了来自温带南部科罗拉多州和北部新墨西哥州上游地区的浅层临界带的占主导地位的贡献。在非灌溉（低流量）季节，当河水储存在象峰水库时，许多下游位置的格兰德河表现出异质化的化学和同位素组成，反映了地下水上升（深层临界带）、浅层地下水置换、农业回流水和城市废水的变量输入。本研究强调了在具有复杂地质和人类影响的河流中，使用多示踪剂方法研究多种溶质和盐度来源的必要性。