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In cooperation with the Edwards Aquifer Authority and
San Antonio Water System

Hydrogeologic Framework and Geochemistry of the Edwards Aquifer Saline-Water Zone, South-Central Texas

By George E. Groschen and Paul M. Buszka

U.S. Geological Survey
Water-Resources Investigations Report 97–4133


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Contents

Abstract

Introduction

Purpose and Scope

Methods of Data Collection and Analysis

Sampling Methods

Sample Preparation and Laboratory Analyses

Well-Numbering System

Acknowledgments

Hydrogeologic Framework

Geology and Stratigraphy of Aquifer Units

Kainer, Person, and Georgetown Formations

Devils River Formation

West Nueces, McKnight, and Salmon Peak Formations

Stuart City Formation

Stratigraphy of Underlying and Overlying Units

Glen Rose Limestone

Del Rio Clay, Buda Limestone, and Eagle Ford Group

Geologic Structure and Faults

Hydrologic Framework

Geochemistry of the Saline-Water Zone

Equilibrium Species Distribution

Application of Equilibrium-Computation Program

Uncertainties in Equilibrium Species Distribution

Temperature and Hydrogen Activity

Dissolved Constituents

Dissolved Solids

Chloride

Calcium and Magnesium

Sulfur

Dissolved, Gas Phase, and Nonaqueous Liquid Phase

Stable Isotopes

Carbon

Dissolved

Stable Isotopes

Other Dissolved Species

Other Isotopes

Stable Isotopes of Hydrogen and Oxygen

Stable Isotopes of Strontium and Boron

Unstable Isotopes of Carbon and Chloride

Summary and Conclusions

References

Figures

1.   Map showing location of the study area
2.   Diagram showing well-numbering system
3.   Chart showing correlation of Cretaceous stratigraphic units in south Texas and Edwards aquifer hydrostratigraphic units in the San Marcos platform area, south-central Texas
4.   Map showing location of selected faults, oil and gas fields, and depositional provinces near the Edwards aquifer saline-water zone, south-central Texas
5.   Generalized hydrogeologic section showing the relation between the Edwards aquifer and the geopressured zone in the subsurface of the Texas Gulf Coast
6–7.   Maps showing:
  6.   Location of wells in the Edwards aquifer saline-water zone, south-central Texas, for which water-sample analyses are available
  7.   Location of sampled wells and monitor-well transects in San Antonio, New Braunfels, and San Marcos, Texas
8–10.   Graphs showing:
  8.   Relation between estimated in-place temperature of ground-water samples and distance of well from the downdip limit of freshwater in the Edwards aquifer, south-central Texas
  9.   Relation between corrected pH and dissolved solids concentration in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
  10.   Relation between dissolved solids concentration in ground-water samples from the saline-water zone and distance of the well from the downdip limit of freshwater in the Edwards aquifer, south-central Texas
11.   Map showing mineral-saturation indices of dolomite and magnesite in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
12.   Graphs showing relations between (A) del sulfur-34 (d34S) of dissolved sulfate and dissolved solids concentration, (B) d34S of dissolved sulfide and dissolved solids concentration, and (C) d34S of dissolved sulfate minus d34S of dissolved sulfide and estimated temperature in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
13.   Map showing distribution of del carbon-13 (d13C) of dissolved inorganic carbon and del sulfur-34 (d34S) of dissolved sulfate and sulfide in ground-water samples, and location of wells that produce water with meteoric del deuterium (dD) and del oxygen-18 (d18O) in the Edwards aquifer saline-water zone, south-central Texas
14–20.   Graphs showing:
  14.   Relation between dissolved iron and dissolved solids concentrations in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
  15.   Relation between dissolved potassium and dissolved solids concentrations in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
  16.   Relation between del deuterium (dD) and del oxygen-18 (d18O) in ground-water samples from the Edwards aquifer saline-water-zone, south-central Texas
  17.   Relation between ratio of strontium-87/strontium-86 (87Sr/86Sr) of dissolved strontium and dissolved strontium concentration in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
  18.   Ranges of del boron-11 (d11B) of dissolved boron in selected natural materials and in water
  19.   Relation between del boron-11 (d11B) of dissolved boron and dissolved solids concentration in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas
  20.   Relations between (A) ratio of chlorine-36/chlorine (36Cl/Cl) of dissolved chloride and 36Cl, and (B) dissolved solids concentration and 36Cl in ground-water samples from the Edwards aquifer saline-water zone, south-central Texas

Tables

1.   Selected physical properties for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
2.   Alkalinity and concentrations of major dissolved ions and dissolved silica for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
3.   Summary of mineral saturation indices computed by SOLMINEQ.88 (Kharaka and others, 1988) for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
4.   Dissolved solids concentrations and anhydrous weight percentages of computed salt-normative mineral assemblages for samples from selected wells in the Edwards aquifer saline-water zone, south-central Texas, collected over various time periods
5.   Concentrations of dissolved organic and inorganic carbon, dissolved bicarbonate, del carbon-13 (d13C) of dissolved inorganic carbon, carbon-14 (14C), and dissolved aliphatic acids for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
6.   Concentrations of selected dissolved gases for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
7.   Concentrations of minor dissolved ions for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
8.   Stable-isotope ratios of selected dissolved species for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990
9.   Ratio of chlorine-36 to chlorine (36Cl/Cl), number of 36Cl atoms per liter, and associated error for samples from wells in the Edwards aquifer saline-water zone, south-central Texas, July–September 1990

Abstract

The Edwards aquifer supplies drinking water for more than 1 million people in south-central Texas. The saline-water zone of the Edwards aquifer extends from the downdip limit of freshwater to the southern and eastern edge of the Stuart City Formation. Water samples from 16 wells in the Edwards aquifer saline-water zone were collected during July–September 1990 and analyzed for major and minor dissolved constituents, selected stable isotopes, and radioisotopes. These data, supplemental data from an extensive water-quality data base, and data from other previous studies were interpreted to clarify the understanding of the saline-water-zone geochemistry.

Most of the isotope and geochemical data indicate at least two distinct hydrological and geochemical regimes in the saline-water zone of the Edwards aquifer. On the basis of hydrogen and oxygen isotopes and radiocarbon data, the shallower updip regime is predominantly meteoric water that has been recharged probably from the freshwater zone within recent geologic time (less than tens of thousands of years). Also, on the basis of hydrogen and oxygen isotope data, water in the hydrologically stagnant regime (downdip) has been thermally altered in reactions with the carbonate rocks of the zone. The deeper water probably is much older than water in the shallow zone and is nearly stagnant relative to that in the shallow zone.

The geochemical grouping observed in the well-water data from well samples in the saline-water zone indicates that the zone is hydrologically compartmentalized, in part because of faults that function as barriers to downdip flow of recharge water. These fault barriers also probably impede updip flow. Flow compartmentalization and the resulting disparity in geochemistry between the two regimes indicate that updip movement of substantial amounts of saline water toward the freshwater zone is unlikely.

Estimated in-place temperature of the samples collected indicates an increase with depth and (or) distance from the downdip limit of freshwater. The pH of the samples decreases with increasing distance from the downdip limit of freshwater, but the decrease is caused partly by the increase in temperature. Dissolved major ions and dissolved solids concentrations all indicate a progressive but monotonic increase in salinity from updip to downdip. The alkalinity of the water samples is predominantly bicarbonate because the low-molecular weight aliphatic-acid anion concentrations are small relative to the bicarbonate concentrations. The dissolved organic carbon concentrations also are lower than expected for an aquifer with economic amounts of oil and gas hydrocarbons.




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