USING BORON ISOTOPES TO TRACE URBAN SALINITY INPUTS TO THE RIO GRANDE RIVER IN SOUTHWEST USA

The Rio Grande is an important source of water for irrigation and municipal use in the arid to semi-arid southwest US. High salinity in the Rio Grande from the greater El Paso region has affected soil health and crop productivity. Thus, identifying salinity inputs to the river is critical to understanding processes that contribute to the degradation of Rio Grande waters. Salinity contributions to the river may come from anthropogenic and natural sources. To assess this issue, we collected monthly river samples at 15 sites along a 200-km stretch of the Rio Grande from Elephant Butte Reservoir, NM to El Paso, TX in 2015 and 2016. 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), ⁸⁷Sr/⁸⁶Sr, δ³⁴S isotope ratios in Rio Grande waters suggest multiple salinity inputs from geological, agricultural, and urban sources. High TDS values and high (²³⁴U/²³⁸U), ⁸⁷Sr/⁸⁶Sr, δ³⁴S suggest that natural upwelling of groundwater is significant in the Rio Grande near Elephant Butte. Rio Grande waters in the Mesilla Valley region have characteristic lower (²³⁴U/²³⁸U), ⁸⁷Sr/⁸⁶Sr, δ³⁴S and are possibly agriculturally-sourced from fertilizers and gypsum which are extensively used in the region. Agricultural practices during flood irrigation intensify evaporation of Rio Grande surface water significantly increasing water salinity. Stable O and H isotope ratios indicate signatures of evaporation and multiple water sources due to agricultural practices and groundwater pumping. 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 recent B isotope data show city wastewater effluents have distinctively lower δ¹¹B values (~3‰) than groundwater end-members (~30‰). Furthermore, δ¹¹B values in Rio Grande waters range from 3 to 30‰ suggesting possible mixing of anthropogenic salinity end-members. Here we combine our new δ¹¹B data with (²³⁴U/²³⁸U), ⁸⁷Sr/⁸⁶Sr, δ³⁴S to distinguish multiple urban and natural salinity inputs to the river.