Tracing Anthropogenic Salinity Inputs to the Semi-Arid Rio Grande River: A Multi-Isotope Tracer Approach
Water is an important component for life and as such water quality and availability are major concerns in the world today. In arid to semi-arid regions, freshwater quality is especially important due to its scarcity. Elevated salinity of rivers in arid regions poses potentially devastating consequences for the communities that depend on these rivers as a source of freshwater. In the Rio Grande region, high salinity has led to severe reductions in crop productivity and accumulation of salts in soils. Salinity inputs into the river may come from both anthropogenic and natural sources. Thus, identifying and quantifying salinity inputs to the river is critical to understanding processes that contribute to the degradation of Rio Grande waters. To assess this issue, we collected monthly river samples in 2015-2016 at 15 sites along a 200-km stretch of the Rio Grande from Elephant Butte Reservoir, NM to El Paso, TX. Irrigation canals, city drains, wastewater effluents, and groundwater in this region were also sampled as possible salinity end-members. Major ion chemistry and (234U/238U), δ 11B, 87Sr/86Sr, and δ34 S isotope ratios in Rio Grande waters suggest multiple salinity inputs from geological, agricultural, and urban sources. High TDS values and high (234U/238U), δ11B, 87Sr/86Sr, and δ34S suggest that natural upwelling of groundwater is significant in the Rio Grande at Williamsburg near Elephant Butte. Rio Grande waters in the Mesilla Valley region have characteristic lower (234U/238U), 87Sr/ 86Sr, δ34S and intermediate δ11 B values that are possibly agriculturally-sourced from fertilizers and gypsum which are extensively used in the region. In addition to introducing anthropogenic salinity, agricultural practices during flood irrigation intensify evaporation of Rio Grande surface water significantly increasing water salinity. Shallow groundwater signatures were identified at several river locations, possibly due to the artificial pumping of local groundwater for irrigation. Measured B concentrations were higher for locations downstream to Las Cruces and El Paso wastewater treatment plants, supporting the use of B isotopes as an urban salinity tracer. Indeed, our B isotope data show city wastewater effluents have distinctively lower δ11B values (~3‰) than groundwater end-members (~30‰). Furthermore, δ11 B values in Rio Grande waters range from 3 to 30‰ suggesting possible mixing of anthropogenic and natural salinity end-members. Indeed, (234U/238U), 87Sr/86Sr, and δ34S values for river locations downstream to Las Cruces and El Paso also suggest multiple sources of salinity from both natural and human activities.
Garcia, Sandra, "Tracing Anthropogenic Salinity Inputs to the Semi-Arid Rio Grande River: A Multi-Isotope Tracer Approach" (2017). ETD Collection for University of Texas, El Paso. AAI10284131.