Hydrogeology of the Northern Segment of the Edwards Aquifer, Austin Region

Geologic mapping and fracture analysis of the Lower Cretaceous Edwards aquifer strata near Georgetown and Round Rock, Texas, were performed to provide a better understanding of the geologic framework of the Balcones Fault Zone and to provide information for assessing ground-water flow characteristics. Cretaceous strata dip gently eastward and are locally overlain by terrace deposits and alluvium. Several major normal faults, downthrown to the east, strike northward across the area. Gentle flexures, possibly related to faulting, parallel the faults. Minor normal faults and joints are most abundant in areas adjacent to major faults and flexures. These fractured-strata zones probably parallel the length of the faults or flexure axes and may be as wide as 1 mi (1.6 km).Water-level, precipitation, and hydrochemical data were studied to characterize the hydrogeology of the northern Edwards aquifer. The potentiometric surfaces for conditions of high flow and low flow indicate a regional flow system that is noticeably affected by Balcones faults along the north and south edges of the northern segment of the Edwards aquifer. In the Georgetown area where faults are less abundant, regional ground water generally flows eastward, following approximately the dip of the stratigraphic units. Ground-water flow circulation is faster in the western part of the aquifer than in the eastern part. Main discharge for the western, fast-flowing system occurs along fractures through springs and seeps at the major creeks and rivers in the Georgetown area. Some recharge water moves farther downdip past these springs, where water levels show large seasonal variations due to reduced transmissivity and storativity of the aquifer.The hydrochemistry of Edwards ground water indicates an evolution of ground water from a Ca-HCO, and Ca-Mg-HCO, to a mixed-cation-HCO, farther downdip to a Na-HCO, and finally to a Na-mixed-anion-type water. The hydrochemical evolution of ground water along the flow path indicates congruent and incongruent reaction of recharge water with limestone, resulting in a decrease in Ca/Mg ratios. Tritium concentrations are relatively high in the unconfined part, indicating that ground water was recently recharged. In the confined part, Ca decreases owing to cation exchange, causing an increase in Na. Gypsum dissolution in addition to influx of Glen Rose ground water causes SO, concentration to increase, and mixing with Na-Cl-type water results in both Na and Cl increase. The Na-Cl component in the Na-mixed-anion-type water in Williamson County, where faults are less abundant, may represent connate, interstitial water, as suggested by Sr data. However, Na increases disproportionately to Cl because of cation exchange. Ca ions, derived from gypsum dissolution, are probably exchanged for Na on clay minerals, although shale occurs only as relatively thin layers within the Edwards Limestone. In the confined part of the aquifer, large fluctuations in the potentiometric surface cause seasonal reversals in the vertical flow component. These thin shale layers within the Edwards aquifer strata represent large surface areas for vertical flow; thus, the high Na concentrations can be attributed to cation exchange.