HYDROGEOLOGY OF THE LOWER GLEN ROSE AQUIFER, SOUTH-CENTRAL TEXAS (GROUNDWATER, WATER RESOURCES, ISOTOPE HYDROLOGY)

The lower member of the Glen Rose Formation contains one of the most productive and laterally extensive aquifers in the region. The lower member of the Glen Rose Formation is composed of biomicrite, dolomite, clayey, micritic marl and reef deposits. The lower member is divided into an upper and lower unit on the basis of lithologic and hydrologic differences. Early Miocene uplift of the eastern Edwards Plateau exposed parts of the lower member of the Glen Rose Formation to subaerial weathering. Preferential dissolution of carbonate in fracture zones developed localized zones of secondary permeability.; The lower unit of the lower member is termed the Lower Glen Rose aquifer. Regional ground-water movement in this aquifer is to the southeast. Ground water in the Lower Glen Rose aquifer is under confined and unconfined conditions. Two types of systems of ground-water flow are present; a regional system dominated by syndepositional permeability and porosity, and many local systems produced by later solutional activity. Aquifer tests indicate transmissivity is 3 to 200 m('2)/d, with a range of 3 to 40 m('2)/d being typical of the regional system. Hydraulic conductivity is 3 to 300 m/d with a range of 3 to 40 m/d in the regional system.; Isotopic dating of samples of ground water from the Lower Glen Rose aquifer by ('14)C yields modern apparent ages for ground water of the local systems and old apparent ages (8,490 to 28,980 years) for ground water of the regional system. Velocity of ground-water movement in the regional system as determined by ('14)C analyses ranges from 4.2 to 4.9 m/year. Velocities obtained from hydrologic parameters compare favorably with these values. A similar range of velocity of ground-water movement has been determined for the Floridan aquifer of Florida and the Pahasapa Limestone of South Dakota and Wyoming.; Recognition of local ground-water systems with localized development of porosity associated with concentrations of fractures is useful in determining the areas which are sensitive to urban development.