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A contribution of the Regional Aquifer-System Analysis Program

Chemical Evolution and Estimated Flow Velocity of Water in the Trinity Aquifer, South-Central Texas

By S.A. Jones, Roger W. Lee, and John F. Busby

U.S. Geological Survey
Water-Resources Investigations Report 97–4078


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Contents

Abstract

Introduction

Geologic Setting

Hydrogeologic Characteristics

Geochemistry of the Trinity Aquifer

Data Compilation and Collection

Petrology and Chemistry of Cores from the Trinity Aquifer

Chemistry of Water in the Trinity Aquifer

Dissolved Solids

Major Ions

pH and Bicarbonate Alkalinity

Environmental Isotopes

Chemical Evolution of Water in the Trinity Aquifer

Hydrochemical Facies and Evolution

Rock-Water Interaction Modeling

Mass-Transfer Modeling

Flow Velocity Estimates

Summary

Selected References

Figures

1.   Map showing potentiometric surface and locations of sampled wells and cores in the Trinity aquifer, south-central Texas
2.   Chart showing relation among stratigraphic units, hydrogeologic units, and water-yielding properties of the Trinity aquifer, south-central Texas
3.   Map showing structural and physiographic features associated with the Trinity aquifer of south-central Texas
4–6.   Graphs showing relation of:
  4.   Dissolved magnesium to dissolved calcium, dissolved magnesium to dissolved sulfate, and dissolved calcium to dissolved sulfate for samples collected from the Trinity aquifer, south-central Texas
  5.   pH to partial pressure of carbon dioxide (PCO2) for samples collected from the Trinity aquifer, south-central Texas
  6.   Deuterium (dD) to oxygen-18 (d18O) for samples collected from the Trinity aquifer, south-central Texas
7.   Trilinear diagrams of waters from the (a) middle and (b) lower permeable zones of the Trinity aquifer, south-central Texas
8.   Graphs showing relation of calcite, dolomite, and gypsum saturation indices to dissolved sulfate for samples collected from the Trinity aquifer, south-central Texas

Tables

1.   Results of petrographic microscope analyses of selected cores, middle permeable zone, Trinity aquifer, south-central Texas
2.   Results of solid-phase analyses by X-ray fluorescence and X-ray diffraction of selected cores, middle permeable zone, Trinity aquifer, south-central Texas
3.   Results of isotopic analyses of selected cores, middle permeable zone, Trinity aquifer, south-central Texas
4.   Summary of well data and results of chemical analyses of ground-water samples, Trinity aquifer, south-central Texas
5.   Results of isotopic analyses of ground-water samples and calculated WATEQF saturation indices for selected minerals and partial pressure of carbon dioxide (PCO2), Trinity aquifer, south-central Texas
6.   Results from NETPATH modeling

Abstract

Three permeable zones with varying lithology and water chemistry compose the Trinity aquifer, a principal source of water in the 5,500-square-mile study area in south-central Texas. The upper permeable zone locally yields small quantities of water to wells and was not included in this study. The middle permeable zone primarily is composed of limestone with minor amounts of dolostone. Terrigenous sand and marine limestone, with minor amounts of dolostone, are the principal lithologic units in the lower permeable zone. Dissolved solids concentrations range from 329 to 1,820 milligrams per liter in water samples from the middle permeable zone and from 518 to 3,030 milligrams per liter in water samples from the lower permeable zone. Principal hydrochemical facies in the middle permeable zone are calcium magnesium bicarbonate and calcium magnesium sulfate. Hydrochemical facies in ground-water samples from the lower permeable zone vary. Tritium concentrations as large as 5.3 tritium units in the southeastern part of the study area are indicative of relatively recent recharge. Results of a geochemical mass balance simulation along a flowpath in the middle permeable zone indicate a mass transfer of 4.25 millimoles per liter of dolomite dissolved, 5.74 millimoles per liter of gypsum dissolved, 0.46 millimole per liter of sodium chloride dissolved, 8.07 millimoles per liter of calcite precipitated, and 0.67 millimole per liter of calcium-for-sodium cation exchange between solid and aqueous phases. These results support dedolomitization as a principal chemical process in the middle permeable zone of the Trinity aquifer. Results of a simulation along a flowpath in the lower permeable zone indicate a mass transfer of 0.41 millimole per liter of dolomite dissolved, 0.001 millimole per liter of gypsum dissolved, 9.58 millimoles per liter of sodium chloride dissolved, 1.09 millimoles per liter of calcite precipitated, and 1.11 millimoles per liter of sodium-for-calcium cation exchange between solid and aqueous phases. Lower permeable zone processes indicate sodium chloride dissolution, dedolomitization, and cation exchange. Ground-water-flow velocities determined from adjusted carbon-14 ages, calculated using NETPATH, for selected flowpaths in the middle and lower permeable zones were about 1.7 feet per year and less than about 4.4 feet per year, respectively.

 


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