CHALLENGES AND OPPORTUNITIES FOR BRACKISH-GROUNDWATER DEVELOPMENT AND AQUIFER STORAGE & RECOVERY (ASR) IN THE BINATIONAL MESILLA BASIN, USA AND MEXICO

A number of aquifer systems New Mexico have large reserves of economically-recoverable water in the fresh to moderately brackish-quality range (<10,000 mg/L tds). However, places where de-watered aquifer spaces can be replenished through application of state-of-the-art aquifer storage & recovery (ASR) methods deserve special attention: 1. Rio Grande rift-basin fill in the Albuquerque, Palomas, Tularosa-Hueco, Mimbres, and Mesilla-Jornada Basins; 2. middle Pecos Valley carbonate-bedrock and alluvial units; and 3. Southern High Plains-Ogallala aquifer. One Rio Grande rift-basin aquifer system with exceptional potential for sustained groundwater utilization is the binational Mesilla Basin that extends southward from Las Cruces, NM into Chihuahua, Mexico west of El Paso del Norte. With a population of about 2,000,000, sustainability requires hydrogeologic familiarity.

With the exception of recharge from the Rio Grande and a few high-mountain areas, groundwater resources of the Mesilla Basin are primarily replenished by underflow from local sources that are predominantly brackish. Thick intermontane-basin fill of the Upper Cenozoic Santa Fe Group and thin riverine deposits of the Late Quaternary Mesilla Valley are the principal aquifers. Locally, however, Paleogene and pre-Cenozoic volcanic, siliciclastic and carbonate rocks also form significant groundwater reservoirs. The most productive basin-fill units comprise as much as 300 m of sand-dominated fluvial and eolian facies that include ancestral Rio Grande deposits. The medium- to course- texture, high permeability, and the relatively compact structure of these sediments indicates that: 1) well-pumping yields will be high (up to 60 L/s); 2) land-subsidence potential low; and 3) suitability for post-/co-development ASR is excellent. 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 (<6,000 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 a few places.