Constraining Conceptual Models of Groundwater Flow Using Results from an Electrical Resistivity Survey

An electrical resistivity survey was performed in the floodplain of the LeonaRiver, south of Uvalde, Texas to determine the depth and extent of the Leona Formationaquifer. Information gained from the survey is used to support an integrated assessmentof the hydrogeological relationship of the Leona Formation aquifer to the Knippa Gapsegment of the Edwards aquifer. Previous investigation of the hydrogeology of the LeonaRiver floodplain suggests significant groundwater flow out of the Edwards aquifer andinto the Leona River floodplain, which is located adjacent to and south of the KnippaGap. Results from the resistivity survey provided information on the vertical andhorizontal extents of the Leona Formation aquifer and the hydraulic character of theaquifer. This information was used to estimate the magnitude of groundwater flow out ofthe Edwards aquifer. In particular, the locations of paleostream channels in the LeonaRiver floodplain were delineated and the transitions of permeable sands and gravels toless permeable silts and clays were correlated with specific resistivity values. Estimatesof the cross-sectional area of the main channels of the paleostream were made using theseresults. This information was combined with borehole logs, a potentiometric map ofgroundwater, and the results from a 10-day aquifer test to determine the groundwaterflow regime of the Leona Formation aquifer in the Leona River floodplain. The outflowof groundwater through the Leona Formation aquifer in the Leona River channel wasestimated using these analyses. The ensemble of this information was assembled toconstrain a conceptual model for groundwater flow through the Knippa Gap portion ofthe Edwards aquifer. Results from the combined surveys suggest that as much as 100,000acre-ft of water exits the Edwards aquifer through the Leona River floodplain per year.