Abstract

The U.S. Geological Survey, in cooperation with the San Antonio Water System, examined groundwater-level altitudes (groundwater levels) and water-quality data pertaining to the Austin Group in Bexar County, Texas, during 2009–11. Hydrologic data collected included daily mean groundwater levels collected at seven sites in the study area. Water-quality samples were collected at six sites in the study area and analyzed for major ions, nutrients, trace elements, organic carbon, and stable isotopes. The resulting datasets were examined for similarities between sites as well as similarities to data from the Edwards aquifer in Bexar County, Tex. Similarities in the groundwater levels between sites completed in the Austin Group and site J (State well AY-68-37-203; hereafter referred to as the “Bexar County index well”) which is completed in the Edwards aquifer might be indicative of groundwater interactions between the two hydrologic units as a result of nearby faulting or conduit flow. The groundwater levels measured at the sites in the study area exhibited varying degrees of similarity to the Bexar County index well. Groundwater levels at site A (State well AY-68-36-136) exhibited similar patterns as those at the Bexar County index well, but the hydrographs of groundwater levels were different in shape and magnitude in response to precipitation and groundwater pumping, and at times slightly offset in time. The groundwater level patterns measured at sites C, D, and E (State wells AY-68-29-513, AY-68-29-514, and AY-68-29-512, respectively) were not similar to those measured at the Bexar County index well. Groundwater levels at site F (State well AY-68-29-819) exhibited general similarities as those observed at the Bexar County index well; however, there were several periods of notable groundwater-level drawdowns at site F that were not evident at the Bexar County index well. These drawdowns were likely because of pumping from the well at site F. The groundwater levels at sites H and I (State wells AY-68-37-205 and AY-68-29-932, respectively) exhibited similar patterns as those at the Bexar County index well (coefficient of determination [R²] of 0.99 at both wells), indicating there might be some degree of hydrologic connectivity to the Edwards aquifer.

In general, the water-quality data indicated that the samples were representative of a calcium carbonate dominated system. The major ion chemistry and relations between magnesium to calcium molar ratios and ⁸⁷Sr/⁸⁶Sr isotopic ratios of samples collected from sites H and I indicated that the groundwater from these sites was most geochemically similar to groundwater collected from site B (State well AY-68-36-134), which is representative of groundwater in the Edwards aquifer. Of the sites sampled in this study, there appears to be varying hydrologic connectivity between groundwater from wells completed in the Austin Group and the Edwards aquifer.

First posted December 21, 2012