Hydrogeology of a Gypsum Playa, Northern Salt Basin, Texas

RI0158. Hydrogeology of a Gypsum Playa, Northern Salt Basin, Texas, by F. M. Boyd and C. W. Kreitler. 37 p., 24 figs., 4 tables, 1 appendix, 1986. ISSN: 0092335X: Print.


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ABSTRACT
The northern Salt Basin in West Texas and New Mexico is a closed hydrologic system in which discharge of ground-water flow occurs in a series of playas, or salt flats. Ground water originating in peripheral consolidated rocks and alluvial fans flows toward the center of the basin and discharges by evaporation from the salt flats. Progressive increases in salinity are characteristic of the waters moving down gradient and are primarily attributed to evaporative concentration. Steady increases in sodium (Na⁺) and chloride (Cl^-) and, to a lesser degree, magnesium (Mg⁺²) and potassium (K⁺) characterize increases in total dissolved solids (TDS). Intense evaporation above the water table in the salt flats concentrates the composition of brine to TDS values that range from 50,000 mg/L to greater than 300,000 mg/L. Precipitation of minerals from solution, primarily alkaline earth carbonates and gypsum, results in depletion of calcium (Ca⁺²) and bicarbonate (HCO₃^-) and relative enrichment of sulfate (SO₄^-2) in the ground water. Inflowing ground water of a Ca-Mg-SO₄ composition undergoes evaporation and evolves to a final composition of a Na-Mg- SO₄-Cl brine in the salt flats. Gypsum currently precipitates in the salt flats at a depositional rate that ranges from 0.02 to 0.09 cm/yr. These deposits do not likely represent relict lakebed sediments.

Evaporative concentration and mineral precipitation are the most important processes in the basin and flats that affect the composition of the shallow subsurface fluids. However, the chemistry of the brines in the salt flats is occasionally modified by surface water that ponds on the playa surface and percolates downward.

Keywords: gypsum, hydrogeology, Salt Basin, Trans-Pecos Texas, water chemistry

CONTENTS

ABSTRACT

INTRODUCTION

Geologic setting

Geomorphic setting

HYDROLOGY

Recharge

Movement

Discharge

PLAYA CHARACTERISTICS

Playa surfaces and microrelief structures

SHALLOW SUBSURFACE STRATIGRAPHY

MINERALOGY

HYDROCHEMISTRY

Data base and analytical methods

Results

DISCUSSION

Brine chemistry evolution

Gypsum and halite

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX: History of salt mining at the salt lakes

Figures

1. Location of the northern Salt Basin

2. Zimpleman Salt Lake

3. Structure map of the northern and central Salt Basin

4. Geomorphic features

5. Location of wells and test holes sampled for this study or for which chemical analyses were available

6. Map of the potentiometric surface of the northern and north-central Salt Basin 7. Playa surfaces of the northern Salt Basin

8. Shallow sediments beneath the playa surface

9. SEM photographs of playa crusts

10. Occurrence of gypsum

11. a. Section A-A'. Ground-water chemical composition from the Delaware Mountains and associated fans to the salt flats.11b. Section B-B'. Ground-water chemical composition from Dell City to the salt flats. 11c. Section C-C'. Ground-water chemical composition from Sierra Diablo Plateau and associated alluvial fans to the salt flats. 11d. Section D-D'. Ground-water chemical composition within the salt flats

12. Piper diagrams of chemical composition of ground waters from the Salt Basin

13. Saturation potential for gypsum

14. Cl^- versus TDS

15. Na⁺ versus K⁺

16. Na⁺/Cl versus TDS

17. Brine evolution and mineral precipitation sequence

18. Ca⁺²/HCO₃^-versus TDS

19. Ca⁺² versus TDS

20. HCO₃^- versus TDS

21. Ca⁺²/SO₄^-2 versus TDS

22 SO₄^-2 versus TDS

23. Mg ⁺²/SO₄^-2 versus TDS

24. Flooding of the salt flats, summer 1981

Tables

1. Geologic and hydrologic characteristics, Northern Salt Basin and surrounding highlands

2. Water levels of wells and test holes, northern Salt Basin

3. Chemical analyses of waters, northern Salt Basin

4. Chemical analyses of Salt Flat pore waters



Citation
Boyd, F. M., and Kreitler, C. W., 1986, Hydrogeology of a Gypsum Playa, Northern Salt Basin, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 158, 37 p.