Tracing regional flow paths to major springs in Trans-Pecos Texas using geochemical data and geochemical models

San Solomon, Giffin, and Phantom Lake Springs, located in Trans-Pecos Texas , have a high TDS, Na-Cl-SO4 baseflow component derived from a regional flo w system and a low TDS, mixed cation-mixed anion stormflow component derive d from local precipitation events. The hypothesis that the regional flow sy stem maintaining baseflow spring discharge originates in the Salt Basin and flows through the Apache Mountains towards the springs is tested with hist orical geochemical data from wells and springs. Data from over 1400 wells i n the study area over a 50-year period were analyzed and used to delineate 11 hydrochemical facies based on the predominant ions. Geochemical data from samples along the hypothesized regional flow path ind icate a trend of increasing dissolved solids and Cl-HCO3 ratios and decreas ing Na-Cl ratios. These are consistent with evolution of groundwater in an unconfined regional system dominated by carbonates and evaporites. In the b icarbonate facies, the waters represent recent recharge modified by mineral dissolution and cation exchange. In the sulfate zones, the hydrochemical f acies are controlled by gypsum, anhydrite, and halite dissolution, cation e xchange, and mixing with Na-Cl waters. In the chloride zones, the hydrochem ical facies are controlled by halite dissolution and irrigation return flow . Spring discharge chemistry is most similar to chloride zone waters; Na-Cl and Ca-SO4 ratios suggest that baseflow is derived from the chloride zone waters upgradient along the hypothesized flow path. PHREEQC modeled groundwater evolution along the hypothesized flow path and spring discharge under stormflow and baseflow conditions. Results indicate that: (1) hydrochemistry along the regional flow path is controlled by diss olution of halite, gypsum, dolomite, and CO2 and by precipitation of calcit e; (2) baseflow spring discharge is derived primarily from this regional fl ow system; and (3) spring discharge after major storm events can constitute as much as 72% local recharge that is further modified by dissolution of c alcite, gypsum, and CO2. Data analysis and model results suggest that cave formation in this system is occurs during major storm events. (C) 2001 Publ ished by Elsevier Science BN.