Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas

RI0141. Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas, by R. K. Senger and C. W. Kreitler. 35 p., 29 figs., 5 tables, 1984. ISSN: 0082335X:print.

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ABSTRACT
The Edwards Formation, on the downthrown side of Mt. Bonnell fault in the Austin, Texas, area (Hays and Travis Counties), is part of the northeastern extension of the Edwards Underground Reservoir, the primary source of water in numerous counties along the Balcones Fault Zone. Recharge to the aquifer is supplied mainly by creeks that cross the Balcones Fault Zone southwest of Austin. Barton Springs is the major point of discharge. Changes in water levels of wells in the area correlate positively with changes in discharge at Barton Springs, suggesting good interconnection. The potentiometric surface of the aquifer changes significantly from high flow to low flow at Barton Springs. During low-flow conditions, ground-water flow lines converge in the eastern part of the Balcones Fault Zone. Water Ievels are also much lower (less than 30 m) and indicate flow from the "bad-water” zone (water with 1,000 mg/L TDS or more from downdip in the Edwards Formation).

Water chemistry at Barton Springs also varies between high and low discharge. Concentrations of sodium, chlorine, sulfate, and strontium increase with decreasing discharge, indicating influx from the "bad-water" zone. This influx of highly saturated "bad-water” into the fresh-water aquifer theoretically results in a decrease in saturation state with respect to calcite and dolomite. The decrease in saturation state would enhance carbonate dissolution at the interface between fresh water and "bad-water" zones, thereby increasing permeabilities in this section of the aquifer. The Edwards aquifer generally contains a consistent calcium bicarbonate water. In some areas of the fresh-water section, however, leakage from the Glen Rose Formation increases the sulfate and strontium concentrations. Leakage occurs across fronts created by large displacements of faults that bring the Edwards Formation into contact with the Glen Rose Formation updip.

Keywords: Barton Springs, Edwards aquifer, Glen Rose Formation, leakage, "bad-water" zone carbonate equilibria, karst hydrology

CONTENTS

ABSTRACT

INTRODUCTION

Previous work

Scope

HYDROGEOLOGIC SETTING

Physiography and climate

Geology related to hydrology

Glen Rose Formation

Walnut Formation

Edwards Formation

Georgetown Formation

Del Rio Clay

Depositional environment of the Edwards Limestone

Hydrogeologic development of the Edwards Limestone aquifer

PHYSICAL HYDROGEOLOGY

Recharge and discharge

Ground-water flow in the aquifer

Interaction between aquifer and springs

Aquifer characteristics

Water volume

Aquifer parameters

HYDROCHEMISTRY

Barton Springs

The "bad-water" zone

Edwards aquifer

Carbonate equilibria

DISCUSSION

SUMMARYACKNOWLEDGMENTS

REFERENCES

Figures
1. Division of the Edwards aquifer according to the Texas Department of Water Resources
2. Location of the study area

3. Geologic map of the study area

4. Regional elements of Texas during the Early Cretaceous
5. Measurements of stream flow showing channel losses in the Balcones Fault Zone

6. Location of major springs of Barton Springs

7. Total recharge to the aquifer compared with total discharge in Barton Springs

8. Potentiometric surface during conditions of high flow, June 1979 and June 1981

9. Potentiometric surface during low-flow conditions, August 1978

10. Location of wells that were measured monthly or were recorded continuously

11. Water-level hydrographs for selected wells in the Austin area

12. Correlation of water levels in well 58-42-903 and total discharge in Barton Springs

13. Water-level response in well 58-42-915 due to draining of Barton pool

14. Water-level response in well 58-50-216 due to draining of Barton pool during low-flow conditions

15. Part of the hydrograph with the recession curve that is analyzed

16. Computed hydraulic heads for well 58-42-915 (isotropic conditions) compared with the observed water level

17. Trilinear diagram of water-chemistry analyses from the Edwards aquifer

18. Chemical composition of water from Barton Springs during varying discharge

19. Increase of sodium and chloride concentrations in Barton Springs water during decreasing discharge

20. Tri linear diagram of water-chemistry analyses from Barton Springs and Lake Austin

21. Strontium versus sodium concentrations in different types of water

22. Areal distribution of sulfate concentration in the Edwards aquifer

23. Trilinear diagram of water chemistry analyses from the Glen Rose Formation

24. Areal distribution of strontium concentrations in the Austin area

25. SO₄/Cl versus sulfate concentration for different types of water

26. Schematic cross section across the Balcones Fault Zone

27. Carbonate equilibria for Edwards aquifer water

28. Carbona1e equilibria for Barton Springs water

29. Carbonate equilibria for surface water from creeks: water samples from channel-loss study

Tables

1. Stratigraphy of geologic units in the Austin area

2. Average annual recharge from different watersheds

3. Transmissivities and storativities obtained from recession-curve analysis of water-level declines in selected wells during 1979 and 1980

4. Results of mixing water from Barton Springs with "bad water"

5. Results of mixing water from Barton Springs with saturated flood water from Barton Creek


Citation
Senger, R. K., and Kreitler, C. W., 1984, Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 141, 35 p.