Three-Dimensional Ground-Water Modeling in Depositional Systems, Wilcox Group, Oakwood Salt Dome Area, East Texas

RI0133. Three-Dimensional Ground-Water Modeling in Depositional Systems, Wilcox Group, Oakwood Salt Dome Area, East Texas, by G. E. Fogg, S. J. Seni, and C. W. Kreitler. 55 p., 43 figs., 4 tables, 1983. ISSN: 0082335X: Print.


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ABSTRACT
A three-dimensional model was constructed of ground-water flow in the Wilcox-Carrizo aquifer system near Oakwood salt dome to facilitate understanding the hydrogeology around salt domes of the Gulf interior region and ultimately to evaluate the hydrologic suitability of Oakwood Dome for storage of high-level nuclear waste. The data base includes not only measurements of hydraulic head and hydraulic conductivity but also lithofacies maps constructed in a previous study of Wilcox depositional systems.

The Carrizo aquifer is a fairly homogeneous sand sheet overlying the much thicker Wilcox Group, a multiple-aquifer system composed primarily of fluvial channel-fill sand bodies distributed among lower permeability interchannel sands and muds. The interconnectedness of the channel-fill sands, which have predictable values of hydraulic conductivity, strongly influences the rate and direction of ground-water flow. Lateral interconnectedness may depend largely on frequency distributions of channel-fill sands (that is, sand percent). Vertical interconnectedness is apparently poor owing to the horizontal stratification of sand and mud. Simulating observed pressure-depth trends by manipulating values of equivalent vertical hydraulic conductivity (K,') demonstrates that the ratio of vertical to horizontal conductivity (K,'/Khr) is very low (approximately 10 ^-3 to 10^-4).   Locally high values of K,' could result in locally rapid vertical flow, which could in turn be detected using pressure-depth data. Ground-water velocities and travel times computed by the model indicate ground-water residence times of 10³ to 10⁴ years in channel-fill facies and 10⁵ to 10⁶ years in interchannel facies. Because Oakwood Dome is apparently surrounded by interchannel facies as a result of syndepositional dome growth, the dome may be essentially isolated from circulating Wilcox ground water. A possible exception is where channel-fill facies appear to touch or come close to the northeast flank, coinciding with a brackish-water plume that apparently results from dissolution of salt or cap rock. The northeast orientation of the plume appears to be caused by sand-body distribution and interconnection.


Keywords: ground-water modeling, hydrogeology, Oakwood Dome, Wilcox Group, Carrizo Sand, salt domes, waste disposal, depositional systems, Texas


CONTENTS
ABSTRACT

INTRODUCTION

HYDROGEOLOGY

Regional setting

Oakwood Dome vicinity

Hydraulic head.

Pressure versus depth

Total dissolved solids

Ground·water chemistry

Hydraulic conductivity

Field pumping tests

Laboratory permeameter tests

Comparison of field and laboratory tests.

Aquifer mapping

Sand-body interconnectedness

COMPUTER PROGRAMS

MODELING PROCEDURE

Lateral boundaries

Integrated finite difference mesh construction

Boundary conditions

Lateral boundary conditions

Vertical·leakage boundary condition

Equivalent hydraulic conductivity

Simulations

RESULTS AND DISCUSSION

Vertical interconnection of sand bodies

Lateral interconnection of sand bodies

Topographic effects and updip flow directions

Hydraulic conductivity of the Reklaw aquitard

Effects of inserting locally high vertical conductivity values at the Trinity River boundary

Comparison of measured and computed heads

Areas of maximum potential for discharge

Ground-water budgets computed by the model

Ground·water travel times

SUMMARY AND CONCLUSIONS

Applicability to salt dome studies in Louisiana and Mississippi

ACKNOWLEDGMENTS

REFERENCES

Figures

1. Location of model area

2. Hydrostratigraphic column showing Tertiary Wilcox and Claiborne Groups and underlying Cretaceous formations (Midway and Navarro Groups)

3. Regional structural cross section through the study area

4. Potentiometric surface map for the Carrizo aquifer over Oakwood salt dome

5. Potentiometric surface map for the Wilcox aquifer in the Oakwood Dome vicinity

6. Potentiometric surface map for the Carrizo aquifer in the Oakwood Dome vicinity

7. Relationship between elevation of the water table and land surface in the Queen City aquifer, Leon and Freestone Counties

8. Pressure-versus-depth relationship for data within 2 mi (3.22 km) of the Carrizo-Reklaw surface contact

9. Pressure-versus-depth relationship based on water level and pressure measurements made at drill site TOH-2, 2,000 ft (610 m) southeast of Oakwood salt dome

10. Electrical-resistivity-log estimates of total dissolved solids (TDS; mg/ L) in Wilcox-Carrizo sands around Oakwood Dome.

11. Histograms showing hydraulic conductivity (K) distributions from pumping tests and laboratory permeameter tests

12. Results of laboratory permeameter tests conducted on Wilcox core and of pumping tests conducted in adjacent water wells

13. Comparison of arithmetic-normal and log-normal frequency distributions

14. Sample calculations of equivalent horizontal hydraulic conductivity (Kh') of a section of the Wilcox aquifer

15. Sand-percent map of channel-fill sands (R., > 20 ohm-m) for upper layer of the model

16. Sand-percent map of channel-fill sands (R., > 20 ohm-m) for middle layer of the model

17. Sand-percent map of channel-fill sands (R., > 20 ohm-m) for lower layer of the model

18. Example of how the integrated finite difference (IFD) mesh is generated with program OGRE

19. Plan view of the IFD mesh and surface geology from Barnes (1967 and 1970)

20. Structure-contour map on top of Wilcox Group (base of Carrizo Sand)

21. Structure-contour map on base of Wilcox Group

22. Three-dimensional perspective from the southwest of the outer surface of the IFD mesh generated from the structure-contour maps using program OGRE

23. Three-dimensional perspective from the southeast of the upper surface of the IFD mesh

24. Map of IFD mesh, values of hydraulic head prescribed on lateral boundaries, and nodal areas where the Reklaw aquitard is absent in the model

25. Schematic cross section showing values of horizontal head differential (Llh) that would occur if pressure-versus-depth (P-D) slopes· (m) were assumed to be uniformly equal to 0.95 and 1.05 at updip and downdip boundaries, respectively

26. Schematic cross section depicting how vertical-leakage boundary condition was prescribed

27. Map of Queen City water table calculated from the observed water-table/ topography relationship shown in figure 7

28. Values of equivalent horizontal hydraulic conductivity (K1,') calculated for each layer of the model

29. Contour maps of vertical hydraulic gradient (ah/oz) computed between upper and middle and middle and lower layers in simulations (a) Bl, (b) A, and (c) B2

30. Contour maps of hydraulic head computed in simulation A

31. Contour maps of hydraulic head computed in simulation C2

32. Maps showing ground-water velocity vectors (specific discharge) computed in simulation A

33. Maps of ground-water velocity vectors (specific discharge) computed in simulation C2

34. Contour maps of hydraulic head computed in simulation D showing effects of reducing hydraulic conductivity of the Reklaw aquitard

35. Contour maps of hydraulic head computed in simulation E showing effects of increasing the value of Kv' at a node location near the Trinity River boundary

36. Maps of velocity vectors computed in simulation E showing effects of increasing the value of Kv' at a node location near the Trinity River boundary

37. Contour maps of vertical hydraulic gradients (Clh/Clz) computed in simulation E depicting effects of increasing the value of Kv' at a node location near the Trinity River boundary

38. Comparisons of measurement-based and model-generated (simulation E) potentiometric surfaces for (a) upper layer and Wilcox and (b) upper layer and Carrizo

39. Comparisons of measurement-based and model-generated (simulation E) potentiometric surfaces for (a) middle layer and Wilcox and (b) middle layer and Carrizo

40. Maps depicting areas of maximum potential for discharge based on simulations A and E

41. Map showing directions of leakage across the Reklaw aquitard based on simulation D

42. Maps showing ground-water travel times for the middle and lower layers based on simulation A

43. Maps showing ground-water travel times for the middle and lower layers based on simulation E

Tables

1. Pressure-versus-depth (P-D) relationships

2. Field pumping test results from the model area and vicinity

3. Summary of conditions imposed during each simulation

4. Ground-water budgets computed by the model


Citation
Fogg, G. E., Seni, S. J., and Kreitler, C. W., 1983, Three-Dimensional Ground-Water Modeling in Depositional Systems, Wilcox Group, Oakwood Salt Dome Area, East Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 133, 55 p.