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

RI0192. Hydrogeology of the Northern Segment of the Edwards Aquifer, Austin Region, by R. K. Senger, E. W. Collins, and C. W. Kreitler. 58 p., 34 figs., 2 tables, 1990. ISSN: 0082335X: Print.

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ABSTRACT
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.

Keywords: Balcones Fault Zone. Edwards aquifer, hydrochemical processes, hydrodynamics

CONTENTS
Abstract

Introduction

Previous studies

Methods

Hydrogeologic, physiographic, and climatic settings

Geologic framework

Physiography

Hydrologic framework

Climate

Geology

Stratigraphy
Glen Rose Formation

Walnut Formation

Comanche Peak Formation

Edwards Limestone

Georgetown Formation

Del Rio Formation

Buda Formation

Eagle Ford Formation

Austin Group

Quaternary deposits

Structure

Fractures in aquifer limestones

Aquifer hydrodynamics

Ground-water flow patterns, Edwards aquifer

Characterization of water-level variations

Long-term water-level variations, Edwards aquifer

Long-term water-level variations, lower Trinity aquifer

Short-term water-level fluctuations, Edwards aquifer

Water-level patterns

Water-level responses to recharge

Water-level responses to discrete rainfall events

Recharge and discharge mechanisms

Hydrochemistry

Edwards aquifer

Trinity aquifer

Discussion

Hydrochemical patterns

Sulfur isotopic composition

Hydrochemical evolution of ground water

Comparison with the southern aquifer segments

Leakage from deeper formations

"Bad-water" line

Hydrochemical implications of flow through the Georgetown Formation

Recommendations for future studies

Conclusions

Acknowledgments

References

Figures

1. Regional structural setting of the Balcones Fault Zone
2. Map of the Edwards aquifer outcrop belt and location of monitoring wells and rain gauges

3. Physiographic provinces of the Central Texas area

4. Stratigraphic column of Cretaceous rocks of the northern segment of the Edwards aquifer, Austin region

5. Dip-oriented cross section A-A' of the Balcones Fault Zone

6. Geologic map and cross section B-B' of study area

7. Distribution and geometries of major faults and flexures, minor faults, and joints exposed along the North Fork of the San Gabriel River

8. Potentiometric surface map representing high-flow conditions of the northern segment of the Edwards aquifer, Austin region

9. Potentiometric surface map representing low-flow conditions of the northern segment of the Edwards aquifer, Austin region

10. Water-level hydrographs of several wells in the study area

11. Water-level hydrograph of well 58-27-305 based on daily measurements and graph of monthly rainfall amounts in the area

12. Monthly pumpage volumes by the cities of Round Rock and Georgetown

13. Pattern of continuous water-level records from wells 58-35-110, 58-19-507, 58-19-205, and 58-19-623

14. Pattern of continuous water-level records from wells 58-27-210, 58-27-305, and 58-20-409

15. Water-level hydrographs based on weekly measurements for wells 58-27-103, 58-27-217, and 58-19-703

16. Daily water-level hydrographs based on continuous recordings for well 58-35-110

17. Daily water-level hydrographs based on weekly measurements for well 58-19-622 and on continuous recordings for well 58-19-623
18. Daily water-level hydrographs based on continuous recordings for wells 58-19-507 and 58-19-205

19. Daily water-level hydrographs based on continuous recordings for wells 58-19-910 and 58-20-409

20. Daily water-level hydrographs based on continuous recordings for wells 58-27-833 and 58-27-305

21. Continuous water-level recordings showing water-level responses in wells 58-19-623, 58-27-210, 58-35-110, and 58-19-507 to a discrete rainfall event

22. Piper diagram of ground water from the Edwards aquifer

23. Geographic distribution of water types and total dissolved solids in the Edwards aquifer

24. Geographic distribution of fluoride concentrations and water types in the Edwards aquifer

25. Geographic distribution of nitrate concentrations and water types in the Edwards aquifer

26. Piper diagram of ground water from the Glen Rose Formation (upper and middle Trinity aquifer)

27. Piper diagram of ground water from the Hosston Formation (lower Trinity aquifer)

28. (a) Molal SO₄/CI ratios versus SO₄ concentrations and (b) concentrations of SO₄ versus δ³⁴S isotopic concentrations for four samples collected during this study compared with samples collected from the Edwards aquifer, San Antonio region
29. Plot of Ca-SO₄ (molal) versus molal HC0₃ concentrations of water samples from the northern segment of the Edwards aquifer

30. Plot of molal concentrations of Na versus Ca

31. Plot of molal concentrations of Na versus Cl

32. Plot of Na-Cl (molal) versus molal concentration of SO₄

33. Log of molal concentrations of Sr versus Cl for the confined part of the central segment in the San Antonio region, the Barton Springs segment, and the northern segment

34. Log of molal Sr concentrations versus Ca/Mg ratios for all sections of the Edwards aquifer

Tables

1. Fracture-spacing data measured in Georgetown Formation, Edwards Limestone, and Comanche Peak Formation

2. Results of chemical analyses of water samples collected during this study


Citation
Senger, R. K., Collins, E. W., and Kreitler, C. W., 1990, Hydrogeology of the Northern Segment of the Edwards Aquifer, Austin Region: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 192, 58 p.