HYDROGEOCHEMICAL ANALYSIS OF THE ROSWELL GROUNDWATER BASIN, NEW MEXICO

A detailed geochemical analysis was conducted on more than 300 water samples collected from the Pecos River and several aquifer systems within the Roswell Groundwater Basin, New Mexico. These systems include unconfined Quaternary alluvium (Q1 and Q2), Tertiary-Quaternary alluvium (AG2), Permian Artesian Group (PGB1), Permian Melange (M), Permian upper San Andres Formation (USA), and Permian San Andres Formation (SA). Alluvial aquifer material is derived from a mixture of sedimentary and igneous source rocks. The M, PGB1, USA, and US aquifer systems dominantly consist of marine-derived dolomite, calcite, and gypsum with variable amounts of anhydrite. Ionic compositions include calcium-sodium-sulfate (Pecos River), calcium-sodium-chloride-sulfate (PGB1), calcium-sulfate (AG2), and calcium-sulfate-bicarbonate (Q1, Q2, USA, and SA). Groundwater mixing between the Q1, Q2, and AG2 aquifers, AG2 and PGB1 aquifers, and PGB1 and M aquifers at specific locations is suggested by similar chloride and nitrate concentrations. Pervasive groundwater mixing through artificial recharge and from improperly installed wells has taken place for decades within the Roswell Groundwater Basin.

Inverse and batch-equilibrium modeling of aquifer material and groundwater using the computer program PHREEQC quantifies geochemical reactions occurring along groundwater flow paths within the Q1, USA, and other aquifer systems. Similar calculations were performed for the Pecos River. Rain, snow, and surface water provide direct recharge to the shallow alluvial Q1 and Q2 aquifers and the USA aquifer exposed west of Roswell. Major ions are calculated to occur mainly as uncomplexed solutes. Varying amounts of sodium, potassium, magnesium, and calcium, however, form complexes with sulfate, bicarbonate, and carbonate ligands. Results of inverse geochemical calculations suggest that dissolution of gypsum, halite, dolomite, calcite, cristobalite(alpha), and/or anhydrite dominantly influences groundwater chemistry within the Q1 and USA aquifer systems and Pecos River. Dissolution of halite from several brine-bearing aquifers occurs east of Roswell, contributing increasing total dissolved solids and salinity along the Pecos River between Acme and Artesia.