GROUND-WATER AGES FROM THE FRESH WATER ZONE OF THE EDWARDS AQUIFER-INSIGHTS INTO GROUND-WATER FLOW AND RECHARGE

As part of a long-term study of ground-water flow and recharge to the Edwards aquifer in south-central Texas, ground-water age data are being collected in order to define both ground-water residence times as well as to provide another tool for the calibration of a ground-water flow model. A suite of tritium-helium-3 samples were recently collected from both the confined and unconfined areas of the Edwards aquifer. The data set consists of discrete samples taken from monitoring wells and integrated well-head samples from both public and private supply wells located in Uvalde County. Preliminary apparent ages show moderate, downward average linear velocities of 3 to 8 m/yr in the Uvalde area, with increasing age to the east along the ground-water flow path. Though the apparent age data show a fairly consistent distribution across the study area, many apparent ages indicate mixing of both modern (< 60 yr) and pre-modern (> 60 yr) waters. This mixing is most evident along the “bad water” line, an arbitrary delineation (1,000 mg/l total dissolved solids) that separates the fresh water zone of the Edwards aquifer from the down-dip saline water transition zone. Mixing of modern and pre-modern waters is also indicated within the unconfined zone of the aquifer by strikingly high excess helium concentrations in young waters. Excess helium anomalies in the unconfined aquifer are consistent with possible subsurface discharge of pre-modern ground water from the underlying Trinity aquifer into the younger ground water of the Edwards aquifer. The data set suggests a fairly uniform ground-water flow system within an otherwise complex geological environment that consists of regionally faulted bedrock units with igneous intrusions, overprinted by classic karstic features.