Regional Hydrodynamics and Hydrochemistry of the Uranium-Bearing Oakville Aquifer (Miocene) of South Texas

RI0124. Regional Hydrodynamics and Hydrochemistry of the Uranium-Bearing Oakville Aquifer (Miocene) of South Texas, by G. E. Smith, W. E. Galloway, and C. D. Henry, assisted by J. K. Gluck, J. F. McIntyre, E. T. Pisasale, and S. D. Spradlin. 31 p., 26 figs., 1 table, 1 appendix, 1982. ISSN: 0082335X:Print.

To purchase this publication as a (PDF) download, please order RI0124D.



ABSTRACT
The Oakville Formation consists of sediments deposited by several major fluvial systems that traversed the Texas Coastal Plain during the Miocene Epoch. Facies geometry and composition, together with superimposed structure and topography, are important determinants of ground-water flow, aquifer transmissivity, and regional hydrochemical evolution.

Topographically high areas along the Oakville outcrop are ground-water recharge zones. Downdip movement of ground water is deflected along strike into major incised stream valleys and modified by local ground-water pumping, resulting in discharge of Oakville water from relatively shallow sections of the aquifer. Discharge from local and intermediate-scale flow cells, as well as influx along growth faults of high-salinity water from deeper stratigraphic horizons, is readily shown by field phenomena. Introduction of exotic evolved connate waters is demonstrable by the distribution of sulfate, chloride, and reduced sulfur species. Shallow Oakville ground waters exhibit a clearly defined compositional change from predominantly sodium chloride waters in the southwest to calcium bicarbonate waters in the northeast.

Redox potential (Eh) of Oakville ground water decreases from +470 mV (oxidizing) to -170 mV (reducing) with increasing depth and coincides with an increase in pH and temperature. Dissolved oxygen content exhibits an inverse relationship to Eh and reduced sulfur. Higher recharge in the northeast results in a deeper subsurface penetration of oxygenated ground waters.

The regional distribution of trace amounts of uranium, molybdenum, selenium, and arsenic in Oakville ground water demonstrates a good correlation with known sites of uranium mineralization, and defines elevated trace metal contents unrelated to known uranium occurrences. These same elements exhibit an overall increased background level in the southwestern section of the study area.


Keywords: hydrochemistry, hydrodynamics, Miocene, Oakville Formation, trace metals, stratigraphy, transmissivity, uranium, South Texas, Texas Coastal Plain


CONTENTS
ABSTRACT

INTRODUCTION

PHYSICAL STRATIGRAPHY

HYDRODYNAMICS

Recharge and discharge

Regional temperature gradients

HYDROCHEMISTRY

Major components

Total dissolved solids

Bicarbonate

Sulfate and chloride

Sodium and calcium

Hydrochemical facies

Electrochemical parameters

Trace metals

Uranium, molybdenum, and selenium

Arsenic and copper

Iron and manganese

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX

Figures

1. Location map of study area

2. Depositional elements of the Oakville Formation

3. Regional transmissivity of the Oakville aquifer

4. Regional hydraulic head within the Oakville Formation

5. Topographic relief along the Oakville outcrop belt

6. Differences in hydraulic head versus elevation

7. Distribution of field phenomena and regional recharge-discharge zones

8. Ground·water temperatures within the Oakville aquifer

9. Total dissolved solids content of Oakville ground water

10. Bicarbonate content of Oakville ground water

11. Sulfate content of Oakville ground water

12. Chloride content of Oakville ground water

13. Sodium-to-calcium ratio of Oakville water

14. Hydrochemical fades patterns within the Oakville aquifer

15. Oxidation potential of Oakville ground water

16. Distribution of dissolved oxygen in Oakville ground water

17. Content of reduced sulfur species in Oakville water

18. Hydrogen ion potentials in Oakville ground water

19. Uranium distribution in Oakville ground water

20. Uranium content as a function of Eh and pH

21. Molybdenum distribution in Oakville ground water

22. Selenium distribution in Oakville ground water

23. Arsenic distribution in Oakville ground water

24. Copper distribution in Oakville ground water

25. Iron distribution in Oakville ground water

26. Manganese distribution in Oakville ground water

Table

1. Summary of vertical hydraulic gradient data


Citation
Smith, G. E., Galloway, W. E., and Henry, C. D., 1982, Regional Hydrodynamics and Hydrochemistry of the Uranium-Bearing Oakville Aquifer (Miocene) of South Texas: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations No. 124, 31 p.