Skip Links

USGS - science for a changing world

Scientific Investigations Report 2012–5285

Prepared in cooperation with the San Antonio Water System

Borehole Geophysical, Fluid, and Hydraulic Properties Within and Surrounding the Freshwater/Saline-Water Transition Zone, San Antonio Segment of the Edwards Aquifer, South-Central Texas, 2010–11

By Jonathan V. Thomas, Gregory P. Stanton, and Rebecca B. Lambert

Thumbnail of and link to report PDF (5.11 MB)Abstract

The freshwater zone of the San Antonio segment of the Edwards aquifer is used by residents of San Antonio and numerous other rapidly growing communities in south-central Texas as their primary water supply source. This freshwater zone is bounded to the south and southeast by a saline-water zone with an intermediate zone transitioning from freshwater to saline water, the transition zone. As demands on this water supply increase, there is concern that the transition zone could potentially move, resulting in more saline water in current supply wells. Since 1985, the U.S. Geological Survey (USGS), San Antonio Water System (SAWS), and other Federal and State agencies have conducted studies to better understand the transition zone.

During 2010 and 2011, the USGS, in cooperation with SAWS, conducted a study to further assess the potential for movement of the transition zone in part of the San Antonio segment of the Edwards aquifer. Equivalent freshwater heads were computed to investigate the transition from saline to freshwater zones in the San Antonio segment and evaluate the potential for lateral flow at the freshwater/saline-water interface. Data were collected within and surrounding the transition zone from 13 wells in four transects (East Uvalde, Tri-County, Fish Hatchery, and Kyle).

Hydraulic head and geophysical log data were used to calculate equivalent freshwater heads and then analyzed to identify possible horizontal gradients across the transition zone and thus flow. Unlike previous studies that used indirect methods to calculate fluid conductivity from fluid resistivity, in this study geophysical tools that directly measured fluid conductivity were used. Electromagnetic (EM) flowmeter logs were collected under both ambient and stressed (pumping) conditions and were processed to identify vertical flow zones within the borehole.

The San Antonio segment of the Edwards aquifer (the study area) is about 175 miles long and extends from the western groundwater divide near Brackettville in Kinney County to the eastern groundwater divide near Kyle in Hays County. The four transects consist of two to five wells per transect and were configured approximately perpendicular to and across the expected trace of the freshwater/saline-water interface.

The deep flow zone indicated by the EM flowmeter data for East Uvalde transect well EU2 corresponds directly with a large, negative deflection of the fluid logs, indicating an inflow of fresher water from the Devils River Limestone. To the southwest, towards the freshwater/saline-water interface, this same flow zone was observed in well EU1, but with a reduction of flow, and displayed no apparent fluid curve deflections.

The highest observed transmissivity of the study area was observed in the saline zone of the Tri-County transect, at well TC3, which had a total transmissivity of 24,900 square feet per day. Zones of high transmissivity throughout the study site were observed to not be continuous and are likely caused by localized secondary porosity such as intersecting faults or karst features.

Although analyses of daily mean equivalent freshwater heads for the East Uvalde transect indicated that the gradient across the freshwater/saline-water interface varied between into and out of the freshwater zone, the data indicate that there was a slightly longer period during which the gradient was out of the freshwater zone. Analyses of all daily mean equivalent freshwater heads for the Tri-County transect indicated that the lateral-head gradients across the freshwater/saline-water interface were typically mixed (not indicative of flow into or out of freshwater zone). Assessment of the daily mean equivalent freshwater heads indicated that, although the lateral-head gradient at the Kyle transect varied between into and out of the freshwater zone, the lateral-head gradient was typically from the transition zone into the freshwater zone.

First posted December 28, 2012

For additional information contact:
Director, Texas Water Science Center
U.S. Geological Survey
1505 Ferguson Lane
Austin, Texas 78754-4501
http://tx.usgs.gov/

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.


Suggested citation:

Thomas, J.V., Stanton, G.P., and Lambert, R.B., 2012, Borehole geophysical, fluid, and hydraulic properties within and surrounding the freshwater/saline-water transition zone, San Antonio segment of the Edwards aquifer, south-central Texas, 2010–11: U.S. Geological Survey Scientific Investigations Report 2012–5285, 65 p., 3 apps.


Contents

Abstract

Introduction

Methods of Analysis

Borehole Geophysical, Fluid, and Hydraulic Properties

Hydraulics of Lateral Flow

Summary

References Cited

Appendix 1. Data for Computation of Equivalent Freshwater Head in Transect Wells (online only)

Appendix 2. Observations of Specific Conductance and Total Dissolved Solids (TDS) Concentration Related by Regression To Yield Threshold Values of Specific Conductance Corresponding to the Threshold Values of TDS Concentrations That Describe Freshwater and Categories of Saline Water (Slightly, Moderately, or Very Saline), 1999–2007 (online only)

Appendix 3. Electromagnetic (EM) Flowmeter Analyses of Transect Wells with Flow-Log Analysis of Single Holes (FLASH) Numerical Model Inputs and Results (online only)

 


Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/sir/2012/5285/index.html
Page Contact Information: GS Pubs Web Contact
Page Last Modified: Thursday, 10-Jan-2013 20:04:00 EST