HYDROCHEMICAL CHARACTERIZATION OF GEOLOGICAL AND ANTHROPOGENIC INPUTS OF SALINITY IN THE EL PASO/JUAREZ VALLEY

High salinity has been recorded in both surface water and shallow groundwater in the El Paso/Juarez Valley since the 1930s. The main theories for the origin of salinity include recycling of irrigation water in heavily irrigated areas, evaporative concentration from phreatophytes, upwelling of saline waters sourced from direct dissolution of evaporite units, and upwelling of deep basinal brines. This study focuses on the dilute and saline groundwater and surface water in the El Paso/Juarez Valley between Clint and Fort Quitman, Texas, a stream reach extending for about 110 km below El Paso, Texas. Historical data indicates dissolved solids in the Rio Grande Aquifer in this reach of stream ranges from 800 to 30,000 mg/L, sulfate ranges from 100 to 4000 mg/L, and chloride ranges up to 10,000 mg/L. Historical and new samples analyzed for chloride/bromide (Cl/Br) weight ratios are up to 2,000 in the Rio Grande Aquifer in the study area. Published data indicate Cl/Br ratios usually range from 50 to 150 in precipitation, 300 to 600 in domestic sewage, and 100 to 300 in agricultural drain water. Ratios are typically greater than 1,000 in waters that react with evaporite-rich strata in basin fill. Our current studies integrate about 80 samples from groundwater drains, springs, and shallow water wells that were sampled from 2017 to 2019. These data sets point to two main sources of salinity between Clint and Fort Quitman; 1) agricultural factors resulting in salinization, and 2) dissolution of geological evaporites in basin fill beneath the Rio Grande alluvium. About 30% of the shallow groundwater samples contain Cl/Br ratios of 1000 to 2000, with Cl concentrations greater than 600 mg/L. These indicate a natural evaporite dissolution source. The geologic source of saline groundwater appears appears most prominently near Fabens, Texas located on the northern edge of the Rio Grande Floodplain about 20 km below Clint. There, a shift in the chloride/sulfate weigh ratios (Cl/SO4) occurs in shallow groundwater from about 0.7 to 0.9 Cl/SO4 to 1.2 to 10.0 Cl/SO4. This is due to dissolution of a halite rich, gypsum deficient evaporite directly beneath the Rio Grande alluvium.