Permeability Structure of the Edwards Aquifer, South Texas: Implications for Aquifer Management. Digital Download

RI0250D. Permeability Structure of the Edwards Aquifer, South Texas: Implications for Aquifer Management, by S. D. Hovorka, R. E. Mace, and E. W. Collins. 55 p., 30 figs., 12 tables, 1998. doi.org/10.23867/RI0250D. Downloadable PDF.


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ABSTRACT
Quantitative and interpretive assessment of the three-dimensional distribution of permeability in the prolific, complex, and heterogeneous Edwards fractured karstic aquifer leads to improved understanding of the dynamic behavior of the aquifer and identifies areas where information is lacking. The study area comprises the Kinney to Comal County section of the aquifer, which supplies the city of San Antonio, Comal and San Marcos Springs, and agriculture. Techniques used in this study include analysis of specific-capacity tests; reinterpretation of research-well results; stratigraphic and porosity interpretation of wireline logs; core, core-plug, and thin-section analysis; subsurface and outcrop mapping; and quantitative analysis of outcrop analogs of the aquifer

Permeability in the Edwards aquifer varies over eight orders of magnitude, and it is multimodal. Multimodal permeability distribution implies that the fastest moving water can travel many times faster than the largest volume of water. This distribution is important to aquifer-usage assessment involving drawdown, traveltimes, or effective porosity. Structurally influenced cave systems contribute the highest hydraulic conductivities (10¹ to 10⁶ ft/d), solution-enhanced fractures and stratigraphically controlled karst contribute intermediate values, and the porous carbonate matrix contributes hydraulic conductivities of 10^-3 to 10¹ ft/d.

Because the interface between the aquifer and saline zone downdip is complex in three dimensions, the risk of water-quality reduction in times of lowered water level needs to be assessed separately in each part of the aquifer. In the narrow north part of the aquifer, the interface is locally fault bounded. In the central part, the deepest penetration of fresh water may be related to high transmissivity on the San Marcos Platform edge. In the west part of the aquifer, structural influences on brine leakage complicate the fresh-saline interface.

Permeability distribution is related to aquifer genesis. Dolomite alteration and dissolution driven by mixing of saline and fresh water have had a strong effect on permeability development in the confined aquifer. The unconfined aquifer in the north part of the study area has a two-order-of-magnitude lower average permeability than does the confined aquifer. The observed aquifer permeability distribution can improve assessments of various spring-flow scenarios. In the west part of the aquifer, geologic data suggest that permeability structure is related to multistage dissolution of evaporites. In this area, most permeability is developed in the fractured upper part of the Edwards Group above the evaporite section, accounting for steeper head gradients in this area. This west-to-east aquifer heterogeneity, which could cause variable responses to drawdown, may be relevant in any attempt to balance or regulate western agricultural or eastern municipal usage.

Keywords: Edwards aquifer, hydrogeology, hydrology, karst, South Texas

CONTENTS

Abstract

Introduction

            Geologic setting and previous work

                        Structural setting

                        Stratigraphy, facies, and diagenesis

                        Hydraulic properties and simulations.

            Methods

                        Structural data set

                        Specific capacity, transmissivity, and hydraulic-conductivity data set

                                    Specific capacity

                                    Transmissivity and hydraulic conductivity

                                    Calculation of transmissivity from specific capacity

                                    Geostatistical analysis

            Matrix-permeability data set

            Quantitative description of faults. fractures. and conduits in outcrop

                        Fracture mapping

                        Fracture aperture and porosity

                        Fracture and conduit roughness

                        Karst features

Geometry of the Edwards aquifer
            Structural framework of the Edwards aquifer

            Geometry of the fresh-saline interface

            Relay ramps and flow pathways

Matrix, fracture, and conduit permeability of the Edwards aquifer

            Rock fabric and facies controls on permeability

                        Porosity-permeability relationships

            Conduit and fracture porosity

                        Structural description of fractures

                        Fracture aperture and porosity

                        Fracture and conduit roughness

                        Solution features

            Transmissivity from aquifer tests

                        Statistical description of data

                        Areal distribution of transmissivity

                        Spatial relationships

                        Vertical variation of hydraulic conductivity

                        Relationship to matrix permeability

            Interrelationships between data sets

                        Vertical variation in permeability

                        Horizontal variation in permeability

                                    Relationships between faults and permeability

                                    Relationships between hydrologic setting and permeability

                                    Relationships between depositional facies and permeability

Results

Implications of permeability structure of the Edwards aquifer for aquifer management

            Spring  flow

            Western area

            Contaminant transport and resource evaluation

            Water-quality degradation as a result of saline invasion

Acknowledgments

References

Figures

1. Map showing structural, hydrologic, and facies setting of the Edwards aquifer

2. Cross section A-A', showing stratigraphic units within the Edwards Group

3. Scattergram showing empirical relationship that was used to estimate transmissivity from specific-capacity data

4. Map showing location of cored wells, outcrops, and logged wells used in this study

5. Map of Balcones Fault Zone structure on the base of the Del Rio Formation

6. Generalized view of a relay ramp

7. Rock and pore-water conductivity map superimposed on generalized structural map

8. Schematic diagram of several common pore structures resulting from different diagenetic histories

9. Semilog scatter plots of porosity versus plug  permeability

10. Semilog scatter plots of plug porosity versus plug permeability 

11. Permeability calculated from porosity log of the USGS Castle Hills well

12. Distribution of average matrix permeability

13. Throws of faults and widths of highly fractured-brecciated zones associated with faults of the Balcones Fault Zone

14. Fracture zones at faults cutting the Kainer Formation of the Edwards Group 

15. Faults, fractures, and solution-enlarged voids at the Wilderness Oak exposure, drawn from a photomosaic

16. Faults, fractures, and solution-enlarged voids at the San Geronimo exposure, drawn from a photomosaic

17. Cumulative distribution functions for fracture apertures measured at the Stone Oak, Wilderness Oak, and San Geronimo exposures

18. Fracture and conduit surfaces measured at the Wilderness Oak exposure

19. Fracture and conduit surfaces measured at the San Geronimo exposure

20. Histograms of karst-feature size distribution for Lake Medina, FM 1604 at Bitters Road, Wilderness Oak, FM 1604 at Blanco Road, Fiesta Texas, New Braunfels, San Geronimo, and Stone Oak

21. Faults, fractures, and solution-enlarged voids at the Lake Medina exposure, drawn from a south-north photomosaic

22. Histograms of specific-capacity, transmissivity, and hydraulic-conductivity data

23. Smoothed and kriged transmissivity of the Edwards aquifer

24. Transmissivity histograms of all data, data from the confined zone, data from the unconfined (or outcrop) zone, data from the confined part of the Bexar County area, and data from the confined part of the Devils River Formation

25. Hydraulic-conductivity histograms of all data, data from the confined zone, data from the unconfined (or outcrop) zone, and matrix data

26. Experimental and theoretical semivariograms of specific-capacity, transmissivity, and hydraulic-conductivity data

27. West-east cross section showing vertical variation of aquifer test and matrix permeability for test wells at New Braunfels

28. Northwest-southeast cross section showing vertical variation of aquifer test and matrix permeability for test wells at San Marcos

29. North-south cross section showing vertical variation of aquifer test and matrix permeability for test wells at San Antonio

30. Vertical variation of aquifer test and matrix permeability for test well in south Medina County

Tables

1. Literature sources of structural data on the Edwards aquifer

2. Hydrologic descriptions of the Edwards aquifer

3. Empirically derived porosity-permeability relationships for various components of the Edwards Group

4. Outcrops analyzed for this study

5. Summary of porosity calculations from aperture measurements at outcrops

6. Specific roughness of fractures and solution features measured at the San Geronimo and Wilderness Oak sites

7. Karst porosities measured using image analysis of selected parts of outcrops

8. Size distribution of karst features

9. Summary of transmissivity values for subdivisions of the aquifer

10. Summary of matrix and aquifer test hydraulic-conductivity values for subdivisions of the aquifer

11. Parameters for theoretical spherical variograms

12. Summary of 50-ft flow tests and packer tests made at the bad-water-line experimental test sites in New Braunfels, San Marcos, San Antonio, and the test well in south Medina County



Citation
Hovorka, S. D., Mace, R. E., and Collins, E. W., 1998, Permeability Structure of the Edwards Aquifer, South Texas: Implications for Aquifer Management: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 250, 55 p. doi.org/10.23867/RI0250D.