MAPPING THE UPPER GLEN ROSE UNIT 3 AQUITARD: IMPLICATIONS FOR RECHARGE TO THE MIDDLE TRINITY AQUIFER, HAYS COUNTY, TEXAS

Developing a detailed understanding of geologic structure, lithology and spatial distribution of geologic units that act as barriers to flow (aquitards) is critical for delineating groundwater movement and recharge in karst aquifers. The Upper Glen Rose (UGR) is the uppermost unit of the Trinity Group and outcrops over most of western Hays County. Surface water/groundwater interactions in the UGR are complex, and the unit contains shallow perched water tables and numerous springs. The UGR overlies the Middle Trinity aquifer, the primary water producer in the area, and is incised by streams along the eastern edge of the Edwards Plateau. While considerable work has been done on the regional lithostratigraphy of the UGR, little is known about the potential for streams flowing over the UGR to recharge the underlying Middle Trinity aquifer. Recent work in the Onion Creek watershed suggests that losing segments of these streams provide recharge to the Middle Trinity. Here we present the results of mapping Unit 3, a UGR sub-unit (Stricklin et al., 1971). Unit 3 is a nodular fossiliferous limestone and clay with abundant Orbitolina texana (foram). Literature and field observations indicate that Unit 3 acts as an aquitard, a barrier to downward vertical flow through the UGR within the Onion Creek watershed. The thinning or absence of the Unit 3 aquitard over sections of Onion Creek, combined with fractures and karst features, provides the potential for recharge into the Middle Trinity aquifer. Remote sensing, subsurface geophysical log interpretation, measured sections, and field mapping methods were used to produce detailed structure and geologic outcrop maps of the study area. These data will be useful in predicting potential areas of recharge from surface water streams within the watershed, and informing future projects aimed at understanding recharge to the Middle Trinity aquifer in western Hays County.