DESALINATION OF BRACKISH GROUNDWATER IN NEW MEXICO: GEOCHEMICAL ASPECTS OF WATER TREATMENT AND CONCENTRATE MANAGEMENT

Desalination of brackish groundwater is one of many strategies that southwestern states are considering to meet the rising demand for drinking water. Unknown factors include 1) quantity and quality of existing brackish groundwater resources, 2) suitability of brackish waters for desalination treatment, and 3) affordable and sustainable methods for concentrate management. By compiling published water chemistry datasets and applying a variety of geochemical modeling approaches, we provide initial assessments of the viability of desalination and of the potential problems that may be encountered if desalination concentrate is injected into deep aquifers.

We have developed a brackish water chemistry database for our study area, and have found that of groundwaters with total dissolved solids (TDS) within an acceptable range for desalination (1,000 - 10,000 mg/l), the most prevalent water type is Ca-SO4,; less common but also important are Na-SO4, and Na-Cl waters. Using PHREEQC, we simulated the desalination process (including standard anti-scaling treatments) for all water types. Our results suggest that approximately 50% of these waters can be successfully treated and the resulting concentrate will be undersaturated with respect to both calcite and gypsum, common scales of concern.

Often the most expensive and problematic aspect of inland desalination is concentrate management; one option is deep well injection. We evaluated the chemical compatibility of simulated concentrate water chemistry with potential receiving formations and found that the large proportion of potential combinations will cause either calcite and/or gypsum precipitation. Previous studies in oil fields have found that the consequence of scale precipitation on formation porosity will depend strongly on the location of precipitation. We developed reactive transport models to evaluate this issue for New Mexico brackish aquifers and to determine the most important geochemical factors influencing degree and location of porosity reduction in the subsurface. We found that of many geochemical factors considered, the longevity of anti-scalants in the subsurface is the dominating variable. Our results suggest that large increases in the longevity of injection wells can be gained by relatively modest increases in the lifetime of anti-scalants.