Using Stable Water Isotopes and Radiogenic Strontium Isotopes to Trace Water and its Salinity Sources in Pecos and Colorado River Basins in Texas

Water stable isotope ratios (Deuterium/hydrogen 2H/1H and Oxygen 18O/16O ratios) and radiogenic strontium isotope ratios (87Sr/86Sr) have been widely used to trace various hydrologic processes and understand surface-groundwater interactions for watershed and critical zone studies. Our project aimed to trace the water and salinity sources in the Colorado and Pecos Rivers, two major rivers that provide essential water resources for water-stressed parts of Texas. In our study, we collected water samples from 25 locations during July (monsoon season), December 2021 (dry season), and May 2022 (pre-monsoon season) along the Pecos and Colorado rivers from central Texas to the Gulf of Mexico. Our project used water-stable isotope ratios to understand water source/age, evaporation, groundwater recharge, and discharge as stable isotope water signatures reflect water source regions\u27 altitude, latitude, and seasonality. Our project also used strontium isotope ratios to trace the salinity sources from water-rock interactions in watersheds. In addition, a trace metals study was included to understand human and industrial presence on both rivers. Water stable isotope ratios were analyzed to identify the water source from each location using Picarro L2130-i Isotope and Gas Concentration Analyzer and then compared to the global meteoric water line (GWML). Sr isotope ratios were analyzed using Nu Plasma Multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS). Trace metals were completed using a Thermo Fisher Scientific iCAP RQ ICP-MS. Field measurements were conducted at each location, including DO, conductivity, TDS, pH, ORP, and temperature, with a YSI ProQuatro Multiparameter Meter and a field alkalinity test. Geology and land use information were obtained from multiple public sources and analyzed by independent research teams on this project. Water stable isotope results indicate highly saline brine upwelling in the upper reaches of the Pecos and Colorado Rivers. The middle portion shows the absence of brine contributions from a carbonate aquifer, resulting in water dilution. The lower portion of the Colorado has more precipitation and brine signatures in drier seasons. Upwelling brines in the upper Pecos and Colorado rivers also contribute to higher 87Sr/86Sr ratios and Arsenic concentrations, with both elements associated with shallow and deep brines in the region. Brines may flow through the middle stream in the main river channel and ultimately reach the lower portion of the river during periods of low to no rainfall. Our study highlights that water stable isotope ratios and strontium isotope ratios provide simple yet significant background information for scientific research and water management strategies, such as understanding primary river water sources, salinity sources, mixing with other sources, as well as how rapidly water is moving and where it is stored in the watershed