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

Abstract

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