PRELIMINARY GEOCHEMICAL ANALYSES OF SPRINGS WITHIN THE OWL MOUNTAIN AND NOLAN CREEK PROVINCES, FORT HOOD MILITARY INSTALLATION, BELL AND CORYELL COUNTIES, TEXAS

The Owl Mountain and Nolan Creek Provinces within the Fort Hood Military Installation are karst landscapes characterized by plateaued outcrops of Cretaceous-age Trinity and Fredericksburg Groups including Glen Rose, Walnut Clay, Comanche Peak, and Edwards limestones and marls. These plateaus have been interpreted as isolated mound structures that existed behind the main Edwards reef trend. Many springs and seeps exist within these provinces, potentially providing valuable information about flow paths, residence time and basic geochemistry of the fluids within the system. A suite of springs have been monitored over a twelve month period in order to understand spatial and temporal physiochemical variations and gain an understanding of the hydrogeologic controls and possible coupling to other parts of the military installation.

The physiochemical properties of springs coupled with the proposed depositional model are the basis for developing a conceptual geochemical model for fluid migration. Permeability varies greatly across the study area; regions where Edwards and Comanche Peak formations interfinger typically have lower permeabilities than regions dominated by only Edwards deposition. Results from this study suggest that the interfingered-nature of Edwards and Comanche Peak sediments has created a partitioned groundwater system where vadose and phreatic components mix in the subsurface. The interfingered formations have likely created a semi-confined aquifer system where deeper phreatic fluids, migrate upwards through low permeability strata along preferential flow paths and communicate with meteoric waters near the ground surface, in response to surface denudation and climate change, attesting to a complex groundwater system that is continuing to evolve.