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Groundwater ages from the freshwater zone of the Edwards aquifer, Uvalde County, Texas—Insights into groundwater flow and recharge

Scientific Investigations Report 2015-5163
By: , , and 
https://doi.org/10.3133/sir20155163

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Abstract

Tritium–helium-3 groundwater ages of the Edwards aquifer in south-central Texas were determined as part of a long-term study of groundwater flow and recharge in the Edwards and Trinity aquifers. These ages help to define groundwater residence times and to provide constraints for calibration of groundwater flow models. A suite of 17 samples from public and private supply wells within Uvalde County were collected for active and noble gases, and for tritium–helium-3 analyses from the confined and unconfined parts of the Edwards aquifer. Samples were collected from monitoring wells at discrete depths in open boreholes as well as from integrated pumped well-head samples. The data indicate a fairly uniform groundwater flow system within an otherwise structurally complex geologic environment comprised of regionally and locally faulted rock units, igneous intrusions, and karst features within carbonate rocks. Apparent ages show moderate, downward average, linear velocities in the Uvalde area with increasing age to the east along a regional groundwater flow path. Though the apparent age data show a fairly consistent distribution across the study area, many apparent ages indicate mixing of both modern (less than 60 years) and premodern (greater than 60 years) waters. This mixing is most evident along the “bad water” line, an arbitrary delineation of 1,000 milligrams per liter dissolved solids that separates the freshwater zone of the Edwards aquifer from the downdip saline water zone. Mixing of modern and premodern waters also is indicated within the unconfined zone of the aquifer by high excess helium concentrations in young waters. Excess helium anomalies in the unconfined aquifer are consistent with possible subsurface discharge of premodern groundwater from the underlying Trinity aquifer into the younger groundwater of the Edwards aquifer.

Suggested Citation

Hunt, A.G., Landis, G.P., and Faith, J.R., 2016, Groundwater ages from the freshwater zone of the Edwards Aquifer, Uvalde County, Texas—Insights into groundwater flow and recharge: U.S. Geological Survey Scientific Investigations Report 2015–5163, 28 p., http://dx.doi.org/10.3133/sir20155163.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Overview of Groundwater Age
  • Uvalde County
  • Sampling
  • Laboratory Analysis
  • Data Analysis
  • Results
  • Summary
  • References Cited
  • Appendix
Publication type Report
Publication Subtype USGS Numbered Series
Title Groundwater ages from the freshwater zone of the Edwards aquifer, Uvalde County, Texas—Insights into groundwater flow and recharge
Series title Scientific Investigations Report
Series number 2015-5163
DOI 10.3133/sir20155163
Year Published 2016
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Crustal Geophysics and Geochemistry Science Center
Description viii, 28 p.
Country United States
State Texas
County Uvalde County
Other Geospatial Edwards Aquifer
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details