NM WRRI Publishes Technical Completion Report

By Carolina Mijares, NM WRRI Program Manager

NM WRRI announces the publication of technical completion report no. 388, a collaborative publication prepared for the Bureau of Reclamation and its Desalination and Water Purification Research Development Program (Report No. NMSU005). In 2017, New Mexico State University (NMSU) faculty member Dr. Kenneth C. Carroll (Department of Plant and Environmental Sciences) received funding through a cooperative agreement between Reclamation and NMSU. The cooperative agreement is a collaborative project that aims to increase scientific knowledge and research expertise in the area of alternative waters for water supply sustainability in New Mexico and the western U.S.

An Integrated Geochemical Approach for Defining Sources of Groundwater Salinity in the Southern Rio Grande Valley of the Mesilla Basin, New Mexico and West Texas, USA by Christopher Kubicki, NMSU Water Science and Management alumnus, Kenneth C. Carroll, James C. Witcher, and Andrew Robertson is available in its entirety on the NM WRRI website by clicking here.

Executive Summary

Salinization of aquifers in arid regions is a growing issue due to increased water use as a result of population growth and increasing agricultural demands (Szynkiewicz et al., 2011). Spatial variability in sources of groundwater salinity may exist due to stratigraphic, geochemical, and hydrologic processes even in an integrated and relatively homogeneous aquifer system. For this reason, methods are needed to determine salinity sources, groundwater flow, and transport of salts in alluvial/fluvial groundwater basins.

Geochemical tracers analyzed from groundwater samples were used to determine the sources of salt contributing to groundwater salinity in the Mesilla Valley of the Mesilla basin located in southern New Mexico and west Texas. Results from southern Mesilla Valley groundwaters show a localized area, plume, of saline groundwater (10,000 to 29,700 mg/L total dissolved solids) near Sunland Park, New Mexico.

Results from this work help to construct a conceptual model of groundwater flow and the source of salinity in the Mesilla basin. Analysis of δ18O and δD isotopes from groundwater samples support previous research that effectively shows groundwater in the Mesilla Valley has been recharged primarily from the Rio Grande. North of Sunland Park, New Mexico, lower groundwater salinity is associated with a spatial transition from sedimentary to volcanic rock underlying the alluvial aquifer, increasing alluvial sediment thickness, and δ34S signatures of groundwater (+2.28 to +5.76‰), indicating a sulfate source that could not originate from Paleozoic bedrock. These results indicate a reduced influence of upward groundwater flow from sedimentary bedrock and increased dilution of brackish groundwater within the lower salinity alluvial aquifer in the central and northern Mesilla Valley.

Results for major ion analysis indicate a general shift from HCO3- recharge waters in the northern Mesilla Valley groundwater toward Cl- and SO42- waters in the southern Mesilla Valley. The transition in water types occurs from north to south and along the groundwater flow path, indicating increased influence from halite and gypsum dissolution and/or cation exchange in southern groundwaters. Evidence presented herein shows that groundwater salinity in the Mesilla Valley is increasingly derived from evaporite mineral dissolution and/or cation exchange as water flows southward. The δ34S signatures of groundwater from the southern Mesilla Valley within the area of high salinity (+12.36 to +12.46‰) are comparable to δ34S signatures of Upper Paleozoic gypsum (+12.5‰), indicating prolonged contact between groundwater and underlying Paleozoic bedrock. Greater than atmospheric concentrations of 39Ar (132 to 134% pM) and high 4He (10-6 to 10-7 ccSTP/g(H2O) in groundwater samples collected from the plume confirm a substantial fraction of the groundwater is old (>1,000 years). This study illustrates how nested well clusters and geochemical tracers can be used to identify salinity sources and processes in geochemical investigations.