Constraining Conceptual Models Of Groundwater Flow Using Results From An Electrical Resistivity Survey

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