GEOTHERMAL ANOMALIES AND COMPARTMENTALIZED GROUNDWATER FLOW AT A MOUNTAIN BLOCK-GRABEN INTERFACE

The eastern portion of the Hueco Bolson aquifer in west Texas is recognized as an area of complex geology, variable water quality, and geothermal heating of shallow groundwater. A recent hydrogeological study of this region identified three compartmentalized groundwater flow systems separated by major bounding faults and local groundwater divides. From west to east the compartmentalized flow systems include; 1) basin fill deposits near the main mountain bounding fault, 2) Paleozoic and Mesozoic carbonate and siliciclastic strata forming the transitional pediment/mountain province opposite the basin bounding fault, and 3) Permian carbonate rocks forming the western Diablo Plateau. Just west of the basin bounding fault, groundwater in basin fill is slightly saline, moderately evaporated, and ambient temperature. Groundwater in basin fill contains < 5 pmc and no detectable tritium. Evaporation of water is presumed to have occurred prior to recharge when shallow water bodies were intermittently present on bolson surfaces during pluvial periods. On the eastern side of the bounding fault, groundwater in the transitional region is fresh (<500 mg/L TDS), not evaporated, and isotopically distinct from groundwater detected in basin-fill wells only 200 meters away. These are dilute, geothermal groundwaters at 35 to 38 degrees C near the fault, or about 5 to 8 degrees C warmer than bolson groundwater opposite the fault. A few kilometers east of the main fault but still in the transitional region, groundwater contains bomb tritium and 30 to 50 pmc. The main basin bounding fault is interpreted to act as a flow barrier for a local groundwater flow cell developing in the middle transitional region. There, groundwater moves downward, then laterally to the west where it is heated before encountering the fault barrier. At the fault barrier groundwater moves upward and then laterally along the main fault. East of the transitional region a groundwater divide separates the Diablo Plateau “compartment” from the compartment identified in the transitional region. Groundwater in the Diablo Plateau has a unique isotopic and hydrochemical character that is distinct from the other two compartments; it is marginally evaporated, slightly saline, non-thermal, and contains a mixture of modern and older groundwater (0.5 to 14.0 TU and 5 to 25 pmc).