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Scientific Investigations Report 2007-5285

In cooperation with the San Antonio Water System

Geologic, Hydrologic, and Geochemical Identification of Flow Paths in the Edwards Aquifer, Northeastern Bexar and Southern Comal Counties, Texas

By Cassi L. Otero

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Scientific Investigations Report
2007-5285 PDF (14.3 MB)

The U.S. Geological Survey, in cooperation with the San Antonio Water System, conducted a 4-year study during 2002–06 to identify major flow paths in the Edwards aquifer in northeastern Bexar and southern Comal Counties (study area). In the study area, faulting directs ground water into three hypothesized flow paths that move water, generally, from the southwest to the northeast. These flow paths are identified as the southern Comal flow path, the central Comal flow path, and the northern Comal flow path. Statistical correlations between water levels for six observation wells and between the water levels and discharges from Comal Springs and Hueco Springs yielded evidence for the hypothesized flow paths. Strong linear correlations were evident between the datasets from wells and springs within the same flow path and the datasets from wells in areas where flow between flow paths was suspected. Geochemical data (major ions, stable isotopes, sulfur hexafluoride, and tritium and helium) were used in graphical analyses to obtain evidence of the flow path from which wells or springs derive water. Major-ion geochemistry in samples from selected wells and springs showed relatively little variation. Samples from the southern Comal flow path were characterized by relatively high sulfate and chloride concentrations, possibly indicating that the water in the flow path was mixing with small amounts of saline water from the freshwater/saline-water transition zone. Samples from the central Comal flow path yielded the most varied major-ion geochemistry of the three hypothesized flow paths. Central Comal flow path samples were characterized, in general, by high calcium concentrations and low magnesium concentrations. Samples from the northern Comal flow path were characterized by relatively low sulfate and chloride concentrations and high magnesium concentrations. The high magnesium concentrations characteristic of northern Comal flow path samples from the recharge zone in Comal County might indicate that water from the Trinity aquifer is entering the Edwards aquifer in the subsurface. A graph of the relation between the stable isotopes deuterium and delta-18 oxygen showed that, except for samples collected following an unusually intense rain storm, there was not much variation in stable isotope values among the flow paths. In the study area deuterium ranged from -36.00 to -20.89 per mil and delta-18 oxygen ranged from -6.03 to -3.70 per mil. Excluding samples collected following the intense rain storm, the deuterium range in the study area was -33.00 to -20.89 per mil and the delta-18 oxygen range was -4.60 to -3.70 per mil. Two ground-water age-dating techniques, sulfur hexafluoride concentrations and tritium/helium-3 isotope ratios, were used to compute apparent ages (time since recharge occurred) of water samples collected in the study area. In general, the apparent ages computed by the two methods do not seem to indicate direction of flow. Apparent ages computed for water samples in northeastern Bexar and southern Comal Counties do not vary greatly except for some very young water in the recharge zone in central Comal County.

Version 1.0

Posted March 2008


Suggested citation:

Otero, C.L., 2007, Geologic, hydrologic, and geochemical identification of flow paths in the Edwards aquifer, northeastern Bexar and southern Comal Counties, Texas: U.S. Geological Survey Scientific Investigations Report 2007–5285, 48 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of the Edwards Aquifer and Study Area

Acknowledgments

Methods of Investigation

Data Collection and Sample Analysis

Quality Control and Quality Assurance of Geochemical Samples

Methods of Data Analysis

Hydrologic Data

Geochemical Data

Geologic and Hydrologic Identification of Flow Paths

Southern Comal Flow Path

Central Comal Flow Path

Northern Comal Flow Path

Correlation of Water Levels

Analysis of Hydrograph Recession Curves

Geochemical Identification of Flow Paths

Major-Ion Chemistry

Stable Isotopes

Ground-Water Ages

Summary

References

Appendix 1—Water-Level (2004–06) and Chemical Data (2003–06)

1.1. Daily mean depth to water at well DX–68–22–913, Comal County, Texas, 2004–06

1.2. Daily mean depth to water at well DX–68–23–502, Comal County, Texas, 2004–06

1.3. Chemical and isotope data in ground-water samples from wells and springs (by flow path) collected for this study, northeastern Bexar and southern Comal Counties, Texas, 2003–06



For additional information contact:
Director, Texas Water Science Center
U.S. Geological Survey
8027 Exchange Drive
Austin, Texas 78754-4733

World Wide Web: http://tx.usgs.gov/
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