Hydrogeochemistry of the Vadose Zone in Unmined and Reclaimed Deposits at Big Brown Lignite Mine, East Texas

RI0160. Hydrogeochemistry of the Vadose Zone in Unmined and Reclaimed Deposits at Big Brown Lignite Mine, East Texas, by A. R. Dutton. 37 p., 23 figs., 7 tables, 1 appendix, 1986. ISSN: 0082335X: Print.

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ABSTRACT
In the subsurface and at the outcrop, waters in argillaceous deposits of the Calvert Bluff Formation (lower Eocene, Wilcox Group) are brackish to saline. Samples of vadose water from the outcrop of argillaceous beds at Big Brown lignite mine in Freestone County, Texas, have chloride concentrations of as much as 3,500 mg/L and total dissolved solids of as much as 8,000 mg/L. The composition of brackish ground water was probably modified from Eocene seawater by sevenfold to ninefold dilution with rain water. Ion exchange, pyrite oxidation, and calcite solution further modified water composition.

Recharge rates through the vadose zone vary with lithology. Outcropping sands annually transmit about 10 cm of recharging vadose water to the water table, two to five times the amount that unmined argillaceous beds transmit. Chemical composition of vadose water in outcropping sand deposits is affected by weathering of feldspar and other silicate minerals in the soil zone. The recharge rate through reclaimed mud deposits is probably about the same as that through unmined sand deposits. The chemical composition of vadose water in reclaimed land is inherited from formation water in unmined mud deposits but is changed by dilution and water-rock reactions.

Argillaceous deposits of low hydraulic conductivity in the Calvert Bluff Formation in the EastTexas Basin most likely function as local confining beds. At their outcrop at Big Brown mine, the confining beds can retard movement of vadose water from reclaimed land to juxtaposed aquifers, limiting the impact of mining on ground-water quality. The lateral extent of the argillaceous deposits varies among lignite mines in East Texas.

Keywords: hydrogeology, lignite surface mine, recharge, soil weathering, vadose zone, water chemistry

CONTENTS

ABSTRACT

INTRODUCTION

HYDROGEOLOGIC SETTING

METHODS

Lithology

Vadose-Zone Hydrology

Hydrochemistry

RESULTS

Unmined Mud Deposits

Lithology

Vadose-zone hydrology

Hydrochemistry

Unmined Sand Deposits

Lithology

Vadose-zone hydrology

Hydrochemistry

Reclaimed Mud Deposits

Lithology

Vadose-zone hydrology

Hydrochemistry

DISCUSSION

Estimation of Recharge Rate

Origin of Brackish Water

Estimation of dilution

Hydrochemistry of Sand Deposits

Changes in Hydrogeochemistry Caused by Mining

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX–Chemical composition of water samples

Figures

1. Location of Big Brown lignite mine in Freestone County and of other lignite mines along the outcrop of the Wilcox Group in Texas

2. Stratigraphic column showing texture, facies types, and hydrologic units in the Eocene Wilcox,

Claiborne, and Jackson Groups

3. Hydrogeologic units at Big Brown lignite mine and location of sampling stations

4. Calibration of volumetric water content by neutron-count ratio based on laboratory and field data

5. Summary of substrate textures and sampling approach in an unmined mud deposit at station 3

6. Cation exchange capacity and ratio of bivalent/monovalent cations adsorbed on clay minerals

7. Variation of vadose water content with depth and time in different substrates

8. Effect of stratification on water-content profile

9. Mean values of total dissolved solids

10. Piper diagram showing chemical composition of vadose water in unmined mud

11. Variation in concentration of dissolved chloride with depth

12. Summary of substrate textures and sampling approach in unmined sand deposits

13. Variation in mineralogy and texture of sand deposits with depth

14. Photomicrographs showing development of secondary porosity and clay molds in sand deposits

15. Piper diagram showing chemical composition of vadose water in unmined sand deposits

16. Variation in water content and ionic concentrations with time in unmined sand deposits at station 5

17. Variation in water content and ionic concentrations with time in unmined sand deposits at station 6

18. Variation of dissolved sodium, bicarbonate, and calcium concentrations with depth in the vadose zone in unmined sand deposits

19. Summary of substrate textures and sampling approach in reclaimed mud deposits

20. Piper diagram showing chemical composition of vadose water in reclaimed mud deposits

21. Relation of sodium and sulfate concentrations

22. Vadose water composition plotted on stability field diagram of NaO-SiO₂-H₂O system at 25°C

23. Vadose water composition plotted on stability field diagram of NaO-CaO-SiO₂-H₂O system at 25°C

Tables

1. Chemical composition of rainfall at Big Brown mine, February 7, 1980

2. Sedimentary units at Big Brown mine

3. Concentrations of dissolved and adsorbed cations

4. Concentrations of dissolved ions for estimation of dilution

5. Comparison of feldspar abundance and composition in outcrop and subsurface samples

6. Possible sources of chloride in the Calvert Bluff Formation

7. Calculation of ground-water recharge



Citation
Dutton, A. R., 1986, Hydrogeochemistry of the Vadose Zone in Unmined and Reclaimed Deposits at Big Brown Lignite Mine, East Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No.  160, 37 p.