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| Texas Water Science Center |
By Allan K. Clark and Celeste A. Journey
U.S. Geological Survey
Scientific Investigations Report 2006–5200
In cooperation with the San Antonio Water System
The U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a 4-year study during 2001–04 to identify major ground-water flow paths in the Edwards aquifer in northern Medina and northeastern Uvalde Counties, Texas. The study involved use of geologic structure, surface-water and ground-water data, and geochemistry to identify ground-water flow paths. Relay ramps and associated faulting in northern Medina County appear to channel ground-water flow along four distinct flow paths that move water toward the southwest. The northwestern Medina flow path is bounded on the north by the Woodard Cave fault and on the south by the Parkers Creek fault. Water moves downdip toward the southwest until the flow encounters a cross fault along Seco Creek. This barrier to flow might force part or most of the flow to the south. Departure hydrographs for two wells and discharge departure for a streamflow-gaging station provide evidence for flow in the northwestern Medina flow path. The north-central Medina flow path (northern part) is bounded by the Parkers Creek fault on the north and the Medina Lake fault on the south. The adjacent north-central Medina flow path (southern part) is bounded on the north by the Medina Lake fault and on the south by the Diversion Lake fault. The north-central Medina flow path is separated into a northern and southern part because of water-level differences. Ground water in both parts of the north-central Medina flow path moves downgradient (and down relay ramp) from eastern Medina County toward the southwest. The north-central Medina flow path is hypothesized to turn south in the vicinity of Seco Creek as it begins to be influenced by structural features. Departure hydrographs for four wells and Medina Lake and discharge departure for a streamflow-gaging station provide evidence for flow in the north-central Medina flow path. The south-central Medina flow path is bounded on the north by the Seco Creek and Diversion Lake faults and on the south by the Haby Crossing fault. Because of bounding faults oriented northeast-southwest and adjacent flow paths directed south by other geologic structures, the south-central Medina flow path follows the configuration of the adjacent flow paths—oriented initially southwest and then south. Immediately after turning south, the south-central Medina flow path turns sharply east. Departure hydrographs for four wells and discharge departure for a streamflow-gaging station provide evidence for flow in the south-central Medina flow path. Statistical correlations between water-level departures for 11 continuously monitored wells provide additional evidence for the hypothesized flow paths. Of the 55 combinations of departure-dataset pairs, the stronger correlations (those greater than .6) are all among wells in the same flow path, with one exception. Simulations of compositional differences in water chemistry along a hypothesized flow path in the Edwards aquifer and between ground-water and surface-water systems near Medina Lake were developed using the geochemical model PHREEQC. Ground-water chemistry for samples from five wells in the Edwards aquifer in the northwestern Medina flow path were used to evaluate the evolution of ground-water chemistry in the northwestern Medina flow path. Seven simulations were done for samples from pairs of these wells collected during 2001–03; three of the seven yielded plausible models. Ground-water samples from 13 wells were used to evaluate the evolution of ground-water chemistry in the north-central Medina flow path (northern and southern parts). Five of the wells in the most upgradient part of the flow path were completed in the Trinity aquifer; the remaining eight were completed in the Edwards aquifer. Nineteen simulations were done for samples from well pairs collected during 1995–2003; eight of the 19 yielded plausible models. Ground-water samples from seven wells were used to evaluate the evolution of ground-water chemistry in the south-central Medina flow path. One well was the Trinity aquifer end-member well upgradient from all flow paths, and another was a Trinity aquifer well in the most upgradient part of the flow path; all other wells were completed in the Edwards aquifer. Nine simulations were done for samples from well pairs collected during 1996–2003; seven of the nine yielded plausible models. The plausible models demonstrate that the four hypothesized flow paths can be partially supported geochemically.
Abstract
Introduction
Purpose and Scope
Structural Controls on Ground-Water Flow Paths
Acknowledgments
Approach and Methods
Data Collection and Sample Analysis
Quality Assurance and Quality Control of Chemical Samples
Methods of Data Analysis
Hydrologic Indicators of Ground-Water Flow Paths
Geochemical Methods to Characterize Water
Geochemical Simulation to Identify Flow Paths
Hydrologic Identification of Ground-Water Flow Paths
Northwestern Medina Flow Path
North-Central Medina Flow Path (Northern and Southern Parts)
South-Central Medina Flow Path
Correlation Between Water Levels in Wells With Continuous Record
Geochemical Characterization
Age of Ground Water
Ground-Water/Surface-Water Mixing Relations
Major-Ion Chemistry
Geochemical Simulation
Northwestern Medina Flow Path
North-Central Medina Flow Path (Northern and Southern Parts)
South-Central Medina Flow Path
Summary
References
| 1–2. | Maps showing: | |
| 1. | Location of study area and relation between Edwards and Trinity aquifers, northern Medina and northeastern Uvalde Counties, Texas | |
| 2. | Surface geology and major faults in the study area, northern Medina and northeastern Uvalde Counties, Texas | |
| 3–4. | Diagrams showing: | |
| 3. | Relay-ramp development | |
| 4. | Idealized view of relay ramps, fractures, faults, and flow paths in the Edwards aquifer, northern Medina County, Texas | |
| 5–6. | Maps showing: | |
| 5. | Locations of continuous water-level monitoring wells, surface-water data-collection sites, and hypothesized ground-water flow paths, northern Medina and northeastern Uvalde Counties, Texas | |
| 6. | Locations of ground-water wells from which samples were obtained and hypothesized ground-water flow paths, northern Medina and northeastern Uvalde Counties, Texas | |
| 7–10. | Hydrographs showing: | |
| 7. | Water-level departure for two wells in the northwestern Medina flow path, northern Medina County, Texas, and discharge departure for Hondo Creek near Tarpley, Texas, March 2002–May 2004 | |
| 8. | Water-level departure for a well in the north-central Medina flow path (northern part) and for Medina Lake, northern Medina County, Texas, and discharge departure for Seco Creek at Miller Ranch, Texas, March 2002–May 2004 | |
| 9. | Water-level departure for three wells in the north-central Medina flow path (southern part) and for Medina Lake, northern Medina County, Texas, March 2002–May 2004 | |
| 10. | Water-level departure for four wells in the south-central Medina flow path, northern Medina County, Texas, and discharge departure for Medina River at Bandera, Texas, March 2002–May 2004 | |
| 11. | Graph showing correlation between water-level departures for 11 monitoring wells, northern Medina County, Texas | |
| 12. | Map showing water-level departure pairs from 11 monitoring wells for which Kendall's tau was greater than .60, northern Medina and northeastern Uvalde Counties, Texas | |
| 13–15. | Graphs showing: | |
| 13. | Variation with depth of selected bulk-rock chemistry, in weight-percent, determined by X-ray fluorescence analysis of cuttings from well Medina5 (TD–68–25–711), northern Medina County, Texas, 2002 | |
| 14. | Apparent ages of ground water in selected wells in the Trinity and Edwards aquifers, northern Medina and northeastern Uvalde Counties, Texas, based on sulfur hexafluoride concentration | |
| 15. | Relation between delta oxygen-18 and delta deuterium ratios for selected ground- and surface-water sites, northern Medina County, Texas, 1995–2003 | |
| 16–17. | Stiff diagrams showing the variability in composition of: | |
| 16. | Ground water and surface water, northern Medina County, Texas | |
| 17. | Ground water in the Edwards aquifer, grouped by year of recharge (apparent age based on sulfur hexafluoride concentration), northern Medina and northeastern Uvalde Counties, Texas | |
| 18. | Piper diagrams showing composition of ground water in the Edwards aquifer in the northwestern, north-central, and south-central Medina flow paths, northern Medina and northeastern Uvalde Counties, Texas | |
| 19. | Map showing wells in flow paths that yielded samples used to evaluate evolution of ground-water chemistry by simulation, northern Medina and northeastern Uvalde Counties, Texas | |
| 20–24. | Graphs showing: | |
| 20. | Saturation indexes (computed with PHREEQC) for selected mineral phases for ground water from wells in the northwestern Medina flow path, northern Medina County, Texas, 1995–2003 | |
| 21. | Saturation indexes (computed with PHREEQC) for selected mineral phases for ground water from wells in the north-central Medina flow path, northern Medina County, Texas, 1995–2003 | |
| 22. | Relation between percentage surface-water contribution to the Trinity and Edwards aquifers and calcite saturation index, north-central Medina flow path, northern Medina County, Texas, 1995–2003 | |
| 23. | Saturation indexes (computed with PHREEQC) for selected mineral phases for ground water from wells in the south-central Medina flow path, northern Medina County, Texas, 1995–2003 | |
| 24. | Relation between percentage surface-water contribution to the Trinity and Edwards aquifers, south-central Medina flow path, and (A) calcite saturation index and (B) dolomite saturation index, northern Medina County, Texas, 1995–2003 | |
| 1. | Ground-water sites (wells) and surface-water sites, northern Medina and northeastern Uvalde Counties, Texas, from which data were obtained for this report |
| 2. | Selected chemical and isotope data for ground water from wells (by flow path) and surface water (by site), northern Medina and northeastern Uvalde Counties, Texas, 1992–2003 |
| 3. | Percentage contribution of surface water to ground water at selected wells in the Trinity and Edwards aquifers, by flow path, northern Medina and northeastern Uvalde Counties, Texas, 1995–2003, based on natural isotopic compositions of oxygen and hydrogen |
| 4. | Results of PHREEQC simulations of ground-water mixing based on initial and final water chemistry at wells, by flow path, northern Medina County, Texas |
| 5. | Saturation indexes computed with PHREEQC for ground water from selected wells, by flow path, and Medina Lake, northern Medina and northeastern Uvalde Counties, Texas, 1995–2003 |
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