Groundwater Sustainability in Mesilla Basin–Rio Grande Valley Area, USA and Mexico—A Hydrogeologic Perspective

With the exception of recharge from the Rio Grande and a few high-mountain areas, sustainable groundwater resources in the binational Mesilla Basin and Valley region of New Mexico, Texas, and Chihuahua (Mexico) are primarily replenished by underflow from local sources that are predominantly brackish. In this respect, all (surface and subsurface) waters in the fresh to moderately brackish range (<10,000 mg/L) are here considered as assets rather than liabilities. Thick intermontane-basin fill of the Upper Cenozoic Santa Fe Group and thin fluvial deposits of the Late Quaternary Mesilla Valley corridor constitute the primary aquifer systems. Locally, however, bedrock terranes also provide significant secondary groundwater reservoirs. The latter include carbonate and evaporitic rocks of Permian and Mesozoic Age, and Lower to Middle Tertiary sedimentary and volcanic rocks. In central-basin and inner river-valley areas of primary interest, the most productive aquifers comprise as much as 300 m of sand-dominated lithofacies that include thick ancestral Rio Grande deposits. For an aquifer system with an area of 2700 km² and 300 m average thickness, a conservative estimate of the amount of economically-recoverable fresh to slightly brackish groundwater (<5,000 mg/L tds) is about 80 km³ (65 million ac-ft). Basic assumptions include: unconfined to leaky-confined aquifer conditions, a specific yield (Sy) of 0.1, long-term increase in storivity (S) from a 1 x 10^-4 base value to one approaching the Sy range, and recognition of a potential for significant land subsidence in some places. These conclusions are based on recent hydrogeologic-framework model updates in the Mesilla Basin and surrounding basin/range areas as part of the ongoing interdisciplinary, multi-institutional Transboundary Aquifer Assessment Project coordinated by the NMWRRI. Emphasis is on digital characterization of lithofacies distribution, hydrostratigraphy, and structural-boundary conditions. Three-D, hydrogeologic map and fence-diagram compilation scale is 1:100,000, and model-base elevation is Mean Sea Level.