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Flow Paths in the Edwards Aquifer, 

Northern Medina and Northeastern 

Uvalde Counties, Texas, Based on 

Hydrologic Identification and 

Geochemical Characterization 

and Simulation 

By Allan K. Clark and Celeste A. Journey 

In cooperation with the San Antonio Water System 

Scientific Investigations Report 2006–5200 

U.S. Department of the Interior 

U.S. Geological Survey 
 

U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2006 

For sale by U.S. Geological Survey, Information Services
 

Box 25286, Denver Federal Center
 

Denver, CO 80225
 

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply
 

endorsement by the U.S. Government.
 

Although this report is in the public domain, permission must be secured from the individual copyright owners to
 

reproduce any copyrighted materials contained within this report.
 

Suggested citation: 

Clark, A.K., and Journey, C.A., 2006, Flow paths in the Edwards aquifer, northern Medina and northeastern Uvalde 

Counties, Texas, based on hydrologic identification and geochemical characterization and simulation: U.S. 

Geological Survey Scientific Investigations Report 2006–5200, 48 p. 
iii 

Contents 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 

Structural Controls on Ground-Water Flow Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 

Approach and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 

Data Collection and SampleAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
 

Quality Assurance and Quality Control of Chemical Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 

Methods of Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 

Hydrologic Indicatorsof Ground-Water Flow Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 

Geochemical Methods to Characterize Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 

Geochemical Simulation to Identify FlowPaths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
 

Hydrologic Identification of Ground-Water Flow Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

NorthwesternMedina Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

North-Central Medina FlowPath (Northern and Southern Parts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
 

South-Central Medina Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
 

Correlation Between Water Levels in Wells With Continuous Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
 

Geochemical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
 

Age of Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
 

Ground-Water/Surface-Water Mixing Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
 

Major-Ion Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
 

Geochemical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 

NorthwesternMedina Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 

North-Central Medina FlowPath (Northern and Southern Parts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 

South-Central Medina Flow Path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
 

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
 

Figures 

1–2. Maps showing: 

1.	 Location of study area and relation between Edwards and Trinity aquifers, 


northernMedina and northeasternUvalde Counties, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
 

2.	 Surface geology and major faults in the study area, northern Medina and
 

northeastern Uvalde Counties, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 

3–4. Diagrams showing:
 

3. 	 Relay-ramp development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
 

4.	 Idealized view of relay ramps, fractures, faults, and flow paths in the Edwards
 

aquifer, northern Medina County, Texas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 
iv 

5–6. Maps showing: 

5.	 Locations of continuous water-level monitoring wells, surface-water datacollection sites, and hypothesized ground-water flow paths, northern Medina 

and northeastern Uvalde Counties, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

6.	 Locations of ground-water wells from which samples were obtained and 

hypothesized ground-water flow paths, northern Medina and northeastern 

Uvalde Counties, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

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 . . . . . . . . . . . . . . . . . . . 15 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

11. Graph showing correlation between water-level departures for 11 monitoring wells, 

northernMedina County, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

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 . . . . . . . . . . . . 19 

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), 

northernMedina County, Texas, 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

14.	 Apparent ages of ground water in selected wells in the Trinity and Edwards 

aquifers, northern Medina and northeastern Uvalde Counties, Texas, based on 

sulfurhexafluoride concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

15.	 Relation between delta oxygen-18 and delta deuterium ratios for selected groundand surface-water sites, northern Medina County, Texas, 1995–2003 . . . . . . . . . . . . . . . . . . . . . . . 22 

16–17. Stiff diagrams showing the variability in composition of: 

16.	 Ground water and surface water, northernMedina County, Texas . . . . . . . . . . . . . . . . . . . . . . . . . 22 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

v 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 29 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

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 . . . . . . . . . . . . . . . . . . 31 

Tables 

1.	 Ground-water sites (wells) and surface-water sites, northern Medina and northeastern 

Uvalde Counties, Texas, from which data were obtained for this report . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

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 . . . . . . . . . . . . . . 43 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

Vertical Datum and Isotope Unit Explanations 

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 


(NGVD 29). 


Per mil: A unit expressing the ratio of stable-isotope abundances of an element in a sample to 


those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotope 


ratios are computed as follows (Kendall and McDonnell, 1998):
 

δX = {(Rsample - Rstandard)/ Rstandard} x 1,000 

where 

δ is the "del" notation, 

X is the heavier stable isotope, and 

R is the ratio of the heavier, less abundant isotope to the lighter stable isotope in a sample or 

standard. 

The δ values for stable-isotope ratios discussed in this report are referenced to the following 

standard materials: 

Element R Standard identity and reference 

hydrogen hydrogen-2/hydrogen-1 Vienna-Standard Mean Ocean Water (Fritz and Fontes, 1980) 

oxygen oxygen-18/oxygen-16 Vienna-Standard Mean Ocean Water (Fritz and Fontes, 1980) 
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Flow Paths in the Edwards Aquifer, Northern Medina 

and Northeastern Uvalde Counties, Texas, Based on 

Hydrologic Identification and Geochemical 

Characterization and Simulation 

By Allan K. Clark and Celeste A. Journey 

Abstract 

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, surfacewater 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 northcentral 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 departuredataset 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 
2 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

Flow Paths in the 

Edwards Aquifer, 

Northern Medina and 

Northeastern Uvalde 

Counties, Texas 

 

 

 

 

 

 

 

 

 

 

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. 

Introduction 

The karstic Edwards aquifer of south-central Texas serves 

as the primary source of water for the city of San Antonio and 

adjacent areas (fig. 1). The Edwards aquifer is composed of several geologic formations (fig. 2) that consist primarily of limestone and dolostone, depending on the location and depositional 

province (Maclay and Small, 1984). In the recharge zone 

(fig. 1), rocks forming the Edwards aquifer are exposed at the 

land surface and receive recharge from direct infiltration of 

rainfall and streamflow losses. The Edwards aquifer also 

receives recharge from the laterally adjacent and underlying 

Trinity aquifer, the amount of which is small relative to the 

amount from other sources. Ground water moves down the formational dip from the recharge zone into the confined zone of 

the aquifer where it follows regional flow paths that convey 

water toward major natural springs (discharge points) in Comal 

and Hays Counties. 

The study area encompasses a complex system of flow 

paths that developed as a result of extensional faulting. The Balcones fault zone (fig. 1) is characterized by an en-echelon network of mostly down-to-the-coast normal faults where Cretaceous strata were displaced vertically, fractured intensively, and 

rotated differentially within a series of southwest-to-northeast 

trending fault blocks (Barker and Ardis, 1996). These 

faults—and the secondary effects of the tectonic forces that 

caused them—contribute to the complexity of the diffuse and 

conduit ground-water flow that occurs. The Balcones faulting 

set the foundation for initial ground- and surface-water flow 

paths that since have been modified by dissolution and erosion. 

Maclay and Land (1988, fig. 22, table 4) delineated and 

described four major flow paths (their term is flow units) in the 

Edwards aquifer in the San Antonio region. The flow paths 

originate in the recharge zone and generally convey water first 

to the southwest and then to the east and northeast toward major 

springs. Two of the four flow paths of Maclay and Land (1988) 

originate in the study area. 

To better understand and refine ground-water flow paths in 

the Edwards aquifer in northern Medina and northeastern 

Uvalde Counties, the U.S. Geological Survey (USGS), in cooperation with the San Antonio Water System, conducted a 4-year 

study during 2001–04. The study involved use of geologic 

structure, surface-water and ground-water data, and geochemistry to indicate ground-water flow paths. For example, groundwater levels in wells that fluctuate similarly in areas bounded by 

faults might indicate flow paths. Geochemical characterization 

of ground water is important in understanding its chemical evolution along hypothesized flow paths. Simulations based on 

chemical and isotope data along hypothesized flow paths might 

be of importance in understanding areas of recharge and the 

degree of communication between surface-water and groundwater systems. The results of the study could be useful to water 

managers with responsibility for protecting sources of recharge, 

identifying areas with potential for recharge enhancement, and 

identifying areas of ground-water discharge. 

Purpose and Scope 

The purpose of this report is to identify major groundwater flow paths in the Edwards aquifer in northern Medina 

and northeastern Uvalde Counties on the basis of geologic 

structure and hydrologic and geochemical indicators. Knowledge of geologic structure and the work of Maclay and Land 

(1988) provided a beginning framework, a basis for hypothesizing flow paths. Hydrologic data were analyzed and flow 

paths were identified using water-level departure (from periodof-record average daily mean) hydrographs and statistical correlation of water-level departure for 11 monitoring wells in the 

study area. Continuous water-level data were recorded at the 

11 monitoring wells during mid-March 2002–May 2004. 

Surface-water discharge data were obtained for three streams 

for the same period. Ground-water-chemistry and isotope data 

were obtained from samples collected at 41 wells during March 

1992–August 2003. Surface-water-chemistry and isotope data 

were obtained from samples collected at two sites in Medina 

Lake during 1995–96; isotope data also were obtained for the 

Medina River below Medina Lake from samples collected during 1995–96. One-dimensional geochemical simulations oriented along hypothesized major flow paths provided further 

evidence to support the existence of the flow paths. These 

geochemical simulations represent the chemical evolution of 

ground water along the flow paths and the degree of influence 

of surface water on the ground-water flow system. 

Structural Controls on Ground-Water Flow Paths 

The Balcones fault zone is an extensional fault system that 

crosses Medina and Uvalde Counties from the southwest to the 

northeast (fig. 1). The faults are en echelon and generally 

downthrown to the coast with primarily normal displacement. 

Cross faults trend southeast to northwest. Many of the faults are 

not single, sharp breaks but rather shatter zones where faulting 

occurs over tens of feet. Examples of normal faults in the study 

area are the Haby Crossing and Parkers Creek faults (fig. 2). 

Faulting has resulted in juxtaposition of stratigraphically older 

rocks against younger rocks of varying lithologies. The amount 

of displacement along a particular fault tends to vary, and thus 

the effectiveness of a fault as a barrier to flow probably changes 

along the fault plane. Near a fault tip no barrier to flow exists; 

as displacement down the fault plane increases, the effectiveness of the fault as a barrier to flow increases. In the study 

area, rocks become progressively younger from northwest to 

southeast and from northeast to southwest because of faulting 

and extension perpendicular to faulting. Extension strongly 
0 2.5 5 7.5 10 MILES

EXPLANATION 

Trinity aquifer Edwards aquifer recharge zone Edwards aquifer confined zone Study area boundary 

0 15 30 45 60 MILES

BEXAR 

ATASCOSA

COUNTY 

WILSON

COUNTY MEDINA COUNTY ZAVALA COUNTY 

UVALDE COUNTY 

REAL 

COUNTY 

BANDERA 

KERR COUNTY 

COMAL

COUNTY

COUNTY 

KENDALL 

COUNTY

 COUNTY 

HAYS 

COUNTY 

TRAVIS 

COUNTY 

FRIO COUNTY 

GILLESPIE COUNTY 

BLANCO

COUNTY 

KINNEY COUNTY 

GUADALUPE

COUNTY 

CALDWELL 

COUNTY 

UVALDE COUNTY MEDINA COUNTY 

BANDERA COUNTY 

99 ̊30' 99 ̊19'30" 99 ̊09' 98 ̊58'30" 

29 ̊21' 

29 ̊31'30" 

29 ̊42' 

Hondo 

Sabinal 

Castroville 

Study area 

0 100 200 MILES 

TEXAS 

LOCATION MAP 

Study area 

Uvalde 

salient 

Balcones

fault

zone 

Area enlarged 

Knippa

gap 

Ranch Rd 1796 

Ranch Rd 187 

State Hwy 127

FM 1796 

FM 2200

Ranch 

State Hwy 173 

Ranch Rd 462

FM 2676 

US Hwy 90 

Ranch Rd 1343

FM 471 

State Hwy 16 

Ranch Rd 470 

Ranch Rd1283 

Rd 1023 

Seco Cr eek 

Sabinal River 

Hondo Creek 

Quihi Creek 

Verde Creek

Medina River

San Geronimo 

Creek 

Medina Lake 

Diversion Lake 

EXPLANATION 

Ground-water source Trinity aquifer Edwards aquifer outcr(recharge zone) Edwards aquifer subcr(confined zone)—Dowboundary from SchultzUnconfined zone (recfrom Ashworth and HBalcones fault zone Fault 

Trinity 

aquifer 

Edwa

(rech

Edwards aquifer (confined zone) 

Generalized relation Trinity aquifers, San 

NORTHWEST 

San Antonio 

Recharge zone from Ashworth and Hopkins, 1995 

Figure 1. Location of study area and relation between Edwards and Trinity aquifers, northern Medina and northeastern Uvalde Counties, Texas. 
Seco Creek 

MEXICO STATES UNITED RIVER 

FORMATION 

DEVILS MAVERICK B

Div

Seco Creek Road fault 

Woodard Cave fault 

Ve r de Creek fault 

Haby Crossing fault 

Parkers Creek fault Bowman fault 

Medina Lake fault 

Lincoln fault Seco Creek fault Vandenburg School fault 

AREA SHOWN IN FIGURE 4 

102O 101O 100O 

31O TEXAS Area enlarged 

EXPLANATION 

HYDROGEOLOGIC AND GEOLOGIC UNITS 

LOWER CRETACEOUS UPPER CRETACEOUS QUATERNARY

TRINITY

AQUIFER EDWARDS AQUIFER UPPER CONFINING UNIT (LOWER

CONFINING DEVILS RIVER FORMATION


UNIT OF
 UNDIVIDED EDWARDS

AQUIFER) 

EDWARDS GROUP


MCKNIGHT AND WEST SALMON PEAK


NUECES FORMATIONS FORMATION
 

KAINER PERSON


FORMATION FORMATION
 

ALLUVIUM 

30O 0 100 200 MILES 

UVALDE GRAVEL REAL

LOCATION MAP 

LEONA FORMATION 

BANDE

MEDIN

ASINUVALDE ESCONDIDO FORMATION 

29O 

ANACACHO LIMESTONE EXPLANATION 

Boundary of Edwards-Trinity aquifer system AUSTIN GROUP 

Approximate stratigraphic boundary (modified from Barker and Ardis, 1996, fig. 11) EAGLE FORD GROUP 

BUDA LIMESTONE DEL RIO CLAY BANDERA COUNTY 

UVALDE MEDINA COUNTY 

GEORGETOWN FORMATION COUNTY CYCLIC AND MARINE MEMBERS, UNDIVIDED (NOT EXPOSED IN STUDY AREA) 99 ̊15' 

99 ̊7'30" 

LEACHED AND COLLAPSED MEMBERS, UNDIVIDED 

99 ̊22'30" 

REGIONAL DENSE MEMBER GRAINSTONE MEMBER KIRSCHBERG EVAPORITE MEMBER DOLOMITIC MEMBER 

99 ̊30' 

29 ̊30'

BASAL NODULAR MEMBER UPPER MEMBER OF THE GLEN ROSE LIMESTONE 

Modified from Small and Clark (2000) and Claon the basis of additional field work. FACIES CHANGE 

FAULT—Dashed where inferred 0 2 4 6 8 MILES 

Figure 2. Surface geology and major faults in the study area, northern Medina and northeastern Uvalde Counties, Texas. 
 

 

 

 

 

 

 

 

 

 

 

Approach and Methods 5 

influenced the orientation of fractures, location of streams, 

areas of recharge, and ground-water flow paths. Fractures generally are parallel to or perpendicular to the main fault trend of 

the Balcones fault zone. 

In this structurally complex area, the Balcones fault zone 

contains relay ramps that form in extensional environments to 

accommodate an increase in deformation. Ramp structures link 

the footwall of a fault segment with the hanging wall of 

an overlapping fault segment. The controls on surface-water 

drainage patterns by relay-ramp formation have been documented by Trudgill (2002) in the Canyonlands of southeastern 

Utah. Figure 3 shows an idealized diagram of relay-ramp 

development and relay-ramp structure in relation to generalized 

ground- and surface-water flow directions, and figure 4 shows 

a block model of faulting. As extension occurs and strain 

increases, rotation and internal fracturing occur along the relay 

ramp (Trudgill, 2002). Continued extension results in the formation of cross faults within the relay ramp structure. The block 

model in figure 4 is an example of a relay ramp breached by a 

cross fault along Seco Creek that downdropped confining units 

against the Edwards aquifer. 

An example of similar controls on surface-water drainage 

patterns might have occurred in the evolution of the Medina 

River. Doyle (2003) reported that the ancestral Medina River 

flowed to the southwest toward the present-day city of Hondo 

and discharged into the ancestral Hondo Creek. Solutionally 

enlarged fractures and conduits might have formed parallel to 

the dip of the relay ramp. These same solutionally enlarged 

fractures and conduits might continue to move ground water to 

the southwest perpendicular to the present-day surface-water 

drainage. 

Streams in the geologic past might have flowed downdip/down structure, following the dip of relay ramps. Over 

geologic time, streams in the study area were intercepted by 

headward erosion (pirated) from the south, which caused 

them to change course to a more southerly direction. Presentday streams that cross the Edwards aquifer recharge zone 

are channelized along fractures (or faults) that are roughly 

perpendicular to the main fault trend. These fractures allow 

streams to lose much if not all of their base flow in the Edwards 

aquifer recharge zone. 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. 

Acknowledgments 

The authors thank personnel of the San Antonio Water 

System, especially Mr. John Waugh and Mr. Kirk Nixon, for 

obtaining easement rights for monitoring wells installed for this 

study and running geophysical logs of the monitoring wells. 

Special thanks are extended to the personnel of the Edwards 

Aquifer Authority (EAA) and the following property owners 

for access to their properties: Mr. Richard Saathoff, Mr. and 

Mrs. Peter Bowman, Mr. Doug Peters, Mr. and Mrs. Steven 

Devlin, Mr. Joe W. Rothe, Mr. Virgil Boles, and Mr. Scott 

Pearson. 

Approach and Methods 

Data Collection and Sample Analysis 

Initially, pertinent historical data were compiled that 

included previously published reports, well locations, historical water levels, geology, hydrogeologic maps, and waterchemistry and isotope data for both ground and surface water. 

Data sources were the USGS, Texas Water Development 

Board, and EAA. 

Eight Edwards aquifer monitoring wells were drilled by 

the USGS in northern Medina County for the study (one of 

which was a dry hole and thus not included in this report) 

(table 1). Each well was drilled to the base of the Edwards aquifer (top of the Glen Rose Limestone [fig. 2]) using primarily air 

rotary methods. Slotted PVC liners were placed in the holes to 

prevent borehole collapse and to facilitate water sampling. 

Water-level recorders were installed on each of the seven new 

wells (Medina1–Medina8 [excluding Medina6]) plus three 

existing wells (Hondo, Castroville, Seco); one additional well 

(Quihi) had a water-level recorder maintained by EAA. Continuous water-level data thus were collected from 11 wells (fig. 5; 

table 1) during March 14, 2002–May 31, 2004. 

The USGS has a long-term stage-monitoring site at 

Medina Lake (Medina Lake near San Antonio) (fig. 5; table 1). 

Lake-stage data were compiled for that site for the same period 

as the continuous ground-water-level data, March 14, 2002– 

May 31, 2004. Surface-water discharge data were compiled for 

three existing USGS streamflow-gaging stations—Medina 

River at Bandera, Hondo Creek near Tarpley, and Seco Creek 

at Miller Ranch—also for the period concurrent with the continuous ground-water-level data. 

Ground-water-chemistry and isotope data were obtained 

from 41 wells (fig. 6; table 1) during March 1992–August 2003. 

Chemistry and isotope samples were obtained from 35 wells, 

chemistry-only samples from two wells, and isotope-only 

samples from four wells) (table 1). Twenty-three wells provided 

historical data (1992–2001 samples from the USGS waterquality database [http://nwis.waterdata.usgs.gov/nwis/ 

qwdata]), 11 wells provided new data (2002–03 samples collected for this study), and seven wells provided both historical 

and new data. Some of the historical data were collected during 

1995 and 1996 and published as part of a Medina Lake area 

study by Lambert and others (2000). 

Ground-water samples were collected from 10 wells during July 2002–February 2003 for analysis of dissolved gases 

(carbon dioxide, oxygen, and argon; not all detected in each 

sample) and sulfur hexafluoride (SF6) concentrations. Dissolved gases and SF6 samples were analyzed by the USGS 

Chlorofluorocarbon Laboratory in Reston, Va., using approved 

methods (U.S. Geological Survey, 2005b). 
A. Stream drainage moves down the relay ramp/downgradient. B. Relay ramp is breached by a cross fault. Subsequently any conduit development would occur parallel to the structure. Down-cutting by stream and small drainages that intersect the footwall. Erosion in footwall is by solution enhancement of fractures. Main stream is intercepted by headward erosio

stream, which pirates the flow into a new primasystem. Main conduits still moving water down parallel to faulting and perpendicular to stream

C. D. Footwall Hanging wall Footwall Hanging wall 

Figure 3. Relay-ramp development (modified from Trudgill, 2002). 
UP 

(a) 

C'

increasing vertical displacement

UP

DOWN 

A'

B'

increasing vertical displacement 

UP

DOWN 

D'

EXPLANATION 

DOWN Fault 

Fracture Generalized direction of ground-water flow 

(b) C FW, footwall; HW, hanging wall 

BEFORE DEFORMATION 

TORSION

TENSION

A

B

D

A'

B'

C'

D'

FW 

HW 

FW 

HW 

Extension 

Extension 

N Verde Creek fault 

Seco Creek Road fa

Area constraine

because of fault

and structure

Recharge concentrated along fractures perpendicular to main faults 

Seco Creek 

LOCATION SHOWN IN FIGURE 2 

EXPLANATION 

Fault Hydrogeologic and geologic units displacement Fa

LOWER EDWARDS CONFINING UPPER CONFINING UNIT AQUIFER UNIT AUSTIN GROUP EAGLE FORD GROUP BUDA LIMESTONE DEL RIO CLAY GEORGETOWN FORMATION (NOT EXPOSED IN STUDY AREA) DEVILS RIVER FORMATION, UNDIVIDED BASAL NODULAR MEMBER OF DEVILS RIVER FORMATION (NOT SHOWN IN STUDY AREA) UPPER MEMBER OF GLEN ROSE LIMESTONE 

Increasing barrier to
 flow as displacement
 increases
 

Fra

GegroSedir

NOTE: Surface geolog

AFTER DEFORMATION 

Figure 4. Idealized view of relay ramps, fractures, faults, and flow paths in the Edwards aquifer, northern Medina County, Texas. 
REAL BANDERA

Frio Rive r

W est SabinalRiver

Cherry Creek

Little DryFrio Creek

Area of idealized view inset 

UVALDE COUNTY MEDINA COUNTY

Uvalde 

Sabinal 

Hondo 

Idealized view of relay ramps dipping to the northeast in Uvalde County and an opposing ramp dipping to the southwest in Medina County and flow paths moving down ramp toward a structural trough near the Medina/Uvalde County line. (FW, footwall; HW, hanging wall) 

FW 

FW 

HW 

HW 

FW 

HW 

Seco 

Sabinal River 

Leona 

Nolton

Verde 

Dry 

River 

Quihi 

Little 

Dinner 

East Elm Creek 

B a l c k C r e e k 

Rancheros 

Onion 

Brie r Branc h 

Creek 

Fush

Live Oak Creek 

Little Seco

Elm Creek 

Parkers Creek 

Elm Slough

Little Live Oak Creek 

East Tehuacana Creek 

Middle Verde Creek 

East Branch Live Oak Creek 

Medina River 

M 

Blanco Creek

Frio River

Little Blanco Creek

Frio 

Creek 

Creek 

Creek 

Creek

Creek

 River 

Creek 

Creek 

Creek 

Creek Hondo 

Bruins Creek 

Creek 

Creek 

Bear 

Ground-water flow path Northwestern Medina North-central Medina (northern part) North-central Medina (southern part) South-central Medina Boundary of aquifer (Edwards aquifer recharge zone from Ashworth and Hopkins, 1995) Inferred flow directions Fault Inferred fault Study boundary Site, by type, and identifier Continuous water-level monitoring well Streamflow-gaging station (and Medina Lake stage) Surface-water sampling site 

EXPLANATION 

Medina8 

TD–69–31–708 

08201500 

!

!

#

0 3 6 

Woodard Cave fault 

Verde Creek fault 

Parkers Creek fault 

Medina Lake fault 

Seco Creek fault 

Haby Crossing fault 

Diversion Lake fault 

TRINITY AQUIFER 

EDWARDS AQUIFER RECHARGE ZONE 

EDWARDS AQUIFER CONFINED ZONE 

!

!

!

!

!

!

!

!

!

#

!

#

#

Seco 

TD–69–38–601 

Hondo TD–69–47–306 

Medina1 TD–69–32–703 

Medina4 

TD–69–40–510 

Quihi 

TD–69–40–102 

MeTD

Medina8 TD–69–31–708 

Medina2 TD–69–31–406 

Medina3 

TD–69–40–209 

08201500 08200000 

08178880 

0817

0817

08179500 (AL) 

29o31'30" 

29o21' 

29o10'30" 

99o30' 99o19'30" 99o09' 98o58'3

Figure 5. Locations of continuous water-level monitoring wells, surface-water data-collection sites, and hypothesized ground-water flow paths, nornortheastern Uvalde Counties, Texas. 
REAL BANDERA

Frio Rive r

W est SabinalRiver

Cherry Creek

Little DryFrio Creek

Area of idealized view inset 

UVALDE COUNTY MEDINA COUNTY

Uvalde 

Sabinal 

Hondo 

Seco 

Sabinal River 

Leona 

Nolton

Verde 

Dry 

River 

Quihi 

Little 

Dinner 

East Elm Creek 

B a l c k C r e e k 

Rancheros 

Onion 

Brie r Branc h 

Creek 

Fush

Live Oak Creek 

Little Seco

Elm Creek 

Parkers Creek 

Elm Slough

Little Live Oak Creek 

East Tehuacana Creek 

Middle Verde Creek 

East Branch Live Oak Creek 

Medina River 

M 

Blanco Creek

Frio River

Little Blanco Creek

Frio 

Creek 

Creek 

Creek 

Creek

Creek

 River 

Creek 

Creek 

Creek 

Creek Hondo 

Bruins Creek 

Creek 

Creek 

Bear 

Ground-water flow path Northwestern Medina North-central Medina (northern part) North-central Medina (southern part) South-central Medina Boundary of aquifer (Edwards aquifer recharge zone from Ashworth and Hopkins, 1995) Study boundary Well and identifier 

EXPLANATION 

Medina8 

!

0 3 6 

TRINITY AQUIFER 

EDWARDS AQUIFER RECHARGE ZONE 

EDWARDS AQUIFER CONFINED ZONE 

99o30' 99o19'30" 99o09' 98o58'

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

TD-69-31-605 

TD-69-30-903 

YP-69-37-101 

TD-69-39-504 TD-69-38-509 

TD-69-39-520 

TD-69-38-709 

TD-69-31-603 

TD-69-37-302 

TD-69-38-106 

TD-69-32-501 

TD-68-

TD-69-32-201 

TD-69-32-303 

TD-68-25-402 

TD-68-2

TD-68-25-70

TD

A

TD-69-40-604 

TD-69-32-905 

TD-68-33-

TD-69-40-202 TD-6

TD-69-40-301 

!

!

!

!

!

!

!

!

Seco 

Medina1 

Medina4 

Quihi 

Medina5 

Medina8 

Medina2 

Medina3 

29o31'30" 

29o21' 

29o10'30" 

Figure 6. Locations of ground-water wells from which samples were obtained and hypothesized ground-water flow paths, northern Medina and nortTexas. 
10 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 1. Ground-water sites (wells) and surface-water sites, northern Medina and northeastern Uvalde Counties, Texas, from which 

data were obtained for this report. 

[Datum for open interval is land surface; USGS, U.S. Geological Survey; NA, not applicable; (s), screened with polyvinyl chloride liner; I, isotope; --, unknown; 

QW, water chemistry; WLC, continuous water level; XRF, X-ray fluorescence; (o), open hole; >, greater than; DIS, discharge; STAGE, water level] 

USGS site 

number 

State well 

number 

USGS 

name Aquifer 

Total 

depth 

(feet) 

Open interval 

(feet) 

(completion type) 

Data type 

Ground-water sites (wells) 

293420098552601 AS–68–25–505 NA Trinity 334 224–334 (s) I 

293249098575101 TD–68–25–402 NA Trinity 486 -­ I, QW 

293255098560401 TD–68–25–510 NA Trinity 200 -­ I, QW 

293309098541901 TD–68–25–602 NA Trinity -­ -­ I, QW 

293334098535401 TD–68–25–603 NA Trinity 535 -­ I 

293127098573701 TD–68–25–701 NA Edwards 615 320–400, 580–614 (s) I, QW 

1

293026099585401 TD–68–25–711 Medina5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 380 35–370 (s) I, QW, WLC, XRF 

293033098571601 TD–68–25–806 NA Edwards 360 200–360 (o) I, QW 

293158098560401 TD–68–25–807 NA Trinity 252 -­ I, QW 

293229098553402 TD–68–25–809 NA Trinity >450 >450 (o) I, QW 

293119098561501 TD–68–25–810 NA Trinity >350 >350 (o) I, QW 

293040098572901 TD–68–25–822 NA Edwards 380 336–380 (o) I, QW 

293034098540901 TD–68–25–904 NA Trinity -­ -­ I, QW 

292820098574801 TD–68–33–102 NA Edwards 625 10–625 (o) I, QW 

292825098574501 TD–68–33–103 NA Edwards 350 -­ I, QW 

292843098564601 TD–68–33–204 NA Edwards 360 -­ I, QW 

1292929098561801 TD–68–33–213 Medina7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 300 80–290 (s) I, QW, WLC 

292117098524701 TD–68–41–301 Castroville . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 710 -­ WLC 

293101099171601 TD–69–30–903 NA Edwards 440 260–440 (o) I, QW 

1293318099140501 TD–69–31–406 Medina2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 210 30–200 (s) I, QW, WLC 

293241099092901 TD–69–31–603 NA Edwards 300 -­ I, QW 

293324099074601 TD–69–31–605 NA Trinity -­ -­ I, QW 

1293125099135701 TD–69–31–708 Medina8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 300 80–290 (s) I, QW, WLC 

293710099032201 TD–69–32–201 NA Trinity 180 -­ I, QW 

293640099020001 TD–69–32–303 NA Trinity 660 -­ I, QW 

293438099041201 TD–69–32–501 NA Trinity -­ -­ I, QW 

1293202099063501 TD–69–32–703 Medina1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 290 50–280 (s) I, QW, WLC 

293044099021101 TD–69–32–905 NA Edwards 358 180–358 (o) QW 

292927099231101 TD–69–37–302 NA Edwards 410 -­ I, QW 

292820099212801 TD–69–38–106 NA Edwards 478 160–478 (o) I, QW 

292638099185501 TD–69–38–509 NA Edwards 380 30–380 (s) I, QW 

292618099165901 TD–69–38–601 Seco . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 538 74–538 (o) I, QW, WLC 

292423099212901 TD–69–38–709 NA Edwards 420 -­ I, QW 

292704099114201 TD–69–39–504 NA Edwards 653 84–653 (o) I 

292600099111201 TD–69–39–520 NA Edwards 290 -­ QW 

292740099070201 TD–69–40–102 Quihi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 559 79–559 (o) I, WLC2 

292803099042601 TD–69–40–202 NA Edwards 400 -­ I, QW 

1292843099040201 TD–69–40–209 Medina3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 550 40–520 (s) I, QW, WLC 

292841099012001 TD–69–40–301 NA Edwards 328 -­ I, QW 

1292536099041001 TD–69–40–510 Medina4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 960 420–950 (s) I, QW, WLC 

292727099003201 TD–69–40–604 NA Edwards 505 10–505 (o) I, QW 

293438099041201 TD–69–47–306 Hondo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Edwards 1,500 -­ WLC 

292821099283201 YP–69–37–101 NA Edwards 500 -­ I, QW 

Surface-water sites 

08178880 NA Medina River at Bandera . . . . . . . . . . . . . . . NA NA NA DIS 

08179500 NA Medina Lake near San Antonio (sites AL NA NA NA STAGE, I, QW 

and DC) . . . . . . . . . . . . . . . . . . . . . . . . . . . (AL and DC) 

08179520 NA Medina River below Medina Lake . . . . . . . NA NA NA I 

08200000 NA Hondo Creek near Tarpley . . . . . . . . . . . . . NA NA NA DIS 

08201500 NA Seco Creek at Miller Ranch near Utopia . . NA NA NA DIS 

1 Well drilled for this study.
 

2 Data from Edwards Aquifer Authority.
 
 

 

 

 

 

 

 

 

Approach and Methods 11 

Historical (1995–96) surface-water-chemistry and isotope 

data were obtained for two sites at Medina Lake (AL and DC) 

(fig. 5; table 1). Historical (1995–96) isotope data also were 

obtained for one other surface-water site, Medina River below 

Medina Lake (in Diversion Lake). 

Water samples were collected, processed, and preserved 

according to analyte-specific requirements and standard USGS 

protocols described in Wilde and others (1999, 2003, 2004). 

Water-chemistry data comprised field properties, major ions, 

nutrients, and trace elements. Water-chemistry samples were 

analyzed for major ions, nutrients, and trace elements by the 

USGS National Water Quality Laboratory in Denver, Colo., 

using approved methods (U.S. Geological Survey, 2005a). Isotope data comprised naturally occurring stable isotopes of 

hydrogen and oxygen. The ratios of naturally occurring stable 

isotopes of hydrogen (2

H/1H) and oxygen (18O/16O) were analyzed by the USGS Stable Isotope Laboratory in Reston, Va., 

using approved methods (U.S. Geological Survey, 2006). Isotopic results are reported as delta deuterium (δD) and delta 18­

oxygen (δ18O), which represents the relative difference in parts 

per thousand (called per mil) between the sample isotope ratio 

and a known standard isotope ratio (Kendall and McDonnell, 

1998). 

During drilling of well Medina5, rock samples from the 

Edwards aquifer were collected at 10-foot depth intervals and 

sent for X-ray fluorescence (XRF) analysis to SGS Canada, 

Inc., Mineral Services. Samples from well Medina5 were 

selected for analysis because that well provided the most 

complete set of cuttings recovered during drilling. 

Quality Assurance and Quality Control of Chemical 

Samples 

Before water sampling, equipment blanks were collected 

to evaluate the equipment and to ensure that the procedures for 

cleaning the equipment did not contaminate or otherwise affect 

the environmental samples. Results of the equipment-blank 

sampling indicated that the equipment and cleaning procedures 

did not measurably affect the major-ion concentrations in 

the environmental samples that were used in the geochemical 

modeling. 

Duplicate samples were collected from two of the 11 wells 

sampled for major ions and trace elements for this report and 

were available from the water-quality database for two other 

wells (table 2, at end of report). Duplicate samples for SF6 were 

collected from all 10 of the wells sampled for SF6 for this 

report. Duplicate samples also were available from the waterquality database from one well for one concentration pair each 

of δD and δ18O. Duplicates were used to evaluate the sampling 

and laboratory procedures. From the four duplicate samples for 

major ions, relative percent differences (RPDs) in calcium and 

chloride concentration pairs (constituents of interest for this 

report) ranged from 0 to 2.9 and 0 to 4.6, respectively. RPDs for 

magnesium and sodium concentration pairs were more variable 

and ranged from 0.11 to 7.6 and 0.52 to 21, respectively; the 

maximum RPDs for both constituents were from historical concentration pairs. RPDs for potassium and sulfate concentration 

pairs ranged from 0.99 to 4.2 and 0 to 1.2, respectively. For the 

10 SF6 concentration pairs, RPDs ranged from 0.66 to 30, with 

a median RPD of 7.5. For the single duplicate pair of δD concentrations, the RPD was 4.2; and for the single duplicate pair 

of δ18O concentrations, the RPD was 0.31. 

Methods of Data Analysis 

Hydrologic Indicators of Ground-Water Flow Paths 

Water levels relative to a land-surface or NGVD 29 datum 

vary over a wide range in the study area. To view water levels 

over time on a common scale, hydrographs of water-level 

departure were plotted. Water-level departure was computed by 

subtracting the period-of-record average daily mean from the 

daily mean. Plotting water-level departure allowed visual comparison between water levels at different wells, which aided in 

flow path identification. Although amplitude of the hydrographs varies, departure similarities in the hydrographs can 

indicate hydraulic connection between wells, areas of recharge, 

or both. 

Departure hydrographs were created from the discharge 

and stage data from the three streamflow-gaging stations and 

Medina Lake, respectively, which then were compared with 

selected water-level departure hydrographs from wells to identify possible sources of recharge to the ground-water system. 

For example, a rise in measured water level (positive increase 

in departure) in a monitor well in the Seco Creek Basin during 

a period when Medina Lake is filled to capacity and no appreciable rainfall occurs could indicate a hydraulic connection 

along a ground-water flow path from Medina Lake to the well. 

Statistical correlations between water-level departures for 

the 11 continuously monitored wells were done using Kendall’s 

tau (Helsel and Hirsch, 1995) in a statistical software package, 

which in this application measures the strength of the monotonic relation between two time series of water-level departures. 

This rank-based procedure was selected over the more commonly used Pearson’s r to measure correlation because graphs 

of the departure data generally show the data are not normally 

distributed; Pearson’s r, which measures linear correlation, 

requires the assumption of normally distributed data. Kendall’s 

tau for each of the 55 possible combinations of departuredataset pairs was computed to indicate strongly correlated 

water-level pairs and thus evidence for hydraulic connection 

(flow paths) between wells. 

Geochemical Methods to Characterize Water 

XRF analysis of rock samples from well Medina5 provided bulk-rock composition, in weight-percent oxides, of 

major and trace elements of the aquifer. X-ray diffraction 

(XRD) analysis of powder rock samples also was done on these 
12 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

 

 

 

 

 

 

 

 

rock samples to identify the dominant mineral species. These 

rock analyses were conducted to augment the geochemical simulation by verifying the dominant mineral species and major 

elements in the Edwards aquifer in the study area. 

Ages of ground water, ground-water/surface-water 

mixing relations, and major-ion chemistry were evaluated 

using geochemical data collected from selected wells and 

surface-water sites in the study area. Chemical and isotopic 

data from Medina Lake, the Medina River, and selected wells 

in the Edwards and Trinity aquifers were used to evaluate 

the geochemical and isotopic controls on ground-water 

composition and to assess mixing of the surface- and groundwater systems. 

Dissolved gases and SF6 were used to estimate the apparent age or year of recharge for the water in the aquifer. The technique involves comparing the SF6 concentration in the ground 

water to established annual atmospheric concentrations. SF6 

occurs as a gas in the atmosphere in trace concentrations, 

mainly of anthropogenic origin. But SF6 can occur naturally in 

some minerals, volcanic and igneous rocks, and fluids. However, caution is advised in using these apparent ages or years of 

recharge because the procedure does not account for mixing of 

older waters with younger waters. Excess air is introduced into 

ground water when air bubbles dissolve during a rapid rise of 

the water table. The addition of excess air to ground water 

increases the SF6 concentration of the ground water above the 

air-water equilibrium concentration. If the presence of excess 

air is not considered in the computation of an SF6 age, then the 

apparent age will be too young. 

The mixing relations between ground water and surface 

water were quantified using 18O/16O and 2

H/1H. Isotopic composition of end-member waters that represented the Edwards 

aquifer (negligible influence of surface water) and the surfacewater system (average of Medina Lake and Medina River isotope concentrations) were used in geochemical simulation to 

compute the percentage of surface water present in the water of 

a well (Hem, 1992; Gibson and others, 1993; Kendall and 

McDonnell, 1998; Lambert and others, 2000). A sensitivity 

analysis was applied to the mixing calculation method (Payne, 

1983; Gibson and others, 1993; Torres and others, 2001) for 

ground and surface waters. The sensitivity analysis is based on 

the variability of the individual end-member water chemistry 

and the difference in the isotope concentrations between the two 

end-member waters (surface and ground). 

Major-ion data were analyzed graphically and statistically 

to identify relations among constituents and differences among 

flow paths and ground-water/surface-water systems. Piper trilinear diagrams and Stiff diagrams were used to graphically 

classify the dominant water composition in the aquifers and 

flow paths (Piper, 1944; Stiff, 1951; Hem, 1992). Piper trilinear 

diagrams often are used to indicate whether particular water 

might be a simple mixture of other waters or whether it is 

affected by dissolution or precipitation of a single salt. The Stiff 

diagrams provide a relatively distinct shape to illustrate differences and similarities in water composition. The width of the 

pattern approximates the total ionic content. 

Geochemical Simulation to Identify Flow Paths 

Geochemical simulation is a two-phase process that first 

requires determination of saturation conditions in the ground 

water in each well to identify potential reactions that are 

geochemically valid (and thus plausible in the field). Second, 

an inverse simulation identifies changes in ground-water 

chemistry between two wells along a hypothesized flow path to 

account for the changes in aqueous ion concentrations (water 

composition) by moving moles of different ions from solid to 

liquid state (dissolve) or liquid to solid state (precipitate). The 

objective of inverse simulation is to find sets of minerals and 

gases that, when reacted in appropriate amounts, quantitatively 

account for the differences in composition between water samples from two wells along a flow path with the potential of surface-water interaction. 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 mole-balance, or inverse, model PHREEQC (Parkhurst and 

Appelo, 1999). 

The overlying assumption in this type of simulation is that 

the ground-water chemistry in the downgradient well evolved 

from the ground-water chemistry in the upgradient well by reaction with the proposed minerals and gases or by input from the 

surface-water system. If this assumption is shown by simulation 

results to be plausible, then the existence of the hypothesized 

flow path is supported. Additional assumptions are that chemical reactions are controlled by thermodynamic equilibrium, the 

chemical composition of the ground water is nearly at steady 

state, and the mineralogy of the aquifer is well defined. Also, an 

assumption in this application was that the ground-water 

chemistry remained relatively stable during the periods 

represented by the samples and that seasonal differences were 

negligible. However, a lack of temporal data prevented accurate 

quantification of seasonal differences in water chemistry, thus 

the validity of this assumption is unknown. Potential reactions 

for geochemical modeling were identified on the basis of XRF, 

XRD, and existing mineralogical data. Simulated reactions 

include precipitation (calcite, amorphous silica), dissolution 

(calcite, dolomite, gypsum, halite), and ion exchange (cation, 

carbon dioxide). All PHREEQC simulations were constrained 

by holding constant the masses of sulfur, calcium, sodium, magnesium, inorganic carbon, and chloride. 

The saturation index (SI) is a dimensionless measure of the 

departure from equilibrium of the water with respect to various 

mineral phases. An SI of zero indicates that the water is at equilibrium (saturated) with respect to the mineral phase. A negative 

SI indicates undersaturation (dissolution of the mineral is possible), and a positive SI indicates supersaturation (precipitation 

is possible). As the chemistry in the ground water changes from 

well to well, the SI changes. Model-calculated SIs were compared among various wells to show which geochemical reactions could or could not occur as water moves along a flow path 

between a pair of wells. For example, if a possible simulation 

indicates that calcite is dissolving at a particular well, but the 
Hydrologic Identification of Ground-Water Flow Paths 13 

 

 

 

 

 

 

 

 

 

 

 

SI calculated for water from that well shows calcite supersaturation, then the model must be rejected. 

The simulation results related the extent to which the input 

data (element concentrations) were adjusted by the model in a 

simulation. Unknowns for the simulation include the mixing 

fraction of each aqueous solution; the mole transfers of minerals 

and gases into or out of the aqueous solution; and a set of uncertainty terms that accounts for uncertainties in element concentrations as evidenced by charge (cation-anion) imbalances in 

the water analyses. The uncertainty terms represent uncertainties associated with analytical error and spatial or temporal variability in concentration of each element. The results included 

(1) the sum of residuals, which is the sum of the uncertainty of 

the unknowns weighted by the inverse of the uncertainty limit, 

which is a user-supplied estimate of uncertainty of each element 

that limits deviation from the chemical input data (for this application it was set to 0.10); (2) the sum of delta/uncertainty, which 

is the sum of the adjustments to each element concentration 

weighted by the inverse of the uncertainty limit; and (3) maximum fractional error in element concentration, which is the 

adjustment to any element or isotopic composition in any solution. If no adjustments were made, all three quantities would be 

zero. For a given mole-balance simulation, if no simpler inverse 

simulation is obtained with any proper subset of the solutions 

and phases of the simulation, the statement “Simulation contains minimum number of phases” is given for that simulation. 

Hydrologic Identification of Ground-Water 

Flow Paths 

Northwestern Medina Flow Path 

The northwestern Medina flow path (NWMFP) is bounded 

on the north by the Woodard Cave fault and on the south by 

the Parkers Creek fault (fig. 5). The NWMFP likely receives 

recharge from Seco, Hondo, and Verde Creeks and from the 

adjacent Trinity aquifer on the basis of its proximity to the 

Edwards aquifer. Water moves downdip toward the southwest—primarily because of structural controls associated with 

a relay ramp—until the flow encounters a cross fault along Seco 

Creek (figs. 2, 5). The cross fault along Seco Creek probably 

forms a barrier or a partial barrier to flow because the fault 

might have offset several hundred feet of the Edwards aquifer 

and juxtaposed confining units against it. This barrier to flow 

might force part or most of the flow to the south. During times 

of relatively large recharge, subsurface “back-flooding” might 

occur, up ramp to the northeast toward Hondo Creek. Southwestern flow in the NWMFP not turned south by cross faulting 

near Seco Creek ultimately is turned south near the Uvalde/ 

Medina County line by geologic structure. At the apex or culminating point of relay-ramp structures, the southwesterly subsurface flow from northwestern Medina County likely joins 

some southeasterly subsurface flow from northeastern Uvalde 

County (inset, fig. 5). Also influencing subsurface flow is a 

structural high in central Uvalde County, the Uvalde salient 

(Clark and Small, 1997; Clark, 2003), and a generally northsouth trending structural trough in eastern Uvalde County, the 

Knippa gap (fig. 1) (Maclay and Land, 1988). The result is 

water being conveyed generally south to join regional southwest-to-northeast flow paths in the southern part of the Edwards 

aquifer. 

Departure hydrographs for wells Medina2 and Medina8 

and discharge departure for station Hondo Creek near Tarpley 

(fig. 7) provide evidence for flow in the NWMFP. The observed 

relation between water-level departures at Medina2 and 

Medina8 with discharge departure for Hondo Creek near Tarpley could indicate that ground water is moving to the southwest 

through Medina8 and toward Seco Creek. Medina8 showed 

rapid responses to recharge events (to positive increases in flow 

departure) occurring along Hondo Creek. Medina2, upgradient 

from Medina8, showed rises in water level during larger 

recharge events, possibly because the subsurface zone downdip 

was filled to capacity. Smaller recharge events along Hondo 

Creek caused little if any corresponding increases in water-level 

departure at Medina2, which could indicate that the subsurface 

zone downdip did not fill to capacity. 

The departure hydrograph for Medina8 also indicates at 

least one extended period of proportionately more diffuse flow 

in the aquifer—water stored in fractures, bedding planes, vugs, 

and so forth—after recharge. A transition to more diffuse flow 

can be identified on the departure hydrograph by a change in 

recession slope to a less steep decline; for example, the period 

from about mid-January to about mid-February 2003. 

North-Central Medina Flow Path (Northern and 

Southern Parts) 

The north-central Medina flow path (NCMFP) is separated 

into a northern part (NCMFPn) and a southern part (NCMFPs) 

(fig. 5). The NCMFPn is bounded by the Parkers Creek fault on 

the north and the Medina Lake fault on the south. The adjacent 

NCMFPs is bounded on the north by the Medina Lake fault 

and on the south by the Diversion Lake fault. The NCMFP is 

separated into a northern and southern part because water levels 

at well Seco are higher than water levels at wells Medina3, 

Medina5, and Quihi. On average, daily mean water levels at 

well Seco are about 39 feet higher than daily mean water levels 

at well Quihi, the closest monitoring well to well Seco. This 

head difference is hypothesized to be the result of the Medina 

Lake fault acting as a barrier to hydraulic communication (connection) across the fault. 

Recharge to the NCMFPn probably occurs as infiltration 

from Seco Creek, as underflow from the adjacent Trinity aquifer, and as leakage from Medina Lake. The NCMFPs probably 

receives recharge as underflow from the Trinity aquifer and by 

leakage from Medina Lake. Ground water in the NCMFPn and 

NCMFPs moves downgradient (and down relay ramp) from 

eastern Medina County toward the southwest. The NCMFPn 
14 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

60 1,000 

800 

DEPARTURE FROM AVERAGE WATER LEVEL, IN FEET 40 

20 

0

-20 

-40 

-60 

600 

400 

200 

0 

-200 

-400 

-600 

-800 

-1,000 HONDO CREEK DEPARTURE FROM AVERAGE DISCHARGE,


IN CUBIC FEET PER SECOND
 

3/14/024/14/025/14/026/14/027/14/028/14/029/14/02

10/14/0211/14/0212/14/02

1/14/032/14/033/14/034/14/035/14/036/14/037/14/038/14/039/14/03

10/14/0311/14/0312/14/03

1/14/042/14/043/14/044/14/045/14/04 

DATE 

EXPLANATION 

Medina2 (TD–69–31–406)
 

Medina8 (TD–69–31–708)
 

Hondo Creek near Tarpley (08200000)
 

Figure 7. Hydrograph showing 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. 

and NCMFPs are hypothesized to turn south in the vicinity of 

Seco Creek as they begin to be influenced by the structural features that focus ground-water flow into the Knippa gap (fig. 1). 

Recent studies of water-budget analyses of the Medina 

Lake/Diversion Lake system (Lambert and others, 2000; 

Slattery and Miller, 2004) indicate that the lake system leaks 

appreciable amounts of water to the Edwards aquifer and that 

the rate of leakage is related to the stage of Medina Lake. Computed rates of leakage from Medina Lake to the Edwards aquifer 

are about 5 to 178 acre-feet per day when Medina Lake stages 

range from 963 to 1,064 feet above NGVD 29 (Slattery and 

Miller, 2004, p. 17). 

Water-level departure hydrographs for well Seco and 

Medina Lake and discharge departure for station Seco Creek 

at Miller Ranch (fig. 8), and water-level departure hydrographs 

for wells Medina3, Medina5, and Quihi and for Medina 

Lake (fig. 9) provide evidence for flow in the NCMFP. The 

observed relation between the well water-level departure hydrographs and the lake water-level departure hydrograph could 

indicate ground water is being recharged by the Medina Lake 

system and is moving down ramp away from Medina Lake 

toward the southwest. This interpretation is based on the similarity between the well water-level and lake-stage departure 

hydrographs. During the period July 2, 2002–April 24, 2003 

(except for 1 week), the lake stage was above the spillway at the 

Medina Lake dam, which resulted in a relatively flat departure 

hydrograph for the period. During this period that the lake was 

at capacity, water-level departures at all four downgradient 

wells increased positively (water levels rose), which indicates 

the lake was recharging the aquifer. Comparison of water-level 

departure hydrographs and rainfall data from several gages in 

the study area during the July 2, 2002–April 24, 2003, period of 

rising water levels did not indicate a relation between rain 

events and departure-hydrograph fluctuations. In general, both 

the frequency and magnitude of rain events decreased during 

the period. 
Hydrologic Identification of Ground-Water Flow Paths 15 

 

 

 

50 800 

40 

600 

DEPARTURE FROM AVERAGE WATER LEVEL, IN FEET

30 

400 

20 

200 

10 

0 0 

-10 

-200 

-20 

-400 

-30 

-600 

-40 

SECO CREEK DEPARTURE FROM AVERAGE DISCHARGE,


IN CUBIC FEET PER SECOND
 

-50 

3/14/025/14/027/14/029/14/02

11/14/02

1/14/033/14/035/14/037/14/039/14/03

11/14/03

1/14/043/14/045/14/04

-800 

DATE EXPLANATION 

Seco (TD–69–38–601)
 

Medina Lake (08179500)
 

Seco Creek at Miller Ranch (08201500)
 

Figure 8. Hydrograph showing 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. 

South-Central Medina Flow Path 

The south-central Medina flow path (SCMFP) is bounded 

on the north by the Seco Creek and Diversion Lake faults and 

on the south by the Haby Crossing fault (fig. 5). The SCMFP 

likely receives recharge from the Medina River and from the 

adjacent Trinity aquifer. Because of bounding faults oriented 

northeast-southwest and adjacent flow paths directed south by 

other geologic structures, the SCMFP follows the configuration 

of the adjacent flow paths—oriented initially southwest and 

then south. Immediately after turning south, the SCMFP turns 

sharply east. The SCMFP is forced east, as are all Edwards 

aquifer flow paths that originate in the recharge zone in and 

west of the study area, by the series of generally northeastsouthwest trending faults that make it easier for ground water to 

flow toward major natural discharge points (Comal and San 

Marcos Springs) east of the study area than downdip to the 

southeast. 

Departure hydrographs for wells Medina7, Castroville, 

Medina4, and Hondo, and discharge departure for station 

Medina River at Bandera (fig. 10) provide evidence for flow 

in the SCMFP. The similarity in configuration among the 

four well-departure hydrographs, which could indicate hydraulic communication, is clear; and the correspondence in dates 
16 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

20 

6 

30 10 

8 

DEPARTURE FROM AVERAGE WATER LEVEL, IN FEET

4 

10 

2 

0 0 

-2 

-10 

-4 

-6 

-20 

-8 

-30 -10 3/14/02 5/14/02 7/14/02 9/14/02 11/14/02 1/14/03 3/14/03 5/14/03 7/14/03 9/14/03 11/14/03 1/14/04 3/14/04 5/14/04

DATE EXPLANATION 

Medina3 (TD–69–40–209) 

Quihi (TD–69–40–102)—Based on daily
 

water-level high values as reported by the
 

Edwards Aquifer Authority
 

Medina5 (TD-68–25–711)
 

Medina Lake (08179500)
 

MEDINA LAKE DEPARTURE FROM AVERAGE STAGE, IN FEET 

Figure 9. Hydrograph showing 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. 

of positive increases in departure between ground-water 

levels and Medina River flow (despite the distance of the gaging 

station from the wells) could indicate hydraulic communication 

between the Medina River and the ground-water flow system. 

The steep negative increases in the Medina4, Hondo, and 

Castroville departure hydrographs (but not in the Medina7 

water-level and Medina River streamflow departure hydrographs) are caused by withdrawals. The relation between 

the water-level departures and the flow departure could indicate hydraulic communication between the Edwards and 

Trinity aquifers—and thus evidence for underflow from 

the Trinity aquifer to the SCMFP (and other flow paths), as 

the Medina River at Bandera station overlies the Trinity 

aquifer. 
Hydrologic Identification of Ground-Water Flow Paths 17 

 

 

 

 

 

50 1,000 

40 800 

DEPARTURE FROM AVERAGE WATER LEVEL, IN FEET

30 

20 

10 

0 

-10 

-20 

-30 

-40 

-50 

600 

400 

200 

0 

-200 

-400 

-600 

-800 

-1,000 MEDINA RIVER DEPARTURE FROM AVERAGE DISCHARGE,


IN CUBIC FEET PER SECOND
 

3/14/025/14/027/14/029/14/02

11/14/02

1/14/033/14/035/14/037/14/039/14/03

11/14/03

1/14/043/14/045/14/04 

DATE 

EXPLANATION 

Medina4 (TD–69–40–510)
 

Medina7 (TD–68–33–213)
 

Hondo (TD–69–47–306)
 

Castroville (TD–68–41–301)
 

Medina River at Bandera (08178880)
 

Figure 10. Hydrograph showing 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. 

Correlation Between Water Levels in Wells With 

Continuous Record 

Statistical correlations between water-level departures 

for the 11 continuously monitored wells provide additional 

evidence for the hypothesized flow paths. Figure 11 graphically 

summarizes the monotonic relations between each of the 

55 combinations of departure-dataset pairs. The Kendall’s 

tau correlation coefficient for each graph indicates the strength 

of the correlation between departure (and thus water-level) 

pairs—the larger the absolute value of tau within the range 

of -1 to +1, the stronger the correlation. The strength of 

each correlation is indicated by the background color of the 

graph. The stronger correlations (those greater than .6) are all 

among wells in the same flow path (fig. 12), with one exception 

(not shown in fig. 12): the strong correlation between departures 

for Medina8 (NWMFP) and Medina7 (SCMFP), for which 

there is no explanation. Also, there is a strong correlation 

between departures for Seco (NCMFPn) and those for Quihi, 

Medina5, and Medina3 (NCMFPs). Although it is hypothesized 

that the Medina Lake fault restricts hydraulic communication 

between the NCMFPn and NCMFPs, water levels in both parts 

of the NCMFP apparently are influenced by the same areas of 

recharge. 
WELL NAME AND NUMBER 

Medina1 69–32–703 EXPLANATION 

Kendall's tau equal to or greater than .70 

Medina2 69–31–406 Kendall's tau greater than .60 but less than .70 

Kendall's tau less than .60 

Medina3 Point of correlation 69–40–209 

Medina4 69–40–510 

Medina5 68–25–711 

Medina7 68–33–213 

Medina8 69–31–708 

Seco 69–38–601 

Quihi 69–40–102 

Hondo 69–47

Figure 11. Correlation between water-level departures for 11 monitoring wells, northern Medina County, Texas. 
Frio Rive r

W est SabinalRiver

Cherry Creek

Little DryFrio Creek

REAL BANDERA

99o30' 99o19'30" 99o09' 

29o31'30" 

29o21' 

29o10'30" 

Uvalde 

Sabinal Hondo 

Seco 

Sabinal River 

Leona 

Nolton 

Verde

Dry 

River 

Quihi 

Little 

Dinner 

East Elm Creek 

Rancheros 

West Squirrel Creek 

Onion 

Brie r Branch 

Bear 

Fush 

Live Oak Creek 

Little Seco 

East Squirrel Creek 

Elm Creek 

Parkers Creek 

Elm Slough

Little Live Oak Creek 

East Tehuacana Creek 

Middle Verde Creek 

East Branch Live Oak Creek 

Medina River 

Blanco Creek

Frio River

Little Blanco Creek

Frio 

Creek 

Creek 

Creek 

Creek

Creek

 River 

Creek 

Creek 

Creek 

Creek Hondo 

Bruins Creek 

Creek 

Cre

Cr 

UVALDE COUNTY MEDINA COUNTY

Ground-water flow path Northwestern Medina North-central Medina (northern part) North-central Medina (southern part) South-central Medina Boundary of aquifer (Edwards aquifer recharge zone from Ashworth and Hopkins, 1995) Inferred flow directions Study boundary Link showing paired wells and Kendall's tau Tau equal to or greater than .70 Tau greater than .60 but less than .70 Continuous water-level monitoring site and name 

EXPLANATION 

Medina1 

!

.74 

.61 

0 3 6 

.86 

.72 

.76 

.81

.83 

.74 

.80 

.74 

.64 

TRINITY AQUIFER 

EDWARDS AQUIFER RECHARGE ZONE 

EDWARDS AQUIFER CONFINED ZONE 

!

!

!

!

!

!

!

!

Seco 

Hondo 

Medina1 

Medina4 

Quihi 

Medina8 Medina

Medina2 

Medina3 

Figure 12. Water-level departure pairs from 11 monitoring wells for which Kendall's tau was greater than .60, northern Medina and northeastern Uva
20 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

60 

WEIGHT-PERCENT 

50 

40 

30 

20 

10 

0 

MISSING DATA

MISSING DATA 

Calcium oxide 

Silicon dioxide 

Magnesium 

oxide 

Loss on ignition (LOI) 

20 40 60 80 120 140 160 180 200 220 240 260 300 320 340 

DEPTH, IN FEET 

100 280 

Figure 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. 

Geochemical Characterization 

Dominant mineral phases and bulk-rock chemistry of 

the Edwards aquifer were evaluated using well cuttings 

from well Medina5. XRD analysis identified calcite as the 

predominant mineral at all depth intervals. No evaporite or 

clay minerals were identified. Bulk-rock compositions were 

determined from XRF analysis. The XRF results were similar 

to the XRD findings such that calcium, in weight-percent oxide, 

and loss on ignition, which usually is indicative of carbon 

dioxide losses from carbonates, accounted for more than 90 percent of the bulk chemistry (fig. 13). Minor silica enrichment 

(greater than 5-percent oxide) was identified in five 10-foot 

depth zones. Magnesium enrichment, which is indicative of 

dolomitic limestone, also was identified in three 10-foot depth 

zones. 

Age of Ground Water 

Apparent ages of ground water in the Edwards aquifer 

(fig. 14) ranged from 1969 in well TD–69–38–709, in the confined zone of the Edwards aquifer in the NCMFPn (fig. 6), to 

present day (2003) in well Medina3, in the recharge zone of the 

Edwards aquifer in the NCMFPs. 

As would be expected in the more downgradient part of 

a flow path, the apparent age of water in confined-zone wells 

(TD–69–38–709, TD–69–38–106, and TD–69–38–509) 

generally was older (1969, 1975, and 1978 respectively) than 

water in the more upgradient parts. An exception was water in 

Medina4, which was relatively younger (1992) than water in 

other wells in the confined zone. Medina4 was the most upgradient well sampled in the SCMFP that yielded a reliable groundwater age. Greater variability in apparent age characterized 

water in wells in the recharge zone compared with water in the 
Geochemical Characterization 21 

 

 

 

 

SULFUR HEXAFLUORIDE CONCENTRATION,

IN PARTS PER TRILLION BY VOLUME 

6 

5 

4 

3 

2 

1 

0 

TD–69–38–709 

TD–69–38–106 

TD–69–38–509 

TD–69–25–806 

TD–69–31–603 

YP–69–37–101 

TD–69–30–903 TD–68–25–810

Medina4 

TD–69–40–510 

Medina3 

Monitoring well, by source TD–69–40–209

Trinity aquifer well 

Edwards aquifer well 

1950 1960 1970 1980 1990 2000 2010 

YEAR 

Figure 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 (modified from Busenberg and Plummer, 2000, fig. 6A). 

confined zone, probably because of more frequent mixing with 

rainfall and surface water. Well TD–68–25–806 yielded the 

oldest apparent age for ground water in the recharge zone. 

Ground-Water/Surface-Water Mixing Relations 

Global and local meteoric water lines (GMWL and 

LMWL, respectively) were graphed from the equations by 

Craig (1961) and Yurtsever and Gat (1981), respectively 

(fig. 15); δD and δ18O determined from surface-water samples 

collected from Medina Lake and the Medina River below 

Medina Lake (table 2) were superimposed on the graphs. Surface-water isotope data plot below the GMWL and LMWL 

because of fractionation effects from evaporative processes. 

Ground-water isotope data were grouped by hypothesized 

flow path and graphed on the same reference lines. Ground 

water that originated by direct recharge to the aquifer from rainfall and not influenced by surface water should have isotope 

ratios that plot near the meteoric water line. Isotope data from 

wells in the NWMFP plotted near the GMWL. Isotope data 

from wells in NCMFP and SCMFP produced a linear trend 

that extends from the surface-water data to the GMWL, which 

represents a gradient of mixing of the surface water with the 

ground water. Simple linear regression of the isotope data from 

this study produced a local mixing line that accounted for 

96 percent of the variability (R2 = .958) 

Ground-water samples that represented end-member 

Edwards aquifer chemistry were selected from wells that had 

isotope ratios that plotted near the GMWL (fig. 15). Ground 

water from Quihi, Medina2, and TD–68–25–806 were selected, 

and their isotope ratios (δ18O and δD, respectively) were averaged to produce the average Edwards end-member isotope 

composition. Isotope ratios also were averaged for samples 

from stations Medina Lake near San Antonio and Medina River 

below Medina Lake to produce the average surface-water endmember isotope compositions. 

Sensitivity of end-member isotope composition to estimate the percentages of surface water in Edwards aquifer samples ranged from 10 (for δ18O) to 12 percent (for δD) on the 

basis of seasonal or hydrologic variability in the ground-water 

system. The sensitivity of Trinity aquifer samples was about 
22 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas DELTA DEUTERIUM, IN PER MIL

15 

10 

5 

0 

-5 

-10 

-15 

-20 

-25 

-30 

-35 

Medina2 

TD–69–31–406 (NWMFP) 

TD–68–25–806 (NCMFP) 

Quihi 

TD–69–40–102 (NCMFP) 

-6 -5 -4 -3 -2 -1 0 

DELTA OXYGEN-18, IN PER MIL 

Sample, by source 

Northwestern Medina flow path (NWMFP) 

North-central Medina flow path (NCMFP) 

South-central Medina flow path 

Medina River 

Medina Lake 

Global meteoric water line (Craig, 1961) 

Local meteoric water line (Yurtsever and Gat, 1981) 

Linear regression (local mixing line) 

Figure 15. Relation between delta oxygen-18 and delta deuterium ratios for selected ground- and surface-water sites, northern Medina 

County, Texas, 1995–2003. 

one-half that of Edwards aquifer samples (5 and 7 percent for 

δ18O and δD, respectively). Therefore, all mixing computations 

were censored at the 10- and 12-percent levels for the Edwards 

aquifer samples and at the 5- and 7-percent levels for Trinity 

aquifer samples (table 3, at end of report). 

Samples from two wells (TD–68–25–402 and TD–68–25– 

510) in the NCMFP and close to Medina Lake (fig. 6), were 

almost entirely surface water (at least 88 percent) on the basis 

of isotope composition (table 3). Percentages in samples from 

other wells ranged from less than the censored level to about 

60 percent. Isotope data indicate that ground water in the 

SCMFP was the most influenced by surface-water recharge, 

possibly from the Medina River. In the NCMFP, proximity of a 

well to a major water body, such as Medina Lake or the Medina 

River, appears to be the major factor in controlling the percentage of surface water. 

Major-Ion Chemistry 

Analysis of major-ion chemistry indicates an appreciable 

difference in the basic water chemistry between the Edwards 

and Trinity aquifers (fig. 16; table 2) and that the Edwards aquifer receives a minimal amount of water derived from the Trinity 

EXPLANATION 

K+Na 

Mg 

Ca 

SO4 

Cl 

HCO3 

0 30 20 10 0 10 20 30 40 

Trinity aquifer 

Edwards aquifer 

Seco Creek 

Medina River 

Medina Lake 

MILLIEQUIVALENTS PER LITER 

4

Figure 16. Stiff diagram showing the variability in composition of 

ground water and surface water, northern Medina County, Texas. 
Geochemical Characterization 23 

 

 

 

 

7 

Na 

Mg 

Ca 

Cl 

HCO3 Year of 

EXPLANATION 

SO4 recharge 

2003 

1992 

1990 

1989 

1989 

1988 

1982 

1978 

1978 

1978 

1978 

1978 

1975 

1969 

6 5 4 3 2 1 0 1 2 3 4 5 6 7 

MILLIEQUIVALENTS PER LITER 

Figure 17. Stiff diagram showing the variability in composition of 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. 

aquifer in the study area. Surface water from major streams 

and Medina Lake recharge both the Edwards and Trinity aquifers. Water chemistry indicates the amount of surface water 

identifiable in the ground-water system primarily is related to 

the proximity of the sampling point to the source of the surface 

water. 

Relatively minor variability in major-ion chemistry was 

observed in water of different ages in the Edwards aquifer 

(fig. 17). Edwards aquifer water was of the calcium-bicarbonate 

type (fig. 18). Minor variability in magnesium and sulfate concentrations among samples from wells in the Edwards aquifer 

was indicated in NWMFP, NCMFP, and the SCMFP flow 

paths. Specific conductance of Edwards aquifer water in the 

study area ranged from 402 to 710 microsiemens per centimeter 

at 25 degrees Celsius, and pH ranged from 6.5 to 7.4 standard 

units. The temperature of Edwards aquifer water ranged from 

17.5 to 30.5 degrees Celsius. 

Samples from wells in the Trinity aquifer yielded ground 

water of a calcium-magnesium-sulfate type (fig. 16). Some 

minor variability in Trinity aquifer water chemistry was noted, 

depending on the location in the aquifer from which the sample 

was obtained. Specific conductance of water in the Trinity 

aquifer in the study area ranged from 417 to 3,160 microsiemens per centimeter at 25 degrees Celsius, and ph ranged from 

6.4 to 9.4 standard units. The temperature of the Trinity aquifer 

water ranged from 12.5 to 24.9 degrees Celsius. The greater 

mineralization, as indicated by greater specific conductance in 

Trinity aquifer water compared with Edwards aquifer water, 

indicates slower ground-water movement in the Trinity aquifer 

than in the Edwards aquifer. 
24 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 100 

 100 

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

0 

100 

80 

60 

40 

20 

0 

20 

40 

60 

80

80 

60 

40 

20 

0 

Magnesium 

Sulfate+chloride 

Sulfate 

Calcium+magnesium

 0

20

40

60

80

100 

100 

80 

60 

40 

20 

0 

Carbonate+bicarbonate

Sodium+potassium

0 

20 

40 

60 

80 

100 

100

80

60

40

20

0 

20 

40 

60 

80 

100 

0 

Calcium Chloride

 100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

PERCENT MILLIEQUIVALENTS 

Northwestern Medina flow path (NWMFP) 100 

80 

60 

40 

20 

20 

40 

60 

80 

100

100 

80 

60 

40 

20 

0 

Magnesium 

Sulfate+chloride 

Sulfate 

Calcium+magnesium

 0

20

40

60

80

100 

100 

80 

60 

40 

20 

0 

Carbonate+bicarbonate

Sodium+potassium

0 

20 

40 

60 

80 

100 

100

80

60

40

20

0 

0 

20 

40 

60 

80 

100 

0 

Calcium Chloride

 100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

PERCENT MILLIEQUIVALENTS 

North-central Medina flow path (NCMFP)

 100 

80 

60 

40 

20 

0 

20 

40 

60 

80 

100

100 

80 

60 

40 

20 

0 

Magnesium 

Sulfate+chloride 

Sulfate 

Calcium+magnesium

 100 

80 

60 

40 

20 

0 

Carbonate+bicarbonate

Sodium+potassium

0 

20 

40 

60 

80 

100 

20 

40 

60 

80 

100 

0 

100 

80 

60 

40 

20 

0 

0 

20 

40 

60 

80 

100 

0 

Calcium Chloride


PERCENT MILLIEQUIVALENTS
 

South-central Medina flow path (SCMFP)
 

 0 

Figure 18. Piper diagrams showing composition of ground water in the Edwards aquifer in the northwestern, north-central, and southcentral Medina flow paths, northern Medina and northeastern Uvalde Counties, Texas.
 

 

 

 

 

 

 

 

Geochemical Simulation 25 

Geochemical Simulation 

Northwestern Medina Flow Path 

Ground-water chemistry for samples from five wells 

in the Edwards aquifer in the NWMFP (in downgradient 

order, Medina1, TD–69–31–603, Medina2, Medina8, and 

TD–69–30–903) were used to evaluate the evolution of groundwater chemistry in the NWMFP (fig. 19; table 4, at end of 

report). SIs of the samples for calcite, dolomite, carbon dioxide 

(gas), gypsum, halite, and amorphous silica (fig. 20; table 5, at 

end of report) indicate that most mineral phases were near 

equilibrium or able to undergo dissolution. Undersaturated 

conditions predominated. 

Seven simulations were done for samples from well pairs 

collected during 2001–03 (table 4); three of the seven yielded 

plausible models. The mole transfers of the mineral phases 

determined in the simulations were evaluated on the basis of 

plausible processes of dissolution or precipitation indicated by 

SIs. For example, if a simulation indicated that 1 mole of calcite 

needed to be precipitated to explain between-well changes in 

chemistry of water undersaturated with respect to calcite, then 

that simulation was considered not plausible. 

Simulations 5, 6, and 6A used ground-water chemistry 

for well Medina1 from August 2003 as the initial solution with 

the final solutions based on ground-water chemistries from 

nearby wells Medina2 and Medina8. Mixing of the initial water 

with end-member Edwards aquifer water (TD–68–25–806, the 

oldest) was included in simulation 5. The most plausible model 

for simulation 5 required a 58-percent contribution of endmember Edwards aquifer water. Precipitation of 0.48 millimole 

per kilogram (mmol/kg) of calcite, degassing of 0.49 mmol/kg 

of carbon dioxide gas, and dissolution of minor amounts of 

dolomite, gypsum, halite, and silica (less than or equal to 

0.027 mmol/kg) were required to produce the final solution. 

Mixing of the initial water with end-member Edwards aquifer 

water (TD–68–25–806) and surface water (average of Medina 

Lake and Medina River) was included in simulations 6 and 6A. 

Simulation 6 required a 27-percent contribution of surface 

water, 73-percent contribution of end-member Edwards aquifer 

water, and dissolution of 2.24 mmol/kg of carbon dioxide, 

0.102 mmol/kg of dolomite, and minor amounts of halite and 

silica (less than or equal to 0.03 mmol/kg). Simulation 6a 

required a 33-percent contribution from surface water but no 

end-member Edwards aquifer water, and mole transfers similar 

to those in simulation 6. 

The results of these simulations demonstrate that the 

hypothesized NWMFP described in the “Hydrogeologic 

Identification of Ground-Water Flow Paths” section can be 

partially supported geochemically; that is, plausible models—processes of dissolution or precipitation in the aquifer 

between some of the well pairs—were identified. Also, the 

results indicate a considerable contribution of ground water 

from the regional flow path (58 to 73 percent). Lastly, a negligible (0 percent) to appreciable (27 to 33 percent) contribution 

of surface water was indicated in the simulations of the 

NWMFP. 

North-Central Medina Flow Path (Northern and 

Southern Parts) 

Ground-water samples from 13 wells were used to 

evaluate the evolution of ground-water chemistry in the 

NCMFP (fig. 19; table 4). 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. 

SIs of samples from these wells for calcite, dolomite, carbon 

dioxide (gas), gypsum, halite, and amorphous silica (fig. 21; 

table 5) indicate, as in the NWMFP, that most mineral phases 

were at equilibrium or able to undergo dissolution. Undersaturated conditions predominated. 

The estimated contribution of surface water in samples 

from NCMFP wells in the Trinity and Edwards aquifers varied 

over a wide range, from less than 6 to 100 percent (table 3). 

Samples with the largest percentage of surface water generally 

were from wells close to Medina Lake. No linear trend existed 

between the percentage of surface-water contribution and the 

calcite SI in ground water from the Trinity or Edwards aquifers 

for this flow path (fig. 22). 

Nineteen simulations were done for samples from well 

pairs collected during 1995–2003 (table 4); eight of the 19 

yielded plausible models. Initial simulations focused on the 

evolution of the ground-water chemistry in the most upgradient 

part of the NCMFP where influences from the surface-water 

system, specifically Medina Lake, were greatest. Trinity aquifer 

wells TD–68–25–402 and TD–68–25– 510 are near Medina 

Lake (fig. 19). Samples from these wells had isotope compositions that represented contributions of surface water from 88 to 

100 percent (table 3). Simulation 1 for well TD–68–25–402 and 

simulation 2 for TD–68–25–510 (table 4) involved Medina 

Lake water as the initial solution and the ground-water chemistry from the August 1996 samples in the respective wells as the 

final solution. Simulations 1 and 2 required 100-percent Medina 

Lake water as the initial solution prior to the dissolution of 

about 0.3 mmol/kg of carbon dioxide, less than 0.1 mmol/kg of 

halite, and 0.278 mmol/kg of gypsum to reach the final-solution 

chemistry. 

Trinity aquifer wells TD–68–25–810 and TD–68–25–809 

are near Diversion Lake (fig. 19). The isotope compositions of 

ground water from these wells indicated a negligible (less than 

6 percent) contribution of surface water in the August 1996 

TD–68–25–810 sample and a 13-percent contribution in the 

July 1996 TD–68–25–809 sample (table 3). Simulations 3 

and 4 (table 4) modeled the influence of the Edwards aquifer 

and the surface-water system on the ground-water chemistry of 

wells TD–68–25–810 and TD–68–25–809, respectively. Endmember Trinity aquifer water (TD–68–25–602, upgradient 

from all flow paths) was the initial solution, and mixing of 

end-member Edwards aquifer water (TD–68–25–806) and 

surface water was allowed. In simulation 3, the final solution 
Frio Rive r

W est SabinalRiver

Cherry Creek

Little DryFrio Creek

REAL BANDERA

99o30' 99o19'30" 99o09' 98o58'30

29o31'30" 

29o21' 

29o10'30" 

Uvalde 

Sabinal Hondo 

Seco 

Sabinal River 

Leona 

Nolton 

Verde 

Dry 

River 

Quihi 

Little 

Dinner 

East Elm Creek 

B a l c k C r e e k 

Rancheros 

West Squirrel Creek 

Onion 

Brie r Branch 

Bear Creek

Fush 

Live Oak Creek 

Little Seco 

East Squirrel Creek 

Elm Creek 

Parkers Creek 

Elm Slough

Little Live Oak Creek 

East Tehuacana Creek 

Middle Verde Creek 

East Branch Live Oak Creek 

Medina River 

M La

Blanco Creek 

Frio River

Little Blanco Creek

Frio 

Creek 

Creek 

Creek 

Creek

Creek

 River 

Creek 

Creek 

Creek 

Creek Hondo 

Bruins Creek 

Creek 

Creek 

UVALDE COUNTY MEDINA COUNTY

Ground-water flow path Northwestern Medina North-central Medina (northern part) North-central Medina (southern part) South-central Medina Boundary of aquifer (Edwards aquifer recharge zone from Ashworth and Hopkins, 1995) Inferred flow directions Study boundary Monitoring well, by source, and number Trinity aquifer well Edwards aquifer well 

EXPLANATION 

TD–68–25–402 

TD–69–30–903 

!

!

0 3 6 

TRINITY AQUIFER 

EDWARDS AQUIFER RECHARGE ZONE 

EDWARDS AQUIFER CONFINED ZONE 

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

Medina1 

Medina8 

Medina2 

TD–69–31–603 

TD–69–30–903 

TD–69–40–604 

TD–69–38–709 

TD–68–2

TD–68–25–402 

TD–68–25–

TD–68–25–701 T

TD–69–32–905 TD–

TD–68–33–103 TD-69-40-202 

Medina4 

Medina5 

Medina3 TD–69–40–301

Figure 19. Wells in flow paths that yielded samples used to evaluate evolution of ground-water chemistry by simulation, northern Medina and northe

Texas. 
 

 

 

 

1TD–69–32–2011TD–69–32–303 

TD–69–31–603

Medina2Medina2Medina8 Medina1 

TD–69–30–903 

Dolomite 

Halite 

Silica 

Carbon dioxide 

Calcite 

Gypsum 

1 

0 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

SATURATION INDEX 

-10 

WELL 

1 Probably northwestern Medina flow path 

Geochemical Simulation 27 

Figure 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. 

was the ground water in well TD–68–25–810 for the August 

1996 sample that had negligible surface-water contribution on 

the basis of isotope data. The most plausible model required no 

surface-water contribution and a 100-percent contribution from 

the Edwards aquifer to the initial solution. About 3 mmol/kg of 

carbon dioxide, 0.13 mmol/kg of dolomite, and 0.21 mmol/kg 

of halite were dissolved to produce the final solution. In 

simulation 4, the final solution was the ground water in well 

TD–68–25–809 for the July 1996 sample that was estimated to 

have a 13-percent surface-water contribution on the basis of isotope data. That simulation required a 20-percent surface-water 

contribution and an 80-percent contribution from the Edwards 

aquifer to the initial solution, which also assumed negligible 

Trinity aquifer water. As in simulation 3, no plausible model 

was identified that required a contribution from the Trinity 

aquifer. The evolution of ground water to the final-solution 

chemistry required precipitation of 2.36 mmol/kg of calcite, 

degassing of about 0.4 mmol/kg of carbon dioxide, and dissolution of 1.15 mmol/kg of dolomite, about 11 mmol/kg of 

gypsum, and 0.014 mmol/kg of halite. 

Simulations 9, 11, 12, and 12A (table 4) represented the 

evolution of ground water from wells in the hypothesized 

NCMFP during December 2002–October 2003. In simulation 9, 

the evolution of ground water was simulated from rechargezone well Medina5 to well Medina3, at the boundary between 

the recharge zone and the confined zone (fig. 19). Ground water 

from Medina5 (August 2003 sample) was the initial solution 

and had negligible (less than 11 percent) surface-water contribution on the basis of isotope data (table 3). The simulation 

allowed mixing of end-member Edwards aquifer water (TD– 

68–25–806) and surface water with the initial water. Ground 

water from Medina3 (August 2003 sample) represented the 

final solution and had an 18-percent surface-water contribution 

on the basis of isotope data. The PHREEQC simulation allowed 

carbon dioxide degassing and dissolution and dolomite, gypsum, halite, and amorphous silica dissolution only. The most 

plausible model required 29-percent contribution from the 

surface-water system and dissolution of about 1.9 mmol/kg of 

carbon dioxide gas, about 0.2 mmol/kg of dolomite, and less 

than 0.075 mmol/kg of gypsum, halite, and silica. 
28 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

Dolomite 

Halite 

Silica 

Carbon dioxide 

Calcite 

Gypsum 

TD–68–25–402TD–68–25–402TD–68–25–510TD–68–25–510TD–68–25–602TD–68–25–602TD–68–25–809TD–68–25–809TD–68–25–810TD–68–25–810TD–68–25–810TD–68–25–701TD–68–25–701TD–68–25–806TD–68–25–806TD–68–25–806

Medina5

TD–69–32–905TD–69–40–301TD–69–40–301

Medina3

TD–69–38–709TD–69–40–202

TD–69–40–202

Seco 

-11 

1 

2 

Trinity aquifer wells

Edwards aquifer wells

SATURATION INDEX 

0 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

-10 

WELL 

Figure 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. 

Simulation 11 was an attempt to reproduce the results 

of simulation 9 for the evolution of ground water in the 

NCMFP from the recharge zone to (nearly) the confined 

zone of the Edwards aquifer, this time using upgradient well 

TD–69–32–905 (October 2003 sample) and, as in simulation 9, 

downgradient well Medina3 (August 2003 sample). The simulation allowed carbon dioxide degassing and dissolution and 

dolomite, gypsum, halite, and amorphous silica dissolution 

only. The most plausible model for this simulation required a 

23-percent surface-water contribution and dissolution of about 

1.7 mmol/kg of carbon dioxide gas, about 0.25 mmol/kg of 

dolomite, and about 0.09 mmol/kg of halite, results similar to 

those of simulation 9. However, a large difference in the results 

of simulation 11 compared with those of simulation 9 was that 

77 percent of the final solution was from end-member Edwards 

aquifer water (TD–68–25–806), not from the upgradient well. 

No plausible model was identified that allowed contribution 

from the upgradient well TD–69–32–905. 

Simulations 12 and 12A represented the evolution of 

ground water in the hypothesized NCMFP from well Medina3 

(August 2003 sample), which was the downgradient well in 

simulation 11, to the most downgradient well in the hypothesized flow path, TD–69–38–709 (fig. 19) (December 2002 

sample). (Although well Medina3 is in the NCMFPs and well 

TD–69–38–709 is in the NCMFPn, the assumption is made 

for these two simulations that the NCMFP is undivided.) The 

sample from well TD–69–38–709 was undersaturated with 

respect to all mineral species and had a 14-percent surfacewater contribution on the basis of isotope data (table 3). The 

PHREEQC software allowed mixing of end-member Edwards 

aquifer water (TD–68–25–806) and surface water with the initial water and calcite dissolution and precipitation, carbon dioxide degassing and dissolution, and dolomite, gypsum, halite, 

and amorphous silica dissolution only. Two plausible simulations were identified. Simulation 12 required a 15-percent 

surface-water contribution and an 85-percent contribution 

from end-member Edwards aquifer water for the final solution 

(table 4). No ground water was contributed from the initial solution (Medina3). To reach the final solution, the simulation 

required precipitation of 0.65 mmol/kg of calcite and the 

dissolution of about 0.29 mmol/kg of dolomite and about 

0.05 mmol/kg of halite along the flow path. Simulation 12A 
 

 

 

 

SURFACE-WATER CONTRIBUTION, IN PERCENT 

100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

Uncerta

saturati

compu

inty of 

on index 

tation 

Uncertainty of mixing computation 

-1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 

CALCITE SATURATION INDEX 

Geochemical Simulation 29 

Figure 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. 

required a 9-percent surface-water contribution and a 54-percent contribution from end-member Edwards aquifer water to 

reach the final solution. To reach the final solution, the model 

also required precipitation of about 0.58 mmol/kg of calcite, 

degassing of about 0.57 mmol/kg of carbon dioxide gas, and 

dissolution of about 0.18 mmol/kg of dolomite. 

Results of these simulations demonstrate that the hypothesized NCMFP described in the “Hydrogeologic Identification 

of Ground-Water Flow Paths” section can be partially supported geochemically. As with simulations of NWMFP well 

pairs, plausible processes of dissolution or precipitation in the 

aquifer between some of the NCMFP well pairs were identified. 

However, developing plausible simulations of chemical evolution of water in wells sampled during different time periods and 

between wells not in the same aquifer (one in Trinity aquifer 

and one in Edwards aquifer) was problematic. Difficulty associated with the former issue likely arises from the assumption 

that seasonal differences in the ground-water chemistry are negligible. Difficulty associated with the latter issue likely arises 

from well construction, pump placement, depth of penetration 

of the borehole, position of the well along the relay ramp, and 

possible stratification of ground water. An example might be: 

Trinity aquifer water is entering the Edwards aquifer near its 

base; an Edwards aquifer well did not fully penetrate the aquifer 

and therefore Trinity aquifer water is not detected in the well, 

even though it is present in the Edwards aquifer at some depth 

below the well bore. 

Surface-water contribution to the ground water was greatest in the Trinity aquifer in wells adjacent to Medina Lake. 

However, minor (9 to 23 percent) surface-water contributions 

were simulated in wells in the Edwards aquifer relatively 

removed from the lake, which indicates other surface-water 

sources (streams). The simulation results also indicate considerable contributions from regional ground-water flow. 
30 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

 

TD–68–25–904 

TD–68–25–904 

Medina7 

TD–68–33–103 

TD–68–33–204 

TD–68–33–204 

Medina4 

TD–69–40–604 

TD–69–40–604 

1 

0 

-1 

-2 

-3 

-4 

-5 

-6 

-7 

-8 

-9 

-10 

Dolomite 

Halite 

Silica 

Carbon dioxide 

Calcite 

Gypsum 

SATURATION INDEX 

-11 

WELL 

Figure 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. 

South-Central Medina Flow Path 

Ground-water samples from seven wells were used to 

evaluate the evolution of ground-water chemistry in the 

SCMFP (fig. 19; table 4). One well was the Trinity aquifer endmember well (TD–68–25–602, upgradient from all flow paths) 

and another was a Trinity aquifer well in the most upgradient 

part of the flow path (TD–68–25–904); all other wells were 

completed in the Edwards aquifer. SIs of samples from these 

wells for calcite, dolomite, carbon dioxide (gas), gypsum, 

halite, and amorphous silica (figs. 21, 23; table 5) indicate, as in 

the other two hypothesized flow paths, that most mineral phases 

were at equilibrium or able to undergo dissolution. Undersaturated conditions predominated. 

Ground water from all wells in the SCMFP contained an 

estimated contribution from the surface-water system during 

the period represented by the samples, which ranged from 12 to 

42 percent (table 3). A linear trend existed between the percentage of surface-water contribution and the calcite SI in ground 

water from the Edwards aquifer for this flow path (fig. 24a). 

The trend implies that ground water with more than about a 

30-percent surface-water contribution to the ground water 

tended to be undersaturated with respect to calcite and that 

ground water with less than about a 20-percent surface-water 

contribution tended to be oversaturated with respect to calcite. 

Although ground water in the Edwards aquifer consistently was 

undersaturated with respect to dolomite, as the contribution of 

surface water decreased, the SI tended to decrease (fig. 24b). On 

the basis of these trends, the mixing of surface water with 

ground water in the Edwards aquifer might be a more important 

factor than location along the flow path (residence time) for 

equilibrium conditions of calcite and dolomite. Mixing of two 

saturated waters or of saturated and oversaturated water can 

produce undersaturated conditions (Back, 1963; Parkhurst, 

1990). 

Nine simulations were done for samples from well pairs 

collected during 1996–2003 (table 4); seven of the nine yielded 

plausible models. Simulation 1 represented the evolution of the 

ground-water chemistry from Trinity aquifer end-member 

water (TD–68–25–602, upgradient from all flow paths) (July 
 

A
 

45
 

40
 

35
 

30
 

25
 

20
 

15
 

10
 

5
 

0
 

y=-33.387x+25.231

 R2=.852 

Uncerta

saturatio

compu

inty of 

n index 

tation 

Uncertainty of mixing computation 

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3
 

CALCITE SATURATION INDEX
 

B
 

-2 -1 0 1 

DOLOMITE SATURATION INDEX 

SURFACE-WATER CONTRIBUTION, IN PERCENT 

y=-20.381x+12.862

 R2=.542 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

SURFACE-WATER CONTRIBUTION, IN PERCENT

Uncertainty of mixing computation 

Uncertainty of 

saturation index 

computation 

Geochemical Simulation 31 

Figure 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. 

1996 sample) to the most upgradient well in the SCMFP 

(TD–68–25–904) (July 1996 sample), near Diversion Lake 

(fig. 19). Average of δD and δ18O isotope compositions in 

TD–68–25–904 indicated that 13 and 12 percent surface water 

was present in the samples collected in March and July 1996, 

respectively (table 3). 

Simulations 2 and 2A represented the evolution of groundwater chemistry from Trinity aquifer well TD–68–25–904 

(July 1996 sample), which contained an estimated 12-percent 

contribution from surface water, to Edwards aquifer well 

Medina7 (August 2003 sample), which contained an estimated 

14-percent contribution from surface water (table 3). The 
32 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

 

 

 

 

 

 

most plausible model for simulation 2 required a 96-percent 

contribution from end-member Edwards aquifer water 

(TD–68–25–806) and only a 4-percent contribution from 

surface water. Only minor carbon dioxide and dolomite 

dissolution (0.62 and 0.10 mmol/kg, respectively) were 

required to equilibrate the ground-water chemistry. Also, 

extremely small (0.059 mmol/kg or less) amounts of gypsum 

and halite were dissolved. The results of simulation 2A were 

essentially the same (table 4). 

Simulation 3 represented the evolution of groundwater chemistry from the same well as in simulation 2, 

TD–68–25–904 (July 1996 sample), to well TD–68–33–204 

(August 1996 sample), with an estimated surface-water 

contribution of 30 percent (table 3). The most plausible 

model for simulation 3 reduced the initial-solution surfacewater contribution from 30 to 10 percent and required an 

83-percent contribution from end-member Edwards aquifer 

water, with the remaining 7 percent supplied by the upgradient 

Trinity aquifer well (TD–68–25–904). To equilibrate the 

ground-water chemistry required dissolution of 0.41 mmol/kg 

of carbon dioxide and 0.084 mmol/kg of dolomite along the 

flow path. 

Simulations 4 and 4A represented the evolution of groundwater chemistry from the Edwards aquifer recharge zone (well 

Medina7, August 2003 sample) to the confined zone (well 

Medina4, August 2003 sample). Although well Medina4 was in 

the confined zone of the Edwards aquifer, the sample had an 

estimated 23-percent surface-water contribution, which was 

greater than the 14 percent estimated for the sample from the 

upgradient well (Medina7). About 0.15 mmol/kg of carbon 

dioxide and about 0.07 mmol/kg of halite were required to dissolve into solution. Simulation 4 required a 12-percent surfacewater contribution to be mixed with the initial water from well 

Medina7 and no end-member Edwards aquifer water to reach a 

final solution at well Medina4. The results of simulation 4A 

were similar—a slightly smaller (7 percent) surface-water contribution and no end-member Edwards aquifer water were 

required. 

The surface-water contribution provides an explanation 

for the younger apparent age (1992) of water from confinedzone well Medina4, as estimated from SF6 concentration, 

than water from recharge-zone wells TD–68–25–806 (1978) 

and TD–68–25–810 (1990) (fig. 14). Apparently the degree 

of confinement associated with well Medina4, despite its location, is less than that associated with the two recharge-zone 

wells. 

Results of these simulations demonstrate that the hypothesized SCMFP described in the “Hydrogeologic Identification of 

Ground-Water Flow Paths” section can be partially supported 

geochemically. Simulations in the SCMFP involved minimal 

(less than 7 percent) contribution of Trinity aquifer water to 

Edwards aquifer water in this flow path. Simulations produced 

plausible models for the regional Edwards aquifer ground water 

and surface water. Also, some surface-water influence was 

demonstrated at Medina4 in the confined zone of the Edwards 

aquifer. 

Summary 

The U.S. Geological Survey (USGS), 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. The study involved use of geologic structure, surfacewater and ground-water data, and geochemistry to identify 

ground-water flow paths. Knowledge of geologic structure and 

previous USGS work provided a beginning framework, a basis 

for hypothesizing flow paths. Hydrologic data were analyzed 

and flow paths identified using structural controls, water-level 

departure hydrographs, and statistical correlation of water-level 

departure for data from 11 monitoring wells in the study area. 

Chemical data were collected from 41 wells and two lake sites 

in the study area to determine chemical characteristics of 

ground water and surface water. One-dimensional geochemical 

simulations oriented along hypothesized major flow paths provided further evidence to support the existence of the flow 

paths. These geochemical simulations represent the chemical 

evolution of ground water along the flow paths and the degree 

of influence of surface water on the ground-water flow system. 

The results of the study could be useful to water managers with 

responsibility for protecting sources of recharge, identifying 

areas with potential for recharge enhancement, and identifying 

areas of ground-water discharge. 

The Balcones fault zone is an extensional fault system that 

crosses Medina and Uvalde Counties from the southwest to the 

northeast. Extension strongly influenced the orientation of fractures, location of streams, areas of recharge, and ground-water 

flow paths. Fractures generally are parallel to or perpendicular 

to the main fault trend of the Balcones fault zone. In this structurally complex area, the fault zone contains relay ramps which 

form in extensional environments to accommodate an increase 

in deformation. 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 (NWMFP) 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—primarily because of structural controls associated with 

a relay ramp—until the flow encounters a cross fault along Seco 

Creek. The cross fault along Seco Creek probably forms a barrier or a partial barrier to flow because the fault might have offset several hundred feet of the Edwards aquifer and juxtaposed 

confining units against it. This barrier to flow might force part 

or most of the flow to the south. Departure hydrographs for two 

wells and discharge departure for streamflow-gaging station 

Hondo Creek near Tarpley provide evidence for flow in the 

NWMFP. 

The north-central Medina flow path (NCMFP) is separated 

into a northern and southern part because of water-level differences. The northern part of the NCMFP (NCMFPn) is bounded 

by the Parkers Creek fault on the north and the Medina Lake 

fault on the south. The adjacent southern part of the NCMFP 
 

 

 

 

 

 

 

 

 

 

 

 

 

References 33 

(NCMFPs) is bounded on the north by the Medina Lake fault 

and on the south by the Diversion Lake fault. Ground water in 

both the NCMFPn and NCMFPs moves downgradient (and 

down relay ramp) from eastern Medina County toward the 

southwest. The NCMFPn and NCMFPs are hypothesized to 

turn south in the vicinity of Seco Creek as they begin to be 

influenced by structural features. Departure hydrographs for 

four wells and Medina Lake and discharge departure for station 

Seco Creek at Miller Ranch provide evidence for flow in the 

NCMFPn and NCMFPs. 

The south-central Medina flow path (SCMFP) 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 SCMFP follows 

the configuration of the adjacent flow paths—oriented initially 

southwest and then south. Immediately after turning south, the 

SCMFP turns sharply east. Departure hydrographs for four 

wells and discharge departure for station Medina River at Bandera provide evidence for flow in the SCMFP. 

Statistical correlations between water-level departures for 

the 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 mole-balance, or 

inverse, model PHREEQC. The overlying assumption in this 

type of simulation is that the ground-water chemistry in a downgradient well evolved from the ground-water chemistry in an 

upgradient well by reaction with minerals and gases or by input 

from the surface-water system. If this assumption is shown by 

simulation results to be plausible, then the existence of the 

hypothesized flow path is supported. 

Ground-water chemistry for samples from five wells in the 

Edwards aquifer in the NWMFP were used to evaluate the 

evolution of ground-water chemistry in the NWMFP. Most 

mineral phases were near equilibrium or able to undergo 

dissolution. Undersaturated conditions predominated. Seven 

simulations were done for samples from well pairs collected 

during 2001–03; three of the seven yielded plausible models. 

The three plausible models demonstrate that the hypothesized 

NWMFP can be partially supported geochemically. The simulations of the NWMFP samples indicate a considerable contribution of ground water from the regional flow path (58 to 73 

percent) and a negligible (0 percent) to appreciable (27 to 33 

percent) contribution of surface water. 

Ground-water samples from 13 wells were used to 

evaluate the evolution of ground-water chemistry in the 

NCMFP 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. As in the NWMFP, most mineral 

phases were at equilibrium or able to undergo dissolution. 

Undersaturated conditions predominated. Nineteen simulations 

were done for samples from well pairs collected during 1995– 

2003; eight of the 19 yielded plausible models. The plausible 

models demonstrate that the hypothesized NCMFP can be partially supported geochemically. However, developing plausible 

simulations of chemical evolution of water in wells sampled 

during different time periods and between wells not in the same 

aquifer (one in Trinity aquifer and one in Edwards aquifer) was 

problematic. Surface-water contribution to the ground water 

was greatest in the Trinity aquifer in wells adjacent to Medina 

Lake. Minor (9 to 23 percent) surface-water contributions were 

simulated in wells in the Edwards aquifer relatively removed 

from the lake, which indicates other surface-water sources 

(streams). The simulation results also indicate considerable 

contributions from regional ground-water flow. 

Ground-water samples from seven wells were used to 

evaluate the evolution of ground-water chemistry in the 

SCMFP. 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. As with the 

other hypothesized flow paths, the plausible models demonstrate that the hypothesized SCMFP can be partially supported 

geochemically. Simulations in the SCMFP involved minimal 

(less than 7 percent) contribution of Trinity aquifer water to 

Edwards aquifer water in this flow path. 

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Table 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—Continued.

Table 2 

 

 

Table 2 35 

Table 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. 

[USGS, U.S. Geological Survey; wu, water unfiltered; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter at 25 °C; °C, degrees Celsius; CaCO3, 

calcium carbonate; *, sample obtained for this study; dup., duplicate; --, not analyzed or not detected; N/A, not applicable; <, less than; E, estimated; μg/L, 

micrograms per liter; wf, water filtered; M, presence of material verified but not quantified; pptv, parts per trillion by volume; δD, delta deuterium; per mil, parts 

per thousand; δ18O, delta oxygen-18] – 

State well number or USGS 

site number 

(USGS name) 

Aquifer Date Time 

Carbon 

dioxide, 

wu 

(mg/L) 

Oxygen, 

dissolved 

(mg/L) 

Dissolved 

oxygen, wu 

(mg/L) 

Dissolved 

oxygen, 

(percent 

saturation) 

pH 

(standard 

units) 

Specific 

conductance 

(μS/cm) 

Temperature, 

water 

(°C) 

Hardness 

(mg/L as 

CaCO3) 

Calcium 

(mg/L) 

Northwestern Medina flow path 

TD–69–30–903 Edwards 12/17/2002* 1145 22 7.4 6.1 75 7.2 435 23.2 220 81.9 

12/17/2002dup* 1146 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–31–406 Edwards 12/31/2001 1555 -­ -­ -­ -­ 7.4 469 24.3 220 75 

(Medina2) 07/30/2002* 1000 -­ -­ -­ -­ -­ -­ -­ -­ -­

08/15/2003* 1315 -­ -­ -­ -­ -­ -­ -­ 260 87.9 

TD–69–31–603 Edwards 12/12/2001 0930 -­ -­ 8.1 88 -­ 625 17.5 -­ -­

11/26/2002* 1045 19 9.7 7.7 91 7 710 22 310 119 

11/26/2002dup* 1046 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–31–605 Trinity 12/19/2001 1100 -­ -­ 7.8 93 6.7 446 22.5 -­ -­

TD–69–31–708 Edwards 07/30/2002* 1040 -­ -­ -­ -­ 7.1 420 23 -­ -­

(Medina8) 08/21/2003* 1315 -­ -­ 5.8 72 6.5 -­ 24.1 230 75 

08/21/2003dup* 1316 -­ -­ 5.8 72 7.3 412 24.0 240 77.2 1

TD–69–32–201 Trinity 01/31/1996 1400 -­ -­ -­ -­ 7.2 1,140 12.5 670 136 

08/28/1996 1320 -­ -­ -­ -­ 6.9 1,130 23 650 132 1

TD–69–32–303 Trinity 12/11/1995 1510 -­ -­ -­ -­ 9.4 2,640 19.2 1,700 369 

08/15/1996 0945 -­ -­ -­ -­ 7 2,660 23.6 1,900 359 

08/15/1996dup 0946 -­ -­ -­ -­ 6.2 2,660 23.6 1,700 350 

TD–69–32–501 Trinity 12/11/1995 1620 -­ -­ -­ -­ 9.2 2,910 17.1 2,100 512 

08/15/1996 1130 -­ -­ -­ -­ 6.8 3,380 26.6 2,500 541 

TD–69–32–703 Edwards 07/29/2002* 1400 -­ -­ -­ -­ 7.1 416 23.8 -­ -­

(Medina1) 08/13/2003* 1440 -­ -­ -­ -­ -­ -­ -­ 220 85.8 

TD–69–37–302 Edwards 12/11/2001 1440 -­ -­ 6.3 73 6.9 489 22 -­ -­

TD–69–38–106 Edwards 12/11/2001 1210 -­ -­ 8 -­ 7 414 22.5 -­ -­

01/14/2003* 1300 22 5.8 7 83 7.2 446 22.8 220 75 

01/14/2003dup* 1301 -­ 5.8 7 83 7.2 415 22.8 220 75 

TD–69–38–509 Edwards 12/28/2001 1110 -­ -­ 5.8 68 7.1 432 21 -­ -­

11/26/2002* 1245 22 3.3 -­ -­ 7 458 21.5 230 75.7 

11/26/2002dup* 1246 -­ -­ -­ -­ -­ -­ -­ -­ -­

YP–69–37–101 Edwards 04/03/2002* 1200 -­ -­ -­ -­ 6.9 450 25 230 66.3 

12/05/2002* 1250 19 7.8 6.7 79 7.1 460 22.5 230 66.8 

12/05/2002dup* 1251 -­ -­ -­ -­ -­ -­ -­ -­ -­

North-central Medina flow path 

TD–68–25–402 Trinity 01/30/1996 1220 -­ -­ -­ -­ 7.4 -­ 20.5 210 55.9 

08/08/1996 1235 -­ -­ -­ -­ 7.2 454 21.8 230 62.6 

TD–68–25–510 Trinity 11/29/1995 1530 -­ -­ -­ -­ 7.2 495 -­ 260 73.3 

08/08/1996 1350 -­ -­ -­ -­ 7.2 509 20.9 260 73.8 

TD–68–25–701 Edwards 11/30/1995 1320 -­ -­ -­ -­ 6.5 613 24.7 340 68.8 

07/29/1996 1545 -­ -­ -­ -­ 6.8 608 25.5 280 58.4 

TD–68–25–711 Edwards 07/31/2002* 1140 -­ -­ -­ -­ 7 407 24.4 -­ -­

(Medina5) 08/12/2003* 1315 -­ -­ 5.3 74 7.2 440 30.5 220 82.4 

TD–68–25–806 Edwards 12/05/1995 1220 -­ -­ -­ -­ 6.9 415 22.8 210 80 

08/19/1996 1245 -­ -­ -­ -­ 6.8 413 23.9 220 84.5 

12/04/2002* 1325 23 3 4.7 56 7 402 22.5 210 78 

12/04/2002dup* 1326 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–25–807 Trinity 01/25/1996 1250 -­ -­ -­ -­ 7.1 1,360 21.8 930 210 

07/09/1996 1305 -­ -­ -­ -­ 7 2,160 22 1,600 359 

TD–68–25–809 Trinity 12/05/1995 1515 -­ -­ -­ -­ 7.3 2,060 19 1,400 536 

07/26/1996 1315 -­ -­ -­ -­ 7.3 2,050 22.4 1,400 498 

TD–68–25–810 Trinity 03/06/1996 1310 -­ -­ -­ -­ 7.3 444 22.1 220 80.5 

08/19/1996 1545 -­ -­ -­ -­ 6.4 465 22.5 230 83.7 

02/20/2003* 1220 -­ -­ 6.1 75 6.8 417 23 220 73.2 

02/20/2003dup* 1221 -­ -­ -­ -­ -­ -­ -­ -­ -­

1 Probably northwestern Medina flow path. 
36 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

Table 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—Continued. 

State well number or USGS 

site number 

(USGS name) 

Aquifer Date Time 

Carbon 

dioxide, 

wu 

(mg/L) 

Oxygen, 

dissolved 

(mg/L) 

Dissolved 

oxygen, wu 

(mg/L) 

Dissolved 

oxygen, 

(percent 

saturation) 

pH 

(standard 

units) 

Specific 

conductance 

(μS/cm) 

Temperature, 

water 

(°C) 

Hardness 

(mg/L as 

CaCO3) 

Calcium 

(mg/L) 

North-central Medina flow path—Continued 

TD–68–25–822 Edwards 12/29/2001 1400 -­ -­ 5.1 61 6.9 440 23 -­ -­

TD–69–32–905 Edwards 10/14/2003* 1140 -­ -­ -­ -­ -­ -­ -­ 260 97.4 

TD–69–38–601 Edwards 03/17/1992 1340 -­ -­ -­ -­ 7.3 437 22.5 230 75 

(Seco) 12/20/2001 1300 61 -­ -­ -­ 6.8 436 -­ 230 73 

TD–69–38–709 Edwards 12/31/2001 1400 -­ -­ 5.8 64 7 435 18.5 -­ -­

12/12/2002* 1430 18 5.3 5.2 60 7 427 20 220 68.3 

12/12/2002dup* 1431 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–504 Edwards 12/18/2001 1450 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–520 Edwards 10/31/2003* 1100 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–40–102 Edwards 12/20/2001 1230 -­ -­ -­ -­ -­ -­ -­ -­ -­

(Quihi) 

TD–69–40–202 Edwards 03/13/1996 1145 -­ -­ -­ -­ 7.2 450 23.4 240 79.4 

(Medina3) 03/13/1996dup 1146 -­ -­ -­ -­ 7.2 450 23.4 240 79.7 

08/20/1996 1320 -­ -­ -­ -­ 7.1 450 25 250 87.1 

TD–69–40–209 Edwards 07/29/2002* 1415 -­ -­ -­ -­ 7.1 406 24.1 -­ -­

07/29/2002dup* 1416 -­ -­ -­ -­ -­ -­ -­ -­ -­

08/20/2003* 1205 -­ -­ 6.2 77 6.7 496 24.7 260 84.8 

TD–69–40–301 Edwards 01/24/1996 1045 -­ -­ -­ -­ 7.2 428 24.1 230 67.5 

08/21/1996 1440 -­ -­ -­ -­ 7.3 415 25 250 75.7 

South-central Medina flow path 

TD–68–25–904 Trinity 03/06/1996 1130 -­ -­ -­ -­ 7.1 810 21.5 430 81.4 

07/26/1996 1210 -­ -­ -­ -­ 7.3 779 22.9 390 73.2 

TD–68–33–102 Edwards 07/30/2002* 1300 -­ -­ -­ -­ 7.1 431 22.9 -­ -­

TD–68–33–103 Edwards 02/21/1996 1150 -­ -­ -­ -­ 7 486 22.5 250 90.8 

08/06/1996 1510 -­ -­ -­ -­ 7 469 22.8 250 88.3 

TD–68–33–204 Edwards 12/13/1995 1250 -­ -­ -­ -­ 7.2 446 22.7 230 77.7 

08/21/1996 1200 -­ -­ -­ -­ 7.2 410 25.1 240 80 

TD–68–33–213 Edwards 07/26/2002* 1330 -­ -­ -­ -­ 7.3 512 22.2 -­ -­

(Medina7) 08/11/2003* 1352 -­ -­ -­ -­ -­ -­ -­ 250 89.1 

TD–69–40–510 Edwards 07/29/2002* 1230 -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina4) 07/29/2002dup* 1231 -­ -­ -­ -­ -­ -­ -­ -­ -­

08/20/2003* 1630 -­ -­ -­ -­ -­ -­ -­ 260 89.4 

TD–69–40–604 Edwards 12/05/1995 1400 -­ -­ -­ -­ 6.9 451 23.3 230 62.6 

08/21/1996 1150 -­ -­ -­ -­ 6.9 455 23 250 66.6 

Flow path unknown (well east of Medina Lake/River) 

AS–68–25–505 Trinity 12/15/1995 1100 -­ -­ -­ -­ -­ -­ -­ -­ -­

07/30/1996 1200 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–25–602 Trinity 02/08/1996 1110 -­ -­ -­ -­ 7 3,160 23.9 2,300 466 

07/30/1996 1340 -­ -­ -­ -­ 6.7 3,160 24.9 2,200 491 

TD–68–25–603 Trinity 02/08/1995 1200 -­ -­ -­ -­ -­ -­ -­ -­ -­

07/30/1996 1415 -­ -­ -­ -­ -­ -­ -­ -­ -­

Medina Lake near San Antonio 

08179500 (AL) N/A 05/22/1995 1020 -­ -­ -­ -­ 8 403 24 190 51 

05/22/1995dup 1021 -­ -­ -­ -­ 8 403 24 200 49.8 

04/02/1996 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

08179500 (DC) N/A 04/02/1996 1205 -­ -­ -­ -­ 8.2 420 15.5 241 52 

Medina River below Medina Lake 

08179520 N/A 05/18/1995 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

11/30/1995 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

04/03/1996 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

05/31/1996 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

08/06/1996 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­
Table 2 37 

Table 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—Continued. 

State well number or USGS 

site number 

(USGS name) 

Magnesium 

(mg/L) 

Potassium 

(mg/L) 

Sodium 

adsorption 

ratio 

Sodium 

(mg/L) 

Sodium 

(percent) 

Alkalinity 

(mg/L as 

CaCO3) 

Bicarbonate 

(mg/L) 

Carbonate 

(mg/L) 

Bromide 

(mg/L) 

Chloride 

(mg/L) 

Fluoride 

(mg/L) 

Silica 

(mg/L) 

Sulfate 

(mg/L) 

Residue 

(sum of 

constituents) 

(mg/L) 

Dissolved 

ammonia 

plus 

organic 

nitrogen 

(mg/L) 

Dissolved 

ammonia 

nitrogen 

(mg/L) 

Northwestern Medina flow path 

TD–69–30–903 3.63 0.37 0.1 4.79 5 209 254 <1 -­ 8.06 <0.17 11.3 3.8 245 E0.05 <0.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–31–406 9 1.2 .1 4 4 196 239 -­ -­ 6 .2 13 25 251 -­ -­

(Medina2) -­ -­ -­ -­ -­ 190 230 -­ -­ -­ -­ -­ -­ -­ -­ -­

8.67 1.51 .1 4.67 4 217 264 -­ -­ 6.28 <.2 12.2 31.4 -­ <.10 <.04 

TD–69–31–603 -­ -­ -­ -­ -­ 180 218 0 -­ -­ -­ -­ -­ -­ -­ -­

3.39 .9 .3 12 8 168 204 <1 -­ 45.6 <.17 11.4 7.8 415 .1 <.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–31–605 -­ -­ -­ -­ -­ 242 294 0 -­ -­ -­ -­ -­ -­ -­ -­

TD–69–31–708 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina8) 10.2 1.13 .2 5.75 5 187 -­ -­ -­ 8.85 .2 12.4 19 248 <.10 <.04 

10.3 1.16 .2 5.78 5 -­ -­ -­ -­ 8.79 .2 12.5 18.7 -­ <.10 <.04 1

TD–69–32–201 77.1 3.98 .1 8.56 3 270 329 -­ 0.1 19.5 .57 13.2 335 772 .2 .177 

74.2 4.2 .2 10 3 260 317 -­ <.5 21 1.3 14.1 706 1,130 <.10 <.100 1

TD–69–32–303 235 16.6 .3 33.7 -­ 240 293 -­ .7 25.1 5.9 13.4 1,590 2,440 -­ 0 

205 16.3 .35 34.5 -­ 250 305 -­ .45 22 4.54 12.8 1,510 2,335 -­ .335 

190 17.0 .3 28 -­ 240 -­ -­ -­ 22 3.6 11 1,500 2,280 -­ -­

TD–69–32–501 208 19.3 .2 17.5 2 240 -­ -­ .2 14 3.24 11.9 1,740 2,680 .62 .577 

282 22.2 .3 30.4 3 220 -­ -­ .6 22.3 7.2 14.1 2,270 3,330 .1 .16 

TD–69–32–703 -­ -­ -­ -­ -­ 222 270 0 -­ -­ -­ -­ -­ -­ -­ -­

(Medina1) 1.86 .21 .1 4.31 4 220 269 -­ -­ 4.69 <.2 12.2 6.8 -­ <.10 <.04 

TD–69–37–302 -­ -­ -­ -­ -­ 221 269 0 -­ -­ -­ -­ -­ -­ -­ -­

TD–69–38–106 -­ -­ -­ -­ -­ 197 240 0 -­ -­ -­ -­ -­ -­ -­ -­

8.71 1.02 .2 5.96 5 196 238 <1 -­ 8.28 <.17 12.3 12.2 247 <.10 <.04 

8.72 1.01 .2 6.02 6 200 243 <1 -­ 8.67 <.17 12.3 12.3 250 <.10 <.04 

TD–69–38–509 10.5 1.01 .2 5.95 5 202 246 <1 -­ 8.73 <.17 12 17.1 256 <.10 <.04 

-­ -­ -­ -­ -­ 205 250 0 -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

YP–69–37–101 14.5 1.08 .2 6.83 6 202 246 0 -­ 9.82 .2 12.5 21.6 259 <.10 <.04 

15.3 1.1 .2 7.09 6 199 242 <1 -­ 10.9 .17 12.4 20.8 258 <.10 <.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

North-central Medina flow path 

TD–68–25–402 16.9 1.71 .2 7.72 7 140 171 -­ .1 11.1 .15 9 55.4 243 .11 .074 

17.2 1.67 .3 9.4 8 160 195 -­ .1 13 .2 11.6 58.9 270 <.01 <.010 

TD–68–25–510 17.4 1.72 .2 8.63 7 160 195 -­ .1 13.7 .17 15.2 79 308 .04 .017 

17.2 1.59 .2 9.02 7 160 195 -­ .1 15 .21 13 80.8 308 .04 <.010 

TD–68–25–701 39.1 2.12 0 .27 0 200 244 -­ .1 8.5 .64 12.1 127 380 <.01 <.010 

31.2 1.48 .1 2.87 2 190 232 -­ -­ 8.2 .68 12.6 112 347 <.01 <.010 

TD–68–25–711 -­ -­ -­ -­ -­ 216 262 -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina5) 3.25 .83 .1 4.39 4 214 260 0 -­ 7.7 <.2 13.4 2.1 242 -­ -­

TD–68–25–806 2.55 <1.00 .1 2.1 -­ 200 244 -­ <.1 6.5 <.05 12 3.7 -­ <.01 <.010 

2.5 <1.00 .2 5.3 -­ 190 232 -­ <.5 7.2 <1.00 14.1 6 -­ <.10 .18 

2.52 .45 .1 4.17 4 205 249 <1 -­ 6.26 <.17 11.7 7.7 237 <.10 <.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–25–807 97 6.94 .2 13 3 180 220 -­ .1 13.4 1.05 9.7 698 1,170 <.01 1.14 

168 12.1 .2 18.3 2 180 220 -­ .1 12 2.91 11.6 1,330 2,030 .39 -­

TD–68–25–809 25.1 1.41 0 4.13 0 180 220 -­ .1 14.2 1.63 11.6 1,120 1,830 <.01 <.010 

33.5 1.72 0 4.2 0 190 232 -­ -­ 9.9 .29 12.5 1,070 1,750 .04 .584 

TD–68–25–810 5.19 <1.00 .2 7.11 -­ 190 232 -­ .1 13.3 .06 10.4 7.9 -­ <.01 <.010 

6 <1.00 .2 7.8 -­ 190 232 -­ <.5 15 <1.00 11.9 15 -­ <.10 .17 

8.01 .38 .2 5.11 5 191 232 <1 -­ 7.49 .11 12.1 7.2 232 E.05 <.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

1 Probably northwestern Medina flow path. 
38 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

Table 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—Continued. 

State well number or USGS 

site number 

(USGS name) 

Magnesium 

(mg/L) 

Potassium 

(mg/L) 

Sodium 

adsorption 

ratio 

Sodium 

(mg/L) 

Sodium 

(percent) 

Alkalinity 

(mg/L as 

CaCO3) 

Bicarbonate 

(mg/L) 

Carbonate 

(mg/L) 

Bromide 

(mg/L) 

Chloride 

(mg/L) 

Fluoride 

(mg/L) 

Silica 

(mg/L) 

Sulfate 

(mg/L) 

Residue 

(sum of 

constituents) 

(mg/L) 

Dissolved 

ammonia 

plus 

organic 

nitrogen 

(mg/L) 

Dissolved 

ammonia 

nitrogen 

(mg/L) 

North-central Medina flow path—Continued 

TD–68–25–822 -­ -­ -­ -­ -­ 217 264 0 -­ -­ -­ -­ -­ -­ -­ -­

TD–69–32–905 4.52 1.41 0.2 6.66 5 206 251 -­ -­ 16.3 <0.2 13.7 12.7 -­ E0.07 <0.04 

TD–69–38–601 12 .8 .2 6.5 6 -­ -­ -­ -­ 11 .1 11 18 257 -­ -­

(Seco) 10 .9 .2 5.3 5 210 256 -­ -­ 10 .2 11 17 253 -­ -­

TD–69–38–709 -­ -­ -­ -­ -­ 198 241 0 -­ -­ -­ -­ -­ -­ -­ -­

13 .92 .2 5.39 5 205 249 <1 -­ 9.75 <.17 11.5 13.6 250 <.10 <.04 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–504 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–520 -­ -­ -­ -­ -­ 217 265 -­ -­ -­ -­ -­ -­ -­ E.06 <.04 

TD–69–40–102 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

(Quihi) 

TD–69–40–202 9.5 1.02 .2 5.45 5 190 232 -­ 0.1 9.1 .11 11.3 23.1 256 <.01 <.010 

(Medina3) 9.52 <1.00 .2 6.28 -­ 190 -­ -­ .1 9.2 .11 11.2 23.1 -­ <.10 -­

8.5 <1.00 .2 8.5 -­ 202 246 -­ <.5 14 <1.00 16 9.5 -­ <.10 <.100 

TD–69–40–209 -­ -­ -­ -­ -­ 195 237 0 -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

12.3 .97 .2 7.82 6 218 265 0 -­ 10.1 .2 13.1 17.5 292 <.10 <.04 

TD–69–40–301 15.4 <1.00 .2 5.44 -­ 210 256 -­ .1 8.2 .11 12.6 12.2 -­ <.01 <.010 

14.6 <1.00 .1 5.4 -­ 210 256 -­ <.5 10.6 <1.00 13.6 15.2 -­ <.10 <.100 

South-central Medina flow path 

TD–68–25–904 54.9 3.28 .2 9.19 4 200 244 -­ .1 9.1 .75 11.2 263 557 .07 <.010 

50.4 2.66 .1 5.91 3 200 244 -­ -­ 8.1 .46 12.5 198 475 .04 .12 

TD–68–33–102 -­ -­ -­ -­ -­ 228 277 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–33–103 5.32 -­ -­ -­ -­ 210 256 -­ .1 12.9 .09 12.9 17.9 -­ .06 .017 

7.52 <1.00 -­ <6.17 -­ 190 232 -­ .1 11.6 .08 11.9 24 -­ <.01 <.010 

TD–68–33–204 9.45 <1.00 .2 5.48 -­ 200 244 -­ .1 11.2 .08 12.1 20.9 -­ <.01 <.010 

10 <1.00 .2 6.5 -­ 190 232 -­ <.5 12.3 <1.00 14.1 22.2 -­ <.10 <.100 

TD–68–33–213 -­ -­ -­ -­ -­ 228 277 -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina7) 5.56 .56 .1 5.3 4 224 273 -­ -­ 9.58 <.2 11.4 13.3 -­ -­ -­

TD–69–40–510 -­ -­ -­ -­ -­ 154 186 -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina4) -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

8.42 1.17 .2 7.13 6 218 266 -­ -­ 11.5 <.2 12.1 16.2 -­ <.10 <.04 

TD–69–40–604 17.9 <1.00 0 .99 -­ 180 220 -­ .1 10.1 .13 11.8 34.8 -­ <.01 <.010 

19.1 1.1 .2 6.1 5 180 220 -­ <.5 11.3 <1.00 11.9 34.8 261 <.10 <.100 

Flow path unknown (well east of Medina Lake/River) 

AS–68–25–505 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–25–602 275 15.6 .1 12.7 1 180 220 -­ .1 12.4 5.69 10.9 2,030 2,950 .53 .687 

238 14.1 .1 10.5 1 180 220 -­ -­ 10 4.21 12.2 1,750 2,650 .03 .666 

TD–68–25–603 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

Medina Lake near San Antonio 

08179500 (AL) 16 1.7 .2 7.7 -­ -­ -­ -­ -­ 12 .2 8.1 49 -­ -­ .1 

17.1 1.68 .3 7.7 -­ -­ -­ -­ .1 12 .25 9 57 236 -­ .01 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

08179500 (DC) 16 1.4 -­ 3.8 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

Medina River below Medina Lake 

08179520 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­
 

 

State well number or USGS 

site number 

(USGS name) 

Dissolved	 

nitrite plus 

nitrate	 

nitrogen 

(mg/L)	 

Dissolved	 

organic	 

nitrogen	 

(mg/L)	 

Total 

nitrogen 

(mg/L) 

Dissolved 

orthophosphate phosphorus 

(mg/L) 

Dissolved 

phosphorus 

(mg/L) 

Aluminum 

(μg/L) 

Antimony 

(μg/L) 

Argon 

(mg/L) 

Arsenic 

(μg/L) 

Barium,	 

wf 

(μg/L)	 

Beryllium, 

wf 

(μg/L) 

Cadmium, 

wf 

(μg/L) 

Chromium, 

wf

(μg/L) 

Cobalt, 

wf

(μg/L) 

Northwestern Medina flow path 

TD–69–30–903 1.1 -- 19.4 <0.02 <0.04 <2 <0.30 0.617 -- 28 <0.06 0.51 <0.8 0.16 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

TD–69–31–406 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

(Medina2) -- -- -- -- -- -- -- -- -- -- -- -- -- -­

1.25 -- -- <.02 <.04 M <.30 -- -- 32 <.06 <.04 <.8 .31 

TD–69–31–603 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

25.8 --	 23.4 <.02 <.04 23 <.30 .681 -- 59 <.06 <.04 <.8 .21 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

TD–69–31–605 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

TD–69–31–708 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

(Medina8) .8 -- -- <.18 <.04 <2 <.30 -- -- 25 <.06 <.04 <.8 .16 

.8 -- -- <.18 <.04 <2 <.30 -- -- 25 <.06 <.04 <.8 .16 1TD–69–32–201 -- 0.02 -- -- -- -- -- -- <1.0 27 <1 <1 10 M 

-- -- -- -- -- -- -- -- <5.0 28 <1 <1 40 <1 1TD–69–32–303 -- -- -- -- -- -- -- -- -- 12 <1 <1 210 M 

-- -- -- -- -- -- -- -- <5.0 9.9 -- <1 210 M 

-- -- -- -- -- -- -- -- <1 <9 1.5 <24 <15 M 

TD–69–32–501 -- .05 -- -- -- -- -- -- <1.0 5.2 <1 <1 M M 

-- -- -- -- -- -- -- -- <5.0 2.3 <1 <1 320 M 

TD–69–32–703 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

(Medina1) 1.23 -- -- <.18 <.04 <2 <.30 -- -- 25 <.06 <.04 <.8 .36 

TD–69–37–302 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

TD–69–38–106 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

1.39 -- 19.9 <.02 <.04 <2 <.30 .641 -- 31 <.06 <.04 <.8 .14 

1.38 --	 -- <.02 <.04 <2 <.30 -- -- 31 <.06 <.04 <.8 .14 

TD–69–38–509	 -- -- -- -- -- -- -- -- -- -- -- -- -- -­

.96 -- 19.4 E.01 <.04 <2 <.30 .638 -- 30 <.06 <.04 <.8 .15 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

YP–69–37–101	 .97 -- -- <.02 <.06 <1 <.05 -- -- 35 <.06 E.03 <.8 .17 

.99 -- 23.2 E.01 <.04 <2 <.30 .707 -- 34 <.06 E.03 <.8 .11 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

North-central Medina flow path 

TD–68–25–402 -- .04 -- -- -- -- -- -- <1.0 25.6 <1 <1 M <1 

-- -- -- -- -- -- -- -- 1 26.2 <1 <1 M <1 

TD–68–25–510 -- .02 -- -- -- -- -- -- 2 26.6 <1 <1 10 <1 

-- -- -- -- -- -- -- -- 1.7 27.5 <1 <1 M <1 

TD–68–25–701 -- -- -- -- -- -- -- -- <1.0 32.6 <1 <1 30 -­

-- -- -- -- -- -- -- -- <1.0 34.2 <1 <1 M 1.37 

TD–68–25–711 -- -- -- -- -- -- -- -- -- -- -- -- -- <1 

(Medina5) -- -- -- -- -- M <.30 -- -- 28 <.06 .04 <.8 <1 

TD–68–25–806	 -- -- -- -- -- -- -- -- <1.0 26 <1 <1 20 <1 

-- -- -- -- -- -- -- -- <5.0 28 <1 <1 <1 <1 

.69 -- 20.2 <.02 <.04 <2 <.30 .629 -- 29 <.06 <.04 <.8 .15 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

TD–68–25–807 -- -- -- -- -- -- -- -- <1.0 9.9 <1 <1 M M 

-- -- -- -- -- -- -- -- 1.3 11.2 -- <1 -- -­

TD–68–25–809 -- -- -- -- -- -- -- -- <1.0 50.6 <1 <1 20 M 

-- -- -- -- -- -- -- -- <1.0 48.8 <1 <1 M M 

TD–68–25–810	 -- -- -- -- -- -- <1 -- <1.1 30.8 <1 <1 M <1 

-- -- -- -- -- -- -- -- <5.0 32 <1 <1 <1 <1 

.89 -- -- <.02 <.04 <2 <.30 -- -- 27 <.06 E.02 <.8 .16 

-- -- -- -- -- -- -- -- -- -- -- -- -- --

Table 2 39 

Table 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—Continued. 

1 Probably northwestern Medina flow path. 
40 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

Table 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—Continued. 

State well number or USGS 

site number 

(USGS name) 

Dissolved 

nitrite plus 

nitrate 

nitrogen 

(mg/L) 

Dissolved 

organic 

nitrogen 

(mg/L) 

Total 

nitrogen 

(mg/L) 

Dissolved 

orthophosphate phosphorus 

(mg/L) 

Dissolved 

phosphorus 

(mg/L) 

Aluminum 

(μg/L) 

Antimony 

(μg/L) 

Argon 

(mg/L) 

Arsenic 

(μg/L) 

Barium, 

wf 

(μg/L) 

Beryllium, 

wf 

(μg/L) 

Cadmium, 

wf 

(μg/L) 

Chromium, 

wf 

(μg/L) 

Cobalt, 

wf 

(μg/L) 

North-central Medina flow path—Continued 

TD–68–25–822 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–32–905 7.94 -­ -­ <0.02 <0.04 M <0.20 -­ -­ 54 <0.06 0.1 <0.8 0.21 

TD–69–38–601 -­ -­ -­ -­ -­ -­ -­ -­ <1 27 -­ <1.0 <1 -­

(Seco) 1.4 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–38–709 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

1.21 -­ 21.6 E.01 <.04 <2 <.30 0.663 -­ 49 <.06 .08 <.8 .11 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–504 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–39–520 5.24 -­ -­ <.02 <.04 -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–40–102 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

(Quihi) 

TD–69–40–202 -­ -­ -­ -­ -­ -­ <1 -­ <1.0 31.2 <1 <1 20 <1 

(Medina3) -­ -­ -­ -­ -­ -­ <1 -­ <1.0 30.6 <1 <1 20 <1 

-­ -­ -­ -­ -­ -­ -­ -­ <5.0 38 <1 <1 <1 <1 

TD–69–40–209 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

3.39 -­ -­ <.18 <.04 <2 <.30 -­ -­ 37 <.06 <.04 <.8 .18 

TD–69–40–301 -­ -­ -­ -­ -­ -­ -­ -­ <1.0 35.4 <1 <1 M <1 

-­ -­ -­ -­ -­ -­ -­ -­ <5.0 38 <1 <1 <1 <1 

South-central Medina flow path 

TD–68–25–904 -­ -­ -­ -­ -­ -­ <1 -­ <1.0 26.9 <1 <1 M <1 

-­ -­ -­ -­ -­ -­ -­ -­ <1.0 24.7 <1 <1 M <1 

TD–68–33–102 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–33–103 -­ 0.04 -­ -­ -­ -­ <1 -­ <1.0 53.4 <1 <1 M <1 

-­ -­ -­ -­ -­ -­ -­ -­ <1.0 37.5 <1 <1 M <1 

TD–68–33–204 -­ -­ -­ -­ -­ -­ -­ -­ 1.2 32.8 <1 <1 M <1 

-­ -­ -­ -­ -­ -­ -­ -­ <5.0 37 <1 <1 <1 <1 

TD–68–33–213 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina7) -­ -­ -­ -­ -­ <2 <.30 -­ -­ 35 <.06 <.04 <.8 .34 

TD–69–40–510 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

(Medina4) -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

1.5 -­ -­ <.18 <.04 <2 <.30 -­ -­ 34 <.06 <.04 <.8 .18 

TD–69–40–604 -­ -­ -­ -­ -­ -­ -­ -­ <1.0 27.1 <1 <1 20 <1 

-­ -­ -­ -­ -­ -­ -­ -­ <5.0 30 <1 <1 <1 <1 

Flow path unknown (well east of Medina Lake/River) 

AS–68–25–505 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–68–25–602 -­ -­ -­ -­ -­ -­ -­ -­ <2.0 8.5 <2 <2 M M 

-­ -­ -­ -­ -­ -­ -­ -­ <1.0 8.4 <1 <1 M M 

TD–68–25–603 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

Medina Lake near San Antonio 

08179500 (AL) <.05 <.01 .20 -­ -­ -­ -­ -­ <1 27 <.5 <8 <5 <3 

-­ <.01 .29 -­ -­ -­ <8 -­ <2 24 <1 <2 <8 <8 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

08179500 (DC) -­ -­ -­ -­ -­ -­ -­ -­ -­ 26 <1 <1 27 1.8 

Medina River below Medina Lake 

08179520 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­
 

Table 2 41 

Table 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—Continued. 

State well number or USGS 

site number 

(USGS name) 

Copper, 

wf 

(μg/L) 

Iron, 

wf 

(μg/L) 

Lead, 

wf 

(μg/L) 

Lithium, 

wf 

(μg/L) 

Manganese, 

wf 

(μg/L) 

Mercury, 

wf 

(mg/L) 

Molybdenum, 

wf 

(μg/L) 

Nickel, 

wf 

(μg/L) 

Selenium, 

wf 

(μg/L) 

Silver, 

wf 

(μg/L) 

Strontium, 

wf 

(μg/L) 

Vanadium, 

wf 

(μg/L) 

Zinc, 

wf 

(μg/L) 

Sulfur 

hexafluoride 

(pptv) 

δD 

(per mil) 

δ18

O 

(per mil) 

Uranium, 

wf 

(μg/L) 

Northwestern Medina flow path 

TD–69–30–903 1.2 -­ 4.82 -­ 0.2 -­ 0.4 2.21 -­ <0.20 -­ -­ 868 2.25 -25.4 -4.28 0.42 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 2.03 -­ -­ -­

TD–69–31–406 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.1 -4.33 -­

(Medina2) -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -30.1 -5.12 -­

.8 -­ <.08 -­ 9.4 -­ 1.7 5.69 -­ <.20 -­ -­ 1 -­ -28 -4.81 .49 

TD–69–31–603 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -23.1 -4.02 -­

1.4 -­ .27 -­ .4 -­ <.3 3.79 -­ <.20 -­ -­ 23 .94 -26.8 -4.64 .42 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 1.16 -­ -­ -­

TD–69–31–605 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -25 -4.38 -­

TD–69–31–708 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.5 -4.21 -­

(Medina8) .3 -­ <.08 -­ <.2 -­ .8 2.75 -­ <.20 -­ -­ <1 -­ -23.4 -3.91 .54 

.3 -­ <.8 -­ <.2 -­ .8 2.62 -­ <.2 -­ -­ 1.4 -­ -­ -­ -­ 1

TD–69–32–201 10 50 <1 M 20 -­ 4 M -­ <1 14,500 3.2 1,060 -­ -27 -4.51 -­

10 170 <1 <20 10 -­ 4 M -­ <1 15,400 <1.0 550 -­ -23.5 -4.31 -­ 1

TD–69–32–303 <2 760 <1 80 50 -­ <1 M -­ <1 10,900 <1 70 -­ -24.7 -4.63 -­

<2 3,015 -­ 75 55 -­ <1 M -­ <1 11,100 3.1 110 -­ -27.1 -4.63 -­

-­ 720 <300 69 46 -­ <30 <30 -­ <1 9,500 <18 45 -­ -­ -­ -­

TD–69–32–501 <1 5,010 <1 80 30 -­ <1 M <5 <1 11,700 2.8 290 -­ -27.8 -4.69 -­

<2 810 <1 110 10 -­ <1 M -­ <1 11,600 <1.0 140 -­ -24.7 -4.79 -­

TD–69–32–703 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -22.2 -4.51 -­

(Medina1) .4 -­ <.08 -­ .8 -­ .8 5.26 -­ <.20 -­ -­ 59 -­ -26 -4.53 .57 

TD–69–37–302 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.3 -4.52 -­

TD–69–38–106 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.6 -4.27 -­

1.5 -­ .17 -­ <.2 -­ .5 2.52 -­ <.20 -­ -­ 21 .50 -24.1 -4.33 .64 

1.6 -­ .21 -­ <.2 -­ .6 2.52 -­ <.20 -­ -­ 21.5 .37 -­ -­ .64 

TD–69–38–509 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.9 -4.18 -­

26 -­ .83 -­ .4 -­ .5 3.59 -­ <.20 -­ -­ 15 .63 -22.4 -4.2 .64 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ .66 -­ -­ -­

YP–69–37–101 1 -­ 3.14 -­ .1 -­ .5 .72 -­ <1 -­ -­ 426 -­ -24.9 -3.96 .73 

1.1 -­ 2.41 -­ E.1 -­ .5 2.45 -­ <.20 -­ -­ 482 1.51 -21.9 -3.98 .67 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 1.52 -­ -­ -­

North-central Medina flow path 

TD–68–25–402 M <5 <1 <1 <1 -­ 1 M -­ <1 600 4 10 -­ -9.9 -1.08 -­

<1 <5 <1 M M -­ 2 M -­ <1 630 3.7 30 -­ -7 -.88 -­

TD–68–25–510 M 40 <1 M 10 -­ 2 M <5 <1 630 5.7 230 -­ -9.1 -1.48 -­

M <5 <1 M M -­ 2 M -­ <1 570 4.3 160 -­ -5.3 -1.06 -­

TD–68–25–701 <1 <5 <1 10 M -­ 2 M -­ <1 3,280 8.8 150 -­ -21.8 -3.99 -­

<1 20 <1 10 M -­ 2 M -­ <1 2,390 2.6 170 -­ -23.2 -3.97 -­

TD–68–25–711 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -23.1 -4.28 -­

(Medina5) .7 -­ <.08 -­ 5.1 -­ 3.2 14.2 -­ <.20 -­ -­ 10 -­ -24.2 -4.25 .45 

TD–68–25–806 M <5 M <1 <1 -­ <1 M -­ <1 80 7.2 2,400 -­ -26.1 -4.31 -­

<2 <2 <1 <20 <1 -­ <1 M -­ <1 80 2.5 660 -­ -25.3 -4.24 -­

3.9 -­ 1.21 -­ 1.3 -­ E.3 3.17 -­ <.20 -­ -­ 1,090 .74 -23.7 -4.37 .62 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ .62 -­ -­ -­

TD–68–25–807 <1 40 <1 30 M -­ 1 M -­ <1 4,970 2.4 10 -­ -15.2 -2.35 -­

<1 30 <1 70 M -­ -­ -­ -­ -­ 7,450 -­ 20 -­ -16 -2.98 -­

TD–68–25–809 <1 1,820 <1 M M -­ 3 M -­ <1 3,590 4.8 90 -­ -22.9 -3.89 -­

<1 1,380 <1 M M -­ 4 M -­ <1 3,020 1.9 70 -­ -23.3 -3.9 -­

TD–68–25–810 M <5 M <1 <1 -­ <1 M -­ <1 80 3.7 240 -­ -26.3 -4.53 -­

M <5 <1 <20 <1 -­ <1 M -­ <1 90 2.6 300 -­ -25 -4.54 -­

.9 -­ .99 -­ .2 -­ .8 3.46 -­ <.20 -­ -­ 285 2.17 -22.7 -4.13 .82 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 2.42 -­ -­ -­

1 Probably northwestern Medina flow path. 
42 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

State well number or USGS 

site number 

(USGS name) 

Copper, 

wf 

(μg/L) 

Iron, 

wf 

(μg/L) 

Lead, 

wf 

(μg/L) 

Lithium, 

wf 

(μg/L) 

Manganese, 

wf 

(μg/L) 

Mercury, 

wf 

(mg/L) 

Molybdenum, 

wf 

(μg/L) 

Nickel, 

wf 

(μg/L) 

Selenium, 

wf 

(μg/L) 

Silver, 

wf 

(μg/L) 

Strontium, 

wf 

(μg/L) 

Vanadium, 

wf 

(μg/L) 

Zinc, 

wf 

(μg/L) 

Sulfur 

hexafluoride 

(pptv) 

δD 

(per mil) 

δ18

O 

(per mil) 

Uranium, 

wf 

(μg/L) 

North-central Medina flow path—Continued 

TD–68–25–822 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.6 -4.35 -­

TD–69–32–905 8.6 -­ 0.98 -­ 0.5 -­ 33.3 1.75 -­ <0.20 -­ -­ 15 -­ -­ -­ 0.59 

TD–69–38–601 7 29 2 -­ 13 -­ -­ -­ <1 <1.0 -­ -­ 17 -­ -­ -­ -­

(Seco) -­ 20 -­ -­ -­ -­ -­ -­ -­ -­ 260 -­ -­ -­ -22 -4.08 -­

TD–69–38–709 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -21.9 -4.14 -­

2.5 -­ .17 -­ .7 -­ .5 3.28 -­ <.20 -­ -­ 90 0.22 -23.2 -4.15 .68 

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ .23 -­ -­ -­

TD–69–39–504 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -23.4 -4.24 -­

TD–69–39–520 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­

TD–69–40–102 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.9 -4.56 -­

(Quihi) 

TD–69–40–202 M <5 <1 <1 <1 -­ <1 M -­ <1 250 6.6 M -­ -19.4 -3.23 -­

(Medina3) M <5 <1 <1 <1 -­ <1 M -­ <1 240 7.2 M -­ -18.6 -3.24 -­

M <5 M <20 <1 -­ <1 M -­ <1 160 3 30 -­ -24.2 -4.01 -­

TD–69–40–209 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 5.5 -24.2 -3.84 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 5.3 -­ -­ -­

.5 -­ <.08 -­ E.1 -­ .5 2.92 -­ <.20 -­ -­ M -­ -22.9 -3.94 .88 

TD–69–40–301 M <5 M <1 <1 -­ <1 M -­ <1 220 4.9 310 -­ -23.7 -3.93 -­

M <5 M <20 M -­ <1 M -­ <1 230 2.5 2,300 -­ -23.3 -3.88 -­

South-central Medina flow path 

TD–68–25–904 <1 20 <1 10 10 -­ 4 M -­ <1 3,650 1.8 40 -­ -21.9 -4.14 -­

<1 <5 <1 10 M -­ 3 M -­ <1 3,180 1.3 70 -­ -22 -4.21 -­

TD–68–33–102 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -20.9 -3.26 -­

TD–68–33–103 M <5 <1 <1 <1 -­ <1 M -­ <1 220 3.7 M -­ -20.9 -3.67 -­

M <5 <1 <1 <1 -­ <1 M -­ <1 280 4.6 M -­ -19.5 -3.46 -­

TD–68–33–204 M 10 <1 M M -­ <1 M <5 <1 410 4.1 360 -­ -21.2 -3.51 -­

<2 <5 <1 <20 M -­ <1 M -­ <1 390 2 510 -­ -21.2 -3.36 -­

TD–68–33–213 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -24.2 -4.06 -­

(Medina7) 1.2 -­ <.08 -­ E.1 -­ .4 3.16 -­ <.20 -­ -­ M -­ -23.9 -4.03 .65 

TD–69–40–510 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 2.8 -20.1 -3.73 -­

(Medina4) -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ 2.7 -­ -­ -­

.3 -­ <.08 -­ .5 -­ .5 3.2 -­ <.20 200 -­ <1 -­ -22 -3.74 .68 

TD–69–40–604 M 20 <1 M M -­ <1 M -­ <1 590 5.4 270 -­ -17.9 -3.11 -­

<2 <5 <1 <20 M -­ <1 M -­ <1 560 1.4 260 -­ -20.2 -3.15 -­

Flow path unknown (well east of Medina Lake/River) 

AS–68–25–505 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -16.1 -2.58 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -14.7 -2.44 -­

TD–68–25–602 <2 180 <2 60 M -­ <2 M -­ <2 10,500 2.5 970 -­ -26.1 -4.7 -­

<1 50 <1 M M -­ 2 M -­ <1 8,500 1.9 570 -­ -25.9 -4.77 -­

TD–68–25–603 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -22.3 -4.44 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -22.4 -4.25 -­

Medina Lake near San Antonio 

08179500 (AL) <10 4 <1 5 <1 -­ <10 <10 <1 1 640 <6 <3 -­ -­ -­ -­

<6 <8 <5 <.01 <2 -­ 3 <20 -­ <6 660 -­ <5 -­ -­ -­ -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -6.60 -1.00 -­

08179500 (DC) 1.3 <5 <1 2.3 <1 <0.2 1.6 41 -­ <1 615 9.2 2 -­ -7.00 -1.20 -­

Medina River below Medina Lake 

08179520 -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -7.80 -1.22 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -8.40 -1.04 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -6.50 -1.05 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -6.00 -.93 -­

-­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -­ -6.00 -.80 -­

Table 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—Continued. 
Table 3 Table 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 Continued. 

 

 

 

Table 3 43 

Table 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. 

[USGS, U.S. Geological Survey; — δD, delta deuterium; δ18O, delta oxygen-18; per mil, parts per thousand; <, censored value] 

State well 

number 

(USGS name) 

TD–69–30–903 

Aquifer 

Edwards 

Date 

12/17/2002 

δD 

(per mil) 

δ18

D 

(per mil) 

Northwestern Medina flow path 

-25.4 -4.28 

Percentage 

surface 

water 

(δD) 

<12 

Percentage 

surface 

water 

(δ18

O) 

<10 

Average 

percentage 

surface 

water 

<11 

TD–69–31–406 

(Medina2) 

Edwards 12/31/2001 

08/15/2003

-24.1 

 -28.0 

-4.33 

-4.81 

12 

<12 

<10 

<10 

<11 

<11 

TD–69–31–708 

(Medina8) 

Edwards 07/30/2002 

08/21/2003 

-24.5 

-24.4 

-4.21 

-3.91 

<12 

15 

10 

19 

<11 

17 

TD–69–32–703 

(Medina1) 

Edwards 07/29/2002 

08/13/2003

-22.2 

 -26.0 

-4.51 

-4.53 

21 

<12 

<10 

<10 

<11 

<11 

YP–69–37–101 Edwards 04/03/2002 

12/05/2002 

-24.9 

-21.9 

-3.96 

-3.98 

<12 

23 

17 

17 

<12 

20 

TD–68–25–402 Trinity 01/30/1996 

08/08/1996 

North-central Medina flow path 

-9.9 -1.08 

-7.0 -.88 

84 

99 

98 

100 

91 

100 

TD–68–25–510 Trinity 11/29/1995 

08/08/1996 

-9.1 

-5.3 

-1.48 

-1.06 

89 

100 

87 

99 

88 

100 

TD–68–25–701 Edwards 11/30/1995 

07/29/1996 

-21.8 

-23.2 

-3.99 

-3.97 

23 

16 

16 

17 

20 

17 

TD–68–25–711 

(Medina5) 

Edwards 07/31/2002 

08/12/2003 

-23.1 

-24.2 

-4.28 

-4.25 

17 

<12 

<10 

<10 

<14 

<11 

TD–68–25–807 Trinity 01/25/1996 

07/09/1996 

-15.2 

-16.0 

-2.35 

-2.98 

57 

53 

63 

45 

60 

49 

TD–68–25–809 Trinity 12/05/1995 

07/26/1996 

-22.9 

-23.3 

-3.89 

-3.9 

10 

8 

18 

18 

14 

13 

TD–68–25–810 Trinity 03/06/1996 

08/19/1996 

-26.3 

-25.0 

-4.53 

-4.54 

-9 

<7 

<5 

<5 

<7 

<6 

TD–69–38–709 Edwards 12/12/2002 -23.2 -4.15 16 12 14 

TD–69–40–202 Edwards 03/13/1996 

08/20/1996 

-19.4 

-24.2 

-3.23 

-4.01 

36 

<12 

38 

16 

37 

<14 

TD–69–40–209 

(Medina3) 

Edwards 07/29/2002 

08/20/2003 

-24.2 

-22.9 

-3.84 

-3.94 

<12 

18 

21 

18 

<16 

18 

TD–69–40–301 Edwards 01/24/1996 

08/21/1996 

-23.7 

-23.3 

-3.93 

-3.88 

14 

16 

18 

19 

16 

18 
44 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

 

 

 

Table 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—Continued. 

State well 

number 

(USGS name) 

TD–68–25–904 

Aquifer 

Trinity 

Date 

03/06/1996 

07/26/1996 

δD 

(per mil) 

δ18

D 

(per mil) 

South-central Medina flow path 

-21.9 -4.14 

-22.0 -4.21 

Percentage 

surface 

water 

(δD) 

16 

15 

Percentage 

surface 

water 

(δ18

O) 

11 

9 

Average 

percentage 

surface 

water 

13 

12 

TD–68–33–102 Edwards 07/30/2002 -20.9 -3.26 28 37 32 

TD–68–33–103 Edwards 02/21/1996 

08/06/1996 

-20.9 

-19.5 

-3.67 

-3.46 

28 

35 

25 

31 

27 

33 

TD–68–33–204 Edwards 12/13/1995 

08/21/1996

-21.2 

 -21.2 

-3.51 

-3.36 

26 

26 

30 

34 

28 

30 

TD–68–33–213 

(Medina7) 

Edwards 07/26/2002 

08/11/2003 

-24.2 

-23.9 

-4.06 

-4.03 

<12 

13 

14 

15 

<13 

14 

TD–69–40–510 

(Medina4) 

Edwards 07/29/2002 

08/20/2003 

-20.1 

-22.0 

-3.73 

-3.74 

32 

22 

24 

23 

28 

23 

TD–69–40–604 Edwards 12/05/1995 

08/21/1996 

-17.9 

-20.2 

-3.11 

-3.15 

43 

32 

41 

40 

42 

36 

AS–68–25–505 Trinity 

Flow path unknown (well east of Medina Lake/River) 

12/15/1995 -16.1 -2.58 48 

07/30/1996 -14.7 -2.44 56 

55 

59 

52 

58 

TD–68–25–603 Trinity 02/08/1996 

07/30/1996 

-22.3 

-22.4 

-4.44 

-4.25 

13 

13 

<5 

8 

9 

10 
Table 4 

 

Solutions Percentage of surface water mixed with initial solution Percentage of 

Simulation 

number 

end-member 

Edwards aquifer 

water mixed with 

initial solution 

(PHREEQC) 

Initial site—State well number 

or USGS site number 

(USGS name) 

Final site—State 

well number 

(USGS name) 

Average 

computed Computed by PHREEQC by mixing 

equation 

Northwestern Medina flow path 

1 TD–69–31–708 (Medina8) TD–69–30–903 <11 No models identified—imbalance in sulfur -­

2 TD–69–31–406 (Medina2) TD–69–30–903 <11 No models identified—imbalance in sulfur -­

3 TD–69–32–703 (Medina1) TD–69–30–903 <11 No models identified—imbalance in sulfur -­

4 TD–69–31–603 TD–69–31–406 (Medina2) <11 Initial well cannot be used—cation-anion -­

balance >17 percent off 

5 TD–69–32–703 (Medina1) TD–69–31–406 (Medina2) <11 0 58 

6 TD–69–32–703 (Medina1) TD–69–31–708 (Medina8) 17 27 73 

6A TD–69–32–703 (Medina1) TD–69–31–708 (Medina8) 17 33 0 

North-central Medina flow path 

1 Medina Lake (average of AL and DC TD–68–25–402 100 100 0 

samples) 

2 Medina Lake (average of AL and DC TD–68–25–510 100 100 0 

samples) 

3 TD–68–25–602 (Trinity end-member) TD–68–25–810 0 0 100 

4 TD–68–25–602 (Trinity end-member) TD–68–25–809 13 20 80 

5 TD–68–25–810 TD–68–25–701 17 No models identified—imbalance in sodium -­

6 TD–68–25–402 TD–68–25–701 17 No models identified—imbalance in sodium, -­

calcium, and chloride 

7 TD–68–25–810 TD–68–25–711 (Medina5) <11 No models identified—imbalance in sulfur -­

8 TD–68–25–806 (Edwards end-member) TD–68–25–711 (Medina5) <11 No models identified—imbalance in sulfur -­

9 TD–68–25–711 (Medina5) TD–69–40–209 (Medina3) 18 29 0 

10 TD–68–25–711 (Medina5) TD–69–32–905 -- No models identified—imbalance in chloride -­

11 TD–69–32–905 TD–69–40–209 (Medina3) 18 23 77 

12 TD–69–40–209 (Medina3) TD–69–38–709 14 15 85 

12A TD–69–40–209 (Medina3) TD–69–38–709 14 9 54 

13 TD–68–25–810 TD–68–25–806 0 No models identified—imbalance in sodium -­

14 TD–68–25–806 (Edwards end-member) TD–68–25–810 16 No models identified—imbalance in sulfur -­

15 TD–68–25–701 TD–69–40–301 18 No models identified—imbalance in sulfur -­

16 TD–69–40–301 TD–69–40–202 13 No models identified—imbalance in sulfur -­

17 TD–69–40–202 TD–69–38–709 14 No models identified—imbalance in sodium, -­

magnesium, chloride, sulfur 

18 TD–69–32–905 TD–69–38–709 14 No models identified—imbalance in chloride, -­

sodium, sulfur 

South-central Medina flow path 

1 TD–68–25–602 (Trinity end-member) TD–68–25–904 13 11 81 

2 TD–68–25–904 TD–68–33–213 (Medina7) 14 4 96 

2A TD–68–25–904 TD–68–33–213 (Medina7) 14 3 94 

3 TD–68–25–904 TD–68–33–204 30 10 83 

4 TD–68–33–213 (Medina7) TD–69–40–510 (Medina4) 23 12 0 

4A TD–68–33–213 (Medina7) TD–69–40–510 (Medina4) 23 7 0 

5 TD–68–33–204 TD–69–40–604 36 No models identified—imbalance in -­

magnesium and sulfur 

6 TD–68–33–204 TD–68–33–103 33 0 44 

7 TD–68–33–103 TD–69–40–604 36 No models identified—imbalance in calcium -­

Table 4 45 

Table 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. 

[USGS, U.S. Geological Survey; <, less than; --, not applicable or no data; >, greater than] 
46 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

Table 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—Continued. 

Simulation 

number 

1 

2 

3 

4 

Calcite 

-­

-­

-­

-­

Carbon 

dioxide 

gas 

-­

-­

-­

-­

Mole transfer 

(millimoles/kilogram) 

Dolomite Gypsum Halite Silica 

Northwestern Medina flow path 

-­ -­ -­ -­

-­ -­ -­ -­

-­ -­ -­ -­

-­ -­ -­ -­

Sum of 

residuals 

-­

-­

-­

-­

Sum of 

delta/ 

uncertainty 

-­

-­

-­

-­

Minimum 

phases 

-­

-­

-­

-­

5 

6 

6A 

-0.480 

-­

-­

-0.490 

2.240 

2.33 

0.027 

.102 

.082 

0.025 

-­

-­

0.002 

.03 

.026 

0.0005 

.021 

.018 

0.5938 

5.593 

7.869 

0.8232 

10.17 

12.95 

-­

-­

-­

1 -­ .291 -­

North-central Medina flow path 

-­ .043 -­ 9.673 9.673 yes 

2 -­ .294 -­ .278 .080 .567 5.403 5.403 -­

3 

4 

-­

-2.36 

2.97 

-.444 

.128 

1.15 

.076 

11.01 

.205 

.014 -­

-­ 8.476 

6.825 

8.476 

12.44 

yes 

-­

5 

6 

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

7 

8 

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

9 

10 

11 

12 

-­

-­

-­

-.65 

1.92 

-­

1.744 

-­

.197 

-­

.245 

.286 

.001 

-­

-­

-­

.071 

-­

.087 

.053 

.013 

-­

-­

-­

7.832 

-­

7.399 

7.487 

12.87 

-­

12.52 

13.39 

-­

-­

yes 

yes 

12A 

13 

14 

15 

-.581 

-­

-­

-­

-.574 

-­

-­

-­

.176 

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

8.809 

-­

-­

-­

18.36 

-­

-­

-­

-­

-­

-­

-­

16 

17 

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

-­

18 -­ -­ -­ -­ -­ -­ -­ -­ -­

1 

2 

2A 

3 

-2.53 

-­

-­

-­

-.653 

-.624 

-.758 

-.410 

1.313 

.104 

.049 

.084 

South-central Medina flow path 

.424 -­ -­

.059 .033 -­

-­ .035 -­

-­ -­ -­

3.149 

7.539 

8.483 

9.982 

8.154 

15.89 

17.66 

22.53 

-­

-­

-­

yes 

4 

4A 

5 

-­

-.277 

-­

.147 

.082 

-­

-­

-­

-­

-­

-­

-­

.066 

.072 

-­

-­

.014 

-­

7.732 

3.067 

-­

14.04 

8.274 

-­

yes 

-­

-­

6 -­ -­ -­ .093 .030 -­ 10.92 16.49 yes 

7 -­ -­ -­ -­ -­ -­ -­ -­ -­
Table 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—Continued. 

Table 5

Table 5 47 

Table 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. 

[In milliequivalents per liter except as indicated; g, grams; --, not applicable or not determined] 

Saturation index 

State well Appar­ (dimensionless) Cationnumber 

(USGS name) 

Aquifer ent 

age 

Date 

Calcium Magnesium Sodium Potassium Chloride Bicarbonate Sulfate Calcite Dolomite 

Carbon 

dioxide 

(g) 

Gypsum Halite Silica 

anion 

balance 

Northwestern Medina flow path 

TD–69–30–903 Edwards 1988 12/17/2002 4.09 0.30 0.21 0 0.23 4.16 0.08 0.05 -0.93 -1.82 -2.90 -8.98 -1.00 1.4 

TD–69–31–406 Edwards -­ 12/31/2001 3.74 .74 .17 .03 .17 3.92 .52 .19 -.20 -2.04 -2.14 -9.19 -.95 .9 

(Medina2) -­ 08/15/2003 4.39 .71 .20 .04 .18 4.33 .65 .29 -.09 -2.00 -1.99 -9.10 -.97 1.8 

1992 08/20/2004 4.46 .69 .31 .03 .32 4.36 .34 .07 -.57 -1.81 -2.27 -8.65 -.96 4.5 

TD–69–31–603 Edwards 1982 11/26/2002 5.94 .28 .52 .02 1.29 3.34 .16 -.12 -1.48 -1.73 -2.48 -7.83 -.98 17.1 

TD–69–31–708 Edwards -­ 08/21/2003 3.74 .84 .25 .03 .25 3.74 .40 -.73 -1.99 -1.16 -2.25 -8.86 -.96 5.2 

(Medina8) 

TD–69–32–201 Trinity -­ 01/31/1996 6.79 6.34 .37 .10 .55 5.40 6.97 -­ -­ -­ -­ -­ -­ 2.6 

-­ 08/28/1996 5.59 6.11 .43 .11 .59 5.20 14.7 -.21 -.35 -1.45 -.78 -8.30 -.90 -21.5 

TD–69–32–303 Trinity -­ 12/11/1995 18.4 14.4 1.54 .41 .53 5.20 29.8 -­ -­ -­ -­ -­ -­ -1.1 

-­ 08/15/1996 17.90 19.30 1.47 .42 .71 4.80 33.10 .16 .46 -1.62 -.27 -7.74 -.92 .7 

TD–69–32–703 Edwards -­ 08/13/2003 4.28 .15 .19 .01 .13 4.41 .14 0 -1.32 -1.69 -2.63 -9.26 -.97 -.6 

(Medina1) 

North-central Medina flow path 

TD–68–25–402 Trinity -­ 01/30/1996 2.80 1.39 .34 .04 .31 2.80 1.15 -.14 -.51 -2.21 -1.91 -8.62 -1.07 3.3 

-­ 08/08/1996 3.12 1.42 .41 .04 .37 3.20 1.23 -.23 -.70 -1.94 -1.85 -8.48 -.98 2.0 

TD–68–25–510 Trinity -­ 11/29/1995 3.66 1.43 .38 .04 .39 3.20 -­ -.18 -.70 -1.95 -1.68 -8.49 -.85 2.6 

-­ 08/08/1996 3.68 1.42 .39 .04 .42 3.20 1.64 -.18 -.70 -1.95 -1.67 -8.43 -.92 2.1 

TD–68–25–602 Trinity -­ 02/08/1996 23.2 22.6 .55 .40 .35 3.60 42.3 .11 .31 -1.75 -.12 -8.49 -1.01 .7 

-­ 07/30/1996 24.5 19.6 .46 .36 .28 3.60 36.4 -.12 -.23 -1.44 -.13 -8.66 -.97 5.4 

TD–68–25–701 Edwards -­ 11/30/1995 3.43 3.22 .01 .05 .24 4.00 2.64 -.79 -1.49 -1.13 -1.57 -10.22 -.98 -1.2 

-­ 07/29/1996 2.91 2.57 .12 .04 .23 3.80 2.33 -.56 -1.04 -1.45 -1.66 -9.21 -.97 -6.0 

TD–68–25–711 Edwards -­ 08/12/2003 4.11 .27 .19 .02 .22 4.26 .04 .16 -.67 -1.76 -3.16 -9.05 -.98 .8 

(Medina5) 

TD–68–25–806 Edwards -­ 12/05/1995 3.99 .21 .09 0 .18 4.00 .08 -.32 -1.81 -1.50 -2.92 -9.43 -.97 .4 

-­ 08/19/1996 4.22 .21 .23 0 .20 3.80 .12 -.36 -1.92 -1.45 -2.69 -8.98 -.91 6.0 

1978 12/04/2002 3.89 .21 .18 .01 .18 4.08 .16 -.19 -1.55 -1.63 -2.61 -9.14 -.98 -1.4 

TD–68–25–809 Trinity -­ 12/05/1995 26.7 2.07 .18 .04 .40 3.60 23.30 .51 -.05 -2.06 -.14 -8.87 -.95 3.0 

-­ 07/26/1996 24.9 2.76 .18 .04 .28 3.80 22.30 .55 .24 -2.02 -.18 -9.03 -.94 2.7 

TD–68–25–810 Trinity -­ 03/06/1996 4.02 .43 .31 0 .38 3.80 1.68 .08 -0.71 -1.96 -2.59 -8.58 -1.03 4.6 

-­ 08/19/1996 4.18 .49 .34 0 .42 3.80 .31 -.80 -2.42 -1.06 -2.31 -8.49 -.97 5.0 

1990 02/20/2003 3.65 .66 .22 .01 .21 3.80 .15 -.45 -1.53 -1.46 -2.67 -8.98 -.96 4.4 
48 Flow Paths in the Edwards Aquifer, Northern Medina and Northeastern Uvalde Counties, Texas 

State well ApparSaturation index 

(dimensionless) Cationanion 

balance 

number 

(USGS name) 

Aquifer ent 

age 

Date 

Calcium Magnesium Sodium Potassium Chloride Bicar­ Sul­ Calbonate fate cite Dolomite 

Carbon 

dioxide 

(g) 

Gypsum Halite Silica 

North-central Medina flow path—Continued 

TD–69–32–905 Edwards -­ 10/12/2003 4.86 0.37 0.29 0.04 0.46 4.11 0.26 -0.11 -1.23 -1.63 -2.33 -8.53 -0.91 6.9 

TD–69–38–601 Edwards -­ 12/20/2001 3.74 .82 .23 .02 .28 4.20 .36 .09 -.37 -1.92 -2.30 -8.84 -1.00 -.2 

(Seco) 

TD–69–38–709 Edwards 1969 12/12/2002 3.41 1.07 .23 .02 .28 4.08 .28 -.29 -1.01 -1.64 -2.43 -8.84 -.96 1.0 

TD–69–40–202 Edwards -­ 03/13/1996 3.96 .78 .37 0 .39 4.04 .48 -.01 -.62 -1.86 -2.15 -8.87 -1.00 5.0 

-­ 08/20/1996 4.35 .70 .24 0 .23 4.20 .20 -.03 -.71 -1.72 -2.51 -8.50 -.86 7.8 

TD–69–40–209 Edwards 2003 08/20/2003 4.23 1.01 .34 .02 .28 4.34 .36 -.41 -1.32 -1.29 -2.26 -8.67 -.95 5.8 

(Medina3) 

TD–69–40–301 Edwards -­ 01/24/1996 3.37 1.27 .24 0 .23 4.20 .25 -.03 -.36 -1.81 -2.50 -8.92 -.96 2.0 

-­ 08/21/1996 3.78 1.20 .23 0 .30 4.20 .32 .13 -.11 -1.91 -2.36 -8.81 -.93 4.0 

South-central Medina flow path 

TD–68–25–904 Trinity -­ 03/06/1996 4.06 4.52 .40 .08 .26 4.00 5.48 -.23 -.32 -1.76 -1.26 -8.67 -.99 -3.6 

-­ 07/26/1996 3.65 4.15 .26 .07 .23 4.00 4.12 -.03 .10 -1.95 -1.40 -8.91 -.95 -1.4 

TD–68–33–103 Edwards -­ 08/06/1996 4.41 .62 .26 .00 .33 3.80 .50 -.28 -1.30 -1.56 -2.10 -8.72 -.97 6.7 

TD–68–33–204 Edwards -­ 12/13/1995 3.88 .78 .24 .00 .32 4.00 .44 -.01 -.61 -1.84 -2.20 -8.78 -.96 1.5 

-­ 08/21/1996 3.99 .82 .28 .00 .35 3.80 .46 .01 -.53 -1.85 -2.17 -8.67 -.92 5.1 

TD–68–33–213 Edwards -­ 08/11/2003 4.45 .46 .23 .01 .27 4.47 .28 .19 -.52 -1.90 -2.35 -8.86 -.99 1.3 

(Medina7) 

TD–69–40–510 Edwards 1992 08/02/2003 4.46 .69 .31 .03 .32 4.36 .34 .07 -.57 -1.81 -2.27 -8.65 -.96 4.5 

(Medina4) 

TD–69–40–604 Edwards -­ 12/05/1995 3.12 1.47 .04 .00 .28 3.60 .72 -.44 -1.10 -1.58 -2.08 -9.56 -.98 .4 

-­ 08/21/1996 3.32 1.57 .27 .03 .32 3.60 .72 -.42 -1.06 -1.58 -2.06 -8.73 -.97 5.6 

Medina Lake1 

-­ -­ -­ -­ 2.71 1.37 .30 .04 .34 2.77 1.19 .21 .19 -2.33 -1.92 -8.58 -1.05 -­

Table 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—Continued. 

1 Average of 08179500 (AL) and 08179500 (DC) samples. 

Prepared by the USGS Texas Water Science Center: 

8027 Exchange Drive 

Austin, TX 78754–4733 

Information regarding water resources in Texas is available at 

http: //tx.usgs.gov/