Hydrogeology of the lower Cretaceous Edwards and Trinity Group formations near Junction (Kimble County) Texas




Allen, Stephen Robert, 1963-

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This study describes ground-water flow in the Lower Cretaceous formations near Junction (Kimble County), Texas. Rock exposures were examined throughout the 150-mile study area to determine the nature and distribution of permeable features. Dominant features include nearly vertical fractures and horizontal bedding planes in carbonate rocks of the Edwards Group formations (Edwards), and coarse-grained fluvial channel deposits in the underlying Hensel Sand Formation (Hensei). Static water levels were measured in over one hundred wells and contoured to reveal the existence of two separate potentiometric surfaces, one overlying the other. Preliminary, but useful estimates of transmissivity, hydraulic conductivity, and ground-water velocity were derived using specific capacities from eighty-three wells completed in both the upper (Edwards) and lower (Hensel) aquifers. At the edge of the Edwards Plateau where the contact between the Edwards and the Hensel is exposed on the face of the erosional escarpment, ground water discharges from the Edwards aquifer through numerous low volume springs and seeps. An even greater proportion of ground-water discharge leaks from the Edwards aquifer to the underlying Hensel aquifer across a thin low permeability bed at the base of the Edwards which consists of marly, unfractured, nodular limestone. The sum of these two components of discharge is approximately equal to precipitation recharge to the Edwards. To gain additional insight into cross-formational flow an analysis of major and minor ions, redox potential, and dissolved oxygen was conducted for twenty-one water wells which were located along three north-south (inferred flow direction) transects. The Edwards waters were found to be a Ca-Mg-HC0₃ facies; the Hensel waters, a mixed facies. This difference in hydrochemical facies was initially thought to be caused by ion evolution along flowpaths, but it more likely reflects the existence of a regional aquifer below a locally constrained aquifer. High values of dissolved oxygen and redox potential in the Edwards aquifer indicate that recharge is predominant; lower values of these parameters in the Hensel aquifer indicate that this water occurs in an intermediate or discharge zone. To test the conceptual model of steady-state ground-water flow, a numerical model was constructed using the MODFLOW finite-difference computer code. Over one hundred trial and error simulations were executed to calculate leakage through the confining bed, discharge from springs, and discharge to the Llano River. In addition, the distribution and magnitude of focused recharge to the Edwards aquifer was calculated, estimates were made for unknown hydrogeologic parameters, and the Edwards aquifer was demonstrated to be fully perched above the Hensel aquifer. The increased understanding of the ground-water flow regime resulting from this study will support range management activities and improve the success rate of water well drilling.