Geomorphology, hydrogeology, geochemistry, and evolution of the karstic Lower Glen Rose aquifer, south-central Texas.

摘要

The Lower Glen Rose is formed in karsted Cretaceous limestone in south-central Texas. This study is a comprehensive characterization of the aquifer within the Cibolo Creek and Guadalupe River drainage basins. Significant hydrogeologic development of the aquifer began about 1.2 Ma B.P. Aquifer hydrogeology is described based on potentiometric mapping, cave morphology, hydrograph studies and geochemical data. Aquifer properties differ in the adjacent drainage basins. Groundwater in the Guadalupe River basin is gravity-drained and discharges at springs near the river. Flow within the Cibolo basin is shown to drain into the artesian Edwards (Balcones Fault Zone) Aquifer. The rising limb of hydrographs is found to estimate the size of karst springs' drainage basins, volumes of base flow storage, and conduit transmissivities. Recession coefficients values are determined that quantify spring discharge as from either diffuse, fissure, or conduit flow.;Cave morphology is characterized and found a function of the hydraulic gradient; stratigraphy and structure are subordinate factors. The relationship between stream gradient and the deposition of rimstone dams is quantified. Tapoff passages are redefined in broader, theoretical terms, and are found to result in conduit groundwater divides in low gradient cave streams saturated with respect to calcite. The dimensions of potentiometric groundwater troughs along conduits are shown to be measures of conduit and diffuse aquifer storage.;The Lower Glen Rose has a predominantly calcium-bicarbonate groundwater of generally good drinking water quality. Groundwater contamination in karst aquifers is determined more by the presence of contaminant sources rather than the presence of conduits. Isochemical maps are constructed and found effective at spatially defining aquifers' chemistry and groundwater circulation patterns.;Lower Glen Rose epikarst is comprised of thin soils and portions of the limestone usually described as the endokarst, and is chemically and hydrologically indistinguishable from the vadose zone. The aquifer's denudation rate is similar but lower than other karst areas, and occurs mostly in the epikarst. However, many previously calculated denudation rates have been based on spring discharge and surface runoff, which may be incompatible unless all runoff ends up recharging the aquifer.