Geochemistry of Ground Water in the Miocene Oakville Sandstone...Texas Coastal Plain

RI0118. Geochemistry of Ground Water in the Miocene Oakville Sandstone--A Major Aquifer and Uranium Host of the Texas Coastal Plain, by C. D. Henry, W. E. Galloway, G. E. Smith, C. L. Ho, J. P. Morton, and J. K. Gluck. 63 p., 21 figs., 9 tables, 3 appendices, 1982. ISSN: 0082335X:Print.


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ABSTRACT
The Oakville Sandstone is a major aquifer and a major host of uranium deposits in the Texas Coastal Plain. This report discusses the geochemistry - including major ions, the trace elements uranium, molybdenum, arsenic, and selenium, and oxidation-reduction potential – of Oakville ground water to evaluate the potential effect of uranium mining on water quality. Ground-water chemistry was investigated both regionally and in two major uranium mining districts.

Major ion composition of Oakville ground water shows a regional variation. East Texas water has low to moderate dissolved solids (350 to 1,100 mg/L) and evolves from a calcium-magnesium-bicarbonate water in near-surface recharge areas to a sodium-bicarbonate composition downdip. The composition and evolution result from calcite solution and cation exchange. South Texas water has higher dissolved solids (up to 3,000 mg/ L) dominated by sodium, chloride, and sulfate. The high dissolved solids result from solution of evaporites within the Oakville and from the discharge along faults into the Oakville of high-dissolved-solids water from deeper aquifers.

There are three oxidation-reduction zones within the Oakville: (1) an oxidizing zone with Eh values greater than 300 mV, controlled by the presence of measurable concentrations of dissolved oxygen, (2) an intermediate zone with Eh values between 110 and 10 mV, probably controlled by ferrous-ferric mineral reactions, and (3) a reducing zone with Eh values less than -40 mV, probably controlled by the presence of FeS₂.

Uranium, molybdenum, arsenic, and selenium show regional variations in concentrations, with trace element values in South Texas water distinctly higher than in East Texas water. The higher concentrations may result either from secondary dispersion from uranium deposits, which are more abundant in South Texas than in East Texas, or from a residual primary source in South Texas. Uranium closely follows behavior predicted from thermodynamic data; equilibrium with coffinite determines uranium concentrations in intermediate-Eh and reducing water. The other three elements do not follow predicted behavior, probably because no data are available on complexing.

Analysis of water chemistry around the two uranium mining districts gives results similar to those found in the regional study. Very high concentrations of trace elements occur only in a few samples within ore zones. These high concentrations probably result from natural dispersal from the deposits, although this is not a widespread phenomenon.

Keywords: Oakville, water chemistry, uranium, aquifers, geochemistry, Texas, East Texas

CONTENTS

ABSTRACT

INTRODUCTION

GEOLOGY OF THE OAKVILLE SANDSTONE

OAKVILLE GROUND-WATER SAMPLING PROGRAM

MODELING OF GROUND-WATER CHEMISTRY

REGIONAL WATER CHEMISTRY

MAJOR ION CHEMISTRY

East Texas

South Texas

OXIDATION AND REDUCTION

Oxidizing zone

Intermediate Eh zone

Reducing zone

Summary of oxidation-reduction patterns

DISTRIBUTION OF TRACE ELEMENTS

Precipitation

Uranium

Molybdenum

Selenium.

Arsenic

Conclusion

Implications of regional distribution

Uranium

Selenium

Molybdenum

Arsenic

WATER CHEMISTRY IN URANIUM DISTRICTS

RAY POINT DISTRICT.

Major ion chemistry

Oxidation-reduction

Trace element concentrations

GEORGE WEST DISTRICT

Major ion chemistry

Oxidation-reduction

Trace element concentrations

CONCLUSIONS

EVALUATION OF GEOCHEMICAL CONTROLS OF TRACE ELEMENT CONCENTRATIONS

EH GRADIENTS

ACKNOWLEDGMENTS

REFERENCES

APPENDICES

A. ANALYTICAL DATA

A-1. Regional samples

A-2. Ray Point area samples

A-3. George West area samples

B. WELL CROSS SECTIONS

C. PROCEDURES FOR ANALYSIS OF ARSENIC, MOLYBDENUM, SELENIUM, AND URANIUM IN WATER

 

FIGURES

1. Depositional elements of the Oakville fluvial system

2. Generalized distribution of faults and diapiric intrusions

3. Piper (trilinear) diagram: Oakville ground water

4. Eh-pH diagram showing hypothesized controls on the oxidation-reduction state of Oakville water

5. Plot of coffinite saturation indices versus dissolved H2S, in ppm, for selected Oakville water samples

6. Eh-pH diagrams for uranium species at 25° C, 1 atm

7. Eh-pH diagrams for molybdenum species at 25° C, 1 atm

8. Eh-pH diagrams for selenium species at 25° C, 1 atm

9. Eh-pH diagram for arsenic species at 25° C, 1 atm

10. Adsorption characteristics of uranium, molybdenum, selenium, and arsenic

11. Ray Point area, South Texas, showing sampling locations, Oakville net sand, and areas of mining

12. Cross sections of Ray Point district, Texas, showing water wells sampled for this study

13. Piper (trilinear) diagram: water of the Ray Point district, Texas

14. Distribution of Eh values and concentrations of dissolved O2 and H2S in water of the Ray Point district, Texas

15. George West area, South Texas, showing sampling locations, Oakville net sand, and areas of mining

16. Cross sections of the George West district, Texas, showing water wells sampled for this study

17. Piper (trilinear) diagram: water of the George West district, Texas

18. Distribution of Eh values and concentrations of dissolved 02 and H2S in water of the George West district, Texas

19. Distribution of arsenic concentrations in water of the George West district, Texas

20. Distribution of molybdenum concentrations in water of the George West district, Texas

21. Distribution of uranium concentrations in water of the George West district, Texas

 

TABLES

1. Saturation indices for major element minerals (regional wells)

2. Measured and calculated Eh, dissolved 02, and H2S (regional wells)

3. Saturation indices for trace element minerals (regional wells)

4. Saturation indices for major element minerals (Ray Point wells)

5. Measured and calculated Eh, dissolved O2, and H2S (Ray Point wells)

6. Saturation indices for trace element minerals (Ray Point wells)

7. Saturation indices for major element minerals (George West wells)

8. Measured and calculated Eh and dissolved O2, and H2S (George West wells)

9. Saturation indices for trace element minerals (George West wells)


Citation
Henry, C. D., Galloway, W. E., Smith, G. E., Ho, C. L., Morton, J. P., and Gluck, J. K., 1982, Geochemistry of Ground Water in the Miocene Oakville Sandstone--A Major Aquifer and Uranium Host of the Texas Coastal Plain: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 118, 63 p.