Sources of Salinity in the Rio Grande and Mesilla Basin Groundwater

An integrated geologic and geochemical investigation of groundwater, coupled with a study of temporal changes in surface water chemical balances for basins along the Rio Grande from San Marcial to Ft Quitman, is used to investigate salinity in the Mesilla Basin of southcentral New Mexico and West Texas. A conceptual hydrogeologic framework delineates the major geologic influences on flow, storage characteristics, and chemical evolution of groundwater in the basin. The conceptual framework is integrated into a GIS and is general enough to be used in adjacent basins and with groundwater flow models. Three basic hydrogeologic features are detailed: 1) lithofacies assemblages (LFAs), 2) hydro-stratigraphic units (HSUs), 3) and bedrock and structural boundaries. LFAs are the basic building blocks for the HSUs. The LFAs are primarily defined by grain size, mineralogy, depositional environment, and diagenetic modifications. LFAs may have distinct geochemical and hydrologic characteristics and can be recognized by diagnostic borehole geophysical responses. HSUs are mappable and informal basin-fill designations with hydrologic attributes that overlap formalized lithostrati-graphic and chronostratigraphic basin-fill units. The primary purpose of a HSU is to highlight and classify an overall hydrogeologic and geochemical behavior of a basin-fill package.

This investigation of salinity sources uses a combined interpretation of groundwater isotopic signatures and major cation and anion compositions. Because aquifer salinity can involve dissolution, non-reversible chemical reactions, ion exchange, near-surface evaporation or evapotranspiration, and various anthro-pogenic processes, special emphasis is given to fingerprinting processes, mixing, and flow paths. Isotopes such as δD, δ ¹⁸0, and   ⁸⁷Sr/ ⁸⁶Sr and conservative anions such as Cl and Br are especially useful. In particular, the systematics of the Cl/Br, δD, δ ¹⁸O provide useful insight into mixing and evaporative salinity processes. The isotopic systems of   ⁸⁷Sr/ ⁸⁶Sr, δ ¹³C, and δ ³⁴S show groundwater flow paths or water chemistry provenance. Saline and brackish water from deeper HSUs and geothermal water have Cl/Br ratios greater than 600 to 800,   ⁸⁷Sr/ ⁸⁶Sr ratios greater than 0.710, heavier δD and δ ¹⁸O than upper HSUs non-thermal water. However, the deeper HSU and geothermal water is lighter than the δD and δ ¹⁸O of water from the Rio Grande.

The salinity balance in the Rio Grande during the last 40 years for the Mesilla Basin is positive, meaning that more salts are entering the basin than are transported by the Rio Grande out of the basin at El Paso. Higher salinity in shallow groundwater and the Rio Grande in the southern and southeastern Mesilla Basin is probably dominated by structurally forced upwelling of brackish and saline water from deep HSUs and by upflow of geothermal water from shallow bedrock structures and bedrock boundaries.