HYDROGEOCHEMISTRY OF AN IGNEOUS AQUIFER IN THE SOUTHERN DAVIS MOUTAINS, TRANS-PECOS, TEXAS

There are at least two major sources of groundwater in the area near Alpine, Texas, in the southern Davis Mountains: Cretaceous limestone and Tertiary alkalic lavas (Pruett Formation, Cottonwood Springs Basalt, and the Decie Formation). Wells used by the city of Alpine in Sunny Glen canyon are completed in igneous rock (including fractured basalt and peralkaline trachyte and rhyolite, and autobreccias between lava flows), but water-rock interactions in these aquifers are poorly understood. Domestic wells in the area of Alpine, Texas, may be completed in either limestone or igneous rock.

Groundwater from the igneous aquifer in Sunny Glen is characterized by high fluorine (1.6 to 3.0 mg/L), compared to aquifers in limestone (mostly <1.3 mg/L). The fluorine may come from the reaction of water with F-rich alkali amphiboles in alkaline mafic and peralkaline felsic rocks. PHREEQ modelling suggests that these waters are approximately saturated with respect to silica gel and slightly oversaturated with respect to chalcedony, and undersaturated with respect to most other minerals. These results are consistent with the abundance of chalcedony amygdules in lavas in the Pruett Formation. Groundwater from the igneous aquifer is lower in Ca (8 to 30 mg/L), Mg (<8 mg/L), Sr (<0.1 mg/L), and total alkalinity (100 to 140 mg/L) than groundwater from limestone aquifers in the region (Ca, 25 to 80 mg/L; Mg, 3 to 40 mg/L; Sr, 0.1 to 1.0 mg/L; total alkalinity 150-350 mg/L). Clearly, this difference reflects the higher solubility of calcite in an aqueous system. There is considerable overlap in pH (6.8 to 8.5) and concentrations of other solutes (including Na, K, Cl, SO4, and NO3) between water samples from the two aquifer types.