U.S. Department of the Interior

U.S. Geological Survey

Geochemical Evolution Processes and Water-Quality 

Observations Based on Results of the National WaterQuality Assessment Program in the San Antonio Segment 

of the Edwards Aquifer, Texas, 1996−2006

U.S. Geological Survey National Water-Quality Assessment Program

Geochemical Evolution Processes and Water-Quality 

Observations Based on Results of the National WaterQuality Assessment Program in the San Antonio Segment 

of the Edwards Aquifer, Texas, 1996−2006

Scientific Investigations Report 2010–5129
Front cover:

Top, Public-supply well completed in San Antonio segment of the Ewards aquifer (photograph by Lynne 

Fahlquist, U.S. Geological Survey).

Left, Surface-water creek, San Antonio segment of the Edwards aquifer (photograph by Libby Stern, 

U.S. Geological Survey).

Bottom, Surface-water creek, San Antonio segment of the Edwards aquifer (photograph by MaryLynn 

Musgrove, U.S. Geological Survey).

Back cover:

Top right, Surface-water creek, San Antonio segment of the Edwards aquifer (photograph by Libby 

Stern, U.S. Geological Survey).

Top left, Public-supply well completed in San Antonio segment of the Ewards aquifer (photograph by 

Libby Stern, U.S. Geological Survey).

Bottom right, Surface-water creek, San Antonio segment of the Edwards aquifer (photograph by Libby 

Stern, U.S. Geological Survey).

Bottom left, Overview of Edwards Plateau, Central Texas (photograph by Libby Stern, U.S. Geological 

Survey). 
Geochemical Evolution Processes and 

Water-Quality Observations Based on 

Results of the National Water-Quality 

Assessment Program in the San Antonio 

Segment of the Edwards Aquifer, Texas, 

1996−2006 

By MaryLynn Musgrove, Lynne Fahlquist, Natalie A. Houston, Richard J. 

Lindgren, and Patricia B. Ging

U.S. Geological Survey National Water-Quality Assessment Program

Scientific Investigations Report 2010–5129

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Musgrove, M., Fahlquist, L., Houston, N.A., Lindgren, R.J., and Ging, P.B., 2009, Geochemical evolution processes and 

water-quality observations based on results of the National Water-Quality Assessment Program in the San Antonio 

segment of the Edwards aquifer, 1996–2006: U.S. Geological Survey Scientific Investigations Report 2010−5129, 93 p. 

(Appendixes available online at http://pubs.usgs.gov/sir/2010/5129/.)
iii

Acknowledgments

The authors thank Bruce D. Lindsey and Barbara J. Mahler, U.S. Geological Survey, and 

Geary Schindel, Edwards Aquifer Authority, for their helpful technical reviews of this report.
Blank Page
v

Contents

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................2

Purpose and Scope .............................................................................................................................3

Background and Previous Studies ...................................................................................................3

Hydrogeologic Setting ........................................................................................................................3

Variability in Water Quality in Karst Aquifers ..................................................................................9

Methods ..........................................................................................................................................................9

Overview of Geochemical Data ................................................................................................................12

Geochemical Evolution Processes ..........................................................................................................16

Major/Trace Element Ratios .............................................................................................................16

Strontium Isotopes .............................................................................................................................20

Stable Isotopes ...................................................................................................................................20

Salinity Sources .................................................................................................................................22

Radon ...................................................................................................................................................24

Water-Quality Observations ......................................................................................................................26

Nitrate: Occurrence and Trends ......................................................................................................26

Historical Nitrate and Chloride Concentrations at Comal Springs ............................................28

Selected Anthropogenic Organic Contaminants: Occurrence and Trends .............................29

Relation Between Geochemistry and Hydrologic Conditions ....................................................33

Temporal Variability in Geochemical Constituents .............................................................35

Historical Major/Trace Element Ratios .................................................................................42

Groundwater Age Tracers and Travel Times .................................................................................42

Tritium .........................................................................................................................................45

Chlorofluorocarbons and Sulfur Hexafluoride .....................................................................46

Flow Model Estimates of Travel Times and Flow Rates ......................................................47

Implications for Monitoring for Water-Quality Trends in Karst ...........................................................50

Summary .......................................................................................................................................................50

References ...................................................................................................................................................53

Appendixes (available online at http://pubs.usgs.gov/sir/2010/5129/)

1 Site information for wells and springs sampled for National Water-Quality 

Assessment studies, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1996−2006 

2. Physicochemical measurements for groundwater samples collected for National 

Water-Quality Assessment studies, San Antonio segment of the Edwards aquifer, 

south-central Texas, 1996−2006

3. Dissolved solids and major ion measurements for groundwater samples collected 

for National Water-Quality Assessment studies, San Antonio segment of the 

Edwards aquifer, south-central Texas, 1996−2006

4. Nutrient measurements for groundwater samples collected for National WaterQuality Assessment studies, San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006

5. Trace element measurements for groundwater samples collected for National 

Water-Quality Assessment studies, San Antonio segment of the Edwards aquifer, 

south-central Texas, 1996−2006
vi

6. Stable isotope and radionuclide measurements for groundwater samples 

collected for National Water-Quality Assessment studies, San Antonio segment 

of the Edwards aquifer, south-central Texas, 1996−2006

7. Volatile organic and fuel oxygenate compounds for groundwater samples 

collected for National Water-Quality Assessment studies, San Antonio segment 

of the Edwards aquifer, south-central Texas, 1996−2006

8A. Soluble pesticide compounds for groundwater samples collected for National 

Water-Quality Assessment studies, San Antonio segment of the Edwards aquifer, 

south-central Texas, 1996−2006

8B. Polar pesticide compounds for groundwater samples collected for National 

Water-Quality Assessment studies, San Antonio segment of the Edwards aquifer, 

south-central Texas, 1996−2006

8C. Acetamide, glyphosate, amino-methyl-phosphoric acid, and glufosinate pesticide 

compounds for groundwater samples collected for National Water-Quality 

Assessment studies, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1996−2006

9. Wastewater indicator compounds for groundwater samples collected for 

National Water-Quality Assessment studies, San Antonio segment of the Edwards 

aquifer, south-central Texas, 1996−2006

10. Apparent age tracer concentrations and piston-flow model apparent ages for 

groundwater samples collected for National Water-Quality Assessment studies, 

San Antonio segment of the Edwards aquifer, south-central Texas, 1998 and 2006

11. Summary of particle-track results for conduit- and diffuse-flow models, San 

Antonio segment of the Edwards aquifer, south-central Texas

Figures

1. Map showing hydrogeologic setting and location of wells (shallow/urban 

unconfined, unconfined, and confined) and springs (confined) sampled for water 

quality in the San Antonio segment of the Edwards aquifer, south-central Texas, 

1996–2006 ......................................................................................................................................4

2. Diagrammatic north-northwest to south-southeast section showing hydrogeologic 

framework and generalized groundwater flow directions, Edwards Plateau to Gulf 

Coastal Plain, San Antonio region, Texas ................................................................................6

3. Graphs showing time series (1996−2006) of (A) Comal Springs discharge and Bexar 

County index well (J−17) water-level altitude, and (B) San Antonio rainfall .....................7

4–5. Maps showing: 

4. Regional land use and location of wells (shallow/urban unconfined, 

unconfined, and confined) and springs (confined) sampled for water quality 

in the San Antonio segment of the Edwards aquifer, south-central Texas, 

1996−2006 .............................................................................................................................8

5. Surface-water sites sampled for water quality in or near the Edwards aquifer 

recharge zone, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1996−2007 ...............................................................................................................13

6. Trilinear diagram showing relations between major cations and anions in 

groundwater collected from shallow/urban unconfined, unconfined, and confined 

parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 

1996−2006 ....................................................................................................................................14
vii

7–10. Graphs showing: 

7. Relation between magnesium to calcium and strontium to calcium (molar 

ratios) for groundwater samples collected from shallow/urban unconfined, 

unconfined, and confined parts of the San Antonio segment of the Edwards 

aquifer, south-central Texas, 1996−2006; (A) full scale, and (B) shaded region 

in (A) ....................................................................................................................................17

8. Relation between magnesium to calcium (molar ratio) and dolomite mineral 

saturation index for groundwater samples collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of 

the Edwards aquifer, south-central Texas, 1996−2006 ................................................18

9. Relation between (A) magnesium to calcium (molar ratio) and calcium 

concentration, and (B) strontium to calcium (molar ratio x 103) and calcium 

concentration for groundwater samples collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of

the Edwards aquifer, 1996−2006 .....................................................................................19

10. Relation between (A) strontium-87/strontium-86 isotopic ratio and magnesium 

to calcium (molar ratio) and (B) strontium-87/strontium-86 isotopic ratio and 

strontium to calcium (molar ratio x 103) for groundwater samples collected 

from shallow/urban unconfined, unconfined, and confined parts of the San 

Antonio segment of the Edwards aquifer, south-central Texas, 1996–2006 ...........21

11. Graph of (A) relation between chloride concentration and delta deuterium for groundwater samples collected from shallow/urban unconfined, unconfined, and confined 

parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 

1996−2006, and (B) location map of sample sites associated with mixing trendlines 

shown in (A) ................................................................................................................................23

12–19. Graphs showing: 

12. Relation between (A) sulfate to chloride ratio and sulfate concentration, 

and (B) magnesium to sodium ratio and magnesium concentration for 

groundwater samples collected from shallow/urban unconfined, unconfined, 

and confined parts of the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996–2006 .................................................................................................25

13. Relation between magnesium to calcium (molar ratio) and radon-222 for 

groundwater samples collected from unconfined and confined parts of the 

San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006 ...26

14. Relation between well depth and (A) radon-222 and (B) magnesium to 

calcium (molar ratio) for groundwater samples collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of 

the Edwards aquifer, south-central Texas, 1996−2006 ................................................27

15. Relation between nitrate concentration and specific conductance for 

groundwater samples collected from shallow/urban unconfined, unconfined, 

and confined parts of the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006 .................................................................................................29

16. (A) Time series of nitrate concentration in Comal Springs discharge, and 

(B) relation between nitrate concentration at Comal Springs and Comal 

Springs discharge, San Antonio segment of the Edwards aquifer, southcentral Texas, 1938−2006 .................................................................................................30

17. Time series of chloride concentration in Comal Springs discharge, San 

Antonio segment of the Edwards aquifer, south-central Texas, 1938−2006 ...........31

18. (A) Detection frequency and (B) median concentration of the most 

frequently detected anthropogenic contaminants in groundwater samples 

collected from shallow/urban unconfined, unconfined, and confined parts of 

the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−

2006 .....................................................................................................................................32
viii

19. Most frequently detected anthropogenic contaminants, detection frequency 

in (A) shallow/urban unconfined category samples collected in 1998 and 2006, 

and (B) unconfined category samples collected in 1996 and 2006; median 

concentration in (C) shallow/urban unconfined category samples collected in 

1998 and 2006, and (D) unconfined category samples collected in 1996 and 

2006, San Antonio segment of the Edwards aquifer, south-central Texas ..............34

20. Map showing location of five shallow/urban unconfined monitoring wells with 

time-series data, and regional wells (shallow/urban unconfined, unconfined, and 

confined) and springs (confined) sampled for water quality in the San Antonio 

segment of the Edwards aquifer, south-central Texas, 1996−2006 ....................................36

21–24. Graphs showing: 

21. Time-series comparison of (A) depth to water in five shallow/urban 

unconfined category monitoring wells; (B) daily rainfall at San Antonio 

International Airport, and (C) discharge at Comal Springs, San Antonio 

segment of the Edwards aquifer, south-central Texas, 1998−2006 ...........................37

22. Time series comparison of (A) nitrate concentration, (B) atrazine 

concentration, (C) magnesium to calcium (molar ratio) for five shallow/urban 

unconfined category monitoring wells, and (D) discharge at Comal Springs, 

San Antonio segment of the Edwards aquifer, south-central Texas, 1998−

2006 .....................................................................................................................................38

23. Relation between Comal Springs discharge and (A) nitrate concentration, 

(B) atrazine concentration, and (C) magnesium to calcium (molar ratio) for 

five shallow/urban unconfined category monitoring wells, San Antonio 

segment of the Edwards aquifer, south-central Texas, 1998−2006 ...........................39

24. Time-series comparison of depth to water and (A) nitrate concentration, 

(B) atrazine concentration, and (C) magnesium to calcium (molar ratio) for 

shallow/urban unconfined category monitoring well AY−28−28−314, San 

Antonio segment of the Edwards aquifer, south-central Texas, 1998−2006 ...........40

25. Map showing location of unconfined and confined category wells with historical 

water-quality data, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1957−2006 ........................................................................................................................43

26–29. Graphs showing: 

26. Comparison of (A) time series of magnesium to calcium (molar ratio) for 

13 wells (two unconfined category and 11 confined category); (B) historical 

relation between Comal Springs discharge and depth to water at well 

TD−69−46−601 (confined category, Medina County); and (C) time series of 

magnesium to calcium (molar ratio) (note inverted scale) and depth to water 

at well TD−69−46−601 (confined category, Medina County), San Antonio 

segment of the Edwards aquifer, south-central Texas, 1970−2007 ...........................44

27. Relation between groundwater apparent ages for chlorofluorocarbon (CFC) 

(CFC−12 or CFC−113) and sulfur hexafluoride tracers for groundwater 

samples collected from shallow/urban unconfined category wells, San 

Antonio segment of the Edwards aquifer, south-central Texas, 2006 ......................46

28. Relation between magnesium to calcium (molar ratio) and apparent age for 

groundwater samples collected from shallow/urban unconfined and 

unconfined parts of the San Antonio segment of the Edwards aquifer, southcentral Texas, (A) 1998 and (B) 2006 ..............................................................................48

29. Comparison of apparent ages determined from geochemical age tracers 

(sulfur hexafluoride, chlorofluorocarbons, and tritium) for groundwater 

samples collected from shallow/urban unconfined, unconfined, and confined 

parts of the San Antonio segment of the Edwards aquifer, south-central 

Texas, 1996−2006, with mean particle-track travel-time estimates derived 

from conduit-flow and diffuse-flow models for the Edwards aquifer ......................49
ix

Tables

1. Summary of constituents by group for groundwater samples collected for National 

Water-Quality Assessment studies, San Antonio segment of the Edwards aquifer, 

south-central Texas, 1996−2006 ...............................................................................................10

2. Surface-water sites sampled for water quality in or near the Edwards aquifer 

recharge zone, San Antonio segment of the Edwards aquifer, south-central Texas, 

1996−2007 ....................................................................................................................................15

3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and 

stable isotope constituents analyzed in groundwater samples collected from the 

San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006 ............61

4. Summary statistics by group for organic constituents analyzed in groundwater 

samples collected from the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006 ..........................................................................................................69

5. Unconfined and confined category wells with historical water-quality data, San 

Antonio segment of the Edwards aquifer, south central Texas, 1957–2006 .....................91

6. Summary of age tracer data and interpreted apparent ages for groundwater 

samples collected from the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006 ..........................................................................................................92

7. Summary of model-calculated particle-track information for groundwater 

sample categories, San Antonio segment of the Edwards aquifer, south-central 

Texas ............................................................................................................................................93
x

Conversion Factors, Datums, Water-Quality Units, 

Abbreviations, and Isotope Unit Explanations

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

foot per day (ft/d) 0.3048 meter per day (m/d)

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.39370 inch (in.)

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft)

Flow rate

meter per day (m/d) 3.281 foot per day (ft/d)

Datums

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Altitude, as used in this report, refers to distance above the vertical datum.

Water-Quality Units

Chemical concentrations are reported in milligrams per liter (mg/L), micrograms per liter 

(μg/L), and milliequivalents per liter (meq/L). Milligrams per liter and micrograms per liter are 

units expressing the concentration of chemical constituents in solution as weight of solute 

(milligrams or micrograms) per unit volume (liter) of water. Milliequivalents per liter is a unit 

expressing the concentration of chemical constituents in solution as 1/1000 the molecular 

weight, in milligrams, divided by the valence of one molecule of the constituent per unit volume 

(liter) of water. Specific conductance is given in microsiemens per centimeter at 25 degrees 

Celsius (μS/cm at 25 °C).
xi

Abbreviations

femtoMol/kg, femtomoles per kilogram

μg/L, micrograms per liter

μS/cm, microsiemens per centimeter at 25 degrees Celsius

meq/L, milliequivalents per liter 

mg/L, milligrams per liter

NTRU, nephelometric turbidity ratiometric units

pCi/L, picocurie per liter

pk/kg, picograms per kilogram

pMol/kg, picomoles per kilogram

ppm, parts per million

pptv, parts per trillion per volume

SI, saturation index

TU, tritium units

Isotope Unit Explanations

Per mil (‰): A unit expressing the ratio of isotope abundances of an element in a sample to 

those of a standard material. Per mil units are equivalent to parts per thousand. Stable-isotope 

ratios are computed as follows (Kendall and McDonnell, 1998): 

 δX = {(Rsample -Rstandard)/ Rstandard} x 1,000, 

where 

	 δ is the “delta” notation, 

X is the heavier stable isotope, and

R is the ratio of the heavier, less abundant isotope to the lighter, stable isotope in a sample 

or standard. 

The δ values for stable-isotope ratios discussed in this report are referenced to the following 

standard materials: 

Element R Standard identity and reference 

hydrogen hydrogen-2/hydrogen-1 Vienna Standard Mean Ocean Water (VSMOW) (Fritz and 

Fontes, 1980) 

oxygen oxygen-18/oxygen-16 Vienna Standard Mean Ocean Water (VSMOW) (Fritz and 

Fontes, 1980) 
Blank Page
Abstract

As part of the National Water-Quality Assessment 

Program, the U.S. Geological Survey collected and analyzed 

groundwater samples during 1996–2006 from the San Antonio 

segment of the Edwards aquifer of central Texas, a productive 

karst aquifer developed in Cretaceous-age carbonate rocks. 

These National Water-Quality Assessment Program studies 

provide an extensive dataset of groundwater geochemistry and 

water quality, consisting of 249 groundwater samples collected from 136 sites (wells and springs), including (1) wells 

completed in the shallow, unconfined, and urbanized part 

of the aquifer in the vicinity of San Antonio (shallow/urban 

unconfined category), (2) wells completed in the unconfined (outcrop area) part of the regional aquifer (unconfined 

category), and (3) wells completed in and springs discharging from the confined part of the regional aquifer (confined 

category). This report evaluates these data to assess geochemical evolution processes, including local- and regional-scale 

processes controlling groundwater geochemistry, and to make 

water-quality observations pertaining to sources and distribution of natural constituents and anthropogenic contaminants, 

the relation between geochemistry and hydrologic conditions, 

and groundwater age tracers and travel time. Implications for 

monitoring water-quality trends in karst are also discussed.

Geochemical and isotopic data are useful tracers of 

recharge, groundwater flow, fluid mixing, and water-rock 

interaction processes that affect water quality. Sources of dissolved constituents to Edwards aquifer groundwater include 

dissolution of and geochemical interaction with overlying 

soils and calcite and dolomite minerals that compose the 

aquifer. Geochemical tracers such as magnesium to calcium 

and strontium to calcium ratios and strontium isotope compositions are used to evaluate and constrain progressive fluidevolution processes. Molar ratios of magnesium to calcium 

and strontium to calcium in groundwater typically increase 

along flow paths; results for samples of Edwards aquifer 

groundwater show an increase from shallow/urban unconfined, 

to unconfined, to confined groundwater categories. These 

differences are consistent with longer residence times and 

greater extents of water-rock interaction controlling fluid 

compositions as groundwater evolves from shallow unconfined groundwater to deeper confined groundwater. Results 

for stable isotopes of hydrogen and oxygen indicate specific 

geochemical processes affect some groundwater samples, 

including mixing with downdip saline water, mixing with 

recent recharge associated with tropical cyclonic storms, or 

mixing with recharge water than has undergone evaporation. 

The composition of surface water recharging the aquifer, as 

well as mixing with downdip water from the Trinity aquifer or 

the saline zone, also might affect water quality. 

A time-series record (1938−2006) of discharge at Comal 

Springs, one of the major aquifer discharge points, indicates 

an upward trend for nitrate and chloride concentrations, which 

likely reflects anthropogenic activities. A small number of 

organic contaminants were routinely or frequently detected in 

Edwards aquifer groundwater samples. These were the pesticides atrazine, its degradate deethylatrazine, and simazine; 

the drinking-water disinfection byproduct chloroform; and the 

solvent tetrachloroethene. Detection of these contaminants 

was most frequent in samples of the shallow/urban unconfined groundwater category and least frequent in samples 

of the unconfined groundwater category. Results indicate 

that the shallow/urban unconfined part of the aquifer is most 

affected by anthropogenic contaminants and the unconfined 

part of the aquifer is the least affected. The high frequency of 

detection for these anthropogenic contaminants aquifer-wide 

and in samples of deep, confined groundwater indicates that 

the entire aquifer is susceptible to water-quality changes as a 

result of anthropogenic activities.

Like many karst aquifers, the Edwards aquifer responds 

rapidly to changes in hydrologic conditions, with accompanying changes in water-level altitudes, spring discharge rates, 

and water quality. Samples of groundwater were collected one 

or two times during the study period (1996–2006) from most 

sites, a sampling frequency which does not allow for evaluation of temporal trends. Five of the shallow/urban unconfined 

Geochemical Evolution Processes and Water-Quality 

Observations Based on Results of the National WaterQuality Assessment Program in the San Antonio Segment 

of the Edwards Aquifer, Texas, 1996−2006

By MaryLynn Musgrove, Lynne Fahlquist, Natalie A. Houston, Richard J. Lindgren, and Patricia B. Ging
2 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

category wells had a higher frequency of sampling, with a total 

of eight samples collected from each well over the 10-year 

study period. These data indicate that changes in hydrologic 

conditions, as demonstrated by changes in rainfall, aquifer 

recharge, water-level altitudes in wells, and springflow at 

major aquifer discharge points, affect water quality. Variations 

in selected dissolved constituents or ratios of dissolved constituents (atrazine, nitrate, and magnesium to calcium molar 

ratios) at these five wells provide insight into the sources and 

transport of dissolved constituents and contaminants in the 

aquifer, and the relation between water quality and hydrologic conditions. These results suggest that atrazine enters 

the aquifer with surface-water recharge, whereas background 

nitrate concentrations in the aquifer are diluted by surfacewater recharge. Higher nitrate concentrations and magnesium 

to calcium ratios occur in conjunction with lower water-level 

altitudes during drier hydrologic conditions. 

Geochemical age tracers, including tritium, chlorofluorocarbons, and sulfur hexafluoride, were measured in a majority 

of the groundwater samples. Results for apparent groundwater 

ages indicated that samples of groundwater from the Edwards 

aquifer are dominated by young (that is, water recharged 

within approximately the past 50 years) and (or) modern water 

(in this report, samples with apparent ages that belong to the 

present day, that is within 10 years of the time of sampling), 

with extensive groundwater mixing. These results are consistent with the regional hydrogeology and previous studies 

that document a large range of travel times and groundwater 

mixing. Apparent ages for all uncontaminated groundwater 

samples (many of the chlorofluorocarbon and sulfur hexafluoride results indicated that samples were contaminated by 

non-atmospheric sources of these tracers) ranged from 1 to 52 

years, with a median of 20 years. These results are based on a 

piston-flow model, which might not adequately describe the 

range of ages in a mixed sample, which is a relevant consideration for karst aquifers such as the Edwards aquifer. In spite of 

uncertainties in age interpretation, age tracers provide insight 

into how changing hydrologic conditions affect groundwater 

recharge and mixing processes. Apparent age results for 

samples collected from the relatively shallow unconfined part 

of the aquifer were younger than those for samples collected 

from the deeper unconfined part of the aquifer.

A comparison of apparent groundwater ages with calculated particle-track travel times from existing hydrogeologic 

models indicates that these flow models do not accurately 

represent the fast-flow (conduit-dominated) component of this 

karst aquifer. The model results, however, do show a progression toward longer travel times from the unconfined to the 

confined parts of the aquifer, which is consistent with geochemical interpretations and aquifer hydrogeology. The results 

of this evaluation of NAWQA data have implications for the 

design of monitoring programs in karst aquifers. When monitoring programs are intended to provide the data necessary to 

assess long-term trends and increase understanding of karst 

aquifer functioning, time scales of hydrologic and geochemical variability need to be carefully considered.

Introduction

Forty percent of the Nation’s groundwater used for 

drinking water is obtained from karst aquifers (Quinlan and 

Ewers, 1989; Karst Waters Institute, 2008). Karst groundwater 

systems are subject to rapid changes in flow and discharge 

rates, water-level altitudes, and water quality, and as a result, 

are susceptible to contamination (White, 1988). The San 

Antonio segment of the Edwards aquifer in south-central 

Texas, one of the most productive karst aquifers in the world, 

is the sole source water supply for more than 2 million people 

in a rapidly urbanizing region (Sharp and Banner, 1997; U.S. 

Environmental Protection Agency, 2006). The San Antonio 

segment of the Edwards aquifer is in a part of Texas characterized by numerous sinkholes, caves, and losing streams that 

supply recharge to the aquifer and provide direct interaction 

between surface water and groundwater (Sharp and Banner, 

1997). Springs issuing from the Edwards aquifer provide habitat for several threatened and endangered species (Edwards 

Aquifer Research and Data Center, 2010). An understanding 

of the relation between variability in water quality and hydrologic conditions on different time scales including geochemical evolution processes, spatial and temporal variability of 

groundwater geochemistry, and natural and anthropogenic 

effects on water quality is necessary for resource management 

as well as for sustainability of springflow for threatened and 

endangered species habitat. 

Karst aquifers are composed of soluble host rocks, usually carbonates, that have dissolved preferentially to form 

large voids and conduits (White, 1988). As a result, karst 

aquifers are extremely heterogeneous, with a large range of 

porosity and permeability spanning many orders of magnitude. 

Aquifer systems with these features are susceptible to contamination because of the rapid transport of groundwater through 

high porosity voids and conduits with little opportunity for 

subsurface filtration, sorption, or degradation of dissolved or 

particulate constituents (White, 1988). Karst aquifer permeability ranges from matrix permeability, characterized by high 

storage and low transmissivity, to conduit permeability, characterized by low storage and high transmissivity (Schuster and 

White, 1971; Atkinson, 1977). Karst sometimes is considered 

in terms of a triple porosity/permeability model that includes 

matrix permeability (intergranular permeability of the unfractured rock), fracture permeability (permeability associated 

with joints and bedding plane partings, which are sometimes 

enlarged by dissolution), and conduit permeability (permeability associated with pipe-like openings ranging from 1 

centimeter to tens of meters (White, 2002). Most groundwater 

storage occurs within the aquifer matrix, but most transport 

occurs within conduits, which often dominate groundwater 

flow where present (White, 2002). Recharge to karst aquifers occurs as direct infiltration into karst features such as 

sinkholes and fractures (focused recharge), and as distributed 

infiltration into the rock matrix (diffuse recharge); the proportion of focused and diffuse recharge can vary greatly in time 

and space depending on a number of variables (Worthington, 
Introduction 3

2003; Mahler and others, 2006). In wells and springs that are 

dominantly affected by conduit flow, physicochemical properties, concentrations of geochemical constituents, and discharge 

(springs) or water-level altitudes (wells) can change rapidly 

(minutes to hours to days) following rainfall and aquifer 

recharge. In contrast, water-level altitudes in wells that are 

dominantly affected by matrix flow might show little response 

to recharge events and less variability in physicochemical properties and geochemistry. Natural discharge in karst 

aquifers often occurs through large springs, which represent a 

composite of all water moving through the aquifer; as a result, 

variations in spring discharge, physicochemical constituents, 

and geochemistry provide insight into karst aquifer hydrogeology (White, 2002). 

Purpose and Scope

The combined results of previous National Water-Quality 

Assessment (NAWQA) studies done in the San Antonio 

segment of the Edwards aquifer (1996−2006) provide an 

extensive dataset for analysis. The purpose of this report is 

to evaluate those previously collected and quality-assured 

data to assess geochemical evolution processes and to make 

water-quality observations. One of the long-term goals of the 

NAWQA Program is to assess status and trends in the quality of the Nation’s water resources. Toward those goals, this 

report evaluates (1) hydrologic processes in the Edwards 

aquifer such as mineral-solution reactions and groundwater 

geochemical evolution, (2) spatial and temporal (time-series) 

data for natural and anthropogenic constituents, (3) the relation between geochemistry and water quality with hydrologic 

conditions, (4) the utility of geochemical age tracers, and 

(5) a comparison of age-tracer data with existing flow models, 

to develop an integrated understanding of karst and aquifer 

processes. Implications for monitoring water-quality trends in 

karst are also discussed. Given the large number of analytical 

constituents that have been analyzed for samples of groundwater collected throughout the Edwards aquifer, this report 

presents an overview of variability in groundwater geochemistry and water quality, and controlling processes in the San 

Antonio segment of the aquifer. 

Background and Previous Studies

Knowledge of water resources and water quality is 

important for reasons of human and aquatic health as well as 

for costs and concerns associated with water management. In 

1991, the U.S. Geological Survey (USGS) implemented the 

NAWQA Program to characterize, in a nationally consistent 

manner, water quality of major surface-water and groundwater resources of the Nation, to determine natural and human 

factors that affect water quality, and to define trends in water 

quality (Gilliom and others, 1995). The NAWQA Program 

includes efforts to systematically assess the occurrence and 

distribution of natural and anthropogenic compounds in 

aquifers of regional importance. Another component of the 

NAWQA Program is to evaluate trends in water quality in 

these aquifers. Toward these ends, the NAWQA Program collected and analyzed 249 groundwater samples from 136 sites 

during 1996–2006 throughout the San Antonio segment of the 

Edwards aquifer in the south-central Texas (SCTX) study unit. 

The study unit includes the extent of the San Antonio segment 

of the aquifer, which is the area of focus of this report (fig. 1).

Previous studies of the Edwards aquifer provide 

insight into the hydrology and geochemistry of this system. 

Geochemical processes that affect groundwater geochemistry include interaction with overlying soils, mineral-solution 

reactions, and mixing with saline waters from downdip and 

underlying hydrostratigraphic units (Sharp and Clement, 

1988; Oetting and others, 1996; Groschen and Buszka, 1997; 

Musgrove and Banner, 2004). Geochemical tracers of mineralsolution reactions (such as the molar ratios of magnesium to 

calcium [Mg/Ca] concentrations and strontium to calcium 

[Sr/Ca] concentrations) can be indicative of groundwater residence time, which is a function of both flow path and water 

flux (Musgrove and Banner, 2004; Wong, 2008). Studies in 

the Barton Springs segment of the aquifer, a smaller segment 

of this large regional aquifer system (fig. 1), have documented 

extensive surface water-groundwater interaction, relations 

between variable hydrologic conditions and water quality in 

the aquifer, and movement of anthropogenic contaminants 

and dissolved constituents through the aquifer (Hauwert, 

Johns, and others, 2004; Hauwert, Sansom, and others, 2004; 

Mahler and others, 2006; Garner and Mahler, 2007; Mahler 

and Massei, 2007; Mahler and Garner, 2009). Groundwater 

resources susceptible to contamination can be affected by a 

variety of anthropogenic contaminants, including pesticides, 

volatile organic compounds (VOCs), and nutrients. Analyses 

of water samples for anthropogenic contaminants in the San 

Antonio segment of the Edwards aquifer in recent years have 

detected numerous anthropogenic contaminants, mostly at low 

concentrations (less than 1 microgram per liter [μg/L]) (Bush 

and others, 2000; Fahlquist and Ardis, 2004). 

Hydrogeologic Setting

The central Texas Edwards aquifer developed in Early 

Cretaceous-age limestone and dolomite rocks that are extensively faulted, fractured, and karstified. The Edwards aquifer, 

like many karst aquifers, is characterized by hydrologic variability—water-level altitudes in the aquifer can rise rapidly in 

response to rainfall and corresponding recharge, accompanied 

by increases in springflow; conversely, water-level altitudes 

and springflow decrease during periods of low rainfall and 

recharge. The San Antonio segment of the Edwards aquifer is 

present in a narrow band along the Balcones fault zone from 

the groundwater divide near Las Moras Springs in Kinney 

County to the groundwater divide north of San Marcos 

Springs in Hays County (fig. 1). Late Cenozoic faulting 

of the predominantly flat-lying region along the Balcones 

fault zone formed a series of high-angle normal en echelon 
4 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, TexasFigure 1. Hydrogeologic setting and location of wells (shallow/urban unconfined, unconfined, and confined) and springs (confined) sampled for water quality in the San Antonio 

segment of the Edwards aquifer, south-central Texas, 1996–2006. 

San Antonio

segment of the

Edwards aquifer

Barton Springs

segment of the

 Edwards aquifer

TEXAS Balcones 

escarpment

LOCATION MAP

Las MorasSprings

D

San Pedro Springs

MedinaLake

X X X X X X X X

98°

99°

100°

30°

29°

Austin

San Antonio

D

D

D

##

## # # ## ## # #

#

#

##

# #

#

#

"

" " # #

EXPLANATION

Edwards aquiferUnconfined aquiferConfined aquiferGroundwater divide(modified from Maclay,1995, plate 1; Mahler andothers, 2006, fig. 1)

X

#

"

D

Freshwater/saline-waterinterface (1,000 milligramsper liter dissolved solidsconcentration) (Schultz, 1994)Balcones escarpment (modifiedfrom Abbott and Woodruff,

1986, fig. 1)

Groundwater wellShallow/urban unconfinedUnconfinedConfinedConfined spring

" " " " "

" "

" " " " " " "" " " " " " " " " " "

" " " " " " " " " " " "

" " " " " " " " " " " " "

""

"

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office (1999), 

1:24,000Cities: Texas Natural Resources Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

Bexar Countyindex well (J–17)

West Nueces

Nueces

River

Frio River

Sabinal River

Hondo Creek

Verde Creek Guadalupe River

Medina River

Salado Creek

Cibolo Cree

B

k

lanco River

Onion Creek

Creek

San Marcos River

Colorado River

River

Barton

1,000 mg/L

1,000 mg/L

X

X X X X X X X X X X X X X X X X X X X X X X X X X X X

X

X

San Antonio

airport

Shallow/urban unconfined wells

Confined zone

Contributing zone

Recharge zone

Comal Springs

San Marcos Springs

Barton Springs

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

#

" #

##

X

#

#

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15 45
Introduction 5

down-toward-the-coast faults. The Balcones escarpment is the 

surface manifestation of the Balcones fault zone (fig. 1). The 

faulting resulted in a series of blocks of Edwards aquifer rocks 

that are partially to completely offset and divide the confined 

and unconfined parts of the aquifer (Maclay and Small, 1983) 

(fig. 2). The aquifer is composed of carbonate rocks of the 

Georgetown Formation (not shown in fig. 2) and Edwards 

Group (or their stratigraphic equivalents), which range in 

thickness from 350 to 500 feet (ft) (Burchett and others, 1986). 

The Georgetown Formation overlies the Edwards Group and is 

part of the Edwards aquifer. The less permeable upper member 

of the Glen Rose Limestone (not shown in fig. 2), which is 

the uppermost part of the Trinity Group, underlies the aquifer 

and acts as a lower confining unit across much of the region. 

Where confined, the aquifer is overlain by the Del Rio Clay 

and other less permeable units. The aquifer is about 180 miles 

(mi) long from west to east, ranges from 5 to 40 mi wide from 

north to south (Maclay, 1995), and is characterized by relatively high transmissivities (summarized by Lindgren, 2006). 

The northern limit of the recharge zone (outcrop) defines 

the northern aquifer boundary. The aquifer is bounded to the 

south by the downdip limit of potable water in the aquifer, 

below which groundwater salinity rises rapidly downgradient (Abbott, 1975; Sharp and Banner, 1997). The freshwater/

saline-water interface (Schultz, 1994) represents the transition 

(1,000 milligrams per liter [mg/L] dissolved solids concentration) from a zone of more active flow with fresh groundwater 

to a downdip zone of less active flow with saline groundwater. The downdip saline zone is characterized by numerous 

complex saline hydrochemical facies (Clement, 1989; Oetting 

and others, 1996). A small number of volcanic and intrusive 

igneous rocks locally cross-cut the aquifer, possibly affecting 

groundwater geochemistry on a local scale.

The recharge zone consists of approximately 1,250 

square miles (mi2) of Edwards Group limestone exposed at the 

surface (Hamilton and others, 2008) (fig. 1). Streams flowing 

south and east toward the Gulf of Mexico drain the Edwards 

Plateau contributing zone and recharge the aquifer by streamflow losses across the Balcones fault zone, where the porous 

and permeable Edwards Group outcrops (figs. 1 and 2). Most 

stream base flow and much of the stormflow recharges the 

aquifer through open solution channels (Maclay and Land, 

1988). The majority of recharge to the Edwards aquifer occurs 

as losing streams flow across the recharge zone, with estimates of recharge from streams ranging from 60 to 80 percent 

(Klemt and others, 1979; Maclay and Land, 1988; Thorkildsen 

and McElhaney, 1992; Ockerman, 2005). Most of the remaining recharge occurs as direct infiltration on the recharge zone, 

with some recharge occurring by leakage from the underlying 

Trinity aquifer (Sharp and Banner, 1997). This leakage is not 

well quantified, but might constitute as much as 10 percent of 

aquifer recharge (Geary Schindel, Edwards Aquifer Authority, 

written commun., 2009).

Regional groundwater flow is to the east and northeast 

with natural discharge occurring at large springs, predominantly Comal and San Marcos Springs (fig. 1). Domestic, 

livestock, municipal, agricultural, and industrial withdrawals, 

by way of wells, also contribute to groundwater discharge 

throughout the region (Hamilton and others, 2008). Recharge 

and discharge are not evenly distributed across the aquifer; 

most recharge occurs in the westernmost catchments of the 

region, whereas most discharge occurs in the eastern area. The 

proportion of discharge by wells relative to springs varies from 

year to year; springflow typically exceeds well discharge in 

wet years, and well discharge typically exceeds springflow in 

dry years (Hamilton and others, 2008). Comal Springs (fig. 1), 

one of the major aquifer discharge points, is hydrologically 

isolated from large sources of local recharge (Ogden and others, 1986). As such, Comal Springs represents the integration 

of the regional flow system of the San Antonio segment of the 

aquifer and its flow rate and geochemistry provide an indicator 

of regional hydrologic conditions. Several wells in the aquifer, 

such as the Bexar County index well (J−17), intersect major 

flow paths and respond quickly to changes in recharge and 

pumping; water-level altitude variations in well J−17 are also 

indicative of regional hydrologic conditions.

Previous studies indicate that groundwater flow is 

focused in highly permeable units and is affected by faulting 

throughout the aquifer (Abbott, 1975; Woodruff and Abbott, 

1979; Maclay and Small, 1986; Maclay and Land, 1988; 

Sharp, 1990). Water-level altitudes in the aquifer can rise 

rapidly in response to rainfall and corresponding recharge, 

accompanied by increases in springflow (high aquifer flow 

conditions); conversely, water-level altitudes and springflow 

decrease during periods of low rainfall and recharge (low 

aquifer flow conditions). Records of San Antonio rainfall 

(recorded at the San Antonio International Airport [National 

Oceanic and Atmospheric Administration, 2009]), discharge 

at Comal Springs, and water-level altitudes at well J−17 

(Edwards Aquifer Authority, 2009) illustrate the hydrologic 

variability of the Edwards aquifer (fig. 3). 

Mean annual rainfall decreases across the region from 

east to west, equaling 30.3 inches (in.) per year in San 

Antonio. The region is prone to climatic and hydrologic 

extremes (Griffiths and Straus, 1985; Jones, 1991). Some of 

the most extreme 1-day duration storms in the world have 

occurred along the Balcones escarpment (Slade, 1986). During 

the study period (1996−2006) annual rainfall in San Antonio 

ranged from 16.6 in. (1999) to 46.3 in. (2002). Storms often 

produce rapid runoff that provides recharge to the aquifer; 

numerous storms occurred during the 10-year study period. 

Droughts lasting from many months to years have been documented in the region since the earliest settlers began keeping 

records (Texas State Historical Association, 2009). During 

the study period, droughts lasting several months occurred in 

1996, 1998, 2000, and 2006. Wet periods occurred in summer 

and fall 1997, fall and winter 1998, late 2000 and early 2001, 

late 2001 and early 2002, and through most of 2003−05. These 

dry and wet periods were accompanied by changes in spring 

discharge (fig. 3) and water-level altitudes in wells. Comal 

Springs discharge during the study period ranged from 83 to 

509 cubic feet per second (ft3

/s), with a median value of 335 
6 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, TexasFigure 2. Diagrammatic north-northwest to south-southeast section showing hydrogeologic framework and generalized groundwater flow directions, Edwards Plateau to Gulf EXPLANATION

Coastal Plain, San Antonio region, Texas (modified from Barker and Ardis, 1996, plates 1 and 3). 

UPPER CONFINING UNIT

EDWARDS AQUIFER

UNCONFINED AQUIFER

CONFINED AQUIFER

EDWARDS-TRINITYAQUIFER

TRINITY AQUIFER

APPROXIMATE FRESHWATER/SALINEWATER INTERFACE

FAULT—Arrows indicate direction

LIMESTONESAND

UNDERLYING ROCK UNITS

GENERALIZED CONDUIT SYSTEM

GENERALIZED DIRECTION OF GROUNDWATER FLOW

of movement

EDWARDS PLATEAU

HILL COUNTRY

GULF COASTAL

PLAIN

Edwards

Trinity Group

Edwards Group

Trinity

Group

Del Rio Clay, Buda Limestone,

Eagle Ford Group, Austin Chalk, Anacacho Limestone,

and overlying units

Trinity Group

Edwards Group

Edwards Group

Hammett Shale

Group

Balcones

escarpment

NOT TO SCALE

Trinity Group

Edwards aquifer recharge zone

Balcones fault zone

Contributing zone

Unconfined

aquifer Confined aquifer
Introduction 7

ft3/s, which is close to the historical median (1930−2007) of 

308 ft3/s. 

Most of the land use in the region correlates with physiography (fig. 4). The Edwards Plateau is characterized by 

thin soils and is predominantly undeveloped rangeland. There 

is little agriculture in the study area, with agricultural lands 

mostly on the coastal plain to the west of San Antonio, over 

the confined aquifer. Regional land use is characterized as 3 

percent agriculture, 6 percent urban, and 90 percent forest and 

rangeland (including shrub and grassland); the remaining 1 

percent is water, wetlands, and barren land (Homer and others, 

2001) (fig. 4). The city of San Antonio is the principal urban 

area and includes much of Bexar County in the east-central 

part of the aquifer region. San Antonio is the seventh largest 

city in the United States with a 2007 population of about 1.3 

million (U.S. Census Bureau, 2009). Although pumping in 

the San Antonio area has increased about fivefold since the 

1930s, aquifer water-level altitudes have not shown long-term 

declines because the aquifer readily recharges during periods 

of rainfall (Bush and others, 2000). 

1/1/19961/1/19971/1/19981/1/19991/1/20001/1/20011/1/20021/1/20031/1/20041/1/20051/1/2006

620

630

640

650

660

670

680

690

700

710

BEXAR COUNTY INDEX WELL (J–17) WATER-LEVEL 

ALTITUDE, FEET ABOVE MEAN SEA LEVEL

100

150

200

250

300

350

400

450

500

550

COMAL SPRINGS DISCHARGE, 

CUBIC FEET PER SECOND

0

2

4

6

8

10

SAN ANTONIO AIRPORT RAINFALL, INCHES

COMAL SPRINGS

BEXAR COUNTY

INDEX WELL (J–17)

(A)

(B)

610

Figure 3. Time series (1996−2006) of (A) Comal Springs discharge and Bexar County index well (J−17) water-level altitude (Edwards 

Aquifer Authority, 2009), and (B) San Antonio rainfall (recorded at the San Antonio International Airport) (National Oceanic and 

Atmospheric Administration, 2009). 
8 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, TexasFigure 4. Regional land use and location of wells (shallow/urban unconfined, unconfined, and confined) and springs (confined) sampled for water quality in the San Antonio 

segment of the Edwards aquifer, south-central Texas, 1996−2006. 

XY

XY

XY

XY

XY

YX

YX

YX

XY

XY

YX

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

XY

Shallow/urban unconfined wells

XY#*

")

Groundwater wellShallow/urban unconfinedUnconfinedConfinedConfined spring

Land coverUrban/built-upAgricultureRangelandForest

EXPLANATION

WaterWetlandBarren land

Land use from National Land Cover

Database, Homer and others (2001)

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office 

(1999), 1:24,000Cities: Texas Natural Resources Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

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FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

West Nueces

Nueces

River

Frio River

Sabinal River

Hondo Creek

Verde Creek Guadalupe River

Medina River

Salado Creek

Cibolo Cree

B

k

lanco River

Onion Creek Barton Creek

San Marcos River

Colorado River

River

West Nueces

Nueces

River

Frio River

Sabinal River

Hondo Creek

Verde Creek Guadalupe River

Medina River

Salado Creek

Cibolo Cree

B

k

lanco River

Onion Creek Barton Creek

San Marcos River

Colorado River

River

Austin

San Antonio

98°

99°

100°

30°

29°

Comal Springs

San Marcos Springs

Las MorasSprings San Pedro 

Springs

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15 45

Confined aquifer

Unconfined aquifer

Confined zone

Contributing zone

Recharge zone
Methods 9

Water-quality concerns associated with land use include 

contaminants resulting from development and urbanization on 

the recharge zone (anthropogenic contaminants), abandoned 

or poorly completed water wells, and urban nonpoint runoff 

(Hamilton and others, 2008). Like many karst aquifers, high 

porosity and permeability and rapid flow of recharging surface 

water through conduits, with little to no filtration, render the 

Edwards aquifer highly susceptible to contamination.

Variability in Water Quality in Karst Aquifers

Karst aquifers are characterized by extreme variability 

and rapid response to changes in environmental, climatic, 

and hydrologic conditions, which exert a fundamental control 

on groundwater geochemistry and water quality. Temporal 

variability in hydrologic conditions for the Edwards aquifer 

is apparent in data for rainfall, aquifer recharge, water-level 

altitudes in wells, and springflow (for example, fig. 3). 

Temporal changes in hydrologic conditions such as waterlevel altitudes, recharge, groundwater travel times, and flow 

routing affect hydrogeologic processes, including the extent 

of variability in water-rock interaction processes, dilution 

of recharge, and associated geochemistry. Rapidly changing hydrologic conditions and a continuum of flow pathways from matrix to conduit flow imply that hydrogeologic 

processes might operate on different temporal and spatial 

scales in different parts of the aquifer. For example, groundwater dominated by matrix/diffuse flow or less affected by 

focused recharge might exhibit less temporal variability than 

groundwater affected by focused recharge and conduit flow. 

Numerous studies have documented temporal variability 

in karst groundwater in response to changes in hydrologic 

conditions, storms, and recharge, and variable mixtures of 

base flow and stormflow (for example, Hess and White, 1993; 

Desmarais and Rojstaczer, 2002; Vesper and White, 2004; 

Winston and Criss, 2004).

Previous work in the Edwards aquifer has used temporal variability in geochemistry to investigate transport and 

the influence of hydrologic conditions. Mahler and Massei 

(2007) describe rapid changes in concentrations of anthropogenic contaminants in springflow in response to storms in the 

Barton Springs segment of the Edwards aquifer. Musgrove 

and Banner (2004) describe temporal variations in unsaturated-zone groundwater geochemistry that record changes in 

hydrologic conditions. Mahler (2008) investigates the relation between rainfall and aquifer water level on groundwater 

geochemistry for a group of wells transecting the freshwater/

saline-water interface in San Antonio, showing a clear relation 

for some wells and constituents.

Methods

The USGS collected groundwater samples from Edwards 

aquifer groundwater wells and springs during 1996−2006 

for studies done as part of the NAWQA Program (table 1; 

appendix 1). Sampling primarily occurred during 1996−98 and 

again during 2005−06. A small subset of wells was sampled 

repeatedly during 1999−2004. The resulting groundwater 

dataset encompasses a total of 249 samples collected from 

136 sites between June 1996 and December 2006. For this 

analysis, groundwater samples are grouped into categories of 

“shallow/urban unconfined,” “unconfined,” and “confined.” 

These categories are representative of groundwater evolution 

along a hypothetical flow path encompassing endmembers 

of (1) shallow, unconfined, younger, less geochemically 

evolved groundwater (shallow/urban unconfined category) 

and (2) deep, confined, older, more geochemically evolved 

groundwater (confined category). Unconfined groundwater is intermediate between these two endmembers in this 

conceptual flow path. Monitoring wells from which samples 

in the shallow/urban unconfined category were collected 

are relatively shallow (median well depth of 263 ft below 

land surface). Wells from which samples in the unconfined 

and confined categories were collected, in comparison, have 

median well depths of 385 and 859 ft below land surface, 

respectively (excluding those with unknown well depths). 

Groundwater samples collected from four springs discharging 

from the confined part of the Edwards aquifer (Las Moras, San 

Pedro, Comal, and San Marcos Springs) are included in the 

confined category. 

NAWQA studies of the San Antonio segment of the 

Edwards aquifer during 1996−2006 provide groundwater 

geochemistry data with extensive spatial coverage of the 

Edwards aquifer. Time-series data, for the most part, were 

limited to one or two samples over the 10-year study period. 

A small number of the shallow/urban unconfined groundwater 

sampling sites had a higher frequency of sampling and these 

results provide a more extensive time series of geochemical variability. Samples were collected eight times over the 

10-year study period from five shallow/urban unconfined 

sampling sites in Bexar County (appendix 1). Time-series 

water-level and water-quality data for these five wells allow 

for analysis of controls on water-quality variations (environmental, climatic, and hydrologic conditions), sources of 

anthropogenic contaminants, and the relation between hydrologic conditions and water quality. High-resolution time-series 

data for regional hydrologic conditions, including rainfall, 

discharge at Comal Springs, and water-level altitudes in the 

confined Bexar County index well (J−17) are readily available 

for comparison (Edwards Aquifer Authority, 2009; National 

Oceanic and Atmospheric Administration, 2009).

Different groups of wells and springs were sampled. 

The first group consisted of 30 water-level monitoring wells 

completed in 1998 in the urban San Antonio recharge zone 

(outcrop) of the Edwards aquifer in northern Bexar County; 

the recharge zone is the unconfined part of the aquifer. All of 

these wells are relatively shallow (median well depth of 263 ft 

below land surface); their locations were chosen randomly as 

part of a statistical sampling design throughout the urban areal 

extent of the aquifer. The land-use setting of each of these 

wells is classified as either residential or commercial; about 
10 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

30 years ago, all of this area was outside the urban footprint 

of San Antonio on ranch or farmland. These wells and the 

samples collected from them are categorized hereinafter as 

the “shallow/urban unconfined” groundwater category. The 

shallow/urban unconfined wells were sampled in 1998 and 

again in 2006. Different hydrologic conditions were associated with the two sampling periods. Samples were collected 

after a large storm in fall 1998, with a correspondingly larger 

amount of aquifer recharge, higher water-level altitudes, and 

larger spring discharges compared to 2006, a relatively dry 

year. Median springflow at Comal Springs during the 1998 

sampling period was 429 ft3/s, compared to 247 ft3/s during 

the 2006 sampling period for these wells. Additionally, five 

of the 30 shallow/urban unconfined wells were designated 

for more frequent sampling to document long-term geochemical trends and were sampled as many as six additional 

times between 1998 and 2006, under varying hydrologic 

conditions. 

The second group of wells included primarily domesticsupply wells completed in the San Antonio segment of the 

recharge zone (outcrop area) of the Edwards aquifer in a 

variety of land-use settings but at greater depths compared to 

those in the shallow/urban unconfined category. The median 

depth of wells in the second group was 385 ft below land 

surface (excluding those with unknown well depths). Wells 

in the second group and the samples collected from them are 

categorized hereinafter as the “unconfined” groundwater category. Wells in the unconfined groundwater category, located 

randomly as part of a statistical sampling design throughout 

the areal extent of the unconfined aquifer, were sampled in 

1996 and 2006. Twenty-eight wells in this category were 

sampled in 1996 and 23 of these were resampled in 2006, 

as well as 7 additional wells in this category. Five of the 

unconfined category wells were also sampled in 2002 and 

2005. Different hydrologic conditions prevailed during the 

two main sampling intervals for unconfined category wells; 

conditions were exceptionally dry in 1996 (median discharge 

at Comal Springs was 92 ft3

/s) and moderately dry in 2006 

(median discharge at Comal Springs was 269 ft3/s). A few 

of the unconfined groundwater category wells are located 

south of the land-surface delineation of the unconfined aquifer 

(fig. 1). These wells are located near the aquifer unconfined 

Table 1. Summary of constituents by group for groundwater samples collected for National Water-Quality Assessment studies, San 

Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006.

[

87Sr/86Sr, strontium-87/strontium-86; δD, delta deuterium; δ18O, delta oxygen-18; δ13C, delta carbon-13]

Well type

Year 

sampled

Number of 

wells 

sampled

Physicochemical 

constituents

Major 

ions

Trace 

elements

Nutrients

Dissolved 

organic 

carbon

Volatile 

organic 

compounds

Pesticide 

compounds

Wastewater 

indicator 

compounds

Age 

tracers1

87Sr/

86Sr

δD, 

δ18O δ13C Radon Radium

Shallow/urban unconfined

Monitor 1998 30 x x x x x x x x x x x

Monitor 2006 30 x x x x x x x x x

Monitor 1999, 

2000, 

2001, 

2002, 

2004

5 x x x x x x

Unconfined

Mostly 

domestic

1996 28 x x x x x x x x x x

Mostly 

domestic

2006 30 x x x x x x x x x x x

Domestic 2002, 

2005

5 x x x x x x

Public 2005 3 x x x x x x x x

Confined

Mostly 

public2

1997 36 x x x x x x x x x x

Public 2005 8 x x x x

Public 2004, 

2005

34 x x x x x x x x

1

 More information on age tracers in table 6.

2

 Includes samples from four springs (Las Moras, San Pedro, Comal, and San Marcos).
Methods 11

(outcrop) area and are classified as unconfined groundwater 

category wells on the basis of water-level altitude fluctuations 

similar to those observed in other unconfined (subcrop) wells. 

The third group of sampling locations consisted of mostly 

public-supply wells completed in the confined part of the 

Edwards aquifer and springs discharging from the confined 

part of the aquifer. These wells and springs and the samples 

collected from them are categorized hereinafter as the “confined” groundwater category. Wells from which samples in the 

confined category were collected have median well depths of 

859 ft below land surface (excluding those with unknown well 

depths). Thirty-six samples were collected in 1997 from wells 

and springs in the confined groundwater category. Sampling 

sites were mostly wells; six samples were collected from four 

springs discharging from the confined part of the aquifer (Las 

Moras, San Pedro [two orifices], Comal, and San Marcos [two 

orifices] Springs). Hydrologic conditions during the 1997 sampling period, as represented by discharge at Comal Springs, 

were close to the historical median value of 308 ft3

/s (median 

discharge during the sampling period was 296 ft3/s). 

Thirty-nine randomly selected public-supply wells 

throughout the aquifer were sampled during 2004−05 as part 

of NAWQA source-water quality assessment and transport of 

anthropogenic and natural contaminants (TANC) to publicsupply wells studies. Of these 39 wells, 34 were completed in 

the confined part of the aquifer and 5 were completed in the 

unconfined part of the aquifer. Samples collected from these 

wells are included in the confined and unconfined categories, respectively. Fifteen of these public-supply wells were 

large-capacity wells selected from the top quartile of pumping 

(76th to 100th percentile), 13 of which are in Bexar County. 

The remaining 24 public-supply wells that were sampled 

included 8 each from the lower 3 quartiles of pumping. A few 

of these confined category wells were also sampled in 1997. 

Hydrologic conditions during the 2004−05 sampling period 

were relatively wet (median Comal Springs discharge of 

399 ft3/s).

The aquifer sample categories of shallow/urban unconfined, unconfined, and confined groundwater are useful for 

contrasting different regional hydrologic and land-use effects 

on water quality. The hypothesis that land use affects water 

quality is well established and has been explored in a variety 

of settings (Eckhardt and Stackelberg, 1995; Honisch and 

others, 2002). The water-quality data associated with the categories of shallow/urban unconfined, unconfined, and confined 

groundwater represent different types of land use that can help 

explain differences in water quality. Wells in the shallow/urban 

unconfined category are in northern Bexar County, a rapidly urbanizing area. Groundwater samples from these wells 

provide information about urban influences on water quality in 

contrast to other hydrologic settings. Wells in the unconfined 

category, while randomly located throughout the unconfined 

part of the aquifer, were mostly in undeveloped and rangeland 

land-use settings. Wells and springs in the confined category, 

randomly located throughout the confined part of the aquifer, 

are in both developed and undeveloped land-use settings. 

Physicochemical properties (depth to water, air and water 

temperature, air pressure, dissolved oxygen, specific conductance, pH, turbidity, and alkalinity) were measured in the field 

at the time of sample collection. Samples were analyzed at 

USGS laboratories (with a few noted exceptions) for numerous geochemical constituents, including major ions; nutrients; 

trace elements; isotopes (hydrogen [hydrogen-2/hydrogen-1 

(

2

H/1H)], oxygen [oxygen-18/oxygen-16 (18O/16O)], carbon 

[carbon-13/carbon-12 (13C/12C)], and strontium [strontium-87/

strontium-86 (87Sr/86Sr)]); tritium (3H) and other geochemical age tracers; radon (Rn); radium (Ra); dissolved organic 

carbon; pesticide compounds; and volatile organic compounds 

(VOCs) (table 1). Results for hydrogen and oxygen isotopes in water are reported as delta deuterium (δD) and delta 

oxygen-18 (δ18O), respectively, which represent the relative 

difference in parts per thousand (per mil) between the sample 

isotope ratio and the isotope ratio of a known standard (Vienna 

Standard Mean Ocean Water [VSMOW]) (Hagemann and 

others, 1970; Baertschi, 1976). Not all constituents were measured on all samples; some samples were analyzed for fewer 

constituents depending on specific study objectives. Some 

analyses for 87Sr/86Sr were done at the University of Texas at 

Austin (Musgrove, 2000). 

Site selection for pre-existing and newly drilled wells 

followed protocols described by Gilliom and others (1995), 

Lapham and others (1995), and Squillace and Price (1996). 

All samples were collected according to protocols developed 

for the NAWQA Program (Koterba and others, 1995; Lapham 

and others, 1995; Koterba, 1998; U.S. Geological Survey, 

variously dated). In addition to laboratory quality-control 

samples, field quality-control samples were collected in the 

form of replicate, organic spike, and blank samples to aid in 

evaluating data quality. All of the analytical data, including 

results for quality-control samples, are in the USGS National 

Water Information System (NWIS) (U.S. Geological Survey, 

2009). A list of references describing analytical methods 

is provided in Fahlquist (2003). Results for groundwater 

samples collected from the Edwards aquifer during 1996−98 

as part of the NAWQA Program are discussed by Bush and 

others (2000) and Fahlquist and Ardis (2004). USGS analytical method reporting conventions are defined in Childress and 

others (1999) and include laboratory reporting level, minimum reporting level, method detection limit, and long-term 

method detection level. The types of reporting levels for data 

considered herein varied depending on constituent, analytical 

methods, and time period. In this report, laboratory reporting 

level is used inclusively to describe reporting levels.

Also considered in the analyses for this report are data for 

groundwater quality, water level, and spring discharge from 

the Edwards Aquifer Authority (2009) and San Antonio Water 

System (Hydrogeologic Studies and Assessment Division, 

written commun., 2009). Comal Springs discharge and Bexar 

County index well (J−17) water-level altitudes, as well as data 

included in annual “Hydrogeologic Data Reports,” are available online from the Edwards Aquifer Authority (2009). Some 

water-level altitude, spring discharge, and surface-water and 
12 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

groundwater-quality data (1960−2006) from other studies and 

(or) sites were acquired from the USGS NWIS database (U.S. 

Geological Survey, 2009). Available historical geochemical 

data largely consist of inorganic data, for example major ions, 

trace elements, and (or) nutrients. Although few multidecadal 

historical data are available, a small number of groundwater wells distributed across the aquifer have geochemical 

data extending back to the 1970s. Rainfall data measured 

at the San Antonio International Airport during 1996−2006 

(Coop ID 417945) (fig. 3) were acquired from the National 

Climatic Data Center (National Oceanic and Atmospheric 

Administration, 2009). 

NAWQA surface-water studies have included sampling of 

some of the streams that directly recharge the Edwards aquifer; many of these sites have been sampled multiple times by 

the USGS and the data are available in the USGS NWIS database (U.S. Geological Survey, 2009). Geochemical data for 

24 surface-water sites across the San Antonio segment of the 

aquifer and within or in close proximity to the recharge zone 

were compiled for comparison with groundwater data to assess 

potential surface-water contributions to groundwater (fig. 5; 

table 2). Data collected for these sites during 1996–2007, the 

same approximate sampling period as NAWQA groundwater 

studies, were considered. These surface-water sites are in a 

variety of land-use settings. 

Nonparametric statistical tests were used to interpret the 

data. A non-normal distribution is a common characteristic 

of water-resources data (Helsel and Hirsch, 2002); as a result, 

non-parametric methods are usually better suited to evaluate water-resources data than traditional parametric methods, 

which require a normal distribution. The Kendall’s tau coefficient is a non-parametric, rank-based test used to measure the 

strength of the monotonic relation between x and y (linear and 

nonlinear) and is resistant to the effects of outliers (Helsel and 

Hirsch, 2002). The tau coefficient ranges from -1 to 1; a value 

of 0 indicates no correlation, values approaching -1 or 1 indicate an increasing strength of correlation. The tau coefficient 

values generally will be lower than values of traditional correlation coefficients for linear associations of the same strength; 

for example, a strong linear correlation of greater than or equal 

to .9 corresponds to a tau value of greater than or equal to 

about .7 (Helsel and Hirsch, 2002). The Kendall’s tau coefficient also was used as a tool to evaluate temporal trends, with 

one ranking representing time. The variety of data collected 

by the NAWQA Program, represented by different sample 

categories that were collected during specific time intervals, 

does not result in the type of time-series dataset appropriate 

for trend testing. Instead, the Mann-Whitney U test, a nonparametric test for comparing two independent groups of data, 

was used to test for differences between the three sample categories (samples collected from the shallow/urban unconfined, 

unconfined, and confined parts of the Edwards aquifer) and 

groups of samples collected over different time intervals (for 

example, unconfined category wells sampled in 1996 and in 

2006). For this report, statistical results with p-values less than 

.05 are considered statistically significant and are reported. 

The geochemical modeling program PHREEQC (Parkhurst, 

1995) was used to calculate ion speciation and mineral saturation states.

Overview of Geochemical Data

Using previously published NAWQA data (U.S. 

Geological Survey, 2009), summary statistics (minimum, maximum, median, 25th and 75th percentiles) were calculated for 

all Edwards aquifer groundwater samples and for the shallow/

urban unconfined, unconfined, and confined sample categories 

(table 3 [at end of report], physicochemical, field properties, 

inorganic, and isotopic properties and constituents; and table 4 

[at end of report], organic constituents). Site information for 

wells and springs sampled by the NAWQA Program during 

1996–2006 is provided in appendix 1. All of the data used in 

this report are listed in appendixes 2–11.

Edwards aquifer groundwater exhibits a relatively 

narrow range in major ion compositions across the region 

and is mostly a dilute calcium-bicarbonate facies; the term 

“facies” refers to a classification scheme used to describe 

the water in terms of the major anions and cations that the 

aquifer contains (Renken, 1998) (fig. 6). Specific conductance, 

which reflects the total dissolved solids content of the water, 

ranges from 377 to 1,200 microsiemens per centimeter at 

25 degrees Celsius (μS/cm) with a median value of 551 μS/

cm. A small number of groundwater samples had relatively 

high specific conductance values associated with relatively 

high concentrations of selected major ions, including chloride 

(Cl), sulfate (SO4

), sodium (Na), and (or) magnesium (Mg). 

Samples of shallow/urban unconfined groundwater have 

(1) higher median values for specific conductance, dissolved 

oxygen, calcium (Ca), and alkalinity concentrations and higher 

strontium isotope ratios; and (2) lower median values for Mg, 

strontium (Sr), and fluoride (F) concentrations and Mg/Ca and 

Sr/Ca molar ratios, than do unconfined and confined groundwater samples. Median values for potassium (K), bromide 

(Br), Na, Cl, SO4, and many trace metal concentrations (boron, 

chromium, copper, lithium, lead, and uranium) are lower in 

shallow/urban unconfined and (or) unconfined groundwater 

relative to confined groundwater. Median nitrate (NO3

) concentrations are similar in shallow/urban unconfined and confined groundwater samples (1.85 and 1.84 mg/L, respectively), 

and lower in unconfined groundwater samples (1.09 mg/L).

Most Edwards aquifer groundwater samples were close 

to saturation with respect to calcite (median saturation index 

[SI] = -0.12), but undersaturated with respect to dolomite 

(median SI = -0.83) and gypsum (median SI = -2.36) (table 

3). Confined groundwater samples are generally closer to 

equilibrium with respect to calcite, dolomite, and to a lesser 

degree, gypsum and anhydrite, than are unconfined and 

shallow/urban unconfined groundwater samples. Dolomite 

mineral SIs, in particular, show a progression toward greater 

saturation from shallow/urban unconfined to unconfined to 

confined groundwater samples (median SI = -0.97, -0.78, and 
Overview of Geochemical Data 13 Figure 5. Surface-water sites sampled for water quality in or near the Edwards aquifer recharge zone, San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2007. 

08198000 08179520

08180500

08201500

08202450

08200000 08200977

08180945

08180941

08180942

08178627

08178625

08181400

08181050

08180947

08178610 08177600

08178585

08178602

08178595

98°

99°

100°

30°

29°

EXPLANATION

Edwards aquiferUnconfined aquiferConfined aquiferSurface-water sample site and number (table 2)

#

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office 

(1999), 1:24,000Cities: Texas Natural Resources Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

West Nueces

Nueces

River

Frio River

Sabinal River

Hondo Creek

Verde Creek Guadalupe River

Medina River

Salado Creek

Cibolo Cree

B

k

lanco River

Onion Creek Barton Creek

San Marcos River

Colorado River

River

Confined zone

Contributing zone

Recharge zone

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15 45

08167800

08171000

08195000

08195500

08167800

#*

#*

#*

#*

#*#*

#*

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#*

#*

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#*

#*#*

#*

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14 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

-0.47, respectively). Calcite SIs for shallow/urban unconfined 

groundwater samples collected in 2006 during moderately 

dry hydrologic conditions are closer to equilibrium (median 

SI = -0.05) and statistically different (on the basis of MannWhitney U test results) than SIs for samples collected in 1998 

when conditions were wetter (median SI = -0.16). 

NAWQA studies detected a total of 63 anthropogenic 

organic contaminants in groundwater from the San Antonio 

segment of the Edwards aquifer (Bush and others, 2000; 

Fahlquist and Ardis, 2004). Concentrations of all detected 

anthropogenic contaminants were routinely low, typically 

much less than 1 μg/L, and less than U.S. Environmental 

Protection Agency (EPA) public drinking-water standards 

(specifically, maximum contaminant levels [MCLs]). The 

majority of anthropogenic contaminants were detected infrequently (that is, detected in less than 5 percent of samples). A 

small number of anthropogenic contaminants were routinely 

or frequently detected (that is, detected in 30 percent or more 

80 60 40 20 20 40 60 80

20

40

60

80 80

60

40

20

20

40

60

80

20

40

60

80

Shallow/urban unconfined 

Unconfined

Confined

EXPLANATION

X

#

"

BICARBONATE

SODIUM

CALCIUM CHLORIDE

MILLIEQUIVALENTS, PERCENT 

MAGNESIUM

CALCIUM + MAGNESIUM

SULFATE

SULFATE + CHLORIDE

100

0

100 100

100

100100

0

0

0

0 0

0

20

20

20

20

100

100

0

100

100

40

40

40

40

0

60 60

60 60

80 80

80 80

Groundwater

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Figure 6. Trilinear diagram showing relations between major cations and anions in groundwater collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. 
Overview of Geochemical Data 15

of samples). These frequently detected contaminants include 

the pesticides atrazine, its degradate deethylatrazine (DEA), 

and simazine; the drinking-water disinfection byproduct 

chloroform (a trihalomethane); and the solvent tetrachloroethene (PCE) (commonly used in dry cleaning). Less frequently 

detected constituents (that is, detected in 5 to 20 percent of 

samples) include the pesticide prometon and the trihalomethanes bromodichloromethane and tribromomethane (bromoform). Other atrazine degradates, CEAT (deisopropylatrazine 

or 2-chloro-6-ethylamino-4-amino-s-triazine) and OIET 

(hydroxyatrazine or 2-hydroxy-4-isopropylamino-6-ethylamino-s-triazine), were analyzed only for samples from confined wells (2004–05) and were detected (either one or both) 

in 30 percent of these samples. Atrazine is the most heavily used herbicide in the United States, with both extensive 

agricultural application and urban application on residential 

lawns and ornamentals. Simazine and prometon are commonly 

found in herbicides used to control weeds in urban areas; 

simazine is also sometimes used in agricultural applications; 

prometon is used for nonagricultural weed control around 

fences, buildings, and roads (Gilliom and others, 2006). The 

EPA MCLs for atrazine and simazine are 3 and 4 μg/L, respectively. There is currently no MCL for prometon. Atrazine, 

simazine, and prometon are among the most frequently 

detected herbicides in U.S. surface water and groundwater 

(Gilliom and others, 2006). Laboratory studies have indicated that low concentrations of atrazine in the environment 

result in endocrine disruption in amphibians, although this 

research is controversial (Hayes, 2004). Chloroform and other 

trihalomethanes are byproducts of drinking-water treatment 

Table 2. Surface-water sites sampled for water quality in or near the Edwards aquifer recharge zone, San Antonio segment of the 

Edwards aquifer, south-central Texas, 1996−2007.

[USGS, U.S. Geological Survey; FM, Farm Road; SH, State Highway]

USGS station name

USGS 

identification 

number 

(fig. 5)

County

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Number of 

samples 

(one or more 

constituents)

Range of 

years for 

collected 

samples

Guadalupe River at Sattler, Tex. 08167800 Comal 29.859108 -98.180011 2 1996−98

Blanco River at Wimberley, Tex. 08171000 Hays 29.994381 -98.088898 42 1996−98

Olmos Creek tributary at FM 1535, Shavano Park, Tex. 08177600 Bexar 29.576617 -98.546130 6 1997−98

Salado Creek at Wilderness Road, San Antonio, Tex. 08178585 Bexar 29.630781 -98.565576 15 1998−2007

Stone Mountain Drainage Channel at Granite Path, San 

Antonio,Tex.

08178595 Bexar 29.648559 -98.501407 12 1996−2008

Camp Creek at Wilderness Trail, San Antonio, Tex. 08178602 Bexar 29.636614 -98.531130 12 1998−2007

Lorence Creek at Donella Drive, San Antonio, Tex. 08178610 Bexar 29.599671 -98.478906 8 1996−98

Elm Waterhole Creek at Evans Road near San Antonio, 

Tex.

08178625 Bexar 29.648003 -98.411405 1 2000

Elm Waterhole tributary at Evans Road near San Antonio, 

Tex.

08178627 Bexar 29.646667 -98.406389 12 2001−07

Medina River below Medina Lake near San Antonio, Tex. 08179520 Medina 29.534119 -98.935308 3 1996

Medina River near Riomedina, Tex. 08180500 Medina 29.498000 -98.905500 5 1996

Government Canyon Creek site 2 near Helotes, Tex. 08180941 Bexar 29.539167 -98.751389 17 1997−2007

Laurel Canyon Creek near Helotes, Tex. 08180942 Bexar 29.556783 -98.745989 5 2004−07

Leon Creek at Scenic Loop Road near Leon Springs, Tex. 08180945 Bexar 29.675556 -98.675833 7 2001−07

Leon Creek at Hausman Road, San Antonio, Tex. 08180947 Bexar 29.571944 -98.619167 9 2001−04

Leon Creek at Prue Road, San Antonio, Tex. 08181050 Bexar 29.541389 -98.631667 15 2000−2007

Helotes Creek at Helotes, Tex. 08181400 Bexar 29.578561 -98.691690 22 1997−2007

Frio River at Concan, Tex. 08195000 Uvalde 29.488565 -99.704776 63 1996−2004

Unnamed tributary near Concan, Tex. 08195500 Uvalde 29.427455 -99.666719 1 1997

Sabinal River near Sabinal, Tex. 08198000 Uvalde 29.491064 -99.492824 33 1996−2003

Hondo Creek near Tarpley, Tex. 08200000 Medina 29.569673 -99.246706 2 2003

Middle Verde Creek at SH 173 near Bandera, Tex. 08200977 Medina 29.567778 -99.096944 4 2001−03

Seco Creek at Miller Ranch near Utopia, Tex. 08201500 Medina 29.573284 -99.403100 25 1996−2003

Seco Creek Reservoir Inflow near Utopia, Tex. 08202450 Medina 29.526341 -99.395322 6 1996−97
16 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

processes. The EPA MCL for total trihalomethanes is 80 μg/L. 

PCE is an industrial solvent primarily used in dry cleaning and 

metal degreasing and is also in household products such as 

paint and spot removers. The EPA MCL for PCE is 5 μg/L.

Results for some organic compounds in USGS National 

Water Quality Laboratory Schedule 1433 (caffeine, phenol, 

benzophenone, and 4-nonylphenol) were not considered 

because of analytical limitations during the study period 

(Zaugg and Leiker, 2006). For example, standard methods for 

laboratory blank detections were not established until January 

2006 for caffeine. Results for DEA were qualified as estimated 

(“E”) by the National Water Quality Laboratory under laboratory reporting conventions used during the study period. A 

remark code of “E” was used when compounds were identified, but concentrations could not be accurately quantified 

for a variety of reasons. Analytical recoveries typically were 

lower than expected for DEA; therefore concentrations were 

qualified (Sandstrom and others, 2001). Discussion of anthropogenic contaminants in this report focuses on the five most 

frequently detected anthropogenic contaminants, atrazine, 

DEA, simazine, chloroform, and PCE. 

Geochemical Evolution Processes

Interactive processes between fluid and rock in porous 

media (water-rock interaction) include mineral-solution reactions, diagenesis, and fluid mixing. Geochemical and isotopic 

data are useful tracers of water-rock interaction processes, 

recharge, groundwater flow, and fluid mixing that might affect 

water quality. Previous work in the Edwards aquifer provides 

a framework of understanding for regional-scale geochemical 

processes that affect and control spatial and temporal variability of fluid geochemistry, especially with respect to major and 

trace elements such as Ca, Mg, and Sr (for example, Oetting, 

1995; Sharp and Banner, 1997; Musgrove and Banner, 2004; 

Wong and others, 2007). An understanding of geochemical 

evolution processes coupled with an evaluation of waterquality data provides information on groundwater residence 

times, endmember compositions and mixing processes, 

flow path length, and aquifer susceptibility to anthropogenic 

contaminants.

Major/Trace Element Ratios

Geochemical processes of water-rock interaction and 

progressive groundwater evolution in carbonate waters (for 

example, by processes of calcite recrystallization, incongruent 

dolomite dissolution, and prior precipitation of calcite along 

flow paths) can result in increases in Mg/Ca and Sr/Ca ratios 

with increasing groundwater residence time over multiple time 

scales (Plummer, 1977; Trudgill and others, 1980; Lohmann, 

1988; Fairchild and others, 2000; Musgrove and Banner, 2004; 

Fairchild and others, 2006). Given this evolution pathway, 

Mg/Ca and Sr/Ca ratios in carbonate groundwater typically 

increase along flow paths. Mg/Ca and Sr/Ca ratios of Edwards 

aquifer groundwater samples were well correlated (Kendall’s 

tau = .61) and increase from shallow/urban unconfined to 

unconfined to confined groundwater sample categories (fig. 7). 

For shallow/urban unconfined, unconfined, and confined 

groundwater sample categories, the median values for Mg/

Ca (0.13, 0.27, 0.35, respectively) and for Sr/Ca (x103) (0.59, 

1.67, 3.08, respectively) are statistically different on the basis 

of Mann-Whitney U test results. These differences among 

Mg/Ca and Sr/Ca ratios are consistent with longer residence 

times and greater extents of water-rock interaction processes 

controlling fluid compositions as groundwater evolves from 

shallow unconfined groundwater to deeper confined groundwater. The ranges of Mg/Ca and Sr/Ca ratios for the three 

sample categories (shallow/urban unconfined, unconfined, 

and confined) overlap, however, suggesting that water-rock 

interaction processes are spatially variable and likely are 

affected by local-scale variations as a result of differences in 

host limestone composition, soil composition, flow routes, and 

residence times. Despite these local complexities that affect 

individual values, a distinct regional pattern is evident.

Water-rock interaction processes can be modeled using 

mass-balance relations, initial fluid and host rock compositions, and distribution coefficients (KD) for Mg and Sr partitioning (Banner and others, 1989; Banner and Hanson, 1990). 

An approach described in Musgrove and Banner (2004) for 

the progressive recrystallization of calcite and (or) dolomite 

was applied to these data; isotopic and elemental exchange 

is simulated by an iterative process of dissolution and reprecipitation of calcite or dolomite (with a constant composition 

that approximates that measured for central Texas carbonates) 

by a fluid of a defined initial composition (with a composition that approximates that measured for leachates of central 

Texas soils). Variations in model input parameters, such as 

trace element concentrations of the host rock, initial fluid 

composition, and trace element KD, alter the resulting modeled 

pathway of fluid composition. Modeled fluid composition is 

based on a range of values measured regionally for soil leachates and surface waters (Bush and others, 2000; Musgrove and 

Banner, 2004). Mesozoic and Paleozoic limestones typically 

contain between 2 and 4 weight percent magnesium carbonate (MgCO3

) (Goldsmith and others, 1955). Measured trace 

element concentrations for Edwards Group limestones exhibit 

a range of Sr and Mg concentrations; mean values for Sr and 

Mg concentration and MgCO3 = 140 parts per million (ppm), 

6,800 ppm, and 2.4 percent, respectively (n = 12) (Kathryn 

M. Conko, U.S. Geological Survey, written commun., 2008). 

Modeled rock compositions are consistent with these measured values and results of other diagenetic studies and analyses of Edwards Group limestones (Fisher and Rodda, 1969; 

Rose, 1972; Petta, 1977; and Ellis, 1985). 

Previous applications of fluid evolution models in the 

Edwards aquifer and in the Pleistocene limestone aquifer of 

Barbados have accounted for variable fluid compositions by 

water-rock interaction between groundwater and host rocks 

of varying composition (Banner and others, 1994; Musgrove 
Geochemical Evolution Processes 17

0

2

4

6

8

10

12

14

16

0.20 0.40 0.60 0.80 1.00

Mg-rich calcite

Calcite 1

Calcite 2

Dolomite

(A)

0

1

2

3

4

5

0.10 0.20 0.30 0.40 0.50

MAGNESIUM TO CALCIUM, MOLAR RATIO

STRONTIUM TO CALCIUM, 

MOLAR RATIO X 103

Calcite 1

Calcite 2

Dolomite

Mg-rich 

Calcite

(B)

Mg-rich calcite

Calcite 1

Dolomite

Shallow/urban unconfined

Unconfined

Confined

Calcite 2

Initial fluid

Water-rock interaction 

model curve

EXPLANATION

Groundwater

Vadose

dripwaters

Soil leachates

Vadose

dripwaters

Soil leachates

MAGNESIUM TO CALCIUM, MOLAR RATIO

STRONTIUM TO CALCIUM, 

MOLAR RATIO X 103

Fields for other aquifer components (soil leachates, vadose dripwaters) from Musgrove and Banner (2004). Water-rock interaction 

model trajectories show the water evolution pathway for magnesium to calcium and strontium to calcium (molar ratios) for water that 

progressively recrystallizes calcite (1 and 2 denote different initial compositions), dolomite, or magnesium-rich (Mg-rich) calcite (as 

labeled). Variations in model input parameters, such as trace element concentrations of host rock, initial water composition, and trace 

element distribution coefficients (KD) will alter the water model pathway; variable model trajectories shown account for majority of 

measured groundwater compositions. 

Water/rock composition Magnesium / strontium / calcium concentration (parts per million)

Calcite 1 water composition 2.0 0.04 60 

Calcite 1 rock composition 3,000 200 400,000

Calcite 2 water composition 2.0 0.02 60

Calcite 2 rock composition 2,500 100 400,000

Dolomite water composition 2.0 0.04 70

Dolomite rock composition 132,000 1,000 217,000

Mg-rich calcite water composition 2.0 0.04 70

Mg-rich calcite rock composition 50,000 1,000 300,000

KD = 0.01 for strontium and magnesium in all model calculations.

Modeled water and rock compositions consistent with measured values based on diagenetic studies and analyses of Edwards Group 

limestones (Kathryn M. Conko, U.S. Geological Survey, written commun., 2008; Fisher and Rodda, 1969; Rose, 1972; Petta, 1977; and 

Ellis, 1985).

0

0

Figure 7. Relation between magnesium to calcium and strontium to calcium (molar ratios) for groundwater samples collected from 

shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 

1996−2006; (A) full scale, and (B) shaded region in (A). 
18 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

and Banner, 2004). Modeling results demonstrate that mineralsolution reactions with aquifer host rocks, shown by variable 

model trajectories, account for the majority of measured 

fluid compositions in Edwards aquifer groundwater (fig. 7). 

Variable initial rock compositions result in different model 

pathways, that is, trajectories with different slopes that better 

represent the range of data observed. These results are consistent with the hypothesis that local-scale variability in host 

limestone composition, soil composition, flow routes, and residence times results in a range of Mg/Ca and Sr/Ca ratios for 

Edwards aquifer groundwater. Nonetheless, a distinct regional 

pattern of increasing Mg/Ca and Sr/Ca ratios is evident as 

fluid evolves from shallow unconfined to deeper confined 

groundwater (fig. 7). Edwards aquifer rocks include both 

calcite and dolomite of variable composition; interaction with 

host rocks of variable composition is indicated by data that 

fall along different model pathways (fig. 7). At a local scale, 

water-rock interaction modeling might allow for distinguishing between flow paths.

Dolomite mineral SIs show a progression toward greater 

values, indicative of increasing mineral saturation, from shallow/urban unconfined to unconfined to confined category samples. Median dolomite SI values were -1.0, -0.78, and -0.47 

for the shallow/urban unconfined, unconfined, and confined 

sample categories, respectively, and were statistically different on the basis of Mann-Whitney U test results. Dolomite SI 

results are an additional indicator of water-rock interaction 

processes and relative residence time. A correlation between 

dolomite SI values and Mg/Ca ratios (Kendall’s tau = .52) 

suggests that dolomite saturation in this aquifer is indicative of 

progressive water-rock interaction (fig. 8). High Mg/Ca ratios 

(greater than 0.60) for a few samples from the unconfined and 

confined parts of the Edwards aquifer (fig. 8) are consistent 

with groundwater compositions that have been influenced by 

mixing with more saline groundwater, likely originating from 

the saline zone.

Precipitation of calcite along flow paths is an additional 

process that can result in increasing Mg/Ca and Sr/Ca ratios 

in residual groundwater as a result of the KD for Mg and Sr 

partitioning during mineral precipitation (Fairchild and others, 

2006; Wong, 2008). This process is evident along the evolution pathway from shallow/urban unconfined to unconfined to 

confined Edwards aquifer groundwater with Mg/Ca and Sr/Ca 

ratios increasing as Ca concentrations decrease (fig. 9). Higher 

Mg/Ca and Sr/Ca ratios, in conjunction with lower Ca concentrations, result from the prior precipitation of calcite along 

flow paths and provide an additional indicator of increasing 

water-rock interaction and groundwater evolution. 

Some unconfined groundwater samples had high Mg/Ca 

ratios (greater than 0.60) that do not coincide with the main 

data trends (figs. 7−9). These data reflect multiple samples 

with high Mg concentrations measured in four wells in Hays 

and Comal Counties (18 to 31 mg/L, relative to a median Mg 

concentration of 11.6 mg/L for all groundwater samples). The 

four wells in Hays and Comal Counties are in areas where 

the aquifer is characterized by extensive faulting; associated groundwater might be interacting with aquifer rocks of 

variable lithology (for example, more dolomitic) and (or) 

Shallow/urban 

unconfined

Confined

Unconfined

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

0 0.200 0.400 0.600 0.800 1.000

DOLOMITE, MINERAL SATURATION INDEX

MAGNESIUM TO CALCIUM, MOLAR RATIO

Undersaturated

Supersaturated

EXPLANATION

Groundwater

Equilibrium

Figure 8. Relation between magnesium to calcium (molar ratio) and dolomite mineral saturation index for groundwater samples 

collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006. 
Geochemical Evolution Processes 19

mixing with downdip water moving along faults. As a result, 

groundwater from these wells can evolve along a different 

fluid evolution pathway, although from a similar starting point. 

Groundwater samples from several of the confined wells had 

high Sr/Ca ratios as a result of particularly high Sr concentrations (4,400 to 7,160 μg/L). These high Sr concentrations can 

result from interaction with variable host rock lithologies (for 

example, high-Sr limestone or trace amounts of Sr-rich minerals associated with the host rock, such as celestite [a trace mineral, sometimes associated with carbonate rocks, that is known 

to occur in Edwards aquifer rocks in the saline zone (Maclay 

and Small, 1983)]) and (or) mixing with deeper groundwater 

CALCIUM, MILLIGRAMS PER LITER

30

60

90

120

150

180

0 0.200 0.400 0.600 0.800

MAGNESIUM TO CALCIUM, MOLAR RATIO

30

60

90

120

150

180

0 4.00 8.00 12.00 16.00

STRONTIUM TO CALCIUM, MOLAR RATIO x 103

CALCIUM, MILLIGRAMS PER LITER

Shallow/urban 

unconfined

Unconfined

Confined

EXPLANATION

(A)

(B)

Groundwater

Figure 9. Relation between (A) magnesium to calcium (molar ratio) and calcium concentration, and (B) strontium to calcium (molar 

ratio x 103) and calcium concentration for groundwater samples collected from shallow/urban unconfined, unconfined, and confined 

parts of the San Antonio segment of the Edwards aquifer, 1996−2006. 
20 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

from the saline zone of the Edwards aquifer. Groundwater 

from the saline zone of the Edwards aquifer typically has 

higher Sr, SO4, and F concentrations than groundwater from 

the freshwater zone (Oetting, 1995; Oetting and others, 1996; 

Mahler, 2008; Lambert and others, 2009; U.S. Geological 

Survey, 2009). Higher than average SO4 and F concentrations 

associated with some of these wells is also indicative of mixing with deeper, more saline groundwater. 

Strontium Isotopes

Sr, an alkali earth metal, commonly substitutes for Ca 

in low-temperature aqueous geochemical environments and 

is a common trace element in carbonate rocks (Hem, 1989; 

Banner, 2004). Sr has four isotopes, of which 87Sr is derived 

from the radiogenic decay of rubidium-87; the Sr isotope 

ratio (87Sr/86Sr), a useful tracer in the hydrologic cycle, often 

provides a diagnostic signal of the source of dissolved constituents to a fluid (Banner, 2004). 87Sr/86Sr ratios in hydrology 

have been used to trace flow paths, mineral-solution reactions, and variations in recharge and flow routes in a number 

of different groundwater settings (McNutt and others, 1990; 

Banner and others, 1994). In the Edwards aquifer, Sr isotope 

ratios have been applied previously in conjunction with trace 

element ratios as a tracer of water-rock interaction, groundwater residence time, recharge, unsaturated zone flow-routes, 

and influence of soil composition on groundwater geochemistry (Oetting and others, 1996; Musgrove and Banner, 

2004; Garner, 2005). Sr isotope ratios in the Edwards aquifer 

decrease with increasing water-rock interaction, from soil 

water, to unsaturated zone cave dripwater, to phreatic groundwater, approaching values similar to those of the host rock 

(Oetting and others, 1996; Sharp and Banner, 1997; Musgrove 

and Banner, 2004). Cretaceous-age carbonate host rocks that 

compose the Edwards aquifer have Sr isotope ratios ranging 

from 0.7074 to 0.7077 (Koepnick and others, 1985; Oetting, 

1995). Higher 87Sr/86Sr ratios for groundwater relative to the 

aquifer host rocks are indicative of a source of more radiogenic Sr (enriched in 87Sr) to the groundwater, which has been 

proposed to be a result of chemical interaction of groundwater 

with overlying soils (Musgrove and Banner, 2004). 87Sr/86Sr analyses for NAWQA samples of Edwards 

aquifer groundwater were mostly limited to samples collected in 1996 from the unconfined part of the aquifer and 

samples collected in 2006 from the shallow/urban unconfined 

part of the aquifer. Sr isotope ratios ranged from 0.7076 to 

0.7095 with a median value of 0.7082 (n = 65). 87Sr/86Sr ratios 

for groundwater samples collected from the shallow/urban 

unconfined part of the aquifer are generally higher (median = 

0.7084; n = 35) than 87Sr/86Sr ratios for groundwater samples 

collected from the unconfined part of the aquifer (median = 

0.7079; n = 27) (table 3) and are statistically different on the 

basis of Mann-Whitney U test results. Although the small 

number of analyses for groundwater samples collected from 

the confined part of the aquifer (n = 3) limit a comparison 

of these results with results for samples collected from other 

parts of the aquifer, Sr isotope ratios for confined groundwater samples (median = 0.7078) were lower than for samples 

from the unconfined part of the aquifer and were similar to 

87Sr/86Sr ratios of the carbonate host rocks. Recent (2007) 

results for additional 87Sr/86Sr analyses (n = 17) of samples 

collected from seven confined wells in Bexar County as part 

of the NAWQA TANC study have similar lower values in the 

range of Cretaceous-age host rocks (median = 0.7077) (Lynne 

Fahlquist, U.S. Geological Survey, written commun., 2009). 

Using the modeling approach described by Musgrove 

and Banner (2004), fluid evolution processes are assessed 

using Sr isotopes in Edwards aquifer groundwater (fig. 10). 

Results demonstrate a progression toward lower 87Sr/86Sr ratios 

and higher Mg/Ca ratios with increased mineral-solution 

reaction as allowed by increased residence time and longer 

flow paths. The relation between 87Sr/86Sr ratios and Sr/Ca 

ratios demonstrates a similar progressive evolution pathway. 

These trends are consistent with previous research that also 

includes other aquifer components such as soils (87Sr/86Sr 

= 0.7084 to 0.7093) and unsaturated zone cave dripwaters 

(87Sr/86Sr = 0.7080 to 0.7091; Oetting, 1995; Musgrove, 2000; 

Musgrove and Banner, 2004; Wong, 2008). This evolution 

of aquifer geochemistry might occur at different scales, both 

spatially and temporally, depending on variations in fluid-rock 

interaction, prior precipitation of calcite along flow paths, 

groundwater mixing, aquifer rock mineralogy, and hydrologic 

conditions; nonetheless, geochemical results indicate that 

controlling processes on groundwater evolution are regionally extensive. Higher 87Sr/86Sr ratios for the shallow/urban 

unconfined groundwater relative to deeper unconfined and 

confined groundwater are consistent with a characterization of 

shallow/urban unconfined groundwater as less geochemically 

evolved; shallow/urban unconfined groundwater is similar in 

composition to unsaturated zone cave dripwater (fig. 10). This 

might be accounted for by shorter residence times, shorter 

flow paths, and (or) a smaller component of diffuse flow. The 

consideration of NAWQA studies data for Edwards aquifer 

groundwater samples supports and further refines the interpretation of a regional-scale continuum of fluid evolution processes that control water compositions in the Edwards aquifer 

with fluid compositions evolving from soil water, to unsaturated zone cave dripwater, to shallow unconfined groundwater, 

to deeper unconfined groundwater. 

Stable Isotopes

Stable isotope ratios of hydrogen and oxygen are commonly used to study hydrologic processes and have been 

applied to assess groundwater origin, mixing, and evaporation, hydrograph separation, and the response of karst systems 

to storms, aquifer recharge, subsurface flow routing (diffuse 

relative to conduit), and antecedent conditions (Craig, 1961; 

Gat, 1981; Lakey and Krothe, 1996; Jones and others, 2000; 

Desmarais and Rojstaczer, 2002; Maloszewski and others, 

2002). δD and δ18O for NAWQA samples of Edwards aquifer 

groundwater range from -36.0 to -15.2 per mil and -6.03 to 
Geochemical Evolution Processes 21

Figure 10. Relation between (A) strontium-87/strontium-86 isotopic ratio and magnesium to calcium (molar ratio) and (B) strontium-87/

strontium-86 isotopic ratio and strontium to calcium (molar ratio x 103) for groundwater samples collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 1996–2006. 

0

0.200

0.400

0.600

0.800

0.7076 0.7086 0.7096

STRONTIUM-87/STRONTIUM-86 

ISOTOPIC RATIO

MAGNESIUM TO CALCIUM, MOLAR RATIO

Cretaceous

limestones

Vadose

dripwaters

Soil leachates

Dolomite

Mg-rich 

calcite

Calcite 1

Calcite 2

0.7081 0.7091

1

2

3

4

5

0.7076 0.7086 0.7096

STRONTIUM-87/STRONTIUM-86 

ISOTOPIC RATIO

STRONTIUM TO CALCIUM, MOLAR RATIO X 103

Dolomite and

Mg-rich Calcite

Calcite 1

Calcite 2

0.7081 0.7091

Cretaceous

limestones

Vadose

dripwaters

Soil

leachates

Mg-rich calcite

Calcite 1

Dolomite

Calcite 2

Initial fluid

Water-rock interaction

model curve

Groundwater

Shallow/urban unconfined

Unconfined

Confined (B)

(A)

EXPLANATION

Fields for other aquifer components (soil leachates, vadose dripwaters, and Cretaceous limestones) from Musgrove and Banner (2004). Model 

trajectories show water evolution pathway for water that progressively recrystalizes calcite (1 and 2 denote different initial compositions), 

dolomite, or magnesium-rich (Mg-rich) calcite (as labeled). Inital water composition based on range of values measured for soil leachates and 

surface waters (Bush and others, 2000; Musgrove and Banner, 2004). Variations in model input parameters, such as trace element concentrations 

of host rock, initial water composition, and trace element distribution coefficients (KD) will alter the water model pathway; variable model 

trajectories shown account for the majority of measured groundwater compositions. Initial water and rock concentrations of magnesium, 

strontium, and calcium detailed in figure 7. Initial strontium-87/strontium-86 isotopic ratios are 0.7094 and 0.7077 for water and rock, 

respectively, for all model runs. Modeled water and rock compositions consistent with measured values based on diagenetic studies and analyses 

of Edwards Group limestones (Kathryn M. Conko, U.S. Geological Survey, written commun., 2008; Fisher and Rodda, 1969; Rose, 1972; Petta, 

1977; and Ellis, 1985). 

0
22 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

-2.64 per mil, respectively, and cluster around median values 

of -23.8 and -4.26 per mil (n = 168). A comparison of these 

data with the global meteoric water line (MWL), which represents the isotopic composition of rainfall around the globe 

(Craig, 1961), indicates that Edwards aquifer groundwater is 

composed of meteoric recharge (Fahlquist and Ardis, 2004). 

Confined groundwater shows less variability in δD and δ18O 

values relative to shallow/urban unconfined and unconfined 

groundwater, which is consistent with the characterization of 

confined groundwater as a deeper, well-mixed, phreatic endmember in the aquifer. 

The relation between δD values and Cl concentrations, 

both conservative constituents, has been used to delineate 

mixing (Musgrove and Banner, 1993) and indicates that some 

groundwater samples are influenced by specific geochemical processes (fig. 11). Groundwater samples from wells that 

are affected by mixing with a saline component are indicated 

by elevated Cl concentrations (more than 30 mg/L) (fig. 11). 

Many confined groundwater samples have elevated Cl concentrations (median value for confined groundwater samples = 

17.4 mg/L) relative to the median value for all samples (12.7 

mg/L); this difference is statistically significant on the basis of 

Mann-Whitney U test results. Slightly elevated Cl concentrations for confined groundwater samples likely indicate mixing 

with a component of saline groundwater. Samples with the 

highest Cl concentrations (more than 30 mg/L), however, are 

mostly from unconfined groundwater. These samples represent 

the same small group of wells in Hays and Comal Counties 

discussed earlier that have higher-than-average Mg concentrations. As previously discussed, these wells are located in 

areas where the aquifer is extensively faulted and groundwater 

might be mixing with more saline downdip water moving 

along faults. Some samples collected in 2006 from the shallow/urban unconfined part of the aquifer also have slightly 

elevated Cl concentrations (greater than 20 mg/L to as high as 

45 mg/L), likely a result of anthropogenic sources of Cl. 

A number of samples, mostly categorized as unconfined 

groundwater, had higher (isotopically heavier) δD values than 

the main field of data (greater than or equal to -20 per mil) 

(fig. 11). Higher δD values indicate that some of groundwater 

was subject to more evaporation losses relative to the bulk of 

the groundwater with lower δD values (Welhan, 1987). Higher 

δD values for the samples collected from wells in Uvalde and 

Medina Counties in the western part of the aquifer are indicative of recharge from surface-water streams or reservoirs that 

have undergone evaporation and possibly share a common 

flow path or flow paths. Previous research has proposed a flow 

path in northern Medina and northeastern Uvalde Counties 

with a component of water (recharged by Medina Lake; fig. 1) 

that is characterized by higher stable isotope values as a result 

of evaporation (Clark and Journey, 2006). 

A number of samples collected from shallow/urban 

unconfined category wells were isotopically heavier, or had 

lower δD values than the main field of data (less than or equal 

to 27 per mil) (fig. 11). These lower δD values likely represent recharge dominated by tropical cyclonic storms. Rainfall 

associated with tropical cyclonic storms has been shown to 

have isotopically lighter stable isotope (δD and δ18O) values 

relative to normal summer rainfall (Lawrence and Gedzelman, 

1996; Lawrence, 1998). These samples from shallow/urban 

unconfined category wells were collected in early to midNovember 1998, shortly following a large tropical storm in 

the area (more than 21 in. of rain were recorded at the San 

Antonio airport between October 1, 1998, and November 15, 

1998). The isotopically light δD values for samples collected 

from these wells in urban Bexar County might indicate they 

share a common conduit-influenced flow path or flow paths 

that are dominated by recent surface recharge. These wells 

were resampled in 2006 under different hydrologic conditions; δD values lower than the median δD isotopic composition were not observed. These results are consistent with the 

hypothesis that hydrologic conditions affect groundwater 

geochemistry and water quality. 

Salinity Sources

In the freshwater part of the aquifer, geochemical data 

indicate that groundwater in some wells mixes with more 

saline water. Groundwaters from the downdip saline zone and 

from the underlying Trinity aquifer are potential sources of 

higher salinity water that might mix with fresh Edwards aquifer groundwater. Understanding the source of this saline component, locally and regionally, has implications for hydrologic 

flow paths, water-use management, and understanding of the 

freshwater/saline-water transition zone. Previous studies have 

investigated the origin of saline-zone groundwater and have 

delineated distinct hydrochemical facies on the basis of variations in major element concentrations (Pearson and Rettman, 

1976; Clement and Sharp, 1988; Sharp, 1990; Groschen and 

Buszka, 1997). The integration of variations in Sr isotope 

ratios, major element concentrations, and quantitative geochemical modeling has been used to evaluate models for the 

evolution of saline groundwater in the Edwards aquifer and 

corresponding depositional and structural controls (Oetting 

and others, 1996). Mixing processes involve freshwater, saline 

groundwater from underlying hydrostratigraphic units that 

migrate through faults and fractures, and brines associated 

with the Edwards Group migrating updip from the Cretaceous 

shelf margin (Oetting and others, 1996). 

Edwards aquifer groundwater samples from parts of 

the aquifer that have a component of more saline water have 

higher (than the median) specific conductance values, typically accompanied by higher (than the median) concentrations 

of Cl, SO4

, and (or) Na. Some saline-influenced groundwater 

also might have lower-than-average dissolved oxygen concentrations, and for the confined part of the aquifer, might be 

in close geographic proximity to the freshwater/saline-water 

transition zone. Differences in the geochemical characteristics 

of downdip water from the saline zone compared to that of 

the underlying Trinity aquifer groundwater, which also can 

be more saline than Edwards aquifer groundwater, have been 

used to distinguish between mixing with these two different 
Geochemical Evolution Processes 23 Figure 11. (A) Relation between chloride concentration and delta deuterium for groundwater samples collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006, and (B) location map of sample sites associated with mixing trendlines shown in (A). 

EXPLANATION

Edwards aquiferUnconfined aquiferConfined aquiferGroundwater wellShallow/urban unconfinedUnconfinedConfinedConfined springGeochemical processMixing with underlying/downdip saline groundwaterMixing with evaporated surface-water rechargeMixing with tropical storm recharge

XY#*

")

D

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X

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#

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23

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office (1999), 1:24,000

Cities: Texas Natural Resources Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL

REAL

KENDALL KERR

EDWARDS

HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15 45

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Pathways away from main cluster of data reflect geochemical processes of mixing with (1) underlying/downdip saline groundwater, (2) contribution of recharge that hasundergone surface evaporation, and (3) contribution of recharge from large tropicalstorms to shallow/urban groundwater. One confined sample is located off the scale ofthe plot (chloride concentration = 179 milligrams per liter). [Delta deuterium relative to Vienna Standard Mean Ocean Water (Fritz and Fontes, 1980)]

98°

99°

100°

30°

29°

Groundwater wellShallow/urban unconfinedUnconfinedConfined

-40 -30 -20 -10

DELTA DEUTERIUM, IN PER MIL

8070605040302010

CHLORIDE, IN MILLIGRAMS PER LITER 

3 2

1

0

B

A
24 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

saline sources. Previous studies have indicated that Trinity 

aquifer groundwater, in comparison to downdip saline-zone 

water, is characterized by higher SO4 relative to Cl, and higher 

Mg relative to Na (Sharp and Clement, 1988). Variations in 

the relation between SO4/Cl ratios and SO4 concentration 

and between Mg/Na ratios and Mg concentration have been 

applied in the Barton Springs segment of the Edwards aquifer 

to distinguish between mixing with Trinity aquifer groundwater or with saline-zone groundwater (Garner, 2005; Garner 

and Mahler, 2007). Geochemical consideration of Trinity 

aquifer and saline-zone groundwater from the San Antonio 

segment of the aquifer with respect to these constituents 

indicates mixing pathways with Edwards aquifer groundwater 

(fig. 12). Trinity aquifer groundwater data were acquired from 

the USGS NWIS database (U.S. Geological Survey, 2009); 

these data are from a 1996 NAWQA study that sampled 31 

wells completed in the Trinity aquifer and randomly located 

throughout the areal extent of the aquifer (Fahlquist and Ardis, 

2004). Saline-zone groundwater data also were acquired 

from the USGS NWIS database (U.S. Geological Survey, 

2009) and from San Antonio Water System (Hydrogeologic 

Studies and Assessment Division, written commun., 2009) for 

multiple samples collected from saline-zone transect wells in 

Bexar, Comal, and Hays Counties. Groundwater from several 

Edwards aquifer wells with mixed-saline characteristics (for 

example, high specific conductance, low dissolved oxygen) 

shows evidence for mixing with saline-zone groundwater 

or Trinity aquifer groundwater on the basis of the relation 

between the SO4

/Cl ratio and SO4 concentration and (or) 

between the Mg/Na ratio and Mg concentration (fig. 12). 

High F concentration in Edwards aquifer groundwater 

samples is also indicative of mixing with groundwater from 

either the Trinity aquifer or the downdip saline zone; the 

median F concentration (1.8 mg/L; n = 31) for NAWQA 

samples of Trinity aquifer groundwater is about one order 

of magnitude higher than the median for Edwards aquifer 

groundwater samples (0.14 mg/L; n = 243). The F concentration, however, cannot be used to distinguish between mixing with saline-zone or Trinity aquifer groundwater. Similar 

to groundwater samples collected from the Trinity aquifer, 

samples from the Edwards aquifer saline zone also tended 

to have high F concentrations (median = 3.6 mg/L, n = 529). 

Several of the NAWQA Edwards aquifer groundwater samples 

with characteristics indicative of some mixing with more 

saline water had relatively high F concentrations. 

Mixing with underlying Trinity aquifer groundwater 

appears to be the dominant source of saline water for most 

of the NAWQA groundwater samples that show evidence for 

saline-water mixing (fig. 12). This distinction suggests that, on 

the basis of the relation between SO4

/Cl ratios and SO4 concentrations and between Mg/Na ratios and Mg concentrations, 

mixing with downdip saline groundwater might be an important salinity source only for Edwards aquifer groundwater 

wells near the saline zone. Data for a few NAWQA Edwards 

aquifer groundwater samples are not clearly associated with 

the mixing pathway for either Trinity aquifer groundwater 

or downdip saline-zone groundwater, which might indicate a 

more complex origin for their saline characteristics, such as 

mixing with both Trinity aquifer and saline-zone groundwater. 

Several of the NAWQA groundwater samples that show evidence for mixing with underlying Trinity aquifer groundwater 

are not characterized by higher specific conductance values 

as might be expected. One sample (collected June 5, 2001) 

from well AY−68−28−211, completed in the shallow/urban 

unconfined part of the aquifer and sampled multiple times, 

had an anomalously high SO4

/Cl ratio associated with a high 

SO4 concentration (fig. 12). Given the shallow well construction, urban setting of this well, and lower values associated 

with other samples collected from this well, the anomalously 

high SO4/Cl ratio and SO4 concentration likely resulted from a 

local, transitory source of contamination. 

Extensive water-rock interaction with gypsum, a trace 

mineral present in the Edwards Group, also might affect 

groundwater SO4

/Cl ratios and SO4 concentrations; downdip 

saline-zone groundwater shows evidence of incongruent gypsum dissolution (Oetting and others, 1996). NAWQA sampled 

Edwards aquifer groundwater is highly undersaturated with 

respect to gypsum (mineral SI ranges from -2.96 to -1.49), 

indicating water that has not undergone extensive interaction 

with gypsum. Several samples with geochemical evidence 

for downdip saline-zone/Trinity aquifer groundwater mixing, 

however, are at the upper end of the gypsum SI range, which 

might indicate mixing with saline source waters that have 

undergone interaction with gypsum.

Evaluation of hydrologic conditions, local stratigraphy 

and structure (faults), well construction, and other groundwater geochemical and isotopic constituents can provide 

additional information for sampled wells with no readily 

distinguishable saline source. Proximity to the freshwater/

saline-water interface, however, does not necessarily indicate 

that mixing with downdip saline-zone water occurs. A number 

of Edwards aquifer wells, including high-production publicsupply wells, are near the freshwater/saline-water interface and 

show no geochemical indication of mixing with a saline-water 

component. Complex hydrostratigraphy limits the uniformity 

of mixing with saline-zone water and cross-formational flow 

in the aquifer, although Oetting and others (1996) notes that 

vertical fluid migration and mixing is more substantial in areas 

with reduced aquifer thickness and greater fault displacement.

Radon 

The use of Rn concentration (radon-222) as an indicator 

of fast transport and rapid transit times is a developing application in karst studies (Hunkeler and Mudry, 2007; Falcone 

and others, 2008). Rn concentrations in karst groundwater 

likely originate in soils, and limestone host rocks are typically 

a poor source of Rn (Surbeck and Medici, 1991; Hunkeler and 

Mudry, 2007). Thus, Rn, which decays to low values in older, 

more evolved, confined groundwater might provide an indicator of residence time and (or) mixing with recent recharge. 

Given its short half-life of 3.8 days, Rn entering an aquifer 
Geochemical Evolution Processes 25

1.00

2.00

3.00

4.00

5.00

6.00

10 20 30 40

MAGNESIUM, MILLIGRAMS PER LITER

MAGNESIUM TO SODIUM RATIO

Saline zone

mixing

Trinity aquifer

mixing

0.50

1.00

1.50

2.00

2.50

3.00

3.50

20 40 60 80 100

SULFATE, MILLIGRAMS PER LITER

SULFATE TO CHLORIDE RATIO

Saline zone

mixing

Trinity aquifer

mixing

10.00

30.00

50.00

70.00

0 1,000 2,000 3,000

SULFATE, MILLIGRAMS PER LITER

SULFATE TO CHLORIDE RATIO

2.00

4.00

6.00

8.00

10.00

100 200 300 400 500

MAGNESIUM, MILLIGRAMS PER LITER

MAGNESIUM TO SODIUM RATIO

Confined

Unconfined

Shallow/urban unconfined

Trinity aquifer

Saline-zone water

EXPLANATION

4.00

0

0

0

(B)

(A)

Trinity aquifer data from U.S. Geological Survey (2009) and saline-zone groundwater data from San Antonio Water System (Hydrogeologic 

Studies and Assessment Division, written commun., 2009). Red-bounded boxes in inserts denote scale expanded in (A) and (B). Arrows on 

(A) and (B) highlight mixing pathways with underlying Trinity aquifer groundwater (median sulfate to chloride ratio = 4.9; sulfate = 130 

milligrams per liter (mg/L); magnesium to sodium ratio = 2.2; magnesium = 48 mg/L; n = 31) and downdip saline-zone groundwater in Bexar, 

Comal, and Hays Counties (median sulfate to chloride ratio = 1.04, n = 376; sulfate = 1,649 mg/L, n = 525; magnesium to sodium ratio = 0.34, 

n = 395; magnesium = 371 mg/L, n = 371). 

Edwards aquifer

0

0

0

0

Figure 12. Relation between (A) sulfate to chloride ratio and sulfate concentration, and (B) magnesium to sodium ratio and magnesium 

concentration for groundwater samples collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio 

segment of the Edwards aquifer, south-central Texas, 1996–2006. 
26 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

with recharge water could decay to concentrations less than 

or equal to laboratory reporting levels within weeks to months 

(depending on the initial Rn concentration). Increases in Rn 

concentrations in spring water associated with an antecedent storm event, recent recharge, and a correlation between 

Rn concentrations and spring discharge have been shown for 

springs in the Swiss Jura karst system (Eisenlohr and Surbeck, 

1995). 

Rn concentrations from NAWQA Edwards aquifer 

groundwater samples range from less than 58 to 776 picocuries per liter (pCi/L) with a median value of 116 pCi/L. 

Median values for samples from unconfined wells (151 pCi/L) 

are higher than for samples from confined wells (89 pCi/L) 

and are statistically different on the basis of Mann-Whitney U 

test results. Rn was not measured at shallow/urban unconfined 

sites. Results for Rn concentrations from NAWQA Edwards 

aquifer groundwater samples indicate that Rn in this system 

might provide an indicator of recent recharge and (or) rapid 

flow paths. Mg/Ca ratios were correlated with Rn concentrations for confined groundwater samples (Kendall’s tau = -.37) 

but showed no significant correlation for unconfined groundwater (fig. 13). Both Mg/Ca ratios and Rn concentrations 

were correlated with well depth for confined groundwater 

samples; samples from deeper wells have lower Rn concentrations and higher Mg/Ca ratios (fig. 14; Kendall’s tau = 

-.34 and .42, respectively). There is no significant correlation between well depth and Rn concentration for unconfined 

groundwater samples or between well depth and Mg/Ca ratios 

for shallow/urban unconfined and unconfined groundwater 

samples. 

The interpretation of Rn concentration data for several 

Edwards aquifer confined groundwater samples is likely 

complicated by potential sources from mixing with saline 

waters and (or) proximity to Tertiary volcanics in the western 

part of the aquifer. These sources might account for some 

high Rn concentrations in unconfined groundwater samples, 

as shown on figures 13 and 14. Although no Rn data for 

saline-zone groundwater is available for comparison, NAWQA 

groundwater samples from the Trinity aquifer collected in 

1996 (Fahlquist and Ardis, 2004) have a higher median Rn 

concentration (295 pCi/L) than Edwards aquifer groundwater 

(statistically different on the basis of Mann-Whitney U test 

results). Several unconfined groundwater samples with high 

Rn concentrations correspond to the samples in Hays and 

Comal Counties with other geochemical indicators of salinezone influence. These results suggest that underlying saline 

water might be a potential source of Rn to Edwards aquifer 

groundwater. Future research should provide new insights into 

the utility of Rn as a tracer of recent recharge in the Edwards 

aquifer.

Water-Quality Observations

Nitrate: Occurrence and Trends

Anthropogenic activity has transformed the modern 

global nitrogen cycle, and elevated nutrient concentrations 

Figure 13. Relation between magnesium to calcium (molar ratio) and radon-222 for groundwater samples collected from unconfined 

and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. 

Unconfined

Confined

MAGNESIUM TO CALCIUM, MOLAR RATIO

0

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

0 200 400 600 800 1,000

RADON-222, PICOCURIES PER LITER

Kendall's tau = .37 ( confined)

EXPLANATION

Groundwater
Water-Quality Observations 27

in water resources are an issue of worldwide concern 

(Schlesinger and others, 2006; Galloway and others, 

2008). Nitrogen (N) is the most soluble of these nutrients (Hem, 1989), and nitrate (NO3

) frequently is elevated 

in water resources as a result of anthropogenic activities. 

Anthropogenic NO3 sources include agricultural fertilizers, 

animal wastes, septic systems, municipal sewage-treatment 

systems, leaking sewer lines, and nitrogen oxide emissions. 

Atmospheric deposition, decaying organic matter, and soil 

mineralization processes also are potential NO3 sources. 

500

1,000

1,500

2,000

2,500

3,000

200 400 600 800 1,000

RADON-222, PICOCURIES PER LITER

WELL DEPTH,

FEET BELOW LAND-SURFACE DATUM

Kendall’s tau = .34 (confined)

(A)

Unconfined

Confined

EXPLANATION

Shallow/urban 

unconfined

Groundwater 0

0

0

0

500

1,000

1,500

2,000

2,500

3,000

0.200 0.400 0.600 0.800 1.000

MAGNESIUM TO CALCIUM, MOLAR RATIO

Kendall’s tau = .42 (confined)

WELL DEPTH,

FEET BELOW LAND-SURFACE DATUM

(B)

Figure 14. Relation between well depth and (A) radon-222 and (B) magnesium to calcium (molar ratio) for groundwater samples 

collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, southcentral Texas, 1996−2006. 
28 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Natural NO3 sources are not common in limestone and dolomitic aquifer rocks, thus NO3

 concentrations greater than the 

prevailing background concentration commonly represent 

anthropogenic sources.

NO3 concentrations in NAWQA Edwards aquifer groundwater samples (1996−2006) range from less than 0.05 to 8.23 

mg/L, with a median value of 1.67 mg/L (n = 240). Nitrite 

(NO2) N was detected rarely and when detected was at low 

concentrations (less than 0.01 to 0.06 mg/L), substantially 

less than NO3. NO3 concentrations greater than 3 mg/L were 

measured in less than 5 percent of all samples collected; these 

samples with relatively high NO3 (more than 3 mg/L) likely 

are affected by local NO3 sources. Groundwater samples with 

relatively high NO3 concentrations were not restricted to any 

one sample category, but rather included the shallow/urban 

unconfined, unconfined, and confined categories. Samples of 

groundwater from the shallow/urban unconfined and confined 

categories had higher median NO3

 concentrations (1.85 mg/L, 

n = 90; and 1.84 mg/L, n = 70, respectively) than samples 

from the unconfined category (median NO3 concentration of 

1.09 mg/L, n = 80), and NO3 concentrations for unconfined 

groundwater samples were statistically different on the basis 

of Mann-Whitney U test results. The higher NO3 concentrations for samples in the shallow/urban unconfined category 

likely reflect a greater anthropogenically-derived influx 

of NO3 in the urban environment coinciding with the shallow/urban unconfined part of the aquifer. The median NO3

concentration measured in groundwater samples from the 

confined part of the aquifer was similar to that for samples 

from the shallow/urban unconfined part of the aquifer; some 

of the confined aquifer wells are in areas where the land use 

is agricultural. Short-circuiting through wellheads has been 

shown to provide a potential pathway through confining layers 

or thick overlying units (McMahon and others, 2008; Landon 

and others, 2009). It is unlikely, however, that this process 

would affect the confined aquifer across the region, but rather 

might result in higher NO3 concentrations at a few wells in 

agricultural areas. The concentrations of NO3 observed in the 

confined Edwards aquifer likely result from the long-term 

leaching of NO3

 from soils into the aquifer matrix. Lower 

NO3 concentrations in the unconfined Edwards aquifer might 

reflect the location of these sampling sites in predominantly 

rural and rangeland areas, with little influence from anthropogenic NO3

 sources and dilution of ambient NO3 by surfacewater recharge. 

There is considerable interest in understanding NO3

sources and fate, including an understanding of natural background concentrations and historical trends for many karst 

aquifers that supply drinking water, including the Edwards 

aquifer. Variability in NO3

 concentrations in response to 

hydrologic conditions, however, complicates the interpretation of temporal trends. The median NO3

 concentration for 

groundwater samples collected from shallow/urban unconfined 

category wells in 1998 (1.5 mg/L) was less than concentrations when shallow/urban unconfined category wells were 

resampled in 2006 (1.9 mg/L) and statistically different on the 

basis of Mann-Whitney U testing. These differences, however, 

might result from differences in aquifer flow conditions; aquifer flow conditions were higher in 1998 compared to 2006. 

NO3 concentrations in the Barton Springs segment of the 

aquifer have been previously demonstrated to be affected by 

variable hydrologic conditions, with lower NO3 concentrations 

associated with higher aquifer flow conditions (Mahler and 

others, 2006; Mahler and Garner, 2009). Results for shallow/

urban unconfined groundwater samples from the San Antonio 

segment of the aquifer are consistent with this relation.

Unconfined groundwater category wells were also sampled during two different periods that represent lower-thanaverage aquifer flow conditions: exceptionally low aquifer 

flow conditions in 1996 (median Comal Springs discharge of 

92 ft3/s) and moderately low aquifer flow conditions in 2006 

(median Comal Springs discharge of 269 ft3/s). The median 

NO3 concentrations measured in unconfined groundwater category samples for these two periods are similar (1.15 mg/L for 

1996 and 1.12 mg/L for 2006) and not statistically different on 

the basis of Mann-Whitney U test results. 

NO3

 concentrations for Edwards aquifer NAWQA 

groundwater samples were statistically correlated with 

specific conductance values (Kendall’s tau = .28; fig. 15). In 

the absence of other sources of salinity (for example, mixing with more saline groundwater from the Trinity aquifer or 

downdip saline zone), specific conductance values would be 

expected to generally increases along aquifer flow paths as a 

result of progressive mineral-solution reactions. NO3 concentrations for NAWQA groundwater samples increase from 

relatively low values (median = 1.09 mg/L) in the unconfined 

(recharge zone) part of the aquifer to higher values (median = 

1.84 mg/L) in the confined part of the aquifer. This relation is 

consistent with the hypothesis that ambient NO3 in the aquifer 

is diluted by incoming recharge and correspondingly that 

recharging surface water is not the main source of NO3 to the 

aquifer. Storage of NO3 in soils with slow release to groundwater by diffuse infiltration is the likely source of ambient 

aquifer NO3 to the confined groundwater (Peterson and others, 

2002). Specific conductance values (median = 578 μS/cm), as 

well as NO3 concentrations (median = 1.85 mg/L), are higher 

in shallow/urban unconfined groundwater samples relative to 

both unconfined and confined groundwater samples. These 

results indicate that urban land use affects water quality and 

that urban recharge is a potential source of NO3

, as well as 

other dissolved constituents and anthropogenic contaminants, 

to groundwater.

Historical Nitrate and Chloride Concentrations 

at Comal Springs

A relatively long time series of NO3 concentration data is 

available for Comal Springs (1938−2006) that indicates a multidecadal upward trend for NO3

 concentration (Kendall’s tau 

= .38) (fig. 16A). These data are a compilation of analyses of 

filtered and unfiltered samples; analyses of unfiltered samples 
Water-Quality Observations 29

might be affected by NO3 associated with particulate matter. 

Considering only results for filtered samples, the median NO3

concentration at Comal Springs during 1938−45 was 1.2 mg/L 

(n = 6); the median NO3 concentration during 1993−2003 was 

1.9 mg/L (n = 4), which is similar to the median value for 

NAWQA regional groundwater samples (1996−2006). The 

data for the two time periods are statistically different on the 

basis of Mann-Whitney U test results. Samples for these two 

time periods (1938−45 and 1993−2003) were collected during similar aquifer flow conditions as indicated by discharge 

at Comal Springs; median discharge was 335 ft3

/s during 

1938−45 and 316 ft3/s during 1993−2003. Although analytical 

methods for these data are not available for comparison, these 

results suggest that matrix NO3 concentrations in the aquifer 

have increased over the last 70 years. Considering the large 

volume of water in storage in the aquifer, a 0.7-mg/L increase 

in median NO3 concentration at Comal Springs represents 

a large increase in NO3 load in the aquifer. Historical NO3

concentrations indicate no relation with discharge at Comal 

Springs (fig. 16B). Background NO3

 concentration (that is, 

not anthropogenically influenced) likely results from leaching of soil NO3

 into the aquifer matrix. Soil NO3 can also be 

affected by anthropogenic activities, including spray irrigation 

of wastewater (currently allowed in the aquifer contributing zone but not over the recharge zone [Cary Betz, Texas 

Commission on Environmental Quality, written commun., 

2009]), application of lawn fertilizers, and leakage from septic 

and sewer systems. 

Increased Cl concentrations might be associated with 

anthropogenic sources, including septic, sewage, and wastewater systems (Panno and others, 2006; Kostick and others, 

2007). Similar to NO3

, the Cl concentration during 1938−2006 

at Comal Springs increased (Kendall’s tau = .41) (fig. 17). 

The increases in NO3 and Cl concentrations over time are 

consistent with increases in anthropogenic effects on aquifer 

geochemistry associated with ongoing urbanization over the 

last 70 years.

Selected Anthropogenic Organic Contaminants: 

Occurrence and Trends

Anthropogenic organic contaminants, such as pesticides 

and solvents, are of concern in groundwater and drinkingwater supplies because of their potential detrimental effects 

on the environment and to human health. Many of these 

compounds are relatively soluble and thus can be present in 

groundwater if anthropogenic sources are present. The frequency of detection and spatial distribution of anthropogenic 

contaminants provide insight into effects of anthropogenic 

activities on water quality. Previous studies have documented 

the detection of anthropogenic organic contaminants in the 

1

2

3

4

5

6

7

8

9

10

300 500 700 900 1,100

SPECIFIC CONDUCTANCE, 

MICROSIEMENS PER CENTIMETER AT 25 DEGREES CELSIUS 

NITRATE, MILLIGRAMS PER LITER AS NITROGEN

EXPLANATION

Shallow/urban 

unconfined

Unconfined

Confined

Groundwater

0

Figure 15. Relation between nitrate concentration and specific conductance for groundwater samples collected from shallow/urban 

unconfined, unconfined, and confined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. 
30 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Filtered sample

Unfiltered sample

EXPLANATION

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500 600

NITRATE, MILLIGRAMS PER LITER AS NITROGEN

COMAL SPRINGS DISCHARGE, CUBIC FEET PER SECOND

(B)

1970

(A)

YEAR

NITRATE, MILLIGRAMS PER LITER AS NITROGEN

0.5

1.0

1.5

2.0

2.5

1930 1940 1950 1960 1980 1990 2000

Kendall’s tau = .32 (all data)

Figure 16. (A) Time series of nitrate concentration in Comal Springs discharge, and (B) relation between nitrate concentration at 

Comal Springs and Comal Springs discharge, San Antonio segment of the Edwards aquifer, south-central Texas, 1938−2006. 
Water-Quality Observations 31

San Antonio segment of the Edwards aquifer but have not 

addressed factors that influence their presence, concentration, 

transport, and temporal variability in response to changes in 

hydrologic conditions (Bush and others, 2000; Fahlquist and 

Ardis, 2004). The most frequently detected contaminants 

throughout the aquifer (atrazine, DEA, simazine, PCE, and 

chloroform) were most often detected in samples of shallow/

urban unconfined groundwater (all of these contaminants 

were detected in more than 50 percent of these samples) 

and were least frequently detected in samples of unconfined 

groundwater (fig. 18). These results indicate that samples of 

groundwater from the unconfined part of the aquifer are the 

least affected by these anthropogenic contaminants, which is 

consistent with the location of these sample sites in largely 

undeveloped rural and rangeland areas. This observation, combined with high detection frequencies in samples of shallow/

urban unconfined groundwater, indicates that the urban environment is a source of anthropogenic contaminants to Edwards 

aquifer groundwater.

Concentrations of the three most frequently detected 

pesticides (atrazine, simazine, and prometon) and the most 

frequently detected pesticide degradate compound (DEA, a 

metabolite and common environmental degradation product 

of atrazine [Fallon and others, 2002]) were low throughout the 

aquifer; median concentrations for all compounds were less 

than the laboratory reporting levels for samples of unconfined 

groundwater and greater than the reporting levels for samples 

of shallow/urban unconfined groundwater (fig. 18; table 4). 

Median concentrations of PCE and chloroform were higher 

than medians for pesticide degradate compounds and were 

highest in samples of shallow/urban unconfined groundwater 

(fig. 18; table 4). 

Chloroform detections indicate that this constituent is 

well distributed throughout the Edwards aquifer relative to 

other frequently detected anthropogenic contaminants, with 

high detection frequencies in all parts of the aquifer (fig. 18). 

Detection of chloroform is also common in the Barton Springs 

segment of the Edwards aquifer, where it is detected routinely in springflow from Barton Springs, with concentrations 

increasing during storm events (Mahler and others, 2006). 

These results and the nearly ubiquitous low-level detection 

of chloroform throughout the aquifer, including unconfined 

recharge-zone wells, indicate that low concentrations of 

chloroform are likely stored in the aquifer matrix. Few data 

are available for chloroform analyses of surface waters from 

the recharge zone for the San Antonio segment; available 

concentration results for surface water were mostly less than 

the laboratory reporting levels. Chloroform reporting levels for 

surface-water analyses, however, were commonly higher than 

chloroform concentrations measured for groundwater analyses done as part of the NAWQA Program (U.S. Geological 

Survey, 2009). 

Frequency of detection and concentration of chloroform 

in the Edwards aquifer are similar to those for studies across 

the United States (Bush and others, 2000). The frequent detection of chloroform in groundwater at low concentrations (less 

than 1 μg/L) in domestic, monitoring, and public-supply wells 

in a wide range of land-use settings (rangeland to agriculture 

Figure 17. Time series of chloride concentration in Comal Springs discharge, San Antonio segment of the Edwards aquifer, southcentral Texas, 1938−2006. 

YEAR

CHLORIDE, MILLIGRAMS PER LITER

5

1 0

1 5

2 0

2 5

1930 1940 1950 1960 1970 1980 1990 2000

 

Kendall’s tau = .41 
32 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Figure 18. (A) Detection frequency and (B) median concentration of the most frequently detected anthropogenic contaminants in 

groundwater samples collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment of the 

Edwards aquifer, south-central Texas, 1996−2006. 

CONTAMINANT

CONTAMINANT

Shallow/urban unconfined

Unconfined

Confined

EXPLANATION

MEDIAN CONCENTRATION, 

MICROGRAMS PER LITER

ATRAZINE

DEETHYLATRAZINE (DEA)

SIMAZINE

TRICHLOROMETHANE

(CHLOROFORM)

TETRACHLOROETHENE (PCE)

(B)

0

0.01

0.02

0.03

0.04

0.05

DETECTION FREQUENCY, PERCENT

ATRAZINE

DEETHYLATRAZINE (DEA)

SIMAZINE

TRICHLOROMETHANE

(CHLOROFORM)

TETRACHLOROETHENE (PCE)

(A) 

0

20

40

60

80

100

ND

ND Nondetection at laboratory reporting level

ND

NDND

NDND

ND

Groundwater
Water-Quality Observations 33

to urban) has been widely documented and is attributed, in 

part, to the recycling of chlorinated waters (Moran and others, 

2002; Zogorski and others, 2006). Chloroform is a byproduct 

of drinking-water treatment processes. Sources of chloroform 

in domestic wells include shock chlorination and leakage from 

septic areas. Potential sources in shallow/urban unconfined 

monitoring wells include leaking water and wastewater lines, 

septic systems, and recharge from irrigation with treated water. 

Public water-supply sources also include the possible use of 

treated water as a downhole lubricant of well pumps. In addition to anthropogenic sources, chloroform has some natural 

sources, including production in soils and emission from 

seawater (Hoekstra and others, 1998; McCulloch, 2003). 

Biomass burning is another potential source of chloroform 

to the atmosphere (Rudolph and others, 1995; Rudolph and 

others, 2000; Simpson and others, 2007). 

An assessment of temporal variability, trends, and 

associations for anthropogenic contaminants throughout 

the study period (1996−2006) provides insight into the 

effects of urbanization on the Edwards aquifer. Frequently 

detected anthropogenic contaminants (atrazine, DEA, simazine, chloroform, and PCE) often are detected together, 

but concentrations are not well correlated with major ions, 

nutrients, isotopes, or physicochemical properties. Shallow/

urban unconfined groundwater sites were sampled in both 

1998 (high aquifer flow conditions) and 2006 (moderately 

low aquifer flow conditions). About twice as many contaminants were detected for shallow/urban unconfined sample 

sites during 1998 under high aquifer flow conditions (median 

discharge at Comal Springs of 429 ft3/s), relative to 2006 

(median discharge at Comal Springs of 247 ft3/s). These 

results indicate that recharge water derived from urban areas 

might be a source for a number of anthropogenic contaminants 

to Edwards aquifer groundwater. The most frequently detected 

anthropogenic contaminants were in the majority of samples 

from the shallow/urban unconfined part of the aquifer for 

both the 1998 and 2006 sampling periods, with small differences in detection frequency and median detected concentrations for each constituent (except chloroform, which had a 

higher median concentration in 2006) (fig. 19). On the basis 

of these results, the most frequently detected anthropogenic 

contaminants appear to have been persistent in the shallow/

urban unconfined part of the aquifer during the study period 

(1996–2006).

Unconfined groundwater samples were collected in 1996 

during exceptionally low aquifer flow conditions (median 

Comal Springs discharge of 92 ft3

/s) and in 2006 during 

moderately low aquifer flow conditions (median Comal 

Springs discharge of 269 ft3/s). For these samples, the frequency of detection for all frequently detected compounds was 

higher in 2006 and at least two times higher for all compounds except chloroform (fig. 19). Median concentrations 

for frequently detected compounds were mostly less than the 

laboratory reporting levels, except for chloroform for samples 

collected in 2006 (fig. 19). The observed increase in detection frequency between 1996 and 2006 might partly result 

from increased urbanization in the region in 2006 compared 

to 1996, particularly over the unconfined part of the aquifer 

(Friesen and others, 2004). Differences in the detection frequency of anthropogenic contaminants might also result from 

variations in hydrologic conditions, as discussed further in 

the section “Relation Between Geochemistry and Hydrologic 

Conditions.” 

Relation Between Geochemistry and Hydrologic 

Conditions

A comparison of temporal variations in hydrologic 

conditions, as represented by variations in the amount of 

recharge, water-level altitudes in indicator wells, and springflow at major discharge points, along with time-series data 

for groundwater geochemistry, provide insight into time 

scales and controls on water-quality changes, fluid evolution 

processes, contaminant transport and storage, and temporal 

effects of anthropogenic contaminants on water quality. Five 

of the shallow/urban unconfined category wells sampled by 

NAWQA had a higher frequency of sampling than other sites 

(fig. 20), with a total of eight samples collected at each well 

during the 10-year study. Relatively continuous or frequent 

periodic water-level altitude measurements for these five 

wells allow for comparison between geochemical variability 

and hydrologic conditions at each site. Water-level altitudes 

for these five wells (fig. 21A) vary in a consistent manner 

in direct response to rainfall (fig. 21B). Water-level altitude 

fluctuations for the five wells also are consistent with variations in regional hydrologic conditions, evidenced by discharge at Comal Springs (fig. 21C) and water-level altitudes 

in the confined Bexar County index well (J−17; fig. 3). The 

rapid response to changes in hydrologic conditions observed 

in these wells appears to be typical of the shallow unconfined 

part of the aquifer based on similar results for water-level 

variations at other monitored shallow/urban unconfined 

groundwater wells. 

Temporal variability observed in the five shallow/urban 

unconfined category wells allows for inferences regarding 

controls on water-quality variations and sources of anthropogenic contaminants. The sampling frequency for these wells 

was not based on water-level fluctuations and thus does not 

encompass the complete range of aquifer flow conditions 

over the study period. Nonetheless, samples were collected 

over a large range of aquifer flow conditions, including high, 

moderate, and low. The following discussion focuses on three 

constituents or ratios of constituents that are representative of 

water-quality concerns and geochemical processes: (1) nitrate 

(NO3

), an important nutrient, that in high concentrations might 

be indicative of anthropogenic sources and is a detriment to 

water quality, (2) atrazine, a commonly used pesticide and 

frequently detected anthropogenic contaminant in the Nation’s 

groundwater (Gilliom and others, 2006); and (3) the magnesium to calcium molar ratio (Mg/Ca), an indicator of waterrock interaction and residence time. 
34 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Figure 19. Most frequently detected anthropogenic contaminants, detection frequency in (A) shallow/urban unconfined category samples collected in 1998 and 2006, and (B) unconfined category samples collected in 1996 and 2006; median concentration in (C) shallow/urban unconfined category samples collected in 1998 and 2006, and (D) unconfined category samples collected in 1996 and 2006, San Antonio segment of the Edwards aquifer, south-central Texas. (B) 1996 2006

ATRAZINE

DEETHYLATRAZINE 

(DEA)

SIMAZINE

TETRACHLOROETHENE (PCE)

TRICHLOROMETHANE

(CHLOROFORM)

19962006

(D) 

ATRAZINE

DEETHYLATRAZINE 

(DEA)

SIMAZINE

TETRACHLOROETHENE (PCE)

TRICHLOROMETHANE

(CHLOROFORM)

1998

2006

(A) 

0

80604020

100

DETECTION FREQUENCY, PERCENT

ATRAZINE

DEETHYLATRAZINE 

(DEA)

SIMAZINE

TETRACHLOROETHENE (PCE)

TRICHLOROMETHANE

(CHLOROFORM)

(C) 1998

2006

MEDIAN CONCENTRATION, 

MICROGRAMS PER LITER

0

0.050.040.030.020.01

ATRAZINE

DEETHYLATRAZINE 

(DEA)

SIMAZINE

TETRACHLOROETHENE (PCE)

TRICHLOROMETHANE

(CHLOROFORM)

0

80604020

100

DETECTION FREQUENCY, PERCENT

MEDIAN CONCENTRATION, 

MICROGRAMS PER LITER

0

0.050.040.030.020.01

ND

ND

ND

ND

ND

ND

ND

ND

ND

Nondetection at laboratory reporting level

ND

CONTAMINANT CONTAMINANT

CONTAMINANT CONTAMINANT

Shallow/urban unconfined Unconfined
Water-Quality Observations 35

Temporal Variability in Geochemical Constituents 

A comparison of temporal variability in NO3 and atrazine concentrations and Mg/Ca ratios measured in samples 

collected from the five shallow/urban unconfined groundwater 

wells (fig. 22) indicates there were differences in the response 

of the five wells to changes in hydrologic conditions, as well 

as differences between the responses of the considered geochemical constituents. Time-series data for NO3

 indicate that 

concentrations are less variable than atrazine concentrations 

and also are less consistent among the five wells (fig. 22A). 

Time-series data for atrazine show some consistencies from 

well to well; atrazine concentrations are variable in time and 

show similar patterns of increase and decrease (fig. 22B). For 

example, concentrations are higher at four of the five wells 

in late 2004 relative to lower concentrations at most of the 

wells in mid-1999, late 2002, and late 2006. This consistency 

suggests that some of the observed geochemical variability 

has a common control and might reflect processes affecting 

all five wells. There are, however, differences between the 

five wells, with some showing relatively stable geochemistry 

and little variability in geochemical constituents (for example, 

AY−68−28−517) and others showing larger changes in geochemical constituents (for example, AY−68−29−216). Similar 

to NO3

 concentrations, temporal variations in Mg/Ca ratios 

at these five wells show small ranges in variability, except for 

well AY−68−28−211, and less consistency among the five 

wells (fig. 22C). The well with the greatest variability in both 

NO3 and atrazine concentrations and the lowest Mg/Ca ratios 

was well AY−68−28−314, which also had the most variability 

in water-level altitudes (fig. 21A); these results indicate that 

well AY−68−28−314 was the most responsive of the group to 

fluctuating hydrologic conditions. Measurement of elevated 

atrazine concentration at this well in 2005 (fig. 22B), for 

example, coincides with high water-level altitude in the well 

(fig. 21A).

Using the variability in Comal Springs discharge to 

represent changes in regional hydrologic conditions, the relation between the geochemical variability for these five wells 

and changes in regional hydrologic conditions was assessed 

(fig. 23). NO3 concentration (considering all data for the five 

wells) is not significantly correlated (fig. 23A) with Comal 

Springs discharge. Atrazine concentration (considering all 

data for the five wells), is positively correlated (fig. 23B) with 

Comal Springs discharge (Kendall’s tau = .29), indicating that 

atrazine concentration in samples of shallow/urban unconfined 

groundwater tends to be higher during wetter hydrologic conditions. Mg/Ca ratio is not significantly correlated (fig. 23C) 

with Comal Springs discharge. Data for well AY−68−28−314 

(the well with the greatest variability in geochemical constituents) had a significant positive correlation between atrazine 

concentration and Comal Springs discharge (Kendall’s tau 

= .78) and significant negative correlations between NO3

concentration and Comal Springs discharge and Mg/Ca 

ratio and Comal Springs discharge (Kendall’s tau = -.57 and 

-.71, respectively). Data for the four other wells did not have 

significant correlations between NO3

 concentrations, atrazine 

concentrations, or Mg/Ca ratios and Comal Springs discharge. This lack of statistical correlation might partly reflect 

complex hydrogeology and hysteresis effects (for example, 

a delay between the hydrologic forcing and the geochemical 

response). A comparison of water-level altitude data for these 

five wells with geochemical constituents (considering all data 

for the five wells), indicates a significant correlation between 

atrazine concentration and water-level altitude (Kendall’s tau 

= .39), as well as between the Mg/Ca ratio and water-level 

altitude (Kendall’s tau = .23).

A comparison of variations in NO3 and atrazine concentrations and Mg/Ca ratios with respect to water-level altitudes 

for the most responsive of the five shallow/urban unconfined 

wells (well AY−68−28−314) demonstrates distinct changes 

in geochemical constituents in response to changes in hydrologic conditions and provides insight into processes affecting 

variable water quality and sources and transport of dissolved 

constituents through the aquifer. Higher NO3

 concentrations 

at this well correspond to low water-level altitudes during 

drier hydrologic conditions (fig. 24A) (Kendall’s tau = .64). 

Higher atrazine concentrations correspond to peaks in waterlevel altitude during wetter hydrologic conditions (fig. 24B) 

(Kendall’s tau = .68). Higher Mg/Ca ratios correspond to 

lower water-level altitudes during drier hydrologic conditions (fig. 24C) (Kendall’s tau = -.78). The remaining four 

wells (AY−68−28−211, AY−68−28−517, AY−68−27−612, 

and AY−68−29−216) with time-series data show some 

significant correlations between water-level altitude data and 

geochemical constituents (NO3 and atrazine concentrations 

and Mg/Ca ratios), but results are not consistent. That is, not 

all of the remaining four wells show significant correlations 

between water-level altitude data and any specific geochemical 

constituent, and no specific geochemical constituent shows a 

significant correlation with water-level altitude data for all or 

most of the four remaining wells. Sampling sites such as well 

AY−68−28−314, with a more direct relation between hydrologic conditions and geochemistry likely reflect a greater component of conduit-dominated flow relative to sites with less 

variable (responsive) geochemistry and likely more complex 

hydrogeology. Sampling sites such as well AY−68−28−314 

are well suited for long-term monitoring to assess the relation 

between aquifer geochemistry and hydrologic conditions.

Previous work in the Barton Springs segment of the 

aquifer has addressed links between variability in NO3

 and 

atrazine concentrations and hydrologic conditions using a 

higher resolution geochemical dataset (Mahler and others, 

2006) and provides a useful analog for evaluating the variability of these constituents in the San Antonio segment, especially given the smaller dataset available to assess temporal 

variability in the San Antonio segment. NO3

 variability in the 

Barton Springs segment of the aquifer was characterized by 

background NO3 concentrations that are diluted by incoming 

recharge with lower NO3 concentrations during high aquifer 

flow conditions (Mahler and others, 2006; Mahler and Garner, 

2009). Mahler and others (2006) also documented increases 
36 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, TexasFigure 20. Location of five shallow/urban unconfined monitoring wells with time-series data, and regional wells (shallow/urban unconfined, unconfined, and confined) and 

springs (confined) sampled for water quality in the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. 

98°

99°

100°

30°

29°

EXPLANATION

Edwards aquiferUnconfined aquiferConfined aquifer

D

Groundwater wellShallow/urban unconfinedHigh-frequency sample site and well number (appendix 1)UnconfinedConfinedConfined spring

XYX

#*

")

AY–68–27–612

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office 

(1999), 1:24,000Cities: Texas Natural Resources 

Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15

")

45

")

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XY

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XY

XY

XY

XYXY

X

X

X

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D

D

D

D

AY–68–27–612

AY–68–28–517

AY–68–28–211

AY–68–28–314

AY–68–29–216
Water-Quality Observations 37

1998 1999 2000 2001 2002 2003 2004 2005 2006

YEAR

COMAL SPRINGS DISCHARGE, 

CUBIC FEET PER SECOND

200

300

400

500

50

100

150

200

DEPTH TO WATER, FEET BELOW LAND SURFACE

SAN ANTONIO AIRPORT 

DAILY RAINFALL, INCHES 

2

4

6

8

10

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

(A)

(B)

(C)

250

0

100

Figure 21. Time-series comparison of (A) depth to water in five shallow/urban unconfined category monitoring wells; (B) daily rainfall 

at San Antonio International Airport, and (C) discharge at Comal Springs, San Antonio segment of the Edwards aquifer, south-central 

Texas, 1998−2006. 
38 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Figure 22. Time series comparison of (A) nitrate concentration, (B) atrazine concentration, (C) magnesium to calcium (molar ratio) for 

five shallow/urban unconfined category monitoring wells, and (D) discharge at Comal Springs, San Antonio segment of the Edwards

aquifer, south-central Texas, 1998−2006. (Nondetections at laboratory reporting level shown as open symbols.) 

0.01

0.02

0.03

0.04

0.05

0.06

ATRAZINE, MICROGRAMS PER LITER

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

NITRATE, MILLIGRAMS PER LITER

 AS NITROGEN

0.050

0.100

0.150

0.200

0.250

MAGNESIUM TO CALCIUM, MOLAR RATIO

(A) Nitrate

(B) Atrazine

(C) Magnesium to calcium (molar ratio)

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

0

0

0

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

1998 1999 2000 2001 2002 2003 2004 2005 2006

COMAL SPRINGS DISCHARGE, 

CUBIC FEET PER SECOND 200

300

400

500 (D)

100

2007

YEAR

5.0
Water-Quality Observations 39

Figure 23. Relation between Comal Springs discharge and (A) nitrate concentration, (B) atrazine concentration, and (C) magnesium to 

calcium (molar ratio) for five shallow/urban unconfined category monitoring wells, San Antonio segment of the Edwards aquifer, southcentral Texas, 1998−2006. (Nondetections at laboratory reporting level shown as open symbols.) 

0.01

0.02

0.03

0.04

0.05

0.06

ATRAZINE, 

MICROGRAMS PER LITER

0.050

0.100

0.150

0.200

0.250

100 200 300 400 500 600

COMAL SPRINGS DISCHARGE, CUBIC FEET PER SECOND

MAGNESIUM TO CALCIUM, 

MOLAR RATIO

1

2

3

4

5

6

NITRATE, MILLIGRAMS PER LITER

 AS NITROGEN

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

(A) Nitrate

(B) Atrazine 

(C) Magnesium to calcium (molar ratio)

Kendall's tau = 

.29 (all data) 

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

AY–68–28–211 

AY–68–28–517 

AY–68–28–314

AY–68–27–612

AY–68–29–216 

0

0

0
40 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Figure 24. Time-series comparison of depth to water and (A) nitrate concentration, (B) atrazine concentration, and (C) magnesium 

to calcium (molar ratio) for shallow/urban unconfined category monitoring well AY−28−28−314, San Antonio segment of the Edwards 

aquifer, south-central Texas, 1998−2006. Note inverse scale for nitrate concentration and magnesium to calcium (molar ratio) in (A) 

and (C). 

75

125

175

1

2

3

4

DEPTH TO WATER, FEET BELOW LAND SURFACE

NITRATE, MILLIGRAMS PER LITER

AS NITROGEN

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

YEAR

75

125

175

0.01

0.02

0.03

0.04

0.05

ATRAZINE, MICROGRAMS PER LITER

(A) Nitrate

(B) Atrazine Water level

Atrazine

Water level

Nitrate

75

125

175

0.01

0.02

0.03

0.04

0.05

0.06

MAGNESIUM TO CALCIUM, 

MOLAR RATIO

(C) Magnesium to calcium (molar ratio) Water level

Magnesium to calcium

Kendall’s tau = .64

Kendall’s tau = -.68 

Kendall’s tau = .78

225 0.07

50 0
Water-Quality Observations 41

in atrazine in groundwater associated with high aquifer flow 

conditions as a result of recharging surface water with elevated 

atrazine concentrations. NO3 and atrazine variability at site 

AY−68−28−314 (fig. 24A,B) is consistent with observed relations between these constituents and hydrologic conditions in 

the Barton Springs segment of the aquifer.

Mahler and others (2006) proposed that during low aquifer flow conditions (characterized by low amounts of recharge, 

low amounts of discharge, and low water-level altitudes in 

wells), focused recharge and associated contaminants move 

rapidly though the conduit system of the Barton Springs segment of the Edwards aquifer and subsequently discharge at 

Barton Springs. During high aquifer flow conditions, when 

conduits are already saturated with water, focused recharge is 

diluted by water in the conduits and much of the recharge and 

associated contaminants go into matrix storage. In this conceptual model, contaminants are slowly released from the matrix 

under low-flow conditions, increasing background (low-flow) 

concentrations over the long term (Mahler and Massei, 2007). 

This process of matrix release of accumulated low concentrations of anthropogenic contaminants might account for the 

persistency of some anthropogenic contaminant detections in 

confined wells in the San Antonio segment. Those contaminants most frequently detected in both the Barton Springs 

segment and the San Antonio segment of the Edwards aquifer 

are similar, which points to regional-scale urbanization and 

contaminant sources. The most frequently detected anthropogenic contaminants in the Edwards aquifer are similar to those 

detected in aquifers across the Nation (Gilliom and others, 

2006; Zogorski and others, 2006).

Analysis of available surface-water data from major 

recharging streams in the contributing and recharge zones of 

the Edwards aquifer during the same time period as groundwater sampling (fig. 5, table 2) for commonly occurring 

anthropogenic contaminants indicates that many of the same 

compounds are detected frequently in both surface water 

and groundwater. Few NAWQA (or other) surface-water 

sample sites have been routinely sampled for anthropogenic 

and organic contaminants; available data are mostly limited 

to pesticides with few analyses for VOCs. Atrazine, DEA, 

and simazine were detected in 75 percent, 48 percent, and 

31 percent, respectively, of NAWQA surface-water samples 

analyzed (n = 48). Chloroform and PCE were not detected in 

surface-water samples but were analyzed in only 11 samples 

collected at the considered sites (fig. 5, table 2) during 

1996−2007. Concentrations of atrazine were about one order 

of magnitude higher in surface-water samples (median = 0.015 

μg/L; n = 48) compared to groundwater samples (median = 

0.003 μg/L). Detected concentrations of DEA and simazine in 

surface-water samples tended to be higher than in groundwater 

samples, although median values in surface-water samples 

are less than laboratory reporting levels for these compounds, 

which limits a direct comparison of concentrations. These 

results are consistent with the hypothesis that some frequently 

detected anthropogenic contaminants, such as atrazine, enter 

the groundwater with recharging surface water. 

A specific source, or sources, of atrazine to recharging 

surface water in the study area is not known; the most direct 

source, application of atrazine in the recharge zone, is not 

well documented. The recharge zone is predominantly rural 

with land use characterized by rangeland and forest (fig. 4). 

Atrazine is used in agricultural and urban applications. As a 

result, the application of atrazine might be higher in agricultural land-use areas (which are largely over the confined part 

of the aquifer) or urban land-use areas. Atrazine might also 

be transported through the atmosphere by wind. Atrazine has 

been routinely detected where analyzed in rainwater samples 

collected from across the United States and from agricultural 

land-use areas (Majewski and Capel, 1995); however, no 

rainwater samples from central Texas have been analyzed for 

pesticides through the NAWQA Program. 

Mg/Ca ratios for the five shallow/urban unconfined wells 

that were sampled at a higher frequency had a median value 

of 0.12, which is lower than results for samples of unconfined 

and confined groundwater categories (fig. 7; table 3). Lower 

Mg/Ca ratios likely reflect less geochemically evolved water, 

shorter groundwater residence times, and (or) mixing of 

groundwater with dilute, recent recharge. Geochemical evolution processes and mixing with recent recharge might proceed 

over short time scales in response to hydrologic variability, as 

demonstrated by higher Mg/Ca ratios associated with lower 

water-level altitudes for well AY−68−28−314 (fig. 24C). This 

relation is consistent with drier hydrologic conditions and 

correspondingly longer fluid residence times allowing for 

relatively greater amounts of water-rock interaction.

The time series of variability in NO3

 concentration at 

well AY−68−28−314 indicates that ambient groundwater 

NO3 is diluted by recharge water, a process that is consistent 

with observed variability in NO3 concentrations in the Barton 

Springs segment of the aquifer (Mahler and others, 2006; 

Mahler and Garner, 2009). The median NO3 concentration for 

surface-water streams (fig. 5, table 2) in and near the recharge 

zone for the San Antonio segment during the study period is 

0.36 mg/L (n = 275, range = 0.03 to 1.9 mg/L). This is significantly less (on the basis of Mann-Whitney U test results) 

than the median Edwards aquifer groundwater NO3

 concentration of 1.7 mg/L and is consistent with the hypothesis that, in 

the absence of other distinct NO3 sources, recharging surface 

water will dilute ambient groundwater NO3 concentrations. A 

range of natural and anthropogenic sources likely contribute 

to ambient groundwater NO3

 concentrations. For example, 

NO3 concentrations measured at well AY−68−28−314 were 

consistently higher relative to NO3 concentrations measured 

in other shallow/urban unconfined wells (appendix 4). The 

median NO3 concentration at well AY−68−28−314 was 

2.6 mg/L, compared to a median of 1.9 mg/L for all shallow/

urban unconfined groundwater samples (significantly different 

on the basis of Mann-Whitney U test results). The median NO3

concentration at site AY−68−28−314 during drier hydrologic 

conditions, when the water-level altitude was low, is as high 

as 4.8 mg/L. These results indicate that groundwater samples 

from this well were affected by a local source of NO3. 
42 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Historical Major/Trace Element Ratios

Major ion data extending back several decades (1957–

2006) are available for some groundwater monitoring wells 

(13) in the Edwards aquifer that were sampled by the NAWQA 

Program (fig. 25; table 5, at end of report). Sufficient major 

ion data are available for these wells to assess temporal variability in Mg/Ca ratios. Eleven of the 13 wells are completed 

in the confined part of the aquifer and are located across the 

region, from Uvalde County in the western part of the study 

area to Comal County in the northeastern part of the study 

area. The remaining two wells are completed in the unconfined part of the aquifer in Bexar County. Median Mg/Ca 

ratios were smaller and variability (interquartile range) in 

Mg/Ca ratios was larger in samples collected from the two 

unconfined wells relative to median ratios and variability in 

samples collected from most of the confined wells (table 5). 

Historical Mg/Ca ratios for samples collected from the 11 

confined wells (range of 0.08 to 0.51 with a median value of 

0.27) are similar to Mg/Ca ratios for samples collected from 

NAWQA confined aquifer category sites during 1996−2006 

(range of 0.09 to 0.79 with a median value of 0.35). Historical 

Mg/Ca ratios for samples collected from the 11 confined wells 

varied widely among the wells (interquartile range of 0.035 

to 0.101) (table 5). There is no apparent relation between 

Mg/Ca ratios and the geographic location of these confined 

wells across the aquifer. For example, high Mg/Ca ratios, 

indicative of more geochemically evolved groundwater, were 

measured in samples collected across the study area from 

Uvalde County to Comal County. Historical data for samples 

from several of the confined wells located across the study 

area had relatively small but temporally consistent changes 

in Mg/Ca ratios; that is, they had time series of increasing 

and decreasing Mg/Ca ratios that varied in a similar pattern (for example, time series from wells YP−69−45−405, 

TD−69−46−601, TD−69−47−303, DX−68−23−303, and 

DX−68−23−601) (fig. 26A). This consistent variability likely 

reflects regional-scale changes in hydrologic conditions; 

higher Mg/Ca ratios are associated with longer residence 

time and corresponding drier hydrologic conditions (low 

aquifer flow conditions). The small range of variability for 

samples collected from individual wells is consistent with 

the hypothesis that most confined wells, unless intersecting a 

fast-flow conduit, are generally dominated by longer residence 

time matrix-dominated flow and attenuation of the inputs 

from storm and recharge events. The geochemistry of matrix 

groundwater is more stable and changes slowly in response to 

longer term regional hydrologic conditions. 

Only one of the wells with historical major ion data also 

has water-level altitude data available for the same time period 

(1970–2006, site TD−69−46−601, Medina County), which 

are compared to discharge at Comal Springs (fig. 26B) and 

to Mg/Ca ratios for groundwater from the well (fig. 26C). 

Although geochemical data and water-level altitude data are 

not available for the same dates of measurement or at the 

same time scales, water-level altitude variations in the well 

are similar to discharge values at Comal Springs (fig. 26B). 

Lower water-level altitudes in well TD−69−46−601 are 

associated with higher Mg/Ca ratios, reflecting greater extents 

of mineral-solution reaction and longer residence time in 

response to drier hydrologic conditions (lower aquifer flow 

conditions) (fig. 26C). The relation between Mg/Ca ratios 

and water-level altitude data for this well is consistent with 

the hypothesis that temporal variations in residence time and 

hydrologic conditions control variations in fluid geochemistry 

over multiple time scales. 

Groundwater Age Tracers and Travel Times

Estimating the age of groundwater is useful for understanding selected aspects of hydrogeology, such as groundwater flow and mixing. Groundwater age estimates have been 

used to determine recharge rates, estimate rates of geochemical and microbiological processes, calibrate groundwater 

flow models, evaluate aquifer susceptibility to contamination, 

and improve water-resource management (Plummer, 2005). 

Techniques for dating groundwater estimate the time passed 

since recharge of that groundwater occurred. Groundwater 

age estimates are based on the measurement of a dissolved 

solute, gas, or isotope in groundwater that has been transported through the aquifer to the point of sampling; as such, 

groundwater age estimates are model-dependent, based on an 

interpretation of recharge and flow mechanisms, and might be 

affected by a variety of physical and geochemical processes 

(Cook and Böhlke, 2000; Plummer, 2005). Because it is not 

possible to identify and account for all physical and chemical 

processes that might affect groundwater age-tracer results, the 

“apparent age” of groundwater is most appropriately reported. 

Groundwater age tracers have historically included measurements of tritium (3

H) and carbon-14 (14C); recent advances 

for interpreting groundwater age information in the 0- to 

50-year time scale include concentrations of anthropogenic 

atmospheric gases in groundwater, such as chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6

), and the ratio of the 

parent/daughter isotopic system of tritium/helium-3 (3

H/3He) 

(Plummer, 2005).

Geochemical age tracers, including 3H, CFCs, and SF6, 

have been applied in NAWQA studies of several aquifers, and 

numerous studies have addressed their application and utility 

(Michel, 1989; Busenberg and Plummer, 1991; Plummer and 

others, 1993; Busenberg and Plummer, 2000; Katz and others, 

2001; Plummer and Busenberg, 2005; International Atomic 

Energy Agency, 2006; Busenberg and Plummer, 2008). 3

H, 

CFCs (CFC−11, CFC−12, and CFC−113), and SF6 tracers 

were measured on a majority of NAWQA samples of groundwater from the Edwards aquifer, some in combination. Results 

for apparent age tracers are summarized in table 6 (at end 

of report); details of the analyses are provided in appendix 

6 for 3

H and in appendix 10 for CFCs and SF6. Many of the 

CFC and SF6 results are indicative of contamination of the 

sample by non-atmospheric sources (that is, the measured 

tracer concentration in the groundwater sample was greater 
Water-Quality Observations

43Figure 25. Location of unconfined and confined category wells with historical water-quality data, San Antonio segment of the Edwards aquifer, south-central Texas, 1957−2006. 

98°

99°

100°

30°

29°

EXPLANATION

Edwards aquiferUnconfined aquiferConfined aquifer

Groundwater wellUnconfined historical water-quality site and well number (table 5)Confined historical water-quality site and well number (table 5)

Base fromAquifers: Ashworth and Hopkins (1995), 1:250,000Counties: Texas General Land Office 

(1999), 1:24,000Cities: Texas Natural Resources 

Information System digital data (1995)Projection: Texas Centric Mapping System - Albers equal area Datum: North American Datum of 1983 

FRIO ZAVALA

ATASCOSA

KINNEY

UVALDE

BEXAR

GUADALUPE

BANDERA

COMAL REAL

KENDALL

KERR

EDWARDS HAYS

GILLESPIE BLANCO

TRAVIS

MEDINA

0 30 60 KILOMETERS 15 45

0 30 60 MILES 15 45

AY–68–29–109

AY–68–37–705

AY–68–28–205

DX–68–23–303

DX–68–23–601

DX–68–22–901

TD–69–40–403

TD–69–47–303

TD–69–46–601

YP–69–45–405

YP–69–43–606

YP–69–50–506

YP–69–50–501

AY–68–28–205

DX–68–22–901

"

"

""

"

" " " "

""

" "

""
44 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Figure 26. Comparison of (A) time series of magnesium to calcium (molar ratio) for 13 wells (two unconfined category and 11 

confined category); (B) historical relation between Comal Springs discharge and depth to water at well TD−69−46−601 (confined 

category, Medina County); and (C) time series of magnesium to calcium (molar ratio) (note inverted scale) and depth to water at well 

TD−69−46−601 (confined category, Medina County), San Antonio segment of the Edwards aquifer, south-central Texas, 1970−2007. 

1970 1975 1980 1985 1990 1995 2000 2005 2010

100

200

300

400

500

YEAR

DISCHARGE, 

CUBIC FEET PER SECOND

(30-DAY MOVING AVERAGE)

20

40

60

80

100

120

140

160

180

Comal Springs discharge

Depth to water, well TD–69–46–601

0.32

0.34

0.36

0.38

1970 1975 1980 1985 1990 1995 2000 2005 2010

YEAR

MAGNESIUM TO CALCIUM,

MOLAR RATIO

20

40

60

80

100

120

140

160

180

Depth to water, well TD–69–46–601 

Magnesium to calcium (molar ratio)

0

0.1

0.2

0.3

0.4

0.5

0.6

YEAR

MAGNESIUM TO CALCIUM,

MOLAR RATIO

1970 1975 1980 1985 1990 1995 2000 2005 2010

DEPTH TO WATER

FEET BELOW LAND SURFACE

(A) 

(B) 

(C) 

YP–69–45–405, Uvalde, confined

YP–69–50–501, Uvalde, confined

YP–69–50–506, Uvalde, confined

DX–68–23–601, Comal, confined

DX–68–22–901, Comal, confined

DX–68–23–303, Comal, confined

AY–68–29–109, Bexar, unconfined YP–69–43–606, Uvalde, confined

AY–68–28–205, Bexar, unconfined

AY–68–37–705, Bexar, confined

TD–69–46–601, Medina, confined

TD–69–47–303, Medina, confined

TD–69–40–403, Medina, confined 

Well number, county and category

EXPLANATION

Unconfined 

sites

600

0.30

0.40

0

200

10

200

10

DEPTH TO WATER

FEET BELOW LAND SURFACE
Water-Quality Observations 45

than the theoretically possible tracer concentration, based on 

equilibrium of the groundwater sample with the maximum 

atmospheric concentration) and, as a result, did not yield 

resolvable apparent ages. Apparent ages for all groundwater 

samples ranged from about 1 to 52 years, with a median of 20 

years; results indicate that groundwater in the Edwards aquifer 

is dominated by “young” or “modern” water, with extensive 

groundwater mixing. “Young” groundwater is defined as water 

recharged within approximately the past 50 years, the time 

scale for which is largely defined by the dating range of applicable environmental tracers (Plummer and Friedman, 1999; 

Plummer and Busenberg, 2005). “Modern” groundwater has 

been defined similarly, that is, water that has recharged within 

the past few decades and is part of an active hydrologic cycle 

(Clark and Fritz, 1997). In this report, because all groundwater 

apparent ages are young, modern groundwater is used to represent samples with apparent ages that belong to the present 

day, that is, within 10 years of the time of sampling. Results 

for apparent groundwater age measurements for groundwater 

samples from the Edwards aquifer are consistent with the 

regional hydrogeology, previous studies, and results of dyetracing tests performed in the aquifer that document a large 

range of travel times and groundwater mixing (Pearson and 

others, 1975; Campana and Mahin, 1985; Ogden and others, 

1986; Johnson and Schindel, 2008).

Preliminary apparent age interpretations for CFC and SF6 

tracer results were provided by the USGS Chlorofluorocarbon 

Laboratory, Reston, Va., and are based on a piston-flow model 

(Niel Plummer, U.S. Geological Survey, written commun., 

2009). In spite of the analytical precision available for measurement of many geochemical age tracers, there are a variety 

of uncertainties associated with the interpretation of these 

data and determining the apparent age of a groundwater 

sample (Phillips and Castro, 2003; Bethke and Johnson, 2008). 

Age interpretation for a sample that consists of water of a 

single age might be relatively simple but becomes increasingly complex for a sample that consists of a mixture of water 

of varying ages (Böhlke, 2006). As described by Long and 

Putnam (2006), this might be especially important in karst 

aquifers, where each component of a complex network of 

conduits, fractures, and matrix porosity might have a different groundwater age and a well or spring might discharge 

water from any combination of porosity types (Plummer and 

Busenberg, 2005). Models for the hypothetical concentrations 

of age tracers in water samples with different age distributions that consider the effect of hydrologic processes such as 

dispersion, exponential mixing, and binary mixing of shallow 

and deep groundwater have been developed to facilitate interpretation of geochemical age tracers (Böhlke, 2006; Long 

and Putnam, 2006). A piston-flow model, which is the simplest and most common transport assumption, assumes 

that the tracer concentration was not altered by transport 

processes (Plummer and Busenberg, 2005). This simplified 

approach likely does not adequately describe the range of 

ages in a mixed sample, especially for karst aquifers (Long 

and Putnam, 2006). In spite of these uncertainties in age 

interpretation, piston-flow model ages provide an initial reference point for comparison of different age tracers and time 

scales of flow processes in the aquifer. Age tracers might 

not be well suited to distinguish small differences in groundwater ages measured in samples collected from individual 

wells in the Edwards aquifer, but they provide insight into 

how changing hydrologic conditions affect groundwater 

recharge and mixing. Results indicate that while apparent 

groundwater ages are consistently young, the apparent age 

measured in individual samples varies as a result of changing 

hydrologic conditions. That is, variability in hydrologic conditions results in different mixtures of groundwater of different 

apparent ages. More detailed evaluation of apparent age tracer 

data for the Edwards aquifer with respect to different hydrodynamic and geochemical processes that might affect tracer 

concentrations and groundwater mixtures is a topic of ongoing 

research. 

Contamination can affect apparent groundwater ages 

determined using CFC and SF6 concentrations, particularly in 

urban areas. Results for a majority of the CFC tracer concentrations measured in Edwards aquifer groundwater samples 

indicate that CFC contamination might be widespread—in 

particular for CFC−11 and CFC−12 in shallow/urban unconfined groundwater samples (table 6). This hypothesis is consistent with a previous study of the Edwards aquifer documenting 

CFC contamination (Thompson and Hayes, 1979). As a result, 

resolvable CFC-based groundwater apparent ages might still 

be affected by excess CFC from widespread aquifer contamination; apparent ages based on CFC concentrations might be 

questionable. Results are discussed below only for samples 

that clearly are not contaminated. Contaminant concentrations 

of CFCs, which might be attributed to anthropogenic sources 

such as septic tanks, leaking sewer lines and storage tanks, 

sewage treatment plant effluent, and industrial wastes, would 

tend to bias apparent ages to younger values (Plummer and 

Busenberg, 2005). CFC tracer concentrations measured in 

samples collected from the deeper unconfined part of the aquifer in 2006 and SF6

 tracer concentrations measured in shallow/

urban unconfined groundwater samples collected in 2006 have 

notably less apparent contamination relative to other samples 

(table 6, appendix 10). 

Tritium 

The most frequently measured age tracer was 3H, a 

radioactive isotope of hydrogen with a half-life of 12.43 years 

(Unterweger and others, 1980). 3H in rainfall is from two 

sources, either natural cosmogenic 3H or produced by atmospheric testing of nuclear weapons and nuclear reactors. Given 

its relatively short half-life, the presence of 3

H in groundwater 

is indicative of recent groundwater recharge (Clark and Fritz, 

1997; Plummer, 2005). The input of 3H to the atmosphere as a 

result of nuclear testing dominated the concentrations of 3H in 

rainfall for about 50 years, with concentrations peaking in the 

1960s. Pre-bomb background atmospheric 3H concentrations 

were about 2 to 10 Tritium Units (TU); 3H concentrations in 
46 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

recent rainfall (since 2006) have globally decreased to approximately pre-bomb background levels (Clark and Fritz, 1997; 

Phillips and Castro, 2003; Robert Michel, U.S. Geological 

Survey, written commun., 2009). 

Concentrations of 3H measured in samples of Edwards 

aquifer groundwater collected during 1996−2006 ranged from 

0.3 to 4.6 TU, with a median of 2.6 TU. Twenty-six shallow/

urban unconfined wells were sampled twice for 3H, in 1998 

and again in 2006. 3H concentrations at most of these wells 

were lower in 2006 (median = 2.0 TU) compared to concentrations in 1998 (median = 2.9 TU) (data are statistically different 

on the basis of Mann-Whitney U test results), which is consistent with 3

H decay in the atmosphere and in groundwater 

over this 8-year period. Groundwater 3H concentrations were 

compared to a local 3

H input function derived from a combination of 3

H concentrations measured in rainfall at Waco, 

Tex. (1961−87) and 3H concentrations measured in rainfall 

at Vienna, Austria (1961–2006). Results for 3H concentrations for Edwards aquifer groundwaters indicate a dominant 

component of young and modern water throughout the aquifer, 

with similar values in all parts of the aquifer characterized 

in this report (table 3, appendix 6). Mixtures of water ranging in age from modern to approximately 20 years cannot be 

readily distinguished on the basis of 3H values. Relatively low 

values (less than 1 TU) were measured in samples collected 

in 1997 from three wells in the confined part of the aquifer 

(appendix 6), which might indicate a mix of water of different 

ages, including a component of older sub-modern water (that 

is, water recharged more than 50 years ago). These three wells 

are in the western part of the aquifer (Uvalde and Medina 

Counties), with depths ranging from approximately 500 to 

2,500 ft below land surface. 

Chlorofluorocarbons and Sulfur Hexafluoride

CFC tracer data (CFC−11, CFC−12, and CFC−113) were 

measured for most of the samples collected from shallow/

urban unconfined wells in 1998 (n = 29) and in 2006 (n = 

22) and for samples collected from unconfined groundwater 

wells in 2006 (n = 30) (appendix 10). Additionally, the tracer 

SF6 was measured in 22 samples collected from shallow/

urban unconfined wells in 2006. Analytical results indicate 

that some of the samples were contaminated with excess 

tracer and do not yield resolvable apparent ages. Results of 

apparent age tracers for uncontaminated samples indicate 

that the shallow/urban unconfined part of the aquifer consists 

of young, recently recharged water, ranging in apparent age 

from modern (about 1 year) to about 38 years, with a median 

apparent age of 14.5 years. These young ages are consistent 

with the rapid recharge and high groundwater flow rates of the 

shallow unconfined part of aquifer. When apparent ages could 

be estimated in individual samples using multiple CFC tracers 

(that is, uncontaminated results were available for a combination of two or more of the CFC tracers), the apparent age 

results were generally in good agreement (table 6; appendix 

10). This occurred most often for samples collected from the 

unconfined part of the aquifer in 2006. Where CFC (mostly 

CFC−113) and SF6

 apparent ages were measured in samples 

collected from shallow/urban unconfined wells in 2006 (n = 

9), the results indicate young apparent ages for both tracers, 

although results do not yield concordant ages (fig. 27). This 

lack of precise agreement is not uncommon for groundwater 

apparent age tracer data; it has been proposed that although 

groundwater ages indicated by various tracers might differ, 

each tracer could still provide unique insights into different 

CHLOROFLUOROCARBON

APPARENT AGE, YEARS

SULFUR HEXAFLUORIDE APPARENT AGE, YEARS

5

10

15

20

25

30

35

5 10 15 20 25 30 35

Shallow/urban unconfined, 2006—CFC–113 1:1 correlation

Shallow/urban unconfined, 2006—CFC–12

Groundwater

0

0

40

40

Figure 27. Relation between groundwater apparent ages for chlorofluorocarbon (CFC) (CFC−12 or CFC−113) and sulfur hexafluoride 

tracers for groundwater samples collected from shallow/urban unconfined category wells, San Antonio segment of the Edwards 

aquifer, south-central Texas, 2006. 
Water-Quality Observations 47

components of the mixed water history, and thus multiple 

tracers might have much greater utility in interpreting groundwater apparent age tracer results (Glynn and Plummer 2005; 

Bethke and Johnson, 2008). 

The median apparent groundwater age using CFC 

concentrations measured in shallow/urban unconfined category samples collected in 1998 (12 years) is less than that 

for samples collected from the same group of shallow/urban 

unconfined category wells in 2006 (18 years). Apparent 

groundwater CFC age results for the shallow/unconfined 

category samples collected in 1998 and 2006 were statistically different on the basis of Mann-Whitney U test results. 

These results are consistent with differences in hydrologic 

conditions associated with these two periods, with relatively 

wet conditions in 1998 (larger amount of recharge and larger 

proportion of modern water) and drier conditions in 2006 

(smaller amount of recharge and smaller proportion of modern 

water). These results indicate that apparent groundwater age 

in a dynamic karst system such as the Edwards aquifer reflects 

mixtures of young and modern water, the proportions of which 

vary temporally as a result of changes in hydrologic conditions. Temporal variability in apparent age results for karst 

groundwater as a result of hydrologic conditions has been 

previously demonstrated in the Floridan aquifer (Happell and 

others, 2006) and indicates that a one-time sampling of karst 

groundwater might not provide representative apparent ages.

The apparent age of groundwater has been used to 

estimate flow paths and residence time (for example, Cook 

and Böhlke, 2000). Based on available NAWQA data, this 

application is limited for the Edwards aquifer, partly because 

of the small number of age-tracer measurements in samples 

collected from the more geochemically evolved groundwater 

from the confined part of the aquifer. Apparent age results 

for samples collected from the relatively shallow unconfined 

part of the aquifer (shallow/unconfined category) (median of 

14.5 years) were younger than results for samples collected 

from the deeper unconfined part of the aquifer (unconfined 

category) (median of 24.1 years) and were statistically different on the basis of Mann-Whitney U test results. This 

comparison between apparent ages for samples of relatively 

shallow unconfined groundwater and deeper unconfined 

groundwater might be biased, however, by differences in 

hydrologic conditions during sampling periods. Apparent age 

tracers were measured in samples collected from shallow/

urban unconfined category wells in 1998 (a relatively wet 

year with younger apparent ages) and 2006 (a relatively dry 

year with older apparent ages), but apparent age tracers were 

measured in samples collected from the deeper unconfined 

part of the aquifer only in 2006. Nonetheless, if apparent ages 

for only samples collected in 2006 are compared, the median 

apparent age for shallow/urban unconfined category wells was 

17.9 years, which is slightly younger than for samples from 

the deeper unconfined part of the aquifer (24.1 years); apparent ages for these two sample groups collected in 2006 were 

still statistically different on the basis of Mann-Whitney U test 

results. 

Mg/Ca ratios, which can be used as a qualitative indicator of groundwater residence time, are compared to apparent 

age results for CFC and SF6

 data from 1998 (shallow/urban 

unconfined groundwater category) and 2006 (shallow/urban 

unconfined and unconfined groundwater categories) in fig. 28 

(no apparent age tracer measurements other than 3H were 

collected from the confined part of the aquifer). Results for 

Mg/Ca ratios and apparent groundwater ages are poorly correlated; only one correlation is statistically significant for any 

specific tracer. That correlation, between Mg/Ca ratios and 

the apparent age from SF6 concentrations for shallow/urban 

unconfined category samples collected in 2006 is negative 

(Kendall’s tau = -.41), which is inverse to what would be 

expected on the basis of the previously discussed relation of 

lower Mg/Ca ratios associated with younger groundwater. 

These results indicate that the complexities associated with 

CFC and SF6 age-tracer methods in this aquifer, including 

potential contamination and use of appropriate mixing models, 

might limit their application beyond providing insight into the 

young nature of the groundwater.

Flow Model Estimates of Travel Times and Flow 

Rates

Previous NAWQA studies have compared groundwater 

age-tracer results to estimates of flow rates and travel times 

from hydrogeologic models in other aquifer systems (Clark 

and others 2007; Burrow and others 2008; Jurgens and others, 

2008; Landon and others, 2008). In a similar way, groundwater age tracers measured in samples collected from the 

Edwards aquifer can be compared to estimates of flow rates 

and travel times from two groundwater flow models of the 

Edwards aquifer (Lindgren and others, 2004; Lindgren, 2006) 

done using MODFLOW2000 (Harbaugh and others, 2000). 

The first model includes simulated, continuously connected 

conduits, as well as barrier faults, that strongly influence subregional flow (hereinafter, the conduit-flow model; Lindgren 

and others, 2004). In the second model, without simulated 

conduits (hereinafter, the diffuse-flow model), flow occurs 

through a network of numerous small fractures and openings; 

this model includes zones of upscaled hydraulic conductivity 

(Lindgren, 2006). Both models were calibrated for steadystate and transient conditions. Detailed descriptions of model 

development, calibration, and sensitivity tests are included in 

Lindgren and others (2004) and Lindgren (2006).

Particle-tracking simulations in the models (MODPATH; 

Pollock, 1994) were used to estimate particle travel times and 

areas contributing recharge for a majority of the NAWQA 

wells (these wells were previously included in model calibrations: n = 113 for the conduit-flow model and n = 115 for the 

diffuse-flow model). MODPATH uses the cell-by-cell flow 

values calculated by MODFLOW to calculate the groundwater 

flow velocity distribution in the groundwater system, which is 

then used to determine flow paths of water particles moving 

through the aquifer (Pollock, 1994). Travel times along flow 

paths are computed using the magnitude of the cell-by-cell 
0

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 10 20 30 40 50 60

(B) 2006

 APPARENT AGE, YEARS

0

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 10 20 30 40 50 60

 APPARENT AGE, YEARS

MAGNESIUM TO CALCIUM, 

MOLAR RATIO

Shallow/urban unconfined—-CFC–11

Shallow/urban unconfined—CFC–12

Shallow/urban unconfined—CFC–113

(A) 1998

Shallow/urban unconfined—SF6

Shallow/urban unconfined—CFC–11

Shallow/urban unconfined—CFC–12

Shallow/urban unconfined—CFC–113

Unconfined—CFC–11

Unconfined—CFC–12

Unconfined—CFC–113

EXPLANATION

EXPLANATION

MAGNESIUM TO CALCIUM, 

MOLAR RATIO

[CFC, chlorofluorocarbon (CFC–11, –12, or –113); SF6, sulfur hexafluoride]

Groundwater

Groundwater

48 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Figure 28. Relation between magnesium to calcium (molar ratio) and apparent age for groundwater samples collected from shallow/

urban unconfined and unconfined parts of the San Antonio segment of the Edwards aquifer, south-central Texas, (A) 1998 and (B) 2006. 
0

50

100

150

200

250

300

TRACER DATA

(SF6, CFC, TRITIUM)

CONDUITFLOW MODEL

DIFFUSEFLOW MODEL

APPARENT AGE METHOD

APPARENT AGE, YEARS

All

Shallow/urban unconfined

Unconfined

Confined

Tracer data results for shallow/urban unconfined and unconfined categories represent the median value for each 

sample category based on all sulfur hexafluoride and chlorofluorcarbon apparent ages; results for the confined 

category are based on tritium data and represent the range of likely apparent groundwater ages.

[SF6, sulfur hexafluoride; CFC, chlorofluorocarbon (CFC–11, –12, or –113)]

Water-Quality Observations 49

flows, porosity, and model cell dimensions. MODPATH calculates advective flow only and does not account for effects of 

mechanical dispersion and (or) chemical reaction on groundwater transport. 

Model results can be used to estimate the travel time and, 

correspondingly, an interpreted age of water at each well. The 

steady-state calibrations of both the conduit-flow and diffuseflow models, with an estimated average effective porosity 

value of 0.18 (based on Hovorka and others, 1996), were 

used to determine backward-tracking particle travel times for 

comparison to geochemical age-tracer results. A total of 100 

particles were specified for each well, spaced uniformly along 

the open or screened interval. The resulting travel time for 

each particle might vary, depending on its starting position. All 

resulting particle tracks for each well were used to calculate 

minimum, maximum, and mean travel times (appendix 11). 

Additionally, the mean particle travel distance was computed 

to provide an estimate of flow path length and, in conjunction 

with mean particle travel time, to estimate a mean flow rate 

associated with each well (table 7, at end of report). 

Results of particle tracking for the conduit-flow and 

diffuse-flow models are compared to results for geochemical apparent age tracers collected for these wells (fig. 29). 

There is little agreement between the apparent groundwater 

ages estimated by the two approaches. Geochemical apparent 

age tracer results are approximately one order of magnitude 

younger than mean model particle-track travel-time results. 

For the model results, the diffuse-flow model tends to yield 

slightly shorter travel times than the conduit-flow model. 

These differences result from differences between the hydraulic conductivity distributions of the two models; particle-track 

pathlines and, correspondingly, travel times tend to align with 

zones of high hydraulic conductivity. In the conduit-flow 

model, these zones are a one-model-cell-wide, continuously 

connected network of conduits. In the diffuse-flow model, 

higher hydraulic conductivity zones are generally wider than 

one cell. Particle-track travel times calculated by MODPATH 

are proportionally sensitive to model porosity values. Because 

of the karstic nature of the groundwater flow, including mixing 

between conduit and matrix flow components, the regional 

Figure 29. Comparison of apparent ages determined from geochemical age tracers (sulfur hexafluoride, chlorofluorocarbons, and 

tritium) for groundwater samples collected from shallow/urban unconfined, unconfined, and confined parts of the San Antonio segment 

of the Edwards aquifer, south-central Texas, 1996−2006, with mean particle-track travel-time estimates derived from conduit-flow and 

diffuse-flow models for the Edwards aquifer (Lindgren and others, 2004; Lindgren, 2006). 
50 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

average effective porosity of 0.18 used in the model might not 

be applicable at the scale of local karst conditions. Differences 

between geochemical age-tracer results and model particletrack travel-time calculations raise many research questions 

and are topics of ongoing investigation. The comparison of 

flow model and geochemical age-tracer results in this report 

indicate that existing flow models might not accurately represent the rapid, fast-flow (conduit-dominated) component 

of this karstic aquifer. This is also demonstrated by results 

of dye-tracing tests in northern Bexar County that indicate 

groundwater velocities of 13 to 5,283 meters per day (Johnson 

and others, 2009), which are orders of magnitude higher than 

model results. These fast flow paths might include flow occurring through local-scale karst dissolution features and local- to 

regional-scale conduits. The current limitations of distributed porous media models to simulate flow in a karst system 

including, specifically, limitations with the simulation of rapid 

flow in conduits and travel times for contaminants are well 

recognized (Lindgren and others, 2004) and an area of ongoing research efforts. Although the absolute values for particletrack travel times appear unrealistically long, the model results 

show a clear progression toward longer flow paths and longer 

travel times from the unconfined to the confined part of the 

aquifer, which is consistent with geochemical interpretations 

and aquifer hydrology. 

Implications for Monitoring for WaterQuality Trends in Karst

Over 2 decades, the NAWQA Program has applied 

nationally consistent procedures for sample site selection, 

sampling, analytical methods, and data management. As a 

result, studies are comparable across different aquifer systems 

and have been used to examine natural factors and anthropogenic practices that affect water quality. Application of a 

consistent study approach has allowed examination of natural 

factors and anthropogenic practices that affect water quality 

in a broad range of spatial scales and environmental settings 

and has provided national scale insights into water quality 

(for example, Gilliom and others, 2006; Zogorski and others, 

2006; McMahon and others, 2009). NAWQA studies of the 

San Antonio segment of the Edwards aquifer conducted during 

1996−2006 have yielded extensive water-quality data, including water-quality data from different parts of the aquifer (shallow/urban unconfined, unconfined, and confined categories). 

These data are useful for assessing the status of water quality 

and regional hydrologic processes that affect geochemistry. 

The analysis of temporal trends or evaluation of the relation between groundwater quality and hydrologic conditions 

is constrained by few time-series data. To assess long-term 

changes in groundwater quality, an understanding of the relation between variability in water quality and hydrologic conditions on different time scales is needed, particularly in karst 

settings. Collection of data at selected integrator sites (for 

example, for the Edwards aquifer, major discharge points such 

as Comal Springs and carefully selected wells) on different 

time scales and associated with different hydrologic conditions 

would address multiple time scales of aquifer variability (from 

event-based, short-term to intermediate and long-term) to 

allow for understanding of both aquifer hydrology and shortand long-term trends in water quality. 

An understanding of the relation between variability in 

water quality and hydrologic conditions provides a foundation on which to assess long-term changes in water quality. 

For example, if concentrations of a contaminant increase over 

time, but knowledge of the expected relation between that constituent and hydrologic conditions indicates that constituent 

concentrations should decrease over that same time period, the 

trend might be independent of changes in hydrologic conditions. A variable-interval sampling program might be designed 

to target high or low aquifer flow conditions or to relate 

sampling intervals to periods of similar hydrologic conditions 

to distinguish between short-term/hydrologically induced 

variation and longer-term trends. A monitoring design for a 

karst system should be based directly on monitoring goals 

(Quinlan, 1989). A long-term monitoring program design 

that includes sampling of matrix- and conduit-dominated 

flow components, springs, and surface-water recharge under 

a variety of flow conditions (including base flow and event 

flow) could provide comprehensive understanding of the water 

quality of the Edwards aquifer and variability of water quality 

as a result of different hydrologic conditions. A monitoring 

design that includes specific integrator sites necessitates some 

prior knowledge of aquifer processes and aquifer response to 

recharge events. Continuous monitoring of physicochemical 

conditions such as water-level altitude, specific conductance, 

and turbidity can be used to guide sample site selection and 

frequency of sample collection. A high sampling frequency, 

especially under storm-event conditions, might be needed to 

assess rapid changes in water quality that have been shown 

to occur in karst aquifers (for example, Mahler and Massei, 

2007). An ideal sampling frequency needs to be flexible and 

driven by hydrologic conditions. Such an approach would 

supplement previous NAWQA studies and facilitate assessment of trends in water quality in karst settings such as the 

Edwards aquifer. When monitoring programs are intended 

to provide the data necessary to assess long-term trends and 

increase understanding of karst aquifer functioning, time 

scales of hydrologic and geochemical variability need to be 

carefully considered.

Summary

During 1996–2006, the U.S. Geological Survey collected 

and analyzed groundwater samples from the San Antonio 

segment of the Edwards aquifer as part of the National 

Water-Quality Assessment (NAWQA) Program. This report 

evaluates those data to assess (1) hydrologic processes in 

the Edwards aquifer such as mineral-solution reactions and 
Summary 51

groundwater geochemical evolution, (2) spatial and temporal 

(time-series) data for natural and anthropogenic constituents, 

(3) the relation between geochemistry and water quality with 

hydrologic conditions, (4) the utility of geochemical age tracers, and (5) a comparison of age-tracer data with existing flow 

models, to develop an integrated understanding of karst and 

aquifer processes. Implications for monitoring water-quality 

trends in karst are also discussed. Given the large number of 

analytical constituents that have been analyzed for samples 

of groundwater collected throughout the Edwards aquifer, 

this report presents an overview of variability in groundwater 

geochemistry and water quality, and controlling processes in 

the San Antonio segment of the aquifer. The dataset includes 

249 groundwater samples from 136 sites throughout the southcentral Texas (SCTX) study unit, including samples from four 

springs. Sampling primarily occurred during 1996−98 and 

again during 2005−06. A small subset of wells was sampled 

repeatedly during 1999−2004. For this analysis, groundwater 

samples were grouped into categories of “shallow/urban 

unconfined,” “unconfined,” and “confined.” These categories 

are representative of groundwater evolution along a hypothetical flow path encompassing endmembers of (1) shallow, 

unconfined, younger, less geochemically evolved groundwater 

(shallow/urban unconfined category) and (2) deep, confined, 

older, more geochemically evolved groundwater (confined 

category). Unconfined groundwater is intermediate between 

these two endmembers in this conceptual flow path. Samples 

collected from shallow unconfined (water-level) monitoring wells in residential or commercial land-use settings in 

the urban San Antonio recharge zone (outcrop area) of the 

aquifer (northern Bexar Country) were categorized as shallow/urban unconfined groundwater, representing the shallow 

unconfined part of the aquifer and the urbanized part of the 

aquifer. Samples collected from recharge zone (outcrop area) 

wells located randomly throughout the aquifer were categorized as unconfined groundwater. Samples collected from 

wells completed in the confined part of the aquifer and from 

springs discharging from the confined part of the aquifer were 

categorized as confined groundwater. Monitoring wells from 

which samples in the shallow/urban unconfined category were 

collected are relatively shallow (median well depth of 263 ft 

below land surface). Wells from which samples in the unconfined and confined categories were collected, in comparison, 

have median well depths of 385 and 859 ft below land surface, 

respectively (excluding those with unknown well depths). 

The central Texas Edwards aquifer is a highly productive 

karst aquifer developed in Early Cretaceous carbonate rocks, 

and is an important water resource in a rapidly growing and 

urbanizing region. Karst groundwater systems are subject 

to rapid changes in flow and discharge rates, water-level 

altitudes, and water quality, and as a result, are susceptible 

to contamination. An understanding of the relation between 

variability in water quality and hydrologic conditions on 

different time scales including geochemical evolution processes, the spatial and temporal variability of groundwater 

geochemistry, and natural and anthropogenic effects on water 

quality is necessary for resource management as well as for 

sustainability of springflow for threatened and endangered 

species habitat. The Edwards aquifer, like many karst aquifers, is characterized by hydrologic variability—water-level 

altitudes in the aquifer can rise rapidly in response to rainfall 

and corresponding recharge, accompanied by increases in 

springflow; conversely, water-level altitudes and springflow 

decrease during periods of low rainfall and recharge. Changes 

in environmental, climatic, and hydrologic conditions can 

affect groundwater geochemistry and water quality. Temporal 

changes in hydrologic conditions such as water-level altitudes, 

recharge, travel times, and flow routing affect hydrogeologic 

processes, including the extent of variability in water-rock 

interaction processes, dilution of recharge, and associated 

geochemistry. Rapidly changing hydrologic conditions and 

a continuum of flow pathways from matrix to conduit flow 

imply that hydrogeologic processes might operate on different 

temporal and spatial scales in different parts of the aquifer. For 

example, groundwater dominated by matrix/diffuse flow or 

less affected by focused recharge might exhibit less temporal 

variability than groundwater influenced by focused recharge 

and conduit flow. Previous studies have used temporal variability in geochemistry of groundwater samples to investigate 

transport processes and the effect of changes in hydrologic 

conditions on water quality. 

Geochemical and isotopic data are useful tracers of 

water-rock interaction processs, recharge, groundwater flow, 

and fluid mixing that affect water quality. An understanding of geochemical evolution processes provides information 

on groundwater residence times, endmember compositions, 

mixing processes, flow path length, and aquifer susceptibility 

to anthropogenic contaminants. Molar ratios of magnesium 

to calcium and strontium to calcium in groundwater typically 

increase along flow paths, and results for Edwards aquifer 

groundwater show an increase from shallow/urban unconfined, 

to unconfined, to confined groundwater categories. These differences are consistent with longer residence times and greater 

extents of mineral-solution reactions controlling fluid compositions as groundwater evolves from shallow unconfined 

groundwater to deeper confined groundwater. Molar ratios of 

magnesium to calcium and strontium to calcium in samples of 

Edwards aquifer groundwater were well correlated (Kendall’s 

tau was .61). Overlapping ranges between different groundwater categories (shallow/urban unconfined groundwater, 

unconfined groundwater, and confined groundwater) suggest 

that water-rock interaction processes are spatially variable and 

likely are affected by local-scale differences in host limestone 

composition, soil composition, flow routes, and residence 

times. Despite these local-scale complexities, a regional pattern is evident. Strontium isotope results for Edwards aquifer groundwater, mostly for shallow/urban unconfined and 

unconfined samples show a progression toward lower isotopic 

values with higher magnesium to calcium and strontium to 

calcium molar ratios resulting from increased mineral-solution 

reaction as allowed by increased residence time and longer 

flow paths. Mass-balance modeling of progressive water-rock 
52 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

interaction processes supports this conclusion and is consistent 

with previous research in the aquifer that also includes other 

aquifer components such as soils and unsaturated zone cave 

dripwaters.

Stable isotopes of hydrogen and oxygen are commonly 

used in studies of hydrologic processes. Deuterium and 

oxygen isotope data indicate Edwards aquifer groundwater is 

composed of meteoric recharge. The relation between hydrogen isotopes and chloride concentrations indicates that specific 

geochemical processes have affected some groundwater 

samples, including (1) mixing with a component of more 

saline groundwater, (2) mixing with recent recharge associated 

with tropical cyclonic storms, and (3) mixing with recharge 

water that has undergone evaporation. 

Samples of Edwards aquifer groundwater exhibit a 

relatively narrow range in major ion compositions across the 

region and are mostly dilute calcium-bicarbonate water. A 

small number of groundwater samples had elevated specific 

conductance values with relatively high concentrations of specific ions (chloride, sulfate, and (or) sodium). The geochemical composition of these samples indicates that they are influenced by mixing with more-saline water. Sources of saline 

water to Edwards aquifer groundwater are the underlying 

Trinity aquifer and (or) downdip saline-zone water, which 

might move up along faults and mix with fresh aquifer water. 

Variations in the sulfate to chloride ratio in relation to sulfate 

concentration, and the magnesium to sodium ratio in relation 

to magnesium concentration might allow for distinguishing 

between these two saline groundwater sources, and indicate 

mixing pathways with saline-water sources for samples with 

elevated specific conductance values. 

Anthropogenic contaminants, such as nutrients, pesticides, and solvents are a national water-quality concern 

because of their potential detrimental effects on the environment and to human health. Median nitrate concentrations were 

similar in samples of the shallow/urban unconfined groundwater category and samples of the confined groundwater 

category (1.85 and 1.84 mg/L, respectively), and lower in 

samples of the unconfined groundwater category (1.09 mg/L). 

Nitrite was rarely detected in samples of groundwater from 

the Edwards aquifer. Higher nitrate concentrations for samples 

from the shallow/urban unconfined groundwater category 

likely reflect a greater anthropogenically-derived influx of 

nitrate in the urban environment, which has a variety of potential nitrate sources, relative to samples collected from more 

rural unconfined groundwater category wells. A time-series 

record (1938−2006) of discharge at Comal Springs, one of 

the major aquifer discharge points, indicates an upward trend 

for nitrate and chloride concentrations, which likely reflects 

anthropogenic activities. A small number of organic contaminants were routinely or frequently detected in Edwards aquifer 

groundwater samples. These were the pesticides atrazine, its 

degradate deethylatrazine, and simazine; the drinking-water 

disinfection byproduct chloroform (a trihalomethane); and the 

solvent tetrachloroethene (commonly used in dry cleaning). 

Detection of these contaminants was most frequent in samples 

of the shallow/urban unconfined groundwater category and 

least frequent in samples of the unconfined groundwater category. These results indicate that the shallow/urban unconfined 

part of the aquifer is most affected by anthropogenic contaminants and that the urban environment is a source of contaminants to the aquifer. The unconfined part of the aquifer is the 

least affected by anthropogenic contaminants (these samples 

were mostly from undeveloped rural and rangeland areas). The 

high frequency of detection for these anthropogenic contaminants aquifer-wide and in samples of deep, confined groundwater indicate that the entire aquifer is susceptible to waterquality changes as a result of anthropogenic activities.

Changes in hydrologic conditions affect water quality. 

Samples of groundwater were collected one or two times during the study period (1996–2006) from most sites, a sampling 

frequency which does not allow for evaluation of temporal 

trends. Five of the shallow/urban unconfined category wells 

sampled by NAWQA had a higher frequency of sampling 

than other sites, with a total of eight samples collected over 

the 10-year study period. These data provide insights into 

the relation between changes in hydrologic conditions and 

changes in water quality. Relatively continuous or frequent 

periodic water-level altitude measurements for these five 

wells allow for comparison between geochemical variability 

and hydrologic conditions at each site. Time-series changes 

in three indicator constituents at these five wells (that is, 

constituents or ratios of constituents that are representative 

of water-quality concerns and geochemical processes) were 

compared to changes in hydrologic conditions as represented 

by water-level altitudes at wells and spring discharge at Comal 

Springs. Indicator constituents considered were nitrate, atrazine, and the magnesium to calcium molar ratio. A comparison 

of variations in indicator constituents to water-level altitude 

for the most responsive of the five shallow/urban unconfined 

wells demonstrates distinct changes in indicator constituents in 

response to changes in hydrologic conditions. Higher atrazine 

concentrations at this well correspond to peaks in water-level 

altitude during wetter hydrologic conditions, whereas higher 

nitrate concentrations and magnesium to calcium ratios occur 

in conjunction with low water-level altitudes during drier 

hydrologic conditions. These results indicate that atrazine 

enters the aquifer with surface-water recharge, whereas 

background nitrate concentrations in the aquifer are diluted by 

surface-water recharge. Analyses of surface water from in and 

near the recharge zone are consistent with these results, showing higher atrazine and lower nitrate concentrations in samples 

of surface water relative to groundwater.

Estimating the age of groundwater is useful for understanding selected aspects of hydrogeology, such as groundwater flow and mixing. Geochemical age tracers were measured on a majority of groundwater samples from the Edwards 

aquifer, some in combination (including tritium, chlorofluorocarbons, and sulfur hexafluoride). Results for apparent 

groundwater ages indicated that groundwater samples from 

the Edwards aquifer are dominated by young (that is, water 

recharged within approximately the past 50 years) and (or) 
References 53

modern water (in this report, samples with apparent ages that 

belong to the present day, that is within 10 years of the time 

and locality of sampling), with extensive groundwater mixing. 

These results are consistent with the regional hydrogeology 

and previous studies that document a range of travel times and 

groundwater mixing. Many of the chlorofluorocarbon and sulfur hexafluoride results indicated that samples were contaminated by non-atmospheric sources of these tracers. Apparent 

ages were not determined for samples with tracer contamination. Apparent ages for all uncontaminated groundwater 

samples ranged from about 1 to 52 years, with a median of 

20 years. Apparent age interpretations are based on a pistonflow model, which assumes that the tracer concentration was 

not altered by transport processes. This approach might not 

adequately describe the range of ages in a mixed sample, 

which is a relevant consideration for karst aquifers such as the 

Edwards aquifer. In spite of uncertainties in age interpretation, 

piston-flow model ages provide an initial reference point for 

comparison of different age tracers and time scales of flow 

processes in the aquifer. Age tracers might not be well-suited 

to distinguish small differences in groundwater ages measured 

in samples collected from individual wells in the Edwards 

aquifer, but they provide insight into how changing hydrologic 

conditions affect groundwater recharge and mixing. Results 

indicate that variability in hydrologic conditions results in 

different mixtures of groundwater of different apparent ages. 

Apparent age results for samples collected from the relatively 

shallow unconfined part of the aquifer were younger than 

those for samples collected from the deeper unconfined part of 

the aquifer.

NAWQA studies in other aquifer systems have compared groundwater apparent age tracer results to estimates 

of flow rates and travel times from hydrogeologic models. 

Previously developed hydrogeologic models for the Edwards 

aquifer allow for a similar comparison. There is little agreement between the apparent groundwater ages estimated by 

the two approaches. These results indicated that existing 

flow models do not accurately represent the rapid, fast-flow 

(conduit-dominated) component of this karstic aquifer, which 

is a recognized limitation of applications of distributed porous 

media models to karst aquifer systems. Support of this conclusion is also demonstrated by results of dye-tracing tests in 

northern Bexar County that indicate groundwater velocities 

that are orders of magnitude higher than model results. The 

model results, however, do show a progression toward longer 

travel times from the unconfined to the confined parts of the 

aquifer, which is consistent with geochemical interpretations 

and aquifer hydrogeology.

Water-quality data for the San Antonio segment of the 

Edwards aquifer are useful for assessing the status of water 

quality and regional hydrologic processes that affect geochemistry. Analysis of temporal trends or evaluation of the 

relation between groundwater quality and hydrologic conditions is constrained by few time-series data. Collection of 

water-quality data at selected integrator sites (for example, for 

the Edwards aquifer, major discharge points such as Comal 

Springs and carefully selected wells) on different time scales 

and associated with different hydrologic conditions would 

address multiple time scales of aquifer variability (from eventbased, short-term to intermediate and long-term) to allow 

for understanding of both aquifer hydrology and short- and 

long-term trends in water quality. When monitoring programs 

are intended to provide the data necessary to assess long-term 

trends and increase understanding of karst aquifer functioning, 

time scales of hydrologic and geochemical variability need to 

be carefully considered.

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Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Field and physicochemical properties

Land-surface altitude, ft above NAVD 

1988

136 136 600 771 905 998 1,242

Well depth, ft below land surface 136 118 80.0 280 493 999 2,700

Depth to top of open or screened interval, 

ft from land surface

136 113 0 180 245 515 2,163

Depth to bottom of open or screened 

interval, ft from land surface

136 113 80.0 280 485 1,040 2,700

Depth to water, ft below land surface 108 108 22.5 148 196 223 353

Turbidity, NTRU 216 216 .03 .21 .49 1.44 81.0

Air pressure, mm/Hg 244 244 710 736 740 743 768

Dissolved oxygen, mg/L 245 245 .07 5.10 6.24 7.14 11.9

pH, standard units 250 250 6.28 6.84 6.94 7.10 7.43

Specific conductance, μS/cm at 25 °C 250 250 377 495 551 600 1,199

Water temperature, °C 249 249 21.1 22.8 23.5 24.3 32.5

Alkalinity, mg/L as CaCO3 246 246 121 208 245 276 377

Dissolved solids (residue on evaporation, 

dried at 180 °C), mg/L

239 239 201 283 313 343 3,852

Major ions and selected saturation indexes and ion ratios

Calcium, mg/L 243 243 50.3 71.5 89.6 103 153

Magnesium, mg/L 243 243 1.47 7.89 11.6 15.6 31.1

Potassium, mg/L 243 243 .43 .85 1.03 1.22 2.93

Sodium, mg/L 242 242 2.92 5.81 7.28 10.0 55.4

Bromide, mg/L 238 237 <.02 .07 .09 .11 .64

Chloride, mg/L 242 242 4.76 10.1 12.7 17.4 179

Fluoride, mg/L 243 207 <.1 .10 .14 .20 1.97

Iodide, mg/L 39 25 <.002 <.002 .001 .002 .004

Sulfate, mg/L 242 242 3.92 10.4 14.2 19.9 87.6

Silica, mg/L 243 243 9.44 11.5 12.2 13.2 16.4

SI calcite 240 240 -.69 -.20 -.12 -.03 .43

SI dolomite 240 240 -2.42 -1.09 -.83 -.51 .23

SI gypsum 240 240 -2.96 -2.48 -2.36 -2.21 -1.49

Mg/Ca, molar ratio 240 240 .02 .13 .22 .35 .79

Sr/Ca, molar ratio (x103

) 230 230 .17 .64 1.31 2.28 60.0

Nutrients

Nitrate plus nitrite, mg/L as N 243 241 <.05 1.13 1.66 2.11 8.23

Nitrate, mg/L as N 240 238 <.05 1.13 1.67 2.12 8.23

Ammonia, mg/L as N 243 80 <.010 <.02 <.04 .008 .102

Nitrite, mg/L as N 243 21 <.002 <.006 <.008 <.01 .060

Orthophosphate, mg/L as P 243 167 <.01 <.01 .009 .014 .030

Phosphorus, mg/L 119 27 <.004 <.01 <.01 <.05 .208

Organic carbon, mg/L 191 191 .20 .30 .40 .51 15.6

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. 

[Statistics shown for all samples and for sample categories (shallow/urban unconfined, unconfined, and confined). ft, feet; NAVD 1988, North American Vertical 

Datum of 1988; NTRU, nephelometric turbidity ratiometric unit; mmHg, millimeters mercury; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter; 

°C, degrees Celsius; CaCO3, calcium carbonate; <, nondetection less than laboratory reporting level; SI, saturation index; Mg, magnesium; Ca, calcium; Sr, 

strontium; N, nitrogen; P, phosphorus; μg/L, micrograms per liter; δ18O, delta oxygen-18; δD, delta deuterium; δ13C, delta carbon-13; 87Sr/86Sr, strontium-87/

strontium-86; pCi/L, picocuries per liter; TU, tritium unit; --, not analyzed]
62 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Trace elements

Aluminum, μg/L 238 95 <1 <1.6 <1.6 2.84 46.2

Antimony, μg/L 238 8 <.05 <.2 <.2 <1 .06

Arsenic, μg/L 234 136 <.9 <1 .26 .38 1.11

Barium, μg/L 238 238 22.9 33.2 37.8 46.5 223

Beryllium, μg/L 238 3 <.06 <.06 <.06 <1 .04

Boron, μg/L 135 135 18.6 30.5 39.5 52.0 132

Cadmium, μg/L 238 10 <.04 <.04 <.04 <1 .10

Chromium, μg/L 238 158 <.04 <.8 .32 2.34 6.39

Cobalt, μg/L 238 127 <.04 <1 .03 .18 1.61

Copper, μg/L 237 194 <.4 .46 1.82 3.46 57.3

Iron, μg/L 239 44 <3 <6 <6 <10 115

Lead, μg/L 234 132 <.08 <1 .09 .50 8.98

Lithium, μg/L 135 135 1.07 1.73 2.26 3.32 22.8

Manganese, μg/L 242 71 <.1 <.2 <1 .11 67.0

Molybdenum, μg/L 238 135 <.2 <1 .25 .62 33.3

Nickel, μg/L 238 201 <.06 .53 1.45 2.77 14.2

Selenium, μg/L 234 140 <.08 <1 .27 .48 1.56

Silver, μg/L 238 0 <.1 <.20 <.20 <1 <1

Strontium, μg/L 232 231 <.1 141 238 397 44,000

Thallium, μg/L 135 34 <.04 <.04 <.04 .01 .14

Uranium, μg/L 237 138 <.04 <1 .56 .74 1.9

Vanadium, μg/L 135 135 .12 2.23 2.63 3.15 6.08

Zinc, μg/L 237 190 <.6 .42 3.19 13.9 747

Radiogenic and stable isotopes

δ18O, per mil 168 168 -6.03 -4.18 -4.26 -4.18 -2.64

δD, per mil 168 168 -36.0 -22.9 -23.8 -22.9 -15.2

δ13C, per mil 30 30 -15.70 -10.23 -10.65 -10.23 -8.50

87Sr/86Sr, per mil 65 65 .70765 .70843 .70822 .70843 .70952

Radon-222, pCi/L 62 45 <58 194 116 194 776

Radium-226, pCi/L 30 30 .02 .16 .09 .16 .55

Radium-228, pCi/L 25 25 .32 .80 .71 .80 1.13

Tritium, pCi/L 118 118 1.00 10.60 8.30 10.60 14.70

Tritium, TU 118 118 .3 3.3 2.6 3.3 4.6
Table 3 63

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Shallow/urban unconfined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Field and physicochemical properties

Land-surface altitude, ft above NAVD 

1988

30 30 803 891 936 973 1,015

Well depth, ft below land surface 30 30 180 244 261 295 320

Depth to top of open or screened interval, 

ft from land surface

30 30 101 163 190 202 250

Depth to bottom of open or screened 

interval, ft from land surface

30 30 180 244 261 295 320

Depth to water, ft below land surface 90 90 105 162 196 224 270

Turbidity, NTRU 86 86 .19 .65 1.52 3.04 81.0

Air pressure, mm/Hg 88 88 710 736 740 743 746

Dissolved oxygen, mg/L 90 90 3.59 5.89 6.71 7.73 11.9

pH, standard units 90 90 6.28 6.80 6.87 6.92 7.29

Specific conductance, mS/cm at 25 °C 90 90 416 548 578 628 813

Water temperature, °C 90 90 22.1 23.3 23.9 24.4 26.0

Alkalinity, mg/L as CaCO3 85 85 134 250 268 293 338

Dissolved solids (residue on evaporation, 

dried at 180 °C), mg/L

90 90 201 312 329 360 456

Major ions and selected saturation indexes and ion ratios

Calcium, mg/L 90 90 74.0 95.2 101 111 144

Magnesium, mg/L 90 90 1.47 5.06 8.11 9.64 18.3

Potassium, mg/L 90 90 .45 .82 .96 1.11 1.85

Sodium, mg/L 89 89 2.92 5.24 6.76 7.82 13.0

Bromide, mg/L 89 88 <.02 .07 .09 .10 .17

Chloride, mg/L 89 89 4.76 10.0 12.2 15.4 45.0

Fluoride, mg/L 90 60 <.1 <.1 .10 .13 .28

Iodide, mg/L 0 -- -- -- -- -- --

Sulfate, mg/L 89 89 3.92 8.58 11.5 14.6 26.5

Silica, mg/L 90 90 9.46 11.4 12.0 12.7 14.9

SI calcite 90 90 -.69 -.18 -.10 -.04 .26

SI dolomite 90 90 -2.42 -1.17 -.97 -.83 .005

SI gypsum 90 90 -2.82 -2.53 -2.39 -2.27 -1.60

Mg/Ca, molar ratio 90 90 .02 .07 .13 .17 .33

Sr/Ca, molar ratio (x103

) 88 88 .17 .47 .59 .84 9.85

Nutrients

Nitrate plus nitrite, mg/L as N 90 89 <.05 1.44 1.85 2.30 4.76

Nitrate, mg/L as N 90 89 <.05 1.43 1.85 2.30 4.76

Ammonia, mg/L as N 90 42 <.010 <.02 <.04 .010 .048

Nitrite, mg/L as N 90 6 <.002 <.002 <.008 <.01 .011

Orthophosphate, mg/L as P 90 80 <.01 .010 .014 .018 .030

Phosphorus, mg/L 45 16 <.05 <.05 <.05 .007 .208

Organic carbon, mg/L 65 65 .20 .40 .50 .60 3.30
64 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Shallow/urban unconfined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Trace elements

Aluminum, mg/L 87 10 <1 <1 <1.6 <1.6 46.2

Antimony, mg/L 87 3 <.05 <.2 <.30 <1 .06

Arsenic, mg/L 87 54 <.9 <1 .27 .34 1.11

Barium, mg/L 87 87 22.9 32.1 36.9 42.3 69.9

Beryllium, mg/L 87 1 <.06 <.06 <.06 <1 .04

Boron, mg/L 52 52 18.9 28.0 31.3 36.5 62.6

Cadmium, mg/L 87 0 <.04 <.04 <.04 <1 <1

Chromium, mg/L 87 67 <.8 .20 .44 2.09 5.57

Cobalt, mg/L 87 45 <.04 <1 .02 .06 1.61

Copper, mg/L 87 50 <.4 <.4 .33 1.04 57.3

Iron, mg/L 90 21 <6 <6 <10 <10 115

Lead, mg/L 83 24 <.08 <.08 <1 .05 .15

Lithium, mg/L 52 52 1.07 1.43 1.83 2.33 3.82

Manganese, mg/L 89 36 <.2 <1 <1 .16 1.7

Molybdenum, mg/L 87 43 <.33 <1 <1 .36 1.38

Nickel, mg/L 87 75 <.06 .38 1.01 1.62 5.27

Selenium, mg/L 87 56 <.08 <1 .26 .35 1.4

Silver, mg/L 87 0 <.1 <.20 <.20 <1 <1

Strontium, mg/L 88 88 39.7 104 137 188 1,907

Thallium, mg/L 52 4 <.04 <.04 <.04 <.04 .05

Uranium, mg/L 87 48 <.04 <1 .53 .68 .91

Vanadium, mg/L 52 52 1.77 2.15 2.36 2.66 5.35

Zinc, mg/L 87 41 <.6 <1 <1 1 5.18

Radiogenic and stable isotopes

δ18O, per mil 70 70 -6.03 -4.53 -4.34 -4.26 -3.95

δD, per mil 70 70 -36.0 -24.5 -23.6 -23.0 -20.7

δ13C, per mil 30 30 -15.70 -11.68 -10.65 -10.23 -8.50

87Sr/86Sr, per mil 35 35 .70781 .70830 .70842 .70853 .70952

Radon-222, pCi/L 0 -- -- -- -- -- -- 

Radium-226, pCi/L 0 -- -- -- -- -- -- 

Radium-228, pCi/L 0 -- -- -- -- -- -- 

Tritium, pCi/L 56 56 3.70 6.10 7.40 9.30 14.70

Tritium, TU 56 56 1.2 1.9 2.3 2.9 4.6
Table 3 65

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Unconfined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Field and physicochemical properties

Land-surface altitude, ft above NAVD 

1988

38 38 610 905 1,042 1,112 1,242

Well depth, ft below land surface 38 26 80.0 274 367 460 1,400

Depth to top of open or screened interval, 

ft from land surface

38 22 6.00 102 180 239 900

Depth to bottom of open or screened 

interval, ft from land surface

38 22 80.0 251 367 450 1,400

Depth to water, ft below land surface 17 17 36.4 119 182 220 353

Turbidity, NTRU 73 73 .05 .14 .32 .62 8.60

Air pressure, mm/Hg 79 79 723 735 739 743 768

Dissolved oxygen, mg/L 77 77 .10 5.63 6.31 7.25 11.5

pH, standard units 81 81 6.53 6.87 7.00 7.11 7.43

Specific conductance, mS/cm at 25 °C 81 81 377 455 506 581 1,003

Water temperature, °C 80 80 21.1 22.5 22.8 23.3 28.2

Alkalinity, mg/L as CaCO3 82 82 121 203 231 273 377

Dissolved solids (residue on evaporation, 

dried at 180 °C), mg/L

79 79 214 259 294 328 3,852

Major ions and selected saturation indexes and ion ratios

Calcium, mg/L 82 82 51.8 66.1 78.5 94.1 153

Magnesium, mg/L 82 82 3.56 9.11 12.7 16.1 31.1

Potassium, mg/L 82 82 .43 .80 .90 1.10 2.93

Sodium, mg/L 82 82 4.41 5.54 6.49 8.18 29.3

Bromide, mg/L 79 79 .03 .07 .08 .09 .25

Chloride, mg/L 82 82 7.47 9.38 10.6 14.3 75.4

Fluoride, mg/L 82 78 <.1 .11 .15 .20 .46

Iodide, mg/L 5 3 <.002 <.002 .001 .001 .002

Sulfate, mg/L 82 82 4.80 10.0 13.1 20.0 81.8

Silica, mg/L 82 82 9.44 11.4 12.0 13.0 14.7

SI calcite 80 80 -.62 -.23 -.18 -.06 .43

SI dolomite 80 80 -1.80 -1.03 -.78 -.54 .23

SI gypsum 80 80 -2.96 -2.52 -2.45 -2.19 -1.49

Mg/Ca, molar ratio 80 80 .06 .18 .27 .37 .76

Sr/Ca, molar ratio (x103

) 74 74 .19 .94 1.67 2.04 5.40

Nutrients

Nitrate plus nitrite, mg/L as N 82 81 <.06 .79 1.05 1.58 7.94

Nitrate, mg/L as N 80 79 <.06 .78 1.09 1.60 7.94

Ammonia, mg/L as N 82 29 <.010 <.015 <.04 .021 .102

Nitrite, mg/L as N 82 13 <.002 <.006 <.008 <.01 .018

Orthophosphate, mg/L as P 82 54 <.006 <.018 .008 .012 .020

Phosphorus, mg/L 37 10 <.004 <.01 <.01 .003 .028

Organic carbon, mg/L 61 61 .20 .30 .40 .43 2.43
66 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Unconfined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Trace elements

Aluminum, mg/L 82 37 <1 <1.6 <1.6 2.85 6.07

Antimony, mg/L 82 5 <.05 <.2 <.2 <1 .05

Arsenic, mg/L 79 47 <.9 <1 .24 .36 .75

Barium, mg/L 82 82 26.1 32.8 35.7 42.7 64.3

Beryllium, mg/L 82 0 <.06 <.06 <.06 <1 <1

Boron, mg/L 49 49 18.6 33.3 40.2 48.3 132

Cadmium, mg/L 82 9 <.04 <.20 <.20 <1 .10

Chromium, mg/L 82 55 <.04 <.8 .06 1.91 5.42

Cobalt, mg/L 82 47 <.04 <1 .14 .19 .53

Copper, mg/L 82 82 .3 1.56 2.41 3.42 11.1

Iron, mg/L 79 14 <3 <3 <6 <10 72.8

Lead, mg/L 82 63 <1 .09 .27 .93 8.98

Lithium, mg/L 49 49 1.25 1.69 2.12 2.75 22.8

Manganese, mg/L 82 27 <.1 <.2 <1 .13 45.4

Molybdenum, mg/L 82 50 <.2 <1 .26 .63 33.3

Nickel, mg/L 82 80 <.06 1.79 2.52 3.06 14.2

Selenium, mg/L 79 47 <.08 <1 .28 .42 1.3

Silver, mg/L 82 0 <.20 <.20 <.20 <1 <1

Strontium, mg/L 76 75 <.1 187 243 295 945

Thallium, mg/L 49 9 <.04 <.04 <.04 <.04 .10

Uranium, mg/L 81 51 <1 <1 .59 .72 1.69

Vanadium, mg/L 49 49 .12 2.34 2.64 3.32 5.31

Zinc, mg/L 81 81 1.29 5.32 15.2 77.4 747

Radiogenic and stable isotopes

δ18O, per mil 61 61 -4.74 -4.30 -4.18 -3.96 -2.64

δD, per mil 61 61 -27.1 -24.4 -23.5 -22.4 -15.2

δ13C, per mil 0 -- -- -- -- -- -- 

87Sr/86Sr, per mil 27 27 .70765 .70782 .70789 .70796 .70845

Radon-222, pCi/L 32 29 <58 105 151 296 776

Radium-226, pCi/L 30 30 .02 .06 .09 .16 .55

Radium-228, pCi/L 25 25 .32 .46 .71 .80 1.13

Tritium, pCi/L 27 27 3.80 7.20 10.60 12.50 14.70

Tritium, TU 27 27 1.2 2.3 3.3 3.9 4.6
Table 3 67

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Confined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Field and physicochemical properties

Land-surface altitude, ft above NAVD 

1988

68 68 600 201 834 911 1,125

Well depth, ft below land surface 68 62 215 100 859 1,458 2,700

Depth to top of open or screened interval, 

ft from land surface

68 61 0 15.3 476 937 2,163

Depth to bottom of open or screened 

interval, ft from land surface

68 61 215 99.5 848 1,462 2,700

Depth to water, ft below land surface 1 1 22.5 22.5 22.5 22.5 22.5

Turbidity, NTRU 57 57 .03 .15 .24 .39 1.47

Air pressure, mm/Hg 77 77 726 738 742 746 768

Dissolved oxygen, mg/L 78 78 .07 4.35 5.20 6.48 9.87

pH, standard units 79 79 6.61 6.94 7.08 7.17 7.42

Specific conductance, mS/cm at 25 °C 79 79 384 494 525 589 1,199

Water temperature, °C 79 79 21.5 23.0 23.7 24.7 32.5

Alkalinity, mg/L as CaCO3 79 79 155 203 211 250 304

Dissolved solids (residue on evaporation, 

dried at 180 °C), mg/L

70 70 259 284 297 330 891

Major ions and selected saturation indexes and ion ratios

Calcium, mg/L 71 71 50.3 68.9 73.4 87.5 153

Magnesium, mg/L 71 71 5.49 13.7 15.5 16.6 24.15

Potassium, mg/L 71 71 .76 1.10 1.20 1.34 2.59

Sodium, mg/L 71 71 5.11 8.42 10.3 11.0 55.4

Bromide, mg/L 70 70 .03 .09 .11 .13 .64

Chloride, mg/L 71 71 9.12 14.6 17.4 21.4 179

Fluoride, mg/L 71 69 <.1 .19 .21 .23 1.97

Iodide, mg/L 34 22 <.002 <.002 .002 .002 .004

Sulfate, mg/L 71 71 6.66 16.2 18.7 24.8 87.6

Silica, mg/L 71 71 10.2 11.9 12.6 13.4 16.4

SI calcite 70 70 -.42 -.18 -.08 0 .15

SI dolomite 70 70 -1.52 -.78 -.47 -.36 -.05

SI gypsum 70 70 -2.71 -2.36 -2.30 -2.16 -1.50

Mg/Ca, molar ratio 70 70 .09 .24 .35 .38 .79

Sr/Ca, molar ratio (x103

) 68 68 .84 2.14 3.08 5.53 60.0

Nutrients

Nitrate plus nitrite, mg/L as N 71 71 .25 1.54 1.82 2.16 8.23

Nitrate, mg/L as N 70 70 .19 1.53 1.84 2.16 8.23

Ammonia, mg/L as N 71 9 <.010 <.04 <.04 <.04 .060

Nitrite, mg/L as N 71 2 <.008 <.008 <.01 <.01 .060

Orthophosphate, mg/L as P 71 33 <.006 <.01 <.01 .006 .016

Phosphorus, mg/L 37 1 <.01 <.01 <.01 <.01 .012

Organic carbon, mg/L 65 65 .20 .30 .40 .50 15.6
68 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Table 3. Summary statistics by group for field, physicochemical, inorganic, radiogenic, and stable isotope constituents analyzed in 

groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued.

Constituent

Confined

Number of 

samples

Number of 

detections Minimum 25th percentile Median 75th percentile Maximum

Trace elements

Aluminum, mg/L 69 48 <1.6 <1.6 2.04 3.78 4.81

Antimony, mg/L 69 0 <.2 <.2 <.30 <1 <1

Arsenic, mg/L 68 35 <1 <1 .35 .47 1.10

Barium, mg/L 69 69 27.4 37.1 47.0 57.4 223

Beryllium, mg/L 69 2 <.06 <.06 <1 <1 .04

Boron, mg/L 34 34 41.6 51.1 57.7 65.0 89.5

Cadmium, mg/L 69 1 <.04 <.04 <1 <1 .02

Chromium, mg/L 69 36 <.8 <.8 .65 3.25 6.39

Cobalt, mg/L 69 35 <1 <1 .10 .18 .48

Copper, mg/L 68 62 <1 1.84 3.13 5.00 12.0

Iron, mg/L 70 9 <3 <3 <6 <6 24.0

Lead, mg/L 69 45 <1 <1 .30 1.28 5.24

Lithium, mg/L 34 34 2.14 3.14 3.88 4.90 12.5

Manganese, mg/L 71 8 <.18 <.2 <1 <1 67.0

Molybdenum, mg/L 69 42 <1 <1 .62 .77 26.2

Nickel, mg/L 69 46 <1 <1 1.08 3.01 10.5

Selenium, mg/L 68 37 <1 <1 .51 .74 1.56

Silver, mg/L 69 0 <.20 <.20 <.20 <1 <1

Strontium, mg/L 68 68 140 352 540 886 44,000

Thallium, mg/L 34 21 <.04 <.04 .03 .04 .14

Uranium, mg/L 69 39 <1 <1 .72 .84 1.9

Vanadium, mg/L 34 34 2.15 2.82 3.24 3.90 6.08

Zinc, mg/L 69 68 <1 2.91 6.13 11.3 205

Radiogenic and stable isotopes

δ18O, per mil 37 37 -4.49 -4.18 -4.22 -4.18 -2.92

δD, per mil 37 37 -26.7 -23.1 -24.0 -23.1 -17.3

δ13C, per mil 0 -- -- -- -- -- -- 

87Sr/86Sr, per mil 3 3 .70768 .70788 .70784 .70788 .70791

Radon-222, pCi/L 30 16 <80 187 89 187 535

Radium-226, pCi/L 0 -- -- -- -- -- -- 

Radium-228, pCi/L 0 -- -- -- -- -- -- 

Tritium, pCi/L 35 35 1.00 10.40 8.60 10.40 12.80

Tritium, TU 35 35 .3 3.3 2.7 3.3 4.0
Table 4 

69

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Volatile organic and fuel oxygenate compounds

Acrolein, μg/L 62 0 <1.43 <1.43 <1.43 <2 <3.94 0 -- -- -- -- -- -- 

1,1,1,2-Tetrachloroethane, μg/L 243 0 <.03 <.03 <.03 <.044 <.05 90 0 <0.03 <0.03 <0.03 <0.044 <0.044

1,1,1-Trichloroethane, μg/L 243 10 <.032 <.032 <.032 <.032 .026 90 7 <.032 <.032 <.032 <.032 .026

1,1,2,2-Tetrachloroethane, μg/L 243 0 <.08 <.08 <.09 <.13 <.132 90 0 <.08 <.08 <.09 <.13 <.13

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC−113), μg/L 243 7 <.032 <.032 <.038 <.038 .493 90 7 <.032 <.032 <.038 <.04 .493

1,1,2-Trichloroethane, μg/L 243 0 <.04 <.04 <.04 <.064 <.1 90 0 <.04 <.04 <.06 <.064 <.064

1,1-Dichloroethane, μg/L 243 0 <.035 <.035 <.035 <.066 .010 90 0 <.035 <.035 <.035 <.066 <.066

1,1-Dichloroethene, μg/L 243 0 <.02 <.024 <.024 <.044 <.1 90 0 <.02 <.024 <.04 <.044 <.044

1,1-Dichloropropene, μg/L 243 0 <.026 <.026 <.026 <.026 <.05 90 0 <.026 <.026 <.026 <.026 <.05

1,2,3,4-Tetramethylbenzene (prehnitene), μg/L 243 0 <.05 <.14 <.14 <.23 <.23 90 0 <.14 <.14 <.23 <.23 <.23

1,2,3,5-Tetramethylbenzene (isodurene), μg/L 243 0 <.05 <.14 <.18 <.2 <.24 90 0 <.12 <.18 <.2 <.2 <.2

1,2,3-Trichlorobenzene, μg/L 243 0 <.12 <.18 <.18 <.266 <.27 90 0 <.12 <.18 <.27 <.27 <.27

1,2,3-Trichloropropane, μg/L 243 0 <.07 <.16 <.18 <.18 <.2 90 0 <.12 <.16 <.16 <.18 <.18

1,2,3-Trimethylbenzene, μg/L 243 0 <.05 <.06 <.09 <.12 <.124 90 0 <.06 <.09 <.12 <.12 <.12

1,2,4-Trichlorobenzene, μg/L 243 0 <.07 <.12 <.12 <.19 <.2 90 0 <.12 <.12 <.12 <.19 <.19

1,2,4-Trimethylbenzene, μg/L 243 2 <.04 <.056 <.056 <.056 .01 90 0 <.04 <.056 <.056 <.056 <.056

1,2-Dibromo-3-chloropropane, μg/L 243 0 <.21 <.214 <.5 <.51 <.51 90 0 <.21 <.21 <.21 <.51 <.51

1,2-Dibromoethane, μg/L 243 0 <.036 <.036 <.036 <.036 <.1 90 0 <.036 <.036 <.036 <.036 <.04

1,2-Dichlorobenzene, μg/L 243 0 <.031 <.048 <.048 <.048 <.05 90 0 <.031 <.048 <.048 <.048 <.048

1,2-Dichloroethane, μg/L 243 0 <.05 <.13 <.13 <.13 <.134 90 0 <.1 <.13 <.13 <.13 <.13

1,2-Dichloropropane, μg/L 243 0 <.02 <.029 <.029 <.068 .010 90 0 <.02 <.029 <.029 <.068 <.068

1,3,5-Trimethylbenzene, μg/L 243 0 <.04 <.044 <.044 <.044 <.05 90 0 <.04 <.044 <.044 <.044 <.044

1,3-Dichlorobenzene, μg/L 243 1 <.03 <.03 <.03 <.054 .001 90 0 <.03 <.03 <.03 <.054 <.054

1,3-Dichloropropane, μg/L 243 0 <.05 <.06 <.06 <.116 <.12 90 0 <.06 <.06 <.12 <.12 <.12

1,4-Dichlorobenzene, μg/L 243 1 <.034 <.034 <.04 <.05 .006 90 0 <.034 <.034 <.05 <.05 <.05

2,2-Dichloropropane, μg/L 243 0 <.05 <.05 <.05 <.078 <.078 90 0 <.05 <.05 <.05 <.078 <.078

2-Chlorotoluene, μg/L 243 0 <.026 <.04 <.04 <.042 <.05 90 0 <.026 <.04 <.04 <.042 <.042

2-Ethyltoluene, μg/L 243 0 <.04 <.06 <.06 <.1 <.1 90 0 <.04 <.06 <.06 <.1 <.1

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006. [Statistics shown for all samples and for sample categories (shallow/urban unconfined, unconfined, and confined). μg/L, micrograms per liter; <, nondetection less than laboratory reporting level (0.000001 substituted for statistical calculations); --, not analyzed; CFC, chlorofluorocarbon; p,p'-DDE, 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene; compounds in bold were most frequently detected by count or by percent; *, field or lab quality-control information indicates questionable concentration]
70 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent All Shallow/urban unconfined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum Volatile organic and fuel oxygenate compounds—Continued 3-Chloropropene, μg/L 243 0 <0.07 <0.196 <0.2 <0.5 <0.5 90 0 <0.07 <0.2 <0.2 <0.5 <0.5 4-Chlorotoluene, μg/L 243 0 <.04 <.05 <.05 <.056 <.06 90 0 <.04 <.05 <.05 <.056 <.06 4-Isopropyltoluene, μg/L 243 0 <.05 <.08 <.08 <.11 <.12 90 0 <.07 <.08 <.08 <.11 <.12 Acetone, μg/L 243 1 <4.90 <5 <6 <6 .300 90 0 <5 <5 <6 <6 <7 Acrylonitrile, μg/L 243 0 <.4 <.8 <.8 <1.2 <2 90 0 <.4 <.8 <1.2 <1.2 <1.2 Benzene, μg/L 243 1 <.016 <.021 <.021 <.05 .081 90 1 <.016 <.021 <.035 <.1 .081 Bromobenzene, μg/L 243 1 <.02 <.028 <.028 <.036 .003 90 0 <.02 <.028 <.036 <.036 <.036 Bromochloromethane, μg/L 243 0 <.044 <.044 <.1 <.12 <.12 90 0 <.044 <.044 <.044 <.12 <.12 Bromodichloromethane, μg/L 243 27 <.028 <.028 <.048 <.1 1.23 90 18 <.028 <.028 <.048 <.048 .143 Bromoethene, μg/L 243 0 <.1 <.1 <.1 <.1 <.12 90 0 <.1 <.1 <.1 <.1 <.12 Bromomethane, μg/L 243 0 <.1 <.148 <.26 <.33 <.4 90 0 <.15 <.15 <.26 <.33 <.4 Carbon disulfide, μg/L 243 9 <.038 <.038 <.05 <.08 .090 90 0 <.08 <.038 <.07 <.37 <.37 Chlorobenzene, μg/L 243 1 <.02 <.028 <.028 <.028 .004 90 0 <.02 <.028 <.028 <.028 <.028 Chloroethane, μg/L 243 0 <.1 <.12 <.12 <.12 <.12 90 0 <.1 <.12 <.12 <.12 <.12 Chloromethane, μg/L 243 12 <.1 <.17 <.17 <.25 .060 90 2 <.1 <.17 <.17 <.25 .054 cis-1,2-Dichloroethene, μg/L 243 2 <.02 <.024 <.024 <.038 .020 90 1 <.02 <.024 <.038 <.038 .014 cis-1,3-Dichloropropene, μg/L 243 0 <.05 <.05 <.06 <.09 <.1 90 0 <.05 <.05 <.09 <.09 <.09 Dibromochloromethane, μg/L 243 4 <.1 <.1 <.1 <.18 2.89 90 1 <.1 <.1 <.18 <.18 .130 Dibromomethane, μg/L 243 1 <.04 <.05 <.05 <.05 .010 90 0 <.04 <.05 <.05 <.05 <.05 Dichlorodifluoromethane (CFC−12), μg/L 243 5 <.096 <.14 <.18 <.18 .300 90 0 <.14 <.14 <.18 <.18 <.27 Dichloromethane, μg/L 243 3 <.04 <.06 <.06 <.38 .010 90 0 <.04 <.06 <.16 <.38 <.38 Diethyl ether, μg/L 243 0 <.08 <.08 <.08 <.17 <.17 90 0 <.08 <.08 <.17 <.17 <.17 Diisopropyl ether, μg/L 243 0 <.06 <.098 <.1 <.1 <.1 90 0 <.06 <.098 <.1 <.1 <.1 Ethyl methacrylate, μg/L 243 0 <.14 <.18 <.18 <.28 <1 90 0 <.14 <.18 <.18 <.28 <.28 Ethyl methyl ketone, μg/L 243 1 <1.6 <1.65 <2 <2 3.00 90 0 <1.6 <1.6 <1.6 <2 <5 Ethylbenzene, μg/L 243 0 <.02 <.03 <.03 <.03 .020 90 0 <.02 <.03 <.03 <.03 <.03 Hexachlorobutadiene, μg/L 243 0 <.1 <.14 <.14 <.14 <.2 90 0 <.1 <.14 <.14 <.14 <.14 Hexachloroethane, μg/L 243 0 <.05 <.14 <.14 <.36 <.362 90 0 <.14 <.14 <.19 <.36 <.36 Iodomethane, μg/L 243 2 <.05 <.12 <.4 <.5 .009 90 0 <.12 <.21 <.21 <.5 <.5 Isobutyl methyl ketone, μg/L 243 0 <.2 <.37 <.37 <.37 <5 90 0 <.2 <.37 <.37 <.37 <.37 Isopropylbenzene, μg/L 243 1 <.032 <.032 <.038 <.038 .005 90 0 <.032 <.032 <.032 <.038 <.06 Methyl acrylonitrile, μg/L 243 0 <.4 <.4 <.4 <.57 <2 90 0 <.4 <.4 <.57 <.57 <.6
Table 4 

71

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Volatile organic and fuel oxygenate compounds—Continued

Methyl acrylate, μg/L 243 0 <0.4 <1 <1 <1.4 <2 90 0 <0.4 <1 <1.4 <1.4 <2

Methyl methacrylate, μg/L 243 0 <.2 <.2 <.2 <.35 <1 90 0 <.2 <.2 <.35 <.35 <.35

Methyl tert-pentyl ether, μg/L 243 0 <.04 <.04 <.04 <.11 <.112 90 0 <.04 <.04 <.08 <.11 <.11

m- plus p-Xylene, μg/L 243 2 <.05 <.06 <.06 <.06 .040 90 0 <.06 <.06 <.06 <.06 <.08

Naphthalene, μg/L 243 1 <.2 <.25 <.5 <.52 .423 90 1 <.25 <.25 <.25 <.52 .423

n-Butyl methyl ketone, μg/L 243 0 <.4 <.4 <.4 <.7 <5 90 0 <.4 <.4 <.7 <.7 <.7

n-Butylbenzene, μg/L 243 0 <.05 <.12 <.14 <.186 <.19 90 0 <.12 <.14 <.19 <.19 <.19

n-Propylbenzene, μg/L 243 0 <.04 <.042 <.042 <.042 <.05 90 0 <.04 <.042 <.042 <.042 <.042

o-Xylene, μg/L 243 1 <.038 <.038 <.038 <.06 .010 90 0 <.038 <.038 <.038 <.06 <.07

sec-Butylbenzene, μg/L 243 0 <.032 <.048 <.05 <.06 <.06 90 0 <.032 <.048 <.048 <.06 <.06

Styrene, μg/L 243 2 <.04 <.042 <.042 <.042 .013 90 0 <.04 <.042 <.042 <.042 <.042

tert-Amyl alcohol, μg/L 39 0 <1 <1 <1 <1 <1 0 -- -- -- -- -- -- 

tert-Butyl ethyl ether, μg/L 243 2 <.03 <.03 <.04 <.054 .020 90 0 <.03 <.03 <.05 <.054 <.054

Methyl tert-butyl ether, μg/L 243 3 <.1 <.1 <.1 <.17 .824 90 2 <.1 <.1 <.17 <.17 .824

tert-Butylbenzene, μg/L 243 0 <.048 <.06 <.08 <.096 <.1 90 0 <.06 <.08 <.08 <.1 <.1

Tetrachloroethene (PCE), μg/L 243 81 <.027 <.03 <.1 .037 .953 90 67 <.027 .003 .027 .051 .953

Tetrachloromethane, μg/L 243 0 <.05 <.06 <.06 <.088 <.088 90 0 <.06 <.06 <.06 <.088 <.088

Tetrahydrofuran, μg/L 243 1 <1 <1 <1.2 <5 1.00 90 0 <1 <1.2 <2.2 <9 <9

Toluene, μg/L 243 13 <.018 <.02 <.038 <.05 .750 90 7 <.018 <.02 <.05 <.05 .036

trans-1,2-Dichloroethene, μg/L 243 0 <.018 <.032 <.032 <.032 <.05 90 0 <.018 <.032 <.032 <.032 <.032

trans-1,3-Dichloropropene, μg/L 243 0 <.09 <.09 <.09 <.13 <.134 90 0 <.09 <.09 <.09 <.13 <.13

trans-1,4-Dichloro-2-butene, μg/L 243 0 <.6 <.7 <.7 <.7 <.7 90 0 <.6 <.7 <.7 <.7 <.7

Tribromomethane (bromoform), μg/L 243 4 <.06 <.1 <.1 <.104 3.09 90 1 <.06 <.1 <.1 <.1 .213

Trichloroethene (TCE), μg/L 243 6 <.02 <.038 <.038 <.038 .079 90 5 <.02 <.038 <.038 <.038 .079

Trichlorofluoromethane (CFC−11), μg/L 243 2 <.08 <.08 <.09 <.092 .275 90 2 <.08 <.08 <.09 <.09 .021

Trichloromethane (chloroform), μg/L 243 106 <.024 <.05 .019 .0495 5.88 90 68 <.024 .00625 .032 .061 5.88

Vinyl acetate, μg/L 28 0 <5 <5 <5 <5 <5 0 -- -- -- -- -- -- 

Vinyl chloride, μg/L 242 0 <.08 <.08 <.08 <.11 <.112 90 0 <.08 <.08 <.11 <.11 <.11

Soluble pesticide compounds

1-Naphthol, μg/L 146 0 <.007 <.088 <.088 <.088 <.088 38 0 <.088 <.088 <.088 <.088 <.088

2,6-Diethylaniline, μg/L 239 0 <.0017 <.003 <.006 <.006 <.006 87 0 <.0017 <.003 <.003 <.006 <.006

2-Chloro-2,6-diethylacetanilide, μg/L 118 0 <.005 <.005 <.007 <.007 <.007 38 0 <.005 <.007 <.007 <.007 <.007

2-Chloro-4-isopropylamino-6-amino-s-triazine (deethylatrazine 

[DEA]), μg/L

241 166 <.002 <.014 .005 .01 .053 87 85 <.002 .007 .009 .014 .053
72 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent All Shallow/urban unconfined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum Soluble pesticide compounds—Continued 2-Ethyl-6-methylaniline, μg/L 118 0 <0.005 <0.005 <0.010 <0.010 <0.010 38 0 <0.005 <0.010 <0.010 <0.010 <0.010 3,4-Dichloroaniline, μg/L 114 4 <.005 <.005 <.005 <.005 .014 38 3 <.005 <.005 <.005 <.005 .014 3,5-Dichloroaniline, μg/L 75 0 <.004 <.012 <.012 <.012 <.012 33 0 <.012 <.012 <.012 <.012 <.012 4-Chloro-2-methylphenol, μg/L 118 0 <.005 <.005 <.005 <.006 <.006 38 0 <.005 <.005 <.005 <.005 <.006 Acetochlor, μg/L 239 0 <.002 <.002 <.006 <.006 <.006 87 0 <.002 <.002 <.004 <.006 <.006 Alachlor, μg/L 239 0 <.002 <.002 <.005 <.005 <.005 87 0 <.002 <.002 <.002 <.005 <.005 alpha-Endosulfan, μg/L 75 0 <.005 <.011 <.011 <.011 <.011 33 0 <.011 <.011 <.011 <.011 <.011 alpha-Hexachlorocyclohexane (alpha-HCH), μg/L 121 0 <.002 <.002 <.002 <.002 <.005 49 0 <.002 <.002 <.002 <.002 <.005 Atrazine, μg/L 240 131 <.001 <.007 .003 .008 .132 86 74 <.001 .005 .008 .014 .132 Azinphos-methyl oxygen analog, μg/L 117 0 <.042 <.042 <.042 <.070 <.070 38 0 <.042 <.042 <.042 <.042 <.070 Azinphos-methyl, μg/L 238 0 <.001 <.001 <.05 <.05 <.08 87 0 <.001 <.001 <.05 <.05 <.08 Benfluralin, μg/L 239 2 <.002 <.002 <.01 <.01 .004 87 1 <.002 <.002 <.01 <.01 .004 Butylate, μg/L 121 0 <.002 <.002 <.002 <.002 <.002 49 0 <.002 <.002 <.002 <.002 <.002 Carbaryl, μg/L 239 4 <.003 <.003 <.041 <.041 .02 87 3 <.003 <.003 <.041 <.041 .02 Carbofuran, μg/L 196 0 <.003 <.003 <.003 <.02 <.020 82 0 <.003 <.003 <.02 <.02 <.020 Chlorpyrifos, μg/L 239 0 <.004 <.004 <.005 <.005 <.005 87 0 <.004 <.004 <.005 <.005 <.005 cis-Permethrin, μg/L 239 0 <.005 <.005 <.006 <.006 <.01 87 0 <.005 <.005 <.006 <.006 <.01 cis-Propiconazole, μg/L 75 0 <.008 <.013 <.013 <.013 <.013 33 0 <.013 <.013 <.013 <.013 <.013 Cyanazine, μg/L 196 0 <.004 <.004 <.004 <.018 <.018 82 0 <.004 <.004 <.018 <.018 <.018 Cyfluthrin, μg/L 118 0 <.008 <.027 <.053 <.053 <.053 38 0 <.008 <.053 <.053 <.053 <.053 lambda-Cyhalothrin, μg/L 75 0 <.009 <.014 <.014 <.014 <.014 33 0 <.014 <.014 <.014 <.014 <.014 Cypermethrin, μg/L 118 0 <.009 <.009 <.046 <.046 <.046 38 0 <.009 <.046 <.046 <.046 <.046 Dimethyl tetrachloroterephthalate (DCPA), μg/L 239 1 <.002 <.002 <.003 <.003 .001 87 1 <.002 <.002 <.003 <.003 .001 Chlorpyrifos oxygen analog, μg/L 118 0 <.056 <.056 <.056 <.056 <.056 38 0 <.056 <.056 <.056 <.056 <.056 Desulfinylfipronil, μg/L 123 0 <.004 <.012 <.012 <.012 <.012 43 0 <.004 <.012 <.012 <.012 <.012 Diazinon, μg/L 239 12 <.002 <.002 <.005 <.005 .023 87 7 <.002 <.002 <.003 <.005 .023 Diazoxon, μg/L 118 0 <.006 <.006 <.006 <.006 <.006 38 0 <.006 <.006 <.006 <.006 <.006 Dichlorvos, μg/L 118 0 <.012 <.012 <.013 <.013 <.013 38 0 <.012 <.013 <.013 <.013 <.013 Dicrotophos, μg/L 118 0 <.084 <.084 <.084 <.084 <.084 38 0 <.084 <.084 <.084 <.084 <.084 Dieldrin, μg/L 239 2 <.001 <.001 <.005 <.009 .003 87 0 <.001 <.001 <.005 <.009 <.009 Dimethoate, μg/L 118 0 <.006 <.006 <.006 <.006 <.006 38 0 <.006 <.006 <.006 <.006 <.006 Disulfoton sulfone, μg/L 75 0 <.0059 <.014 <.014 <.014 <.014 33 0 <.014 <.014 <.014 <.014 <.014
Table 4 

73

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Soluble pesticide compounds—Continued

Disulfoton, μg/L 196 0 <0.017 <0.017 <0.017 <0.021 <0.021 82 0 <0.017 <0.017 <0.021 <0.021 <0.021

Endosulfan sulfate, μg/L 75 0 <.014 <.022 <.022 <.022 <.022 33 0 <.022 <.022 <.022 <.022 <.022

s-Ethyl dipropylthiocarbamate (EPTC), μg/L 196 0 <.002 <.002 <.002 <.004 <.004 82 0 <.002 <.002 <.004 <.004 <.004

Ethalfluralin, μg/L 121 0 <.004 <.004 <.004 <.004 <.009 49 0 <.004 <.004 <.004 <.004 <.009

Ethion monoxon, μg/L 118 0 <.002 <.002 <.021 <.021 <.021 38 0 <.002 <.021 <.021 <.021 <.021

Ethion, μg/L 118 0 <.004 <.004 <.016 <.016 <.016 38 0 <.004 <.016 <.016 <.016 <.016

Ethoprop, μg/L 196 0 <.003 <.003 <.003 <.012 <.012 82 0 <.003 <.003 <.012 <.012 <.012

Fenamiphos sulfone, μg/L 118 0 <.049 <.049 <.053 <.053 <.053 38 0 <.049 <.053 <.053 <.053 <.053

Fenamiphos sulfoxide, μg/L 107 0 <.039 <.039 <.040 <.040 <.040 36 0 <.039 <.040 <.040 <.040 <.040

Fenamiphos, μg/L 118 0 <.029 <.029 <.029 <.029 <.029 38 0 <.029 <.029 <.029 <.029 <.029

Desulfinylfipronil amide, μg/L 121 0 <.009 <.029 <.029 <.029 <.029 41 0 <.009 <.029 <.029 <.029 <.029

Fipronil sulfide, μg/L 121 1 <.005 <.013 <.013 <.013 .008 41 0 <.005 <.013 <.013 <.013 <.013

Fipronil sulfone, μg/L 121 1 <.005 <.024 <.024 <.024 .009 41 0 <.005 <.024 <.024 <.024 <.024

Fipronil, μg/L 121 1 <.007 <.016 <.016 <.016 .008 41 0 <.007 <.016 <.016 <.016 <.016

Fonofos oxygen analog, μg/L 20 0 <.003 <.003 <.003 <.003 <.003 5 0 <.003 <.003 <.003 <.003 <.003

Fonofos, μg/L 239 0 <.003 <.003 <.003 <.005 <.006 87 0 <.003 <.003 <.003 <.005 <.006

Hexazinone, μg/L 118 5 <.013 <.013 <.026 <.026 .126 38 3 <.013 <.026 <.026 <.026 .126

Iprodione, μg/L 115 0 <.026 <.026 <.026 <.538 <.538 38 0 <.026 <.026 <.026 <.026 <.387

Isofenphos, μg/L 118 0 <.003 <.003 <.011 <.011 <.011 38 0 <.003 <.011 <.011 <.011 <.011

Lindane, μg/L 121 0 <.004 <.004 <.004 <.004 <.004 49 0 <.004 <.004 <.004 <.004 <.004

Linuron, μg/L 121 0 <.002 <.002 <.002 <.002 <.035 49 0 <.002 <.002 <.002 <.002 <.035

Malaoxon, μg/L 118 0 <.030 <.030 <.039 <.039 <.039 38 0 <.030 <.039 <.039 <.039 <.039

Malathion, μg/L 239 0 <.005 <.005 <.027 <.027 <.027 87 0 <.005 <.005 <.027 <.027 <.027

Metalaxyl, μg/L 118 0 <.005 <.005 <.007 <.007 <.007 38 0 <.005 <.007 <.007 <.007 <.007

Methidathion, μg/L 118 0 <.006 <.006 <.009 <.009 <.009 38 0 <.006 <.009 <.009 <.009 <.009

Methyl paraoxon, μg/L 118 0 <.019 <.019 <.019 <.030 <.030 38 0 <.019 <.019 <.019 <.019 <.030

Methyl parathion, μg/L 239 0 <.006 <.006 <.006 <.015 <.015 87 0 <.006 <.006 <.006 <.015 <.015

Metolachlor, μg/L 239 5 <.002 <.002 <.002 <.006 .004 87 0 <.002 <.002 <.006 <.006 <.013

Metribuzin, μg/L 239 1 <.004 <.004 <.004 <.028 .004 87 1 <.004 <.004 <.006 <.028 .004

Molinate, μg/L 196 0 <.002 <.003 <.004 <.004 <.004 82 0 <.002 <.003 <.003 <.004 <.004

Myclobutanil, μg/L 117 0 <.008 <.008 <.033 <.033 <.033 37 0 <.008 <.033 <.033 <.033 <.033

Napropamide, μg/L 121 0 <.003 <.003 <.003 <.003 <.007 49 0 <.003 <.003 <.003 <.003 <.007
74 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent All Shallow/urban unconfined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum Soluble pesticide compounds—Continued Oxyfluorfen, μg/L 75 0 <0.007 <0.017 <0.017 <0.017 <0.017 33 0 <0.017 <0.017 <0.017 <0.017 <0.017 p,p’-DDE, μg/L 121 11 <.003 <.006 <.006 <.006 .003 49 5 <.003 <.006 <.006 <.006 .002 Parathion, μg/L 121 0 <.004 <.004 <.004 <.004 <.010 49 0 <.004 <.004 <.004 <.004 <.010 Pebulate, μg/L 121 0 <.002 <.004 <.004 <.004 <.004 49 0 <.002 <.004 <.004 <.004 <.004 Pendimethalin, μg/L 239 0 <.004 <.004 <.004 <.022 <.04 87 0 <.004 <.004 <.01 <.022 <.04 Phorate oxygen analog, μg/L 118 0 <.027 <.027 <.027 <.105 <.105 38 0 <.027 <.027 <.027 <.027 <.105 Phorate, μg/L 239 0 <.002 <.002 <.002 <.02 <.055 87 0 <.002 <.002 <.011 <.055 <.055 Phosmet oxygen analog, μg/L 85 0 <.051 <.051 <.051 <.051 <.051 34 0 <.051 <.051 <.051 <.051 <.051 Phosmet, μg/L 91 0 <.008 <.008 <.008 <.008 <.008 35 0 <.008 <.008 <.008 <.008 <.008 Prometon, μg/L 239 51 <.01 <.01 <.010 <.018 .022 87 24 <.01 <.01 <.018 .003 .022 Prometryn, μg/L 115 0 <.005 <.005 <.006 <.006 <.006 37 0 <.005 <.006 <.006 <.006 <.006 Propyzamide, μg/L 237 0 <.003 <.003 <.004 <.004 <.004 85 0 <.003 <.003 <.004 <.004 <.004 Propachlor, μg/L 120 0 <.007 <.007 <.007 <.007 <.010 48 0 <.007 <.007 <.007 <.007 <.010 Propanil, μg/L 196 0 <.004 <.004 <.004 <.011 <.011 82 0 <.004 <.004 <.011 <.011 <.011 Propargite, μg/L 196 0 <.013 <.013 <.013 <.023 <.06 82 0 <.013 <.013 <.02 <.023 <.023 Simazine, μg/L 239 87 <.005 <.005 <.005 .005 .029 87 57 <.005 <.005 .004 .006 .029 Tebuconazole, μg/L 75 0 <.014 <.014 <.014 <.014 <.014 33 0 <.014 <.014 <.014 <.014 <.014 Tebuthiuron, μg/L 241 3 <.01 <.01 <.016 <.016 .042 87 3 <.01 <.01 <.016 <.016 .042 Tefluthrin, μg/L 75 0 <.003 <.003 <.003 <.003 <.010 33 0 <.003 <.003 <.003 <.003 <.003 Terbacil, μg/L 121 10 <.007 <.007 <.007 <.007 .028 49 6 <.007 <.007 <.007 <.007 .028 Terbufos oxygen analog sulfone, μg/L 118 0 <.045 <.045 <.045 <.068 <.068 38 0 <.045 <.045 <.045 <.045 <.068 Terbufos, μg/L 239 0 <.012 <.013 <.013 <.017 <.017 87 0 <.012 <.013 <.017 <.017 <.017 Terbuthylazine, μg/L 116 0 <.008 <.008 <.008 <.010 <.010 36 0 <.008 <.008 <.008 <.008 <.010 Thiobencarb, μg/L 196 0 <.002 <.002 <.002 <.010 <.010 82 0 <.002 <.002 <.005 <.010 <.010 trans-Propiconazole, μg/L 75 0 <.013 <.034 <.034 <.034 <.034 33 0 <.034 <.034 <.034 <.034 <.034 Triallate, μg/L 121 0 <.001 <.001 <.001 <.001 <.002 49 0 <.001 <.001 <.001 <.001 <.002 Tribuphos, μg/L 75 0 <.004 <.035 <.035 <.035 <.035 33 0 <.035 <.035 <.035 <.035 <.035 Trifluralin, μg/L 239 1 <.002 <.002 <.009 <.009 .003 87 1 <.002 <.002 <.009 <.009 .003 Polar pesticide compounds 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T), μg/L 57 0 <.035 <.035 <.035 <.035 <.035 29 0 <.035 <.035 <.035 <.035 <.035 2,4-Dichlorophenoxyacetic acid (2,4-D) methyl ester, μg/L 20 0 <.016 <.016 <.016 <.016 <.016 0 -- -- -- -- -- -- 2,4-D, μg/L 77 2 <.035 <.035 <.038 <.15 .07 29 1 <.15 <.15 <.15 <.15 .07 4-(2,4-Dichlorophenoxy)butyric acid (2,4-DB), μg/L 77 0 <.02 <.020 <.035 <.24 <.24 29 0 <.24 <.24 <.24 <.24 <.24
Table 4 

75

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Polar pesticide compounds—Continued

2-Chloro-6-ethyl-amino-4-amino-s-triazine (CEAT), μg/L 20 6 <0.08 <0.08 <0.08 0.0035 0.009 0 -- -- -- -- -- -- 

2-Hydroxy-4-isopropylamino-6-ethylamino-s-triazine (OIET), μg/L 20 3 <.032 <.032 <.032 <.032 .007 0 -- -- -- -- -- -- 

2-Methyl-4,6-di-nitro-phenol (DNOC), μg/L 56 0 <.035 <.035 <.035 <.42 <.42 28 0 <0.42 <0.42 <0.42 <0.42 <0.42

3-Hydroxy carbofuran, μg/L 76 0 <.008 <.008 <.014 <.014 <.014 28 0 <.014 <.014 <.014 <.014 <.014

Acifluorfen, μg/L 76 0 <.028 <.028 <.035 <.035 <.035 28 0 <.035 <.035 <.035 <.035 <.035

Aldicarb sulfone, μg/L 76 0 <.016 <.016 <.018 <.1 <.1 28 0 <.1 <.1 <.1 <.1 <.1

Aldicarb sulfoxide, μg/L 76 0 <.021 <.021 <.021 <.022 <.11 28 0 <.021 <.021 <.021 <.021 <.11

Aldicarb, μg/L 76 0 <.016 <.016 <.04 <.55 <.55 28 0 <.55 <.55 <.55 <.55 <.55

Bendiocarb, μg/L 20 0 <.02 <.02 <.02 <.02 <.020 0 -- -- -- -- -- -- 

Benomyl, μg/L 20 0 <.022 <.022 <.022 <.022 <.022 0 -- -- -- -- -- -- 

Bensulfuron-methyl, μg/L 20 0 <.018 <.018 <.018 <.018 <.018 0 -- -- -- -- -- -- 

Bentazon, μg/L 77 0 <.012 <.012 <.014 <.014 <.014 29 0 <.014 <.014 <.014 <.014 <.014

Bromacil, μg/L 99 1 <.018 <.035 <.035 <.035 .14 29 1 <.035 <.035 <.035 <.035 .14

Bromoxynil, μg/L 77 0 <.028 <.028 <.035 <.035 <.035 29 0 <.035 <.035 <.035 <.035 <.035

Caffeine, μg/L 42 4 <.018 <.018 <.5 <.5 .009 0 -- -- -- -- -- -- 

Carbaryl, μg/L 77 2 <.008 <.008 <.008 <.018 .03 29 2 <.008 <.008 <.008 <.008 .03

Carbofuran, μg/L 77 0 <.016 <.016 <.028 <.12 <.12 29 0 <.12 <.12 <.12 <.12 <.12

Chloramben methyl ester, μg/L 77 0 <.011 <.011 <.024 <.42 <.42 29 0 <.42 <.42 <.42 <.42 <.42

Chlorimuron, μg/L 20 0 <.032 <.032 <.032 <.032 <.032 0 -- -- -- -- -- -- 

Chlorodiamino-s-triazine (CAAT), μg/L 20 0 <.04 <.04 <.04 <.04 <.04 0 -- -- -- -- -- -- 

Chlorothalonil, μg/L 77 0 <.035 <.035 <.035 <.48 <.48 29 0 <.48 <.48 <.48 <.48 <.48

Clopyralid, μg/L 77 0 <.024 <.024 <.05 <.23 <.36 29 0 <.23 <.23 <.23 <.23 <.23

Cycloate, μg/L 20 0 <.014 <.014 <.014 <.014 <.014 0 -- -- -- -- -- -- 

Dacthal monoacid, μg/L 77 0 <.017 <.017 <.017 <.017 <.028 29 0 <.017 <.017 <.017 <.017 <.017

Dicamba, μg/L 77 1 <.035 <.035 <.035 <.036 .02 29 1 <.035 <.035 <.035 <.035 .02

Dichlobenil, μg/L 57 0 <.02 <.02 <.02 <1.2 <1.2 29 0 <1.2 <1.2 <1.2 <1.2 <1.2

Dichlorprop, μg/L 77 0 <.028 <.028 <.032 <.032 <.032 29 0 <.032 <.032 <.032 <.032 <.032

Dinoseb, μg/L 77 0 <.035 <.035 <.035 <.035 <.038 29 0 <.035 <.035 <.035 <.035 <.035

Diphenamid, μg/L 20 0 <.01 <.01 <.01 <.01 <.010 0 -- -- -- -- -- -- 

Diuron, μg/L 77 1 <.015 <.015 <.02 <.02 .2 29 1 <.02 <.02 <.02 <.02 .2

Esfenvalerate, μg/L 28 0 <.019 <.019 <.019 <.019 <.019 0 -- -- -- -- -- -- 

Fenuron, μg/L 77 0 <.013 <.013 <.013 <.013 <.019 29 0 <.013 <.013 <.013 <.013 <.013
76 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent All Shallow/urban unconfined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum Polar pesticide compounds—Continued Flumetsulam, μg/L 20 0 <0.04 <0.04 <0.04 <0.04 <0.040 0 -- -- -- -- -- -- Fluometuron, μg/L 77 0 <.016 <.035 <.035 <.035 <.035 29 0 <0.035 <0.035 <0.035 <0.035 <0.035 Imazaquin, μg/L 20 0 <.036 <.036 <.036 <.036 <.036 0 -- -- -- -- -- -- Imazethapyr, μg/L 20 0 <.038 <.038 <.038 <.038 <.038 0 -- -- -- -- -- -- Imidacloprid, μg/L 20 0 <.02 <.02 <.02 <.02 <.020 0 -- -- -- -- -- -- Linuron, μg/L 77 0 <.014 <.014 <.018 <.018 <.018 29 0 <.018 <.018 <.018 <.018 <.018 2-Methyl-4-chlorophenoxyacetic acid (MCPA), μg/L 69 0 <.03 <.05 <.05 <.17 <.17 29 0 <.17 <.17 <.17 <.17 <.17 4-(4-Chloro-2-methylphenoxy) butanoic acid (MCPB), μg/L 77 0 <.01 <.010 <.035 <.14 <.14 29 0 <.14 <.14 <.14 <.14 <.14 Metalaxyl, μg/L 42 0 <.5 <.5 <.5 <.012 <.012 0 -- -- -- -- -- -- Methio-arb, μg/L 77 0 <.01 <.010 <.026 <.026 <.026 29 0 <.026 <.026 <.026 <.026 <.026 Methomyl, μg/L 76 0 <.017 <.017 <.017 <.02 <.27 29 0 <.017 <.017 <.017 <.017 <.27 Metsulfuron, μg/L 19 1 <.025 <.025 <.025 <.025 .035 0 -- -- -- -- -- -- N-(4-Chlorophenyl)-N’-methyl urea, μg/L 19 0 <.036 <.036 <.036 <.036 <.036 0 -- -- -- -- -- -- Neburon, μg/L 76 0 <.012 <.012 <.015 <.015 <.015 29 0 <.015 <.015 <.015 <.015 <.015 Nicosulfuron, μg/L 19 0 <.04 <.04 <.04 <.04 <.04 0 -- -- -- -- -- -- Norflurazon, μg/L 76 0 <.02 <.020 <.024 <.024 <.024 29 0 <.024 <.024 <.024 <.024 <.024 Oryzalin, μg/L 76 0 <.012 <.012 <.019 <.31 .08 29 1 <.31 <.31 <.31 <.31 .08 Oxamyl, μg/L 76 1 <.018 <.018 <.018 <.018 <.030 29 0 <.018 <.018 <.018 <.018 <.018 Picloram, μg/L 76 0 <.032 <.032 <.05 <.05 <.05 29 0 <.05 <.05 <.05 <.05 <.05 Propham, μg/L 76 0 <.03 <.030 <.035 <.035 <.035 29 0 <.035 <.035 <.035 <.035 <.035 Propiconazole, μg/L 19 0 <.01 <.01 <.01 <.01 <.010 0 -- -- -- -- -- -- Propoxur, μg/L 76 0 <.008 <.008 <.035 <.035 <.035 29 0 <.035 <.035 <.035 <.035 <.035 Siduron, μg/L 19 0 <.02 <.02 <.02 <.02 <.020 0 -- -- -- -- -- -- Silvex, μg/L 57 0 <.021 <.021 <.021 <.021 <.021 29 0 <.021 <.021 <.021 <.021 <.021 Sulfometuron, μg/L 19 0 <.038 <.038 <.038 <.038 <.038 0 -- -- -- -- -- -- Terbacil, μg/L 19 0 <.016 <.016 <.016 <.016 <.016 0 -- -- -- -- -- -- Triclopyr, μg/L 76 0 <.026 <.026 <.05 <.25 <.25 29 0 <.25 <.25 <.25 <.25 <.25 Acetamide pesticide compounds 2-[(2-Ethyl-6-methylphenyl) amino]-2-oxoethane-sulfonic acid (N- (Ethmethphen)ox ESA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 2-Chloro-N-(2,6-diethylphenyl)-acetamide (alachlor 2nd amide), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 2-Chloro-N-(2-ethyl-6-methylphenyl)-acetamid (acetochlor 2nd amide), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

3-Ketocarbofuran, μg/L 20 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 
Table 4 

77

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Acetamide pesticide compounds—Continued

Acetochlor ethanesulfonic acid (acetochlor ESA), μg/L 24 0 <0.02 <0.02 <0.02 <0.02 <0.02 0 -- -- -- -- -- -- 

Acetochlor oxanilic acid (acetochlor OA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Acetochlor sulfynilacetic acid (acetochlor SAA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Alachlor ethanesulfonic acid secondary amide (alachlor ESA SA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Alachlor ethanesulfonic acid (alachlor ESA), μg/L 24 2 <.02 <.02 <.02 <.02 .02 0 -- -- -- -- -- -- 

Alachlor oxanilic acid (alachlor OA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Alachlor sulfynilacetic acid (alachlor SAA), μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dechloroacetochlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dechloroalachlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dechlorodimethenamid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dechlorometolachlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dimethenamid ethanesulfonic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dimethenamid oxanilic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Dimethenamid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Flufenacet ethanesulfonic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Flufenacet oxanilic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Flufenacet, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Hydroxyacetochlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Hydroxyalachlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Hydroxydimethenamid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Hydroxymetolachlor, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Metolachlor ethanesulfonic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Metolachlor ethanesulfonic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Propachlor ethanesulfonic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Propachlor oxanilic acid, μg/L 24 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- 

Pesticide and wastewater indicator compounds

1,4-Dichlorobenzene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

1-Methylnaphthalene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

2,6-Dimethylnaphthalene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

2-Methylnaphthalene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

3-beta-Coprostanol, μg/L 39 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- 

3-Methyl-1H-indole, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 

3-tert-Butyl-4-hydroxyanisole, μg/L 39 0 <5.0 <5.0 <5.0 <5.0 <5.0 0 -- -- -- -- -- -- 

4-Cumylphenol, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 
78 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent All Shallow/urban unconfined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum Pesticide and wastewater indicator compounds—Continued 4-n-Octylphenol, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 4-Nonylphenol (sum of isomers), μg/L 39 15 <5.0 <5.0 <5.0 .715 3.2 0 -- -- -- -- -- -- 4-Nonylphenol dieth-oxylate (sum of isomers), μg/L 39 0 <5.0 <5.0 <5.0 <5.0 <5.0 0 -- -- -- -- -- -- 4-tert-Octylphenol diethoxylate, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 4-tert-Octylphenol monoethoxylate, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 4-tert-Octylphenol, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 5-Methyl-1H-benzotriazole, μg/L 39 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- 9,10-Anthraquinone, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Acetophenone, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Acetyl hexamethyl tetrahydronaphthalene (AHTN), μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Anthracene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Benzo-[a]-pyrene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Benzophenone, μg/L 39 12 <.5 <.5 <.5 .017 .22 0 -- -- -- -- -- -- beta-Sitosterol, μg/L 39 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- beta-Stigmastanol, μg/L 39 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- Bis-phenol A, μg/L 26 1 <1.0 <1.0 <1.0 <1.0 .14 0 -- -- -- -- -- -- Camphor, μg/L 39 2 <.5 <.5 <.5 <.5 .006 0 -- -- -- -- -- -- Carbazole, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Cholesterol, μg/L 39 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- Cotinine, μg/L 39 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- N,N-Diethyl-meta-toluamide (DEET), μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- D-Limonene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Fluoranthene, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Hexahydrohexamethylcyclopentabenzopyran (HHCB), μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Indole, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Isoborneol, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Isophorone, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Isopropylbenzene, μg/L 39 2 <.5 <.5 <.5 <.5 .023 0 -- -- -- -- -- -- Isoquinoline, μg/L 39 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Menthol, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Methyl salicylate, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- Naphthalene, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 
Table 4 

79

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

All Shallow/urban unconfined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th percen- tile Median 75th percen- tile Maxi- mum

Pesticide and wastewater indicator compounds—Continued

p-Cresol, μg/L 38 1 <1.0 <1.0 <1.0 <1.0 0.024 0 -- -- -- -- -- -- 

Pentachlorophenol, μg/L 17 0 <2.0 <2.0 <2.0 <2.0 <2.0 0 -- -- -- -- -- -- 

Phenanthrene, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Phenol*, μg/L 38 28 <.5 .028 .47 1.4 4.0 0 -- -- -- -- -- -- 

Pyrene, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Tetrachloroethene (PCE), μg/L 38 16 <.5 <.5 <.5 .042 .28 0 -- -- -- -- -- -- 

Tribromomethane (bromoform), μg/L 38 10 <.5 <.5 <.5 .017 1.9 0 -- -- -- -- -- -- 

Tributyl phosphate, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Triclosan, μg/L 38 0 <1.0 <1.0 <1.0 <1.0 <1.0 0 -- -- -- -- -- -- 

Triethyl citrate, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Triphenyl phosphate, μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Tris(2-butoxyethyl) phosphate, μg/L 38 1 <.5 <.5 <.5 <.5 .098 0 -- -- -- -- -- -- 

Tris(2-chloroethyl) phosphate (FYROL CEF), μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 

Tris(dichloroisopropyl) phosphate (FYROL FR 2), μg/L 38 0 <.5 <.5 <.5 <.5 <.5 0 -- -- -- -- -- -- 
80 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Volatile organic and fuel oxygenate compounds Acrolein, μg/L 28 0 <2 <2 <2 <2 <2 34 0 <1.43 <1.43 <1.43 <1.43 <3.94 1,1,1,2-Tetrachloroethane, μg/L 79 0 <.03 <.03 <.03 <.05 <.05 74 0 <.03 <.03 <.03 <.044 <.044 1,1,1-Trichloroethane, μg/L 79 3 <.032 <.032 <.032 <.05 .008 74 0 <.032 <.032 <.032 <.032 <.032 1,1,2,2-Tetrachloroethane, μg/L 79 0 <.08 <.08 <.08 <.1 <.13 74 0 <.08 <.08 <.08 <.132 <.132 1,1,2-Trichloro-1,2,2-trifluoroethane (CFC−113), μg/L 79 0 <.032 <.038 <.038 <.05 <.06 74 0 <.032 <.032 <.038 <.038 .038 1,1,2-Trichloroethane, μg/L 79 0 <.04 <.04 <.04 <.1 <.1 74 0 <.04 <.04 <.04 <.064 <.064 1,1-Dichloroethane, μg/L 79 0 <.035 <.035 <.035 <.05 <.066 74 0 <.035 <.035 <.035 <.066 .010 1,1-Dichloroethene, μg/L 79 0 <.024 <.024 <.024 <.1 <.1 74 0 <.024 <.024 <.024 <.044 <.044 1,1-Dichloropropene, μg/L 79 0 <.026 <.026 <.026 <.05 <.05 74 0 <.026 <.026 <.026 <.026 <.026 1,2,3,4-Tetramethylbenzene (prehnitene), μg/L 79 0 <.05 <.05 <.14 <.14 <.23 74 0 <.14 <.14 <.14 <.23 <.23 1,2,3,5-Tetramethylbenzene (isodurene), μg/L 79 0 <.05 <.05 <.18 <.18 <.2 74 0 <.14 <.14 <.14 <.24 <.24 1,2,3-Trichlorobenzene, μg/L 79 0 <.18 <.18 <.18 <.2 <.27 74 0 <.18 <.18 <.18 <.266 <.266 1,2,3-Trichloropropane, μg/L 79 0 <.16 <.18 <.18 <.2 <.2 74 0 <.07 <.07 <.18 <.18 <.18 1,2,3-Trimethylbenzene, μg/L 79 0 <.05 <.05 <.09 <.09 <.12 74 0 <.06 <.06 <.06 <.124 <.124 1,2,4-Trichlorobenzene, μg/L 79 0 <.07 <.12 <.12 <.2 <.2 74 0 <.12 <.12 <.12 <.188 <.188 1,2,4-Trimethylbenzene, μg/L 79 0 <.05 <.05 <.056 <.056 <.056 74 2 <.056 <.056 <.056 <.056 .01 1,2-Dibromo-3-chloropropane, μg/L 79 0 <.21 <.5 <.5 <.51 <.51 74 0 <.214 <.214 <.51 <.51 <.51 1,2-Dibromoethane, μg/L 79 0 <.036 <.036 <.036 <.1 <.1 74 0 <.036 <.036 <.036 <.036 <.036 1,2-Dichlorobenzene, μg/L 79 0 <.031 <.048 <.048 <.05 <.05 74 0 <.048 <.048 <.048 <.048 <.048 1,2-Dichloroethane, μg/L 79 0 <.05 <.05 <.13 <.13 <.13 74 0 <.13 <.13 <.13 <.134 <.134 1,2-Dichloropropane, μg/L 79 0 <.029 <.029 <.029 <.05 <.068 74 0 <.029 <.029 <.029 <.068 .010 1,3,5-Trimethylbenzene, μg/L 79 0 <.044 <.044 <.044 <.05 <.05 74 0 <.044 <.044 <.044 <.044 <.044 1,3-Dichlorobenzene, μg/L 79 1 <.03 <.03 <.03 <.05 .001 74 0 <.03 <.03 <.03 <.054 <.054 1,3-Dichloropropane, μg/L 79 0 <.05 <.05 <.06 <.06 <.12 74 0 <.06 <.06 <.06 <.116 <.116 1,4-Dichlorobenzene, μg/L 79 0 <.034 <.034 <.034 <.05 <.05 74 1 <.034 <.034 <.034 <.05 .006 2,2-Dichloropropane, μg/L 79 0 <.05 <.05 <.05 <.05 <.078 74 0 <.05 <.05 <.05 <.078 <.078 2-Chlorotoluene, μg/L 79 0 <.026 <.04 <.04 <.05 <.05 74 0 <.04 <.04 <.04 <.042 <.042 2-Ethyltoluene, μg/L 79 0 <.05 <.05 <.06 <.06 <.1 74 0 <.06 <.06 <.06 <.1 <.1
Table 4 

81

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

Unconfined Confined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum

Volatile organic and fuel oxygenate compounds—Continued

3-Chloropropene, μg/L 79 0 <0.07 <0.1 <0.2 <0.5 <0.5 74 0 <0.196 <0.196 <0.5 <0.5 <0.5

4-Chlorotoluene, μg/L 79 0 <.05 <.05 <.05 <.05 <.06 74 0 <.05 <.05 <.05 <.056 <.056

4-Isopropyltoluene, μg/L 79 0 <.05 <.05 <.08 <.08 <.11 74 0 <.08 <.08 <.08 <.11 <.11

Acetone, μg/L 79 1 <5 <5 <6 <6 .300 74 0 <4.90 <4.90 <6 <6 <6

Acrylonitrile, μg/L 79 0 <.8 <.8 <.8 <2 <2 74 0 <.8 <.8 <.8 <1.23 <1.23

Benzene, μg/L 79 0 <.021 <.021 <.021 <.05 <.1 74 0 <.021 <.021 <.021 <.032 .010

Bromobenzene, μg/L 79 0 <.028 <.028 <.028 <.05 <.05 74 1 <.028 <.028 <.028 <.036 .003

Bromochloromethane, μg/L 79 0 <.044 <.1 <.1 <.12 <.12 74 0 <.044 <.044 <.12 <.12 <.12

Bromodichloromethane, μg/L 79 9 <.028 <.028 <.048 <.1 .13 74 0 <.028 <.028 <.048 <.048 1.23

Bromoethene, μg/L 79 0 <.1 <.1 <.1 <.1 <.11 74 0 <.1 <.1 <.1 <.1 <.1

Bromomethane, μg/L 79 0 <.1 <.1 <.26 <.33 <.33 74 0 <.148 <.148 <.26 <.26 <.26

Carbon disulfide, μg/L 79 5 <.038 <.038 <.038 <.05 .02 74 4 <.038 <.038 <.038 <.08 .090

Chlorobenzene, μg/L 79 1 <.028 <.028 <.028 <.05 .004 74 0 <.028 <.028 <.028 <.028 <.028

Chloroethane, μg/L 79 0 <.1 <.1 <.12 <.12 <.12 74 0 <.12 <.12 <.12 <.12 <.12

Chloromethane, μg/L 79 10 <.17 <.17 <.17 <.2 .060 74 0 <.17 <.17 <.17 <.254 .010

cis-1,2-Dichloroethene, μg/L 79 0 <.024 <.024 <.024 <.05 <.05 74 1 <.024 <.024 <.024 <.038 .020

cis-1,3-Dichloropropene, μg/L 79 0 <.05 <.05 <.05 <.1 <.1 74 0 <.05 <.05 <.05 <.092 <.092

Dibromochloromethane, μg/L 79 3 <.1 <.1 <.1 <.1 .060 74 0 <.1 <.1 <.182 <.182 2.89

Dibromomethane, μg/L 79 0 <.05 <.05 <.05 <.1 <.1 74 1 <.05 <.05 <.05 <.05 .010

Dichlorodifluoromethane (CFC−12), μg/L 79 1 <.14 <.18 <.18 <.2 .020 74 4 <.096 <.096 <.18 <.18 .300

Dichloromethane, μg/L 79 3 <.06 <.06 <.06 <.1 .010 74 0 <.06 <.06 <.06 <.382 <.382

Diethyl ether, μg/L 79 0 <.08 <.08 <.08 <.1 <.17 74 0 <.08 <.08 <.08 <.17 <.17

Diisopropyl ether, μg/L 79 0 <.098 <.1 <.1 <.1 <.1 74 0 <.098 <.098 <.1 <.1 <.1

Ethyl methacrylate, μg/L 79 0 <.18 <.18 <.18 <1 <1 74 0 <.18 <.18 <.18 <.278 <.278

Ethyl methyl ketone, μg/L 79 1 <1.6 <2 <2 <5 3.00 74 0 <1.65 <1.65 <2 <2 1.00

Ethylbenzene, μg/L 79 0 <.03 <.03 <.03 <.05 <.05 74 0 <.03 <.03 <.03 <.03 .020

Hexachlorobutadiene, μg/L 79 0 <.14 <.14 <.14 <.2 <.2 74 0 <.14 <.14 <.14 <.142 <.142

Hexachloroethane, μg/L 79 0 <.05 <.05 <.14 <.14 <.36 74 0 <.14 <.14 <.14 <.362 <.362

Iodomethane, μg/L 79 2 <.05 <.05 <.5 <.5 .009 74 0 <.076 <.076 <.5 <.5 <.5

Isobutyl methyl ketone, μg/L 79 0 <.37 <.37 <.37 <5 <5 74 0 <.37 <.37 <.37 <.374 <.374

Isopropylbenzene, μg/L 79 0 <.032 <.038 <.038 <.05 <.06 74 1 <.032 <.032 <.038 <.038 .005

Methyl acrylonitrile, μg/L 79 0 <.4 <.4 <.4 <2 <2 74 0 <.4 <.4 <.4 <.57 <.57
82 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Volatile organic and fuel oxygenate compounds—Continued Methyl acrylate, μg/L 79 0 <1 <1 <1 <2 <2 74 0 <0.612 <0.612 <1 <1 <1 Methyl methacrylate, μg/L 79 0 <.2 <.2 <.2 <1 <1 74 0 <.2 <.2 <.2 <.35 <.35 Methyl tert-pentyl ether, μg/L 79 0 <.04 <.04 <.04 <.1 <.11 74 0 <.04 <.04 <.04 <.112 <.112

m- plus p-Xylene, μg/L 79 0 <.05 <.05 <.06 <.06 <.06 74 2 <.06 <.06 <.06 <.064 .040

Naphthalene, μg/L 79 0 <.2 <.2 <.5 <.52 <.52 74 0 <.25 <.25 <.52 <.52 <.52

n-Butyl methyl ketone, μg/L 79 0 <.4 <.4 <.4 <5 <5 74 0 <.4 <.4 <.4 <.746 <.746

n-Butylbenzene, μg/L 79 0 <.05 <.05 <.14 <.14 <.19 74 0 <.12 <.12 <.12 <.186 <.186

n-Propylbenzene, μg/L 79 0 <.042 <.042 <.042 <.05 <.05 74 0 <.042 <.042 <.042 <.042 <.042

o-Xylene, μg/L 79 0 <.038 <.038 <.038 <.05 <.07 74 1 <.038 <.038 <.038 <.064 .010

sec-Butylbenzene, μg/L 79 0 <.032 <.05 <.05 <.06 <.06 74 0 <.048 <.048 <.06 <.06 <.06

Styrene, μg/L 79 2 <.042 <.042 <.042 <.05 .013 74 0 <.042 <.042 <.042 <.042 <.042

tert-Amyl alcohol, μg/L 5 0 <1 <1 <1 <1 <1 34 0 <1 <1 <1 <1 <1

tert-Butyl ethyl ether, μg/L 79 0 <.03 <.03 <.03 <.1 <.1 74 2 <.03 <.03 <.03 <.054 .020

Methyl tert-butyl ether, μg/L 79 0 <.1 <.1 <.1 <.1 <.17 74 1 <.1 <.1 <.1 <.112 .050

tert-Butylbenzene, μg/L 79 0 <.048 <.05 <.06 <.08 <.1 74 0 <.06 <.06 <.06 <.096 <.096

Tetrachloroethene (PCE), μg/L 79 14 <.027 <.03 <.05 <.05 .088 74 0 <.03 <.03 <.038 .064 .830

Tetrachloromethane, μg/L 79 0 <.05 <.05 <.06 <.06 <.088 74 0 <.06 <.06 <.06 <.088 <.088

Tetrahydrofuran, μg/L 79 1 <1 <1.2 <1.2 <5 .600 74 0 <1 <1 <1 <1.1 1.00

Toluene, μg/L 79 6 <.02 <.02 <.05 <.05 .750 74 0 <.02 <.02 <.02 <.038 .040

trans-1,2-Dichloroethene, μg/L 79 0 <.032 <.032 <.032 <.05 <.05 74 0 <.032 <.032 <.032 <.032 <.032

trans-1,3-Dichloropropene, μg/L 79 0 <.09 <.09 <.09 <.1 <.13 74 0 <.09 <.09 <.09 <.134 <.134

trans-1,4-Dichloro-2-butene, μg/L 79 0 <.7 <.7 <.7 <5 <5 74 0 <.692 <.692 <.7 <.7 <.7

Tribromomethane (bromoform), μg/L 79 3 <.06 <.1 <.1 <.2 .120 74 0 <.1 <.1 <.104 <.104 3.09

Trichloroethene (TCE), μg/L 79 1 <.038 <.038 <.038 <.05 .015 74 0 <.038 <.038 <.038 <.038 .040

Trichlorofluoromethane (CFC−11), μg/L 79 0 <.08 <.08 <.08 <.1 <.1 74 0 <.08 <.08 <.08 <.092 .275

Trichloromethane (chloroform), μg/L 79 38 <.024 <.024 <.05 .074 1.30 74 0 <.024 <.052 .011 .025 .477

Vinyl acetate, μg/L 28 0 <5 <5 <5 <5 <5 0 -- -- -- -- -- -- 

Vinyl chloride, μg/L 79 0 <.08 <.08 <.08 <.1 <.11 73 0 <.08 <.08 <.08 <.112 <.112

Soluble pesticide compounds

1-Naphthol, μg/L 68 0 <.007 <.007 <.088 <.088 <.088 40 0 <.088 <.088 <.088 <.088 <.088

2,6-Diethylaniline, μg/L 79 0 <.0017 <.003 <.006 <.006 <.006 73 0 <.003 <.003 <.006 <.006 <.006

2-Chloro-2,6-diethylacetanilide, μg/L 40 0 <.005 <.005 <.007 <.007 <.007 40 0 <.005 <.005 <.005 <.005 <.005

2-Chloro-4-isopropylamino-6-amino-s-triazine (deethylatrazine 

[DEA]), μg/L

79 28 <.002 <.002 <.014 .004 .015 75 53 <.002 <.006 .004 .008 .022
Table 4 

83

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

Unconfined Confined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum

Soluble pesticide compounds—Continued

2-Ethyl-6-methylaniline, μg/L 40 0 <0.005 <0.005 <0.010 <0.010 <0.010 40 0 <0.005 <0.005 <0.005 <0.005 <0.005

3,4-Dichloroaniline, μg/L 40 1 <.005 <.005 <.005 <.005 .003 36 0 <.005 <.005 <.005 <.005 <.005

3,5-Dichloroaniline, μg/L 35 0 <.004 <.012 <.012 <.012 <.012 7 0 <.004 <.004 <.004 <.004 <.004

4-Chloro-2-methylphenol, μg/L 40 0 <.005 <.005 <.005 <.005 <.006 40 0 <.006 <.006 <.006 <.006 <.006

Acetochlor, μg/L 79 0 <.002 <.002 <.006 <.006 <.006 73 0 <.002 <.002 <.006 <.006 <.006

Alachlor, μg/L 79 0 <.002 <.002 <.005 <.005 <.005 73 0 <.002 <.002 <.005 <.005 <.005

alpha-Endosulfan, μg/L 35 0 <.005 <.011 <.011 <.011 <.011 7 0 <.005 <.005 <.005 <.005 <.005

alpha-Hexachlorocyclohexane (alpha-HCH), μg/L 39 0 <.002 <.002 <.002 <.002 <.005 33 0 <.002 <.002 <.002 <.002 <.002

Atrazine, μg/L 79 19 <.001 <.001 <.007 <.007 .021 75 38 <.001 <.001 .002 .006 .045

Azinphos-methyl oxygen analog, μg/L 39 0 <.042 <.042 <.042 <.06 <.070 40 0 <.07 <.070 <.070 <.070 <.070

Azinphos-methyl, μg/L 79 0 <.001 <.001 <.05 <.05 <.050 72 0 <.001 <.001 <.05 <.05 <.05

Benfluralin, μg/L 79 1 <.002 <.002 <.01 <.01 .001 73 0 <.002 <.002 <.01 <.010 <.010

Butylate, μg/L 39 0 <.002 <.002 <.002 <.002 <.002 33 0 <.002 <.002 <.002 <.002 <.002

Carbaryl, μg/L 79 1 <.003 <.003 <.041 <.041 .01 73 0 <.003 <.003 <.041 <.041 <.041

Carbofuran, μg/L 74 0 <.003 <.003 <.02 <.02 <.020 40 0 <.003 <.003 <.003 <.003 <.02

Chlorpyrifos, μg/L 79 0 <.004 <.004 <.005 <.005 <.005 73 0 <.004 <.004 <.005 <.005 <.005

cis-Permethrin, μg/L 79 0 <.005 <.005 <.006 <.006 <.006 73 0 <.005 <.005 <.006 <.006 <.006

cis-Propiconazole, μg/L 35 0 <.008 <.013 <.013 <.013 <.013 7 0 <.008 <.008 <.008 <.008 <.008

Cyanazine, μg/L 74 0 <.004 <.004 <.018 <.018 <.018 40 0 <.004 <.004 <.004 <.004 <.018

Cyfluthrin, μg/L 40 0 <.027 <.027 <.053 <.053 <.053 40 0 <.008 <.008 <.027 <.027 <.027

lambda-Cyhalothrin, μg/L 35 0 <.009 <.014 <.014 <.014 <.014 7 0 <.009 <.009 <.009 <.009 <.009

Cypermethrin, μg/L 40 0 <.009 <.009 <.046 <.046 <.046 40 0 <.009 <.009 <.009 <.009 <.009

Dimethyl tetrachloroterephthalate (DCPA), μg/L 79 0 <.002 <.002 <.003 <.003 <.003 73 0 <.002 <.002 <.003 <.003 <.003

Chlorpyrifos oxygen analog, μg/L 40 0 <.056 <.056 <.056 <.056 <.056 40 0 <.056 <.056 <.056 <.056 <.056

Desulfinylfipronil, μg/L 40 0 <.012 <.012 <.012 <.012 <.012 40 0 <.012 <.012 <.012 <.012 <.012

Diazinon, μg/L 79 5 <.002 <.002 <.005 <.005 .006 73 0 <.002 <.002 <.005 <.005 <.005

Diazoxon, μg/L 40 0 <.006 <.006 <.006 <.006 <.006 40 0 <.006 <.006 <.006 <.006 <.006

Dichlorvos, μg/L 40 0 <.012 <.012 <.013 <.013 <.013 40 0 <.012 <.012 <.012 <.012 <.012

Dicrotophos, μg/L 40 0 <.084 <.084 <.084 <.084 <.084 40 0 <.084 <.084 <.084 <.084 <.084

Dieldrin, μg/L 79 1 <.001 <.001 <.005 <.009 .003 73 1 <.001 <.001 <.009 <.009 <.009

Dimethoate, μg/L 40 0 <.006 <.006 <.006 <.006 <.006 40 0 <.006 <.006 <.006 <.006 <.006

Disulfoton sulfone, μg/L 35 0 <.0059 <.014 <.014 <.014 <.014 7 0 <.006 <.006 <.006 <.006 <.006
84 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Soluble pesticide compounds—Continued Disulfoton, μg/L 74 0 <0.017 <0.017 <0.021 <0.021 <0.021 40 0 <0.017 <0.017 <0.017 <0.017 <0.021 Endosulfan sulfate, μg/L 35 0 <.014 <.022 <.022 <.022 <.022 7 0 <.014 <.014 <.014 <.014 <.014 s-Ethyl dipropylthiocarbamate (EPTC), μg/L 74 0 <.002 <.002 <.002 <.004 <.004 40 0 <.002 <.002 <.002 <.002 <.004 Ethalfluralin, μg/L 39 0 <.004 <.004 <.004 <.004 <.009 33 0 <.004 <.004 <.004 <.004 <.004 Ethion monoxon, μg/L 40 0 <.002 <.002 <.021 <.021 <.021 40 0 <.012 <.012 <.012 <.012 <.012 Ethion, μg/L 40 0 <.004 <.004 <.016 <.016 <.016 40 0 <.012 <.004 <.004 <.004 <.004 Ethoprop, μg/L 74 0 <.003 <.003 <.005 <.012 <.012 40 0 <.003 <.003 <.003 <.003 <.005 Fenamiphos sulfone, μg/L 40 0 <.049 <.049 <.053 <.053 <.053 40 0 <.049 <.049 <.049 <.049 <.049 Fenamiphos sulfoxide, μg/L 40 0 <.039 <.039 <.040 <.040 <.040 31 0 <.039 <.039 <.039 <.039 <.039 Fenamiphos, μg/L 40 0 <.029 <.029 <.029 <.029 <.029 40 0 <.029 <.029 <.029 <.029 <.029 Desulfinylfipronil amide, μg/L 40 0 <.029 <.029 <.029 <.029 <.029 40 0 <.029 <.029 <.029 <.029 <.029 Fipronil sulfide, μg/L 40 0 <.013 <.013 <.013 <.013 <.013 40 1 <.013 <.013 <.013 <.013 .008 Fipronil sulfone, μg/L 40 0 <.024 <.024 <.024 <.024 <.024 40 1 <.024 <.024 <.024 <.024 .009 Fipronil, μg/L 40 0 <.016 <.016 <.016 <.016 <.016 40 1 <.016 <.016 <.016 <.016 .008 Fonofos oxygen analog, μg/L 0 0 -- -- -- -- -- 15 0 <.003 <.003 <.003 <.003 <.003 Fonofos, μg/L 79 0 <.003 <.003 <.003 <.005 <.005 73 0 <.003 <.003 <.003 <.003 <.003 Hexazinone, μg/L 40 2 <.013 <.013 <.026 <.026 .01 40 0 <.013 <.013 <.013 <.013 <.013 Iprodione, μg/L 37 0 <.026 <.026 <.026 <.026 <.538 40 0 <.387 <.387 <.538 <.538 <.538 Isofenphos, μg/L 40 0 <.003 <.003 <.011 <.011 <.011 40 0 <.003 <.003 <.003 <.003 <.003 Lindane, μg/L 39 0 <.004 <.004 <.004 <.004 <.004 33 0 <.004 <.004 <.004 <.004 <.004 Linuron, μg/L 39 0 <.002 <.002 <.002 <.002 <.035 33 0 <.002 <.002 <.002 <.002 <.002 Malaoxon, μg/L 40 0 <.030 <.030 <.039 <.039 <.039 40 0 <.030 <.030 <.030 <.030 <.030 Malathion, μg/L 79 0 <.005 <.005 <.027 <.027 <.027 73 0 <.005 <.005 <.027 <.027 <.027 Metalaxyl, μg/L 40 0 <.005 <.005 <.007 <.007 <.007 40 0 <.005 <.005 <.005 <.005 <.005 Methidathion, μg/L 40 0 <.006 <.006 <.009 <.009 <.009 40 0 <.006 <.006 <.006 <.006 <.006 Methyl paraoxon, μg/L 40 0 <.019 <.019 <.019 <.019 <.030 40 0 <.030 <.030 <.030 <.030 <.030 Methyl parathion, μg/L 79 0 <.006 <.006 <.006 <.015 <.015 73 0 <.006 <.006 <.015 <.015 <.015 Metolachlor, μg/L 79 2 <.002 <.002 <.006 <.006 .004 73 3 <.002 <.002 <.006 <.006 .004 Metribuzin, μg/L 79 0 <.004 <.004 <.006 <.028 <.028 73 0 <.004 <.004 <.006 <.006 <.006 Molinate, μg/L 74 0 <.002 <.002 <.003 <.004 <.004 40 0 <.003 <.004 <.004 <.004 <.004 Myclobutanil, μg/L 40 0 <.008 <.008 <.033 <.033 <.033 40 0 <.008 <.008 <.008 <.008 <.008

Napropamide, μg/L 39 0 <.003 <.003 <.003 <.003 <.007 33 0 <.003 <.003 <.003 <.003 <.003
Table 4 

85

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

Unconfined Confined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum

Soluble pesticide compounds—Continued

Oxyfluorfen, μg/L 35 0 <0.007 <0.017 <0.017 <0.017 <0.017 7 0 <0.007 <.0007 <0.007 <0.007 <0.0160

p,p’-DDE, μg/L 39 4 <.003 <.006 <.006 <.006 .003 33 2 <.006 <.006 <.006 <.006 .002

Parathion, μg/L 39 0 <.004 <.004 <.004 <.004 <.010 33 0 <.004 <.004 <.004 <.004 <.004

Pebulate, μg/L 39 0 <.002 <.004 <.004 <.004 <.004 33 0 <.004 <.004 <.004 <.004 <.004

Pendimethalin, μg/L 79 0 <.004 <.004 <.022 <.022 <.022 73 0 <.004 <.004 <.022 <.022 <.022

Phorate oxygen analog, μg/L 40 0 <.027 <.027 <.027 <.027 <.105 40 0 <.105 <.105 <.105 <.105 <.105

Phorate, μg/L 79 0 <.002 <.002 <.011 <.055 <.055 73 0 <.002 <.002 <.011 <.011 <.011

Phosmet oxygen analog, μg/L 20 0 <.051 <.051 <.051 <.051 <.051 31 0 <.051 <.051 <.051 <.051 <.051

Phosmet, μg/L 21 0 <.008 <.008 <.008 <.008 <.008 35 0 <.008 <.008 <.008 <.008 <.008

Prometon, μg/L 79 9 <.01 <.01 <.015 <.018 .010 73 18 <.01 <.010 <.018 <.018 .013

Prometryn, μg/L 38 0 <.005 <.005 <.006 <.006 <.006 40 0 <.005 <.005 <.005 <.005 <.005

Propyzamide, μg/L 79 0 <.003 <.003 <.004 <.004 <.004 73 0 <.003 <.003 <.004 <.004 <.004

Propachlor, μg/L 39 0 <.007 <.007 <.007 <.007 <.010 33 0 <.007 <.007 <.007 <.007 <.007

Propanil, μg/L 74 0 <.004 <.004 <.011 <.011 <.011 40 0 <.004 <.004 <.004 <.004 <.011

Propargite, μg/L 74 0 <.013 <.013 <.023 <.023 <.023 40 0 <.013 <.013 <.013 <.013 <.023

Simazine, μg/L 79 14 <.005 <.005 <.005 <.005 .010 73 16 <.005 <.005 <.005 <.005 .023

Tebuconazole, μg/L 35 0 <.014 <.014 <.014 <.014 <.014 7 0 <.014 <.014 <.014 <.014 <.014

Tebuthiuron, μg/L 79 0 <.01 <.01 <.016 <.016 <.016 75 0 <.01 <.01 <.016 <.016 <.026

Tefluthrin, μg/L 35 0 <.003 <.003 <.003 <.003 <.010 7 0 <.008 <.008 <.008 <.008 <.010

Terbacil, μg/L 39 4 <.007 <.007 <.007 <.007 .017 33 0 <.007 <.007 <.007 <.007 <.007

Terbufos oxygen analog sulfone, μg/L 40 0 <.045 <.045 <.045 <.045 <.068 40 0 <.068 <.068 <.068 <.068 <.068

Terbufos, μg/L 79 0 <.013 <.013 <.017 <.017 <.017 73 0 <.013 <.013 <.017 <.017 <.017

Terbuthylazine, μg/L 40 0 <.008 <.008 <.008 <.008 <.010 40 0 <.010 <.010 <.010 <.010 <.010

Thiobencarb, μg/L 74 0 <.002 <.002 <.005 <.010 <.010 40 0 <.002 <.002 <.002 <.002 <.010

trans-Propiconazole, μg/L 35 0 <.013 <.034 <.034 <.034 <.034 7 0 <.013 <.013 <.013 <.013 <.013

Triallate, μg/L 39 0 <.001 <.001 <.001 <.001 <.002 33 0 <.001 <.001 <.001 <.001 <.001

Tribuphos, μg/L 35 0 <.004 <.035 <.035 <.035 <.035 7 0 <.004 <.004 <.004 <.004 <.004

Trifluralin, μg/L 79 0 <.002 <.002 <.009 <.009 <.009 73 0 <.002 <.002 <.009 <.009 <.009

Polar pesticide compounds

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T), μg/L 28 0 <.035 <.035 <.035 <.035 <.035 0 -- -- -- -- -- -- 

2,4-Dichlorophenoxyacetic acid (2,4-D) methyl ester, μg/L 0 -- -- -- -- -- -- 20 0 <.016 <.016 <.016 <.016 <.016

2,4-D, μg/L 28 <.035 <.035 <.035 <.035 <.035 20 1 <.038 <.038 <.038 <.038 .009

4-(2,4-Dichlorophenoxy)butyric acid (2,4-DB), μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.02 <.02 <.02 <.02 <.020
86 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Polar pesticide compounds—Continued 2-Chloro-6-ethyl-amino-4-amino-s-triazine (CEAT), μg/L 0 -- -- -- -- -- -- 20 6 <0.08 <0.08 <0.08 0.0035 0.009 2-Hydroxy-4-isopropylamino-6-ethylamino-s-triazine (OIET), μg/L 0 -- -- -- -- -- -- 20 3 <.032 <.032 <.032 <.032 .007 2-Methyl-4,6-di-nitro-phenol (DNOC), μg/L 28 0 <0.035 <0.035 <0.035 <0.035 <0.035 0 -- -- -- -- -- -- 3-Hydroxy carbofuran, μg/L 28 0 <.014 <.014 <.014 <.014 <.014 20 0 <.008 <.008 <.008 <.008 <.008 Acifluorfen, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.028 <.028 <.028 <.028 <.028 Aldicarb sulfone, μg/L 28 0 <.016 <.016 <.016 <.016 <.016 20 0 <.018 <.018 <.018 <.018 <.018 Aldicarb sulfoxide, μg/L 28 0 <.021 <.021 <.021 <.021 <.021 20 0 <.022 <.022 <.022 <.022 <.022 Aldicarb, μg/L 28 0 <.016 <.016 <.016 <.016 <.016 20 0 <.04 <.04 <.04 <.04 <.04 Bendiocarb, μg/L 0 -- -- -- -- -- -- 20 0 <.02 <.02 <.02 <.02 <.020 Benomyl, μg/L 0 -- -- -- -- -- -- 20 0 <.022 <.022 <.022 <.022 <.022 Bensulfuron-methyl, μg/L 0 -- -- -- -- -- -- 20 0 <.018 <.018 <.018 <.018 <.018 Bentazon, μg/L 28 0 <.014 <.014 <.014 <.014 <.014 20 0 <.012 <.012 <.012 <.012 <.012 Bromacil, μg/L 33 0 <.035 <.035 <.035 <.035 <.5 37 0 <.018 <.018 <.018 <.5 <.5 Bromoxynil, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.028 <.028 <.028 <.028 <.028 Caffeine, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 37 4 <.018 <.018 <.5 <.5 .009 Carbaryl, μg/L 28 0 <.008 <.008 <.008 <.008 <.008 20 0 <.018 <.018 <.018 <.018 <.018 Carbofuran, μg/L 28 0 <.028 <.028 <.028 <.028 <.028 20 0 <.016 <.016 <.016 <.016 <.016 Chloramben methyl ester, μg/L 28 0 <.011 <.011 <.011 <.011 <.011 20 0 <.024 <.024 <.024 <.024 <.024 Chlorimuron, μg/L 0 -- -- -- -- -- -- 20 0 <.032 <.032 <.032 <.032 <.032 Chlorodiamino-s-triazine (CAAT), μg/L 0 -- -- -- -- -- -- 20 0 <.04 <.04 <.04 <.04 <.04 Chlorothalonil, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.035 <.035 <.035 <.035 <.035 Clopyralid, μg/L 28 0 <.05 <.05 <.05 <.05 <.05 20 0 <.024 <.024 <.024 <.024 <.024 Cycloate, μg/L 0 -- -- -- -- -- -- 20 0 <.014 <.014 <.014 <.014 <.014 Dacthal monoacid, μg/L 28 0 <.017 <.017 <.017 <.017 <.017 20 0 <.028 <.028 <.028 <.028 <.028 Dicamba, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.036 <.036 <.036 <.036 <.036 Dichlobenil, μg/L 28 0 <.02 <.02 <.02 <.02 <.02 0 -- -- -- -- -- -- Dichlorprop, μg/L 28 0 <.032 <.032 <.032 <.032 <.032 20 0 <.028 <.028 <.028 <.028 <.028 Dinoseb, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.038 <.038 <.038 <.038 <.038 Diphenamid, μg/L 0 -- -- -- -- -- -- 20 0 <.01 <.01 <.01 <.01 <.010 Diuron, μg/L 28 0 <.02 <.02 <.02 <.02 <.02 20 0 <.015 <.015 <.015 <.015 <.015 Esfenvalerate, μg/L 28 0 <.019 <.019 <.019 <.019 <.019 0 -- -- -- -- -- -- 

Fenuron, μg/L 28 0 <.013 <.013 <.013 <.013 <.013 20 0 <.019 <.019 <.019 <.019 <.019
Table 4 

87

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

Unconfined Confined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum

Polar pesticide compounds—Continued

Flumetsulam, μg/L 0 -- -- -- -- -- -- 20 0 <0.04 <0.04 <0.04 <0.04 <0.040Fluometuron, μg/L 28 0 <0.035 <0.035 <0.035 <0.035 <0.035 20 0 <.016 <.016 <.016 <.016 <.016

Imazaquin, μg/L 0 -- -- -- -- -- -- 20 0 <.036 <.036 <.036 <.036 <.036

Imazethapyr, μg/L 0 -- -- -- -- -- -- 20 0 <.038 <.038 <.038 <.038 <.038

Imidacloprid, μg/L 0 -- -- -- -- -- -- 20 0 <.02 <.02 <.02 <.02 <.020

Linuron, μg/L 28 0 <.018 <.018 <.018 <.018 <.018 20 0 <.014 <.014 <.014 <.014 <.014

2-Methyl-4-chlorophenoxyacetic acid (MCPA), μg/L 28 0 <.05 <.05 <.05 <.05 <.05 12 0 <.03 <.03 <.03 <.030 <.030

4-(4-Chloro-2-methylphenoxy) butanoic acid (MCPB), μg/L 28 0 <.035 <.035 <.035 <.035 <.035 20 0 <.01 <.01 <.01 <.01 <.010

Metalaxyl, μg/L 5 0 <.012 <.012 <.012 <.5 <.5 37 0 <.5 <.5 <.5 <.5 <.5

Methio-arb, μg/L 28 0 <.026 <.026 <.026 <.026 <.026 20 0 <.01 <.01 <.01 <.01 <.010

Methomyl, μg/L 28 0 <.017 <.017 <.017 <.017 <.017 19 0 <.02 <.02 <.02 <.02 <.020

Metsulfuron, μg/L 0 -- -- -- -- -- -- 19 1 <.025 <.025 <.025 <.025 .035N-(4-Chlorophenyl)-N’-methyl urea, μg/L 0 -- -- -- -- -- -- 19 0 0 <.036 <.036 <.036 <.036

Neburon, μg/L 28 0 <.015 <.015 <.015 <.015 <.015 19 0 0 <.012 <.012 <.012 <.012

Nicosulfuron, μg/L 0 -- -- -- -- -- -- 19 0 0 <.04 <.04 <.04 <.04

Norflurazon, μg/L 28 0 <.024 <.024 <.024 <.024 <.024 19 0 0 <.02 <.02 <.02 <.020

Oryzalin, μg/L 28 0 <.019 <.019 <.019 <.019 <.019 19 0 0 <.012 <.012 <.012 <.012

Oxamyl, μg/L 28 0 <.018 <.018 <.018 <.018 <.018 19 0 0 <.03 <.03 <.03 <.030

Picloram, μg/L 28 0 <.05 <.05 <.05 <.05 <.05 19 0 0 <.032 <.032 <.032 <.032

Propham, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 19 0 0 <.03 <.03 <.03 <.030

Propiconazole, μg/L 0 -- -- -- -- -- -- 19 0 0 <.01 <.01 <.01 <.010

Propoxur, μg/L 28 0 <.035 <.035 <.035 <.035 <.035 19 0 0 <.008 <.008 <.008 <.008

Siduron, μg/L 0 -- -- -- -- -- -- 19 0 0 <.02 <.02 <.02 <.020

Silvex, μg/L 28 0 <.021 <.021 <.021 <.021 <.021 0 -- -- -- -- -- -- 

Sulfometuron, μg/L 0 -- -- -- -- -- -- 19 0 <.038 <.038 <.038 <.038 <.038Terbacil, μg/L 0 -- -- -- -- -- -- 19 0 <.016 <.016 <.016 <.016 <.016Triclopyr, μg/L 28 0 <.05 <.05 <.05 <.05 <.05 19 0 <.026 <.026 <.026 <.026 <.026

Acetamide pesticide compounds

2-[(2-Ethyl-6-methylphenyl) amino]-2-oxoethane-sulfonic acid (N-

(Ethmethphen)ox ESA), μg/L

5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02

2-Chloro-N-(2,6-diethylphenyl)-acetamide (alachlor 2nd amide), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02

2-Chloro-N-(2-ethyl-6-methylphenyl)-acetamid (acetochlor 2nd amide), 

μg/L

5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02

3-Ketocarbofuran, μg/L 0 -- -- -- -- -- -- 20 0 <.02 <.02 <.02 <.02 <.02
88 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Acetamide pesticide compounds—Continued Acetochlor ethanesulfonic acid (acetochlor ESA), μg/L 5 0 <0.02 <0.02 <0.02 <0.02 <0.02 19 0 <0.02 <0.02 <0.02 <0.02 <0.02 Acetochlor oxanilic acid (acetochlor OA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Acetochlor sulfynilacetic acid (acetochlor SAA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Alachlor ethanesulfonic acid secondary amide (alachlor ESA SA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Alachlor ethanesulfonic acid (alachlor ESA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 2 <.02 <.02 <.02 <.02 .02 Alachlor oxanilic acid (alachlor OA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Alachlor sulfynilacetic acid (alachlor SAA), μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dechloroacetochlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dechloroalachlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dechlorodimethenamid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dechlorometolachlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dimethenamid ethanesulfonic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dimethenamid oxanilic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Dimethenamid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Flufenacet ethanesulfonic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Flufenacet oxanilic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Flufenacet, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Hydroxyacetochlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Hydroxyalachlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Hydroxydimethenamid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Hydroxymetolachlor, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Metolachlor ethanesulfonic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Metolachlor ethanesulfonic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Propachlor ethanesulfonic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Propachlor oxanilic acid, μg/L 5 0 <.02 <.02 <.02 <.02 <.02 19 0 <.02 <.02 <.02 <.02 <.02 Pesticide and wastewater indicator compounds 1,4-Dichlorobenzene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5 1-Methylnaphthalene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5 2,6-Dimethylnaphthalene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5 2-Methylnaphthalene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5 3-beta-Coprostanol, μg/L 5 0 <2.0 <2.0 <2.0 <2.0 <2.0 34 0 <2.0 <2.0 <2.0 <2.0 <2.0 3-Methyl-1H-indole, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0 3-tert-Butyl-4-hydroxyanisole, μg/L 5 0 <5.0 <5.0 <5.0 <5.0 <5.0 34 0 <5.0 <5.0 <5.0 <5.0 <5.0

4-Cumylphenol, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0
Table 4 

89

Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. 

Constituent

Unconfined Confined

Number of 

samples

Number of 

detections

Minimum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum

Pesticide and wastewater indicator compounds—Continued

4-n-Octylphenol, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

4-Nonylphenol (sum of isomers), μg/L 5 2 <5.0 <5.0 <5.0 .98 3.2 34 13 <5.0 <5.0 <5.0 .633 2.0

4-Nonylphenol dieth-oxylate (sum of isomers), μg/L 5 0 <5.0 <5.0 <5.0 <5.0 <5.0 34 0 <5.0 <5.0 <5.0 <5.0 <5.0

4-tert-Octylphenol diethoxylate, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

4-tert-Octylphenol monoethoxylate, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

4-tert-Octylphenol, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

5-Methyl-1H-benzotriazole, μg/L 5 0 <2.0 <2.0 <2.0 <2.0 <2.0 34 0 <2.0 <2.0 <2.0 <2.0 <2.0

9,10-Anthraquinone, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Acetophenone, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Acetyl hexamethyl tetrahydronaphthalene (AHTN), μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Anthracene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Benzo-[a]-pyrene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Benzophenone, μg/L 5 2 <.5 <.5 <.5 .067 .22 34 10 <.5 <.5 <.5 .015 .068

beta-Sitosterol, μg/L 5 0 <2.0 <2.0 <2.0 <2.0 <2.0 34 0 <2.0 <2.0 <2.0 <2.0 <2.0

beta-Stigmastanol, μg/L 5 0 <2.0 <2.0 <2.0 <2.0 <2.0 34 0 <2.0 <2.0 <2.0 <2.0 <2.0

Bis-phenol A, μg/L 4 0 <1.0 <1.0 <1.0 <1.0 <1.0 22 1 <1.0 <1.0 <1.0 <1.0 .14

Camphor, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 2 <.5 <.5 <.5 <.5 .006

Carbazole, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Cholesterol, μg/L 5 0 <2.0 <2.0 <2.0 <2.0 <2.0 34 0 <2.0 <2.0 <2.0 <2.0 <2.0

Cotinine, μg/L 5 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

N,N-Diethyl-meta-toluamide (DEET), μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

D-Limonene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Fluoranthene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Hexahydrohexamethylcyclopentabenzopyran (HHCB), μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Indole, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Isoborneol, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Isophorone, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Isopropylbenzene, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 2 <.5 <.5 <.5 <.5 .023

Isoquinoline, μg/L 5 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Menthol, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Methyl salicylate, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Naphthalene, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5
90 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas Table 4. Summary statistics by group for organic constituents analyzed in groundwater samples collected from the San Antonio segment of the Edwards aquifer, south-central Texas, 1996−2006—Continued. Constituent Unconfined Confined Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Num- ber of sam- ples Num- ber of detec- tions Mini- mum 25th per- centile Median 75th per- centile Maxi- mum Pesticide and wastewater indicator compounds—Continued

p-Cresol, μg/L 4 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 1 <1.0 <1.0 <1.0 <1.0 0.024

Pentachlorophenol, μg/L 0 -- -- -- -- -- -- 17 0 <2.0 <2.0 <2.0 <2.0 <2.0

Phenanthrene, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Phenol*, μg/L 4 4 .85 1.2 1.4 1.5 1.9 34 24 <.5 <.5 .36 1.0 4.0

Pyrene, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Tetrachloroethene (PCE), μg/L 4 1 <.5 <.5 <.5 <.5 .042 34 15 <.5 <.5 <.5 .051 .28

Tribromomethane (bromoform), μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 10 <.5 <.5 <.5 .024 1.9

Tributyl phosphate, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Triclosan, μg/L 4 0 <1.0 <1.0 <1.0 <1.0 <1.0 34 0 <1.0 <1.0 <1.0 <1.0 <1.0

Triethyl citrate, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Triphenyl phosphate, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Tris(2-butoxyethyl) phosphate, μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 1 <.5 <.5 <.5 <.5 .098

Tris(2-chloroethyl) phosphate (FYROL CEF), μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5

Tris(dichloroisopropyl) phosphate (FYROL FR 2), μg/L 4 0 <.5 <.5 <.5 <.5 <.5 34 0 <.5 <.5 <.5 <.5 <.5
Table 5 91

Table 5. Unconfined and confined category wells with historical water-quality data, San Antonio segment of the Edwards aquifer, 

south central Texas, 1957–2006.

[NAD 83, North American Datum of 1983; Mg, magnesium; Ca, calcium]

Category State well 

number County

Latitude 

(decimal 

degrees)

(NAD 83)

Longitude 

(decimal 

degrees)

(NAD 83)

Number of 

samples 

(one or more 

constituents)

Range 

of years 

(collected 

samples)

Mg/Ca 

(molar ratio)

Number of 

samples Median Interquartile range

Unconfined AY–68–28–205 Bexar 29.58444 -98.55722 16 1972−2004 11 0.162 0.198

Unconfined AY–68–29–109 Bexar 29.58667 -98.48806 29 1973−2006 26 .170 .377

Confined AY–68–37–705 Bexar 29.39138 -98.49678 18 1977−2004 16 .425 .075

Confined DX–68–22–901 Comal 29.63550 -98.26501 22 1974−2005 21 .227 .047

Confined DX–68–23–303 Comal 29.71361 -98.13444 27 1974−2003 26 .361 .079

Confined DX–68–23–601 Comal 29.70800 -98.14501 11 1974−2005 10 .324 .043

Confined TD–69–40–403 Medina 29.43194 -99.11611 11 1977−97 11 .211 .066

Confined TD–69–46–601 Medina 29.33218 -99.27837 32 1972−2005 29 .345 .059

Confined TD–69–47–303 Medina 29.34694 -99.13889 20 1975−2004 19 .406 .101

Confined YP–69–43–606 Uvalde 29.31111 -99.64056 30 1979−2005 28 .217 .083

Confined YP–69–45–405 Uvalde 29.32694 -99.46806 21 1979−2004 19 .324 .041

Confined YP–69–50–501 Uvalde 29.19083 -99.83167 43 1959−2004 16 .184 .035

Confined YP–69–50–506 Uvalde 29.20302 -99.79922 20 1974−2004 20 .155 .035
92 Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas

Table 6. Summary of age tracer data and interpreted apparent ages for groundwater samples collected from the San Antonio segment 

of the Edwards aquifer, south-central Texas, 1996−2006.

[Data summarized for all samples and for sample categories (shallow/urban unconfined and unconfined). CFC, chlorofluorocarbon; pg/kg, picograms per kilogram; pptv, parts per trillion per volume; yr, apparent age (year); SF6

, sulfur hexafluoride; femtoMol/kg, femtomoles per kilogram]

Constituent

All Shallow/urban unconfined

Number 

of 

samples

Number of 

detections or 

measurements 

(uncontaminated samples)

Minimum

Median Maximum

Number 

of 

samples

Number of 

detections or 

measurements 

(uncontaminated samples)

Minimum

Median Maximum

CFC−11, pg/kg 82 28 0.1 1.9 3.0 52 2 2.5 2.8 3.0

CFC−12, pg/kg 82 7 .1 1.4 2.1 52 4 .6 1.8 2.1

CFC−113, pg/kg 82 48 .01 .2 .5 52 26 .0 .3 .5

CFC−11, pptv 82 28 5.3 162.6 224.4 52 2 184.2 204.3 224.4

CFC−12, pptv 82 7 19.5 421.9 53.5 52 4 22.8 472.9 53.5

CFC−113, pptv 82 48 1.9 53.7 86.3 52 26 5.3 6.2 86.3

CFC−11, yr 28 28 1956 1980 2003 2 2 1982 1984 1986

CFC−12, yr 7 7 1955 1986 1994 4 4 1975 1988 1994

CFC−113, yr 48 48 1961 1987 1998 26 26 1969 1988 1998

CFC−11, number of years 28 28 12.9 26.4 5.5 2 2 12.9 18.9 24.9

CFC−12, number of years 7 7 9.4 2.3 51.6 4 4 9.4 13.0 32.2

CFC−113, number of years 48 48 2.8 17.6 46.0 26 26 2.8 12.6 37.7

SF6

, femtoMol/kg 22 20 .4 1.0 2.9 22 20 .4 1.0 2.9

SF6

, pptv 22 20 .9 2.0 5.8 22 20 .9 2.0 5.8

SF6

, yr 20 20 1981 1988 2006 20 20 1981 1988 2006

SF6

, number of years 20 20 1.2 18.7 26.2 20 20 1.2 18.7 26.2

Constituent

Unconfined

Number of 

samples

Number of detections 

or measurements 

(uncontaminated samples)

Minimum

Median Maximum

CFC−11, pg/kg 30 26 0.1 1.8 2.8

CFC−12, pg/kg 30 3 .1 .2 1.4

CFC−113, pg/kg 30 22 .0 .2 .3

CFC−11, pptv 30 26 5.3 151.3 223.8

CFC−12, pptv 30 3 19.5 58.0 421.9

CFC−113, pptv 30 22 1.9 49.4 79.0

CFC−11, yr 26 26 1956 1980 2003

CFC−12, yr 3 3 1955 1964 1986

CFC−113, yr 22 22 1961 1986 1992

CFC−11, number of years 26 26 2.1 27.10 5.5

CFC−12, number of years 3 3 2.3 42.73 51.6

CFC−113, number of years 22 22 14.5 2.60 46.0

SF6

, femtoMol/kg 0 -- -- -- --

SF6

, pptv 0 -- -- -- --

SF6

, yr 0 -- -- -- --

SF6

, number of years 0 -- -- -- --
Table 7 93

Table 7. Summary of model-calculated particle-track information for groundwater sample categories, San Antonio segment of the 

Edwards aquifer, south-central Texas.

[Data summarized for all samples and for sample categories (shallow/urban unconfined, unconfined, and confined). yrs, years; mi, miles; ft/d, feet per day]

Sample 

category

Number of 

wells with 

particle-track 

simulations

Conduit-flow model

Minimum 

particle-track 

travel time 

(yrs)

Mean 

particle-track 

travel time 

(yrs)

Maximum 

particle-track 

travel time 

(yrs)

Mean 

particle-track 

distance 

(mi)

Calculated 

mean flow rate 

(ft/d)

All 108−115 0.002 153.8 2,861 23.0 2.8

Shallow/urban unconfined 30 4.3 80.0 362.0 6.0 1.9

Unconfined 36−38 .002 74.3 2,526 7.4 3.2

Confined 40−49 9.0 270.1 2,861 49.2 3.2

Sample 

category

Number of 

wells with 

particle-track 

simulations

Diffuse-flow model

Minimum 

particle-track 

travel time

(yrs)

Mean 

particle-track 

travel time 

(yrs)

Maximum 

particle-track 

travel time 

(yrs)

Mean 

particle-track 

distance 

(mi)

Calculated 

mean flow rate 

(ft/d)

All 108−115 0.002 117.8 3,346 23.6 4.3

Shallow/urban unconfined 30 5.1 47.2 341.4 4.8 4.0

Unconfined 36−38 .002 68.8 3,346 7.5 4.9

Confined 40−49 11.0 197.0 1,445 47.4 4.1

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Musgrove, Fahlquist, Houston, Lindgren, and Ging—Geochemical Evolution Processes and Water-Quality Observations, San Antonio Segment of the Edwards Aquifer, Texas—SIR 2010–5129

I SBN 978- 1- 4113- 2928- 7

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