U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3142

November 2011

National Water-Quality Assessment, Transport of Anthropogenic and Natural Contaminants (TANC) to Public-Supply Wells

Assessing the Vulnerability of Public-Supply Wells to 

Contamination: Edwards Aquifer Near San Antonio, Texas

Printed on recycled paper

The Edwards aquifer in south-central Texas 

is composed primarily of limestone and 

dolostone deposited in a shallow marine 

environment; it is one of the most productive 

aquifers in the United States. Most water 

entering the Edwards aquifer originates 

from streams that drain the uplifted Edwards 

Plateau (also called the contributing zone 

or catchment area) to the west and north of 

San Antonio. As these streams flow across 

the Balcones fault zone, where porous rocks 

of the Edwards aquifer are exposed at land 

surface, much of the streamflow recharges 

the aquifer by infiltrating through fractures 

and karst features (caused by dissolution of 

bedrock). Precipitation that falls between the 

streams in the recharge zone also infiltrates 

into the aquifer, moving rapidly along karst 

features and faults. A relatively small amount 

of water flows into the Edwards aquifer from 

the adjacent and underlying Trinity aquifer. 

Once recharge reaches the Edwards aquifer, 

it typically moves downward and laterally in a southeasterly direction, into the part of the system that is overlain by rocks of the upper 

confining units. Fine-grained, clay-rich layers in the upper confining units isolate this part of the Edwards aquifer from sources of vertical 

recharge. Groundwater in the Edwards aquifer encounters high-angle faults as it moves downgradient. Some of these faults might act as an 

impediment to flow, where layers of less permeable rock have been downdropped and now abut layers of the aquifer, and some faults may 

act as conduits for flow. Groundwater flow in the confined zone turns to the northeast, toward discharge areas at large springs between 

San Antonio and Austin. Flow is parallel to the southwest-to-northeast orientation of many of the faults. In some areas, these faults have 

been widened by dissolution, creating conduits and linear zones of increased permeability through which water can move rapidly.

In addition to natural discharge at springs, discharge from the Edwards aquifer occurs as groundwater withdrawals from wells, primarily for municipal, industrial, and irrigation water supply. Annual discharge by wells more than tripled between 1939 and 2000 in the San 

Antonio region. Increased pumping has affected springflow at some springs over time, but no long-term declines in groundwater levels 

have been observed. 

The U.S. Geological Survey’s 

National Water-Quality Assessment 

(NAWQA) Program found, in studies 

from 1991 to 2001, low levels of mixtures of contaminants in groundwater 

near the water table in urban areas across 

the Nation. Contaminants were detected 

less frequently in deeper groundwater 

typically developed for public supply (Hamilton and others, 2004). The 

proximity of contaminant mixtures to 

underlying public water-supply sources, 

however, prompted the NAWQA 

This fact sheet highlights findings from the vulnerability study of a 

public-supply well field in San Antonio, 

Texas (Lindgren and others, 2011a; 

Musgrove and others, 2011). The well 

field consists of six production wells that 

tap the Edwards aquifer. Although a single well was initially selected for study, 

constraints of well-field operation made 

it necessary for samples to be collected 

from different wells within the well field 

for various components of the study. Individual wells have a pumping capacity of 

NW

Spring

Spring

Fault

EdwardsTrinity aquifer

Trinity 

aquifer

Edwards

aquifer

Edwards Plateau

Saline

Fresh

Not to scale

Upper

confining units

Recharge zone 

Confined zone

Balcones fault zone

Contributing

(Catchment area)

NE

SE

zone

EXPLANATION

Supply well Urban area

Recharge

Discharge

Generalized conduit system

Generalized groundwater flow

Intermittent stream

Perennial stream

TEXAS

Austin

San 

Antonio

Edwards 

aquifer

Program to begin intensive studies in 

2001 to assess the vulnerability of publicsupply wells to contamination. Specifically, pathways and processes by which 

contaminants reach public-supply wells 

in nine aquifer systems across the country 

are being investigated. Scientists are 

studying the processes that occur below 

land surface—whereby contaminants 

are mobilized or attenuated—as well as 

investigating how human activities can 

affect the vulnerability of public-supply 

wells to contamination. 
Study Design

Computer models of groundwater flow and transport were constructed at a regional scale 

(about 16,185 square miles) to estimate the “zone of contribution” to one of the public-supply wells (W3) in the selected well field (Lindgren and others, 2011b). The zone of contribution is the three-dimensional volume of the aquifer material through which groundwater 

flows from the time it enters the groundwater system at the water table in the recharge 

zone until it eventually discharges at the public-supply well. A total of six monitoring wells 

were installed at different depths in two well clusters along general directions of flow to 

W3 to understand groundwater movement and geochemical processes close to the well. 

Water samples were collected from the six monitoring wells, W3, and additional publicsupply wells in the well field during 2007 through 2009. Samples were analyzed for naturally 

occurring contaminants (such as uranium and arsenic) and anthropogenic contaminants 

(such as VOCs, pesticides, and nutrients). Other water-quality constituents (such as major 

ions, age-dating tracers, and selected isotopes) were analyzed for to improve understanding of sources of water and reactions affecting the chemical composition of groundwater. 

Analytical results were compared to regional water quality evaluated at 39 public-supply 

wells in 2004 and 2005 (Musgrove and others, 2011). Additional water samples were collected from public-supply well W4 in the well field at three depth intervals that correspond 

to three zones of high flow in the well (shallow, 380 feet; intermediate, 430 feet; and deep, 

540 feet) under three different pumping conditions to determine where water and contaminants enter the well and to assess the effect of pumping on water quality (Musgrove and 

others, 2011). To assess short-term temporal variations in water quality, selected monitoring 

wells and W4 were sampled several times in response to a rainfall and recharge event. 

Finally, a detailed, local-scale (1,640-square-mile) computer model of steady-state groundwater flow and transport to W4 was constructed and calibrated to water levels, tritium 

concentrations, and spring discharge (Lindgren and others, 2011a). On the basis of existing 

information and preliminary flow measurements, two regionally extensive horizontal zones 

were included in the model that more readily transmit water than the surrounding aquifer 

materials—one in the leached and collapsed members of the Edwards aquifer and one in 

the Kirschberg evaporite member (fig. 1). These highly transmissive zones represent fast 

flow paths created by dissolution of bedrock. The local-scale model was used to calculate 

the traveltime of water particles entering W4 and to forecast water-quality changes that 

might be expected if a contaminant were released to the water table in the area contributing recharge to this well (Musgrove and others, 2011). 

about 8,000 to 14,000 gallons per minute; 

typically, one or two wells are pumped 

at a time, yielding an average of 20–21 

million gallons per day from the well 

field. In construction and operation, the 

wells studied are representative of many 

public-supply wells that tap the Edwards 

aquifer across the region (although not 

all wells in the region produce as much 

water as those in the selected well field). 

Land use in the study area is dominantly 

rangeland and forest, with a lesser 

amount of urban/built-up land (mostly in 

the San Antonio metropolitan area) and 

minor amounts of agriculture. Increasing population is causing ongoing urban 

growth.

Samples collected by USGS personnel in 2004 and 2005 from a publicsupply well in the selected well field 

contained some constituents of concern 

for drinking-water quality, including 

nitrate; the pesticide compounds atrazine, deethylatrazine, and simazine; and 

the volatile organic compounds (VOCs) 

tetrachloroethene (also known as perchloroethene, or PCE), chloroform, bromoform, and dibromochloromethane. These 

constituents were detected in untreated 

water at concentrations much less than 

established drinking-water standards, 

where such standards exist. Uranium, 

radon, and arsenic also were detected, but 

at concentrations that were not of concern 

with respect to drinking-water standards.

Most of the constituents of concern 

detected in the selected well field are 

anthropogenic (introduced by human 

activity). A database of potential sources 

of contaminants (Texas Commission on 

Environmental Quality, 2006) indicates 

that most potential sources of atrazine, 

PCE, and chloroform in the study area 

are in the San Antonio urban area. Potential sources of atrazine include sites of 

manufacture, sales, or application of pesticides to lawns and commercial properties. PCE is commonly used as a solvent 

in industrial and commercial facilities 

and at dry cleaners. Chloroform, bromoFigure 1. Stratigraphy (rock layers) of 

the Edwards aquifer in the San Antonio 

area (modified from Maclay, 1995).

The vortex in this stream is caused by rapid 

drainage of water through a 2-foot-diameter 

karst feature in the streambed. Streams crossing the recharge zone of the Edwards aquifer 

lose much of their flow as water rapidly 

infiltrates to the water table along dissolution 

features such as this one. (Photo by David 

Johns, City of Austin, used with permission.) EDWARDS GROUP

GEORGETOWN

FORMATION

PERSON FORMATION KAINER FORMATION

Cyclic and

marine members

(undivided)

Leached and

collapsed members

(undivided)

Regional dense

member

Grainstone member

Kirschberg

evaporite member

Dolomitic member

Basal nodular member

Erosional hiatus

form, and dibromochloromethane can be 

formed as byproducts of drinking-water 

chlorination. Publicly supplied water 

containing chlorination byproducts may 

enter the groundwater system by way 

of leaking pipes, lawn irrigation, or use 

of treated water as a supply-well pump 

lubricant. Other sources of chloroform 

include metal plating and some manufacturing facilities. Nitrate may come 

from human sources (including fertilizers, septic system leachate, and leaking 
sewerlines) and natural sources (such as 

nitrogen in soil and animal waste) in the 

study area. Uranium, radon, and arsenic 

occur naturally in the rocks that make up 

the aquifer in the area. 

Overall, the study findings point to 

four primary factors that affect the movement and fate of contaminants and the 

vulnerability of the public-supply well 

field in San Antonio, Texas: (1) groundwater age (how long ago water entered, or 

recharged, the aquifer), (2) fast pathways 

for flow of groundwater through features 

formed or enlarged by dissolution of 

bedrock, (3) recharge characteristics of 

the aquifer, and (4) natural geochemical 

processes within the aquifer. Study findings are intended to help water managers, 

drinking-water suppliers, and policymakers understand how and why contamination of public-supply wells occurs and 

anticipate whether water quality will 

improve or degrade. Additionally, study 

findings may be used in evaluating various 

aquifer-development and land-management scenarios.

Well Vulnerability Results from 

Very Young Age of Groundwater 

Groundwater age refers to the 

elapsed time since water entered an aquifer at the water table. Because water in 

an aquifer typically flows downward and 

laterally over time, it is expected that the 

very youngest water will be found near 

the water table and older water will be 

found at greater depths. Groundwater age 

can be estimated by measuring concentrations of chemical tracers. Ratios of two 

such age tracers—tritium and its decay 

product, helium-3—were measured at 

five monitoring wells, at four well-field 

public-supply wells, and in samples from 

shallow, intermediate, and deep depths in 

supply well W4 (fig. 2). Results indicate 

that groundwater in the well field and 

about 1.5 miles upgradient is generally 

young, recently recharged water (<1 to 41 

years old) throughout most of the thickness of the aquifer. Unlike most aquifer 

settings, water age does not increase with 

depth near the well field.

Tritium:helium-3 ratios yield an 

“apparent” groundwater age that is most 

reliable for samples that are collected 

from a discrete depth interval in the 

aquifer (for example, from wells with 

a very short well screen). However, the 

long open interval of a public-supply well 

(about 500 feet in the selected well field) 

typically captures a mixture of water that 

recharged at different times. For such 

wells, some of the water entering the well 

will be younger and some of it will be 

older than what is indicated by a single, 

apparent (tracer-based) age estimate. To 

better understand the blend of water of 

different ages produced by the publicsupply well, a computer-model simulation of groundwater flow and transport 

was used to estimate the traveltime (or 

age) of water particles entering publicsupply well W4 along the length of the 

open interval. Modeled findings show 

that the simulated ages of particles reaching the well range from less than 1 day to 

342 years (fig. 3), with a mean age of 

4 years. Insight into well vulnerability 

can be gained from considering both the 

longest and shortest traveltimes to the 

well. Less than 1 percent of the water 

entering the well is 50 or more years 

old; water in this age range recharged 

the aquifer before the use of manmade 

chemicals became prevalent. Such a 

small percentage of old groundwater 

offers little potential for dilution of young 

water containing contaminants from 

human activities, should changes in land 

use degrade the quality of water recharging the aquifer in the future. 

A more immediate vulnerability is 

indicated by the shortest traveltimes to 

the well. Figure 3 shows that almost half 

of the water reaching the public-supply 

well is less than 2 years old. Such a large 

percentage of very young water indicates 

that (a) contaminants entering the aquifer 

may be transported rapidly to the well, (b) 

there is limited time for chemical reactions 

to occur in the aquifer that may attenuate 

contaminants, and (c) should recharge 

water become contaminated with pathogenic microorganisms (which have limited 

survival times in aquifers), the microorganisms may be able to persist to the well. 

Karst Features Serve as Fast 

Pathways for Movement of 

Contaminants to the PublicSupply Well 

Geophysical logging and flowmeter 

measurements in public-supply well W4 

in the well field and in nearby monitoring wells showed that most of the flow 

volume into and out of the wells occurs 

in three horizontal zones that correspond 

to regional stratigraphic units (fig.1)—a 

shallow zone at the top of the cyclic and 

marine members, an intermediate zone in 

the leached and collapsed members, and 

a deep zone in the Kirschberg evaporite 

member. These high-flow zones are 

likely areas where karst dissolution has 

enlarged bedding planes. Flow rates are 

also higher in these zones. The mean 

simulated particle age for water entering 

the well through the high-flow zones is 

Figure 2. The colored zones on the map show the simulated areas contributing recharge 

to a public-supply well selected for investigation in San Antonio, Texas. These colored zones 

indicate where water that eventually reaches the public-supply well enters the groundwater 

system at the water table. 

98°50' 98°40'

29°36' 29°30'

Public-supply well 

(W4)

CATCHMENT AREA

RECHARGE ZONE

CONFINED ZONE

MEDINA 

BEXAR 

BANDERA 

MEDINA 

San Antonio

Salado Cr

Medina R

San Antonio

Medina

Lake

0

4 MILES

2

0

4 KILOMETERS

2

EXPLANATION

Areas contributing recharge

 

Simulated particle

 pathlines for 

 selected public- 

 supply well

 

 Traveltime, in years

 0.00 to 1.99

 2.00 to 4.99

 5.00 to 9.99

 10.00 to 49.99

 >50.00
3.8 years, whereas the mean simulated 

particle age for water entering the well 

through the aquifer matrix is 9.6 years. 

Rapid flow of water through 

dissolution-enlarged pathways is further 

evidenced by fluctuations in groundwater chemistry that can be correlated to 

individual precipitation events. With rain, 

concentrations of nitrate in public-supply 

well W4 decrease as they are diluted by 

recharging stream water. Conversely, 

concentrations of PCE increase (at the 

selected well field), likely because of 

accelerated movement of water from an 

upgradient source to the well.

Simulated traveltimes in the area 

contributing recharge to the public-supply 

well are shown in figure 2. The colored 

zones on the map indicate where water 

that eventually reaches the public-supply 

well enters the groundwater system at the 

water table. Although the exact locations 

of these zones are unknown due to uncertainty in the location and connectivity of 

sinkholes, conduits, and other solution 

features, the simulated pattern of recharge 

is realistic given that such features allow 

most of the water reaching the well to 

travel rapidly from the recharge zone 

to the supply well along fast pathways 

rather than through the aquifer matrix. 

Simulated recharge to the well originates 

as far as 16 miles from the well, yet most 

of the water reaches the well in less than 

5 years (fig. 3).

Aquifer Recharge Characteristics 

Affect Movement of Contaminants 

to the Public-Supply Well

Analysis of water samples collected 

from shallow, intermediate, and deep 

zones of the Edwards aquifer at publicsupply well W4 in the well field and from 

nearby monitoring wells revealed that 

water in the vicinity of the selected well 

field is notably well mixed throughout the 

sampled thickness of the Edwards aquifer, 

showing little of the chemical variation 

with depth that is commonly seen in other 

aquifers. Contaminants were found at all 

depths, and they did not enter the well 

through a specific horizon. Thus, although 

flowmeter measurements in the publicsupply well indicated that pumping of 

different wells or combinations of wells 

in the well field affects zones or horizons 

from which the largest volumes of water 

enter the well (Musgrove and others, 

2011), concurrent water-quality sampling 

documented that this does not appreciably affect contaminant concentrations 

discharged from the well. 

The well-mixed nature of the 

Edwards aquifer is caused by the recharge 

characteristics of the area combined with 

fast flow paths through karst features: 

(1) recharge enters the Edwards aquifer 

directly, without infiltrating through overlying, chemically dissimilar rock units 

or thick soils (soils in the recharge zone 

are very thin); (2) recharging water (and 

any contaminants present in this water) 

can travel to depth in the aquifer rapidly 

through preferential flow paths, such as 

dissolution-enlarged faults, fractures, and 

bedding planes; and (3) the large volume 

of recharge (due to ongoing streamflow 

contribution combined with periodic precipitation) and steep hydraulic gradient 

(as much as 100 feet per mile) between 

the recharge area and the confined part 

of the aquifer causes water to flush 

rapidly through the conduits and other 

dissolution features in the aquifer. The 

intersection of high-flow zones combined 

with the higher-than-average pumping 

volumes of the selected well field likely 

induces more mixing and perhaps shorter 

traveltimes than are found elsewhere in 

the aquifer. This is suggested by a greater 

proportion of old water in some regional 

wells than was found in the studied 

wells (as simulated with the local-scale 

flow model), as well as by increased 

ratios of magnesium:calcium and 

strontium:calcium with depth in regional 

wells that were not seen in the studied 

wells. These higher ratios indicate greater 

contact time of groundwater with aquifer 

rocks along deeper, longer flow paths.

Natural Geochemical Processes 

in the Aquifer Affect the Fate of 

Contaminants 

Geochemical conditions, such as the 

oxygen content of groundwater, control 

whether specific contaminants are attenuated or mobilized once they enter the 

groundwater system. In the oxic conditions found throughout the Edwards aquifer (dissolved oxygen concentrations of 

at least 1 mg/L (milligram per liter)), the 

anthropogenic contaminants nitrate, PCE, 

chloroform, bromoform, and dibromochloromethane will not readily degrade 

and can persist over long distances. Other 

contaminants are attenuated in the oxic 

conditions that are present throughout 

the aquifer. Atrazine degrades to deethylatrazine, but the detection of both compounds in the studied public-supply wells 

indicates that short traveltimes relative to 

atrazine degradation rates in the aquifer 

do not allow for complete degradation of 

the pesticide before it reaches the publicsupply wells. 

.

SIMULATED AGE, IN YEARS

PERCENTAGE OF WATER ENTERING WELL

<0.5 0.5 to <2 2-4.9 5-9.9 10-19.9 20-50 51-100 >100

0

10

20

30

40

50

Figure 3. A groundwater-flow model was used to simulate the mixture of water of different ages produced by the selected public-supply well in San Antonio. “Water age” refers to 

the elapsed time between water entering the groundwater system at the water table in the 

recharge zone and arrival at the well. The young age of groundwater entering the well from 

deep parts of the aquifer (540 feet below land surface) is unusual relative to other types of aquifers and is attributed to the way water recharges the aquifer combined with rapid travel of large 

volumes of groundwater through fractures, faults, and bedding planes enlarged by dissolution.
Implications for Aquifer Protection

Human activities in the recharge 

zone and catchment area (contributing 

zone) have the potential to affect water 

quality. Because most water enters the 

Edwards aquifer in the recharge zone, 

management of activities (such as urban 

development or industrial activities) that 

might degrade water quality in this area 

is recognized as essential for protecting 

the quality of drinking water produced by 

public-supply wells that tap the aquifer, 

especially given the short estimated 

traveltimes. Of equal importance is the 

protection of stream-water quality in 

the Edwards Plateau catchment area, 

because these streams provide most of 

the recharge to the aquifer as they flow 

across the recharge zone. The vulnerability of the Edwards aquifer to degradation of water quality was recognized 

in 1970, when the first regulations were 

implemented to protect the recharge 

zone and buffer zones (Texas Commission on Environmental Quality, 2011). 

Today, the “Edwards Aquifer Protection 

Program” is codified into State law to 

safeguard both water quality and water 

quantity in the aquifer (Texas Administrative Code, 2011). The Edwards Aquifer 

Authority, established in 1993, assists 

the Texas Commission on Environmental Quality (TCEQ) protection efforts in 

the San Antonio region. The TCEQ and 

Edwards Aquifer Authority have regulatory responsibilities regarding activities 

that could potentially affect surface- and 

recharge-water quality within the aquifer 

recharge zone and catchment area (Texas 

Commission on Environmental Quality, 2011; Edwards Aquifer Authority, 

2011). The protection programs of these 

agencies include such requirements as 

identification and protection of sensitive 

karst features, implementation of best 

management practices for mitigating 

stormwater quality, limitations for activities associated with potential pollutants, 

and requirements for well construction or 

plugging of abandoned wells. Some water 

purveyors have additional watershedprotection and (or) wellhead-protection 

measures. Although protection mechanisms are in place, data from the Edwards 

aquifer indicate that anthropogenic activities have affected water quality: nitrate 

concentrations have increased over recent 

decades, and anthropogenic contaminants 

are more frequently detected in shallow 

urban wells (Musgrove and others, 2010).

A recent study by Mahler and others (2011) (in a location about 80 miles 

northeast of the well-field study area) 

documented the degradation to groundwater quality that can occur due to rapid 

urban development in areas supplying 

recharge to the Edwards aquifer. Nitrate 

concentrations in Barton Springs (a 

groundwater discharge point in Austin) 

and the streams that provide recharge in 

the catchment area for the springs have 

increased in recent years, in part because 

of increased nitrogen loading caused by 

rapid development in the recharge zone 

(fig. 4). Isotopic analysis indicates that 

a likely source of some of the nitrate is 

human or animal waste. “Increases in 

nitrate concentrations coincide with rapid 

increases in the number of septic systems 

and volume of land application of treated 

wastewater” in the parts of the catchment 

area and recharge zone that supply water 

to Barton Springs (Mahler and others, 

2011). Although nitrate concentrations 

are still well below public drinking-water 

standards in the Barton Springs area and 

in the well-field study area, a trend of 

increasing nitrate concentration is cause 

for concern in an aquifer in which nitrate 

does not readily degrade. 

Contaminant concentrations in the 

public-supply well will respond rapidly 

to changes in land use in the recharge 

zone and catchment area. Constituents 

of concern in the Edwards aquifer for the 

long-term sustainability of the groundwater resource include the nutrient nitrate 

and anthropogenic contaminants such 

as atrazine, PCE, and chloroform. A 

scenario of hypothetical contaminant 

loading was evaluated by using results 

from groundwater-flow-model particle 

tracking to assess the response of the 

aquifer to potential contamination. Contaminant concentrations in the publicsupply well over time are a result of (1) 

the mixture of ages of water reaching the 

well combined with (2) variation in contaminant inputs at land surface over time 

related to changing land use, and (3) degradation rates in the oxic environment of 

the Edwards aquifer. The effect of these 

three factors on concentrations of a hypothetical contaminant in public-supply 

well W4 was estimated by using the age 

mixture of water produced by the well as 

simulated by the local-scale groundwater 

model (fig. 3). The input concentration 

of a hypothetical contaminant applied to 

the water table in the area contributing 

recharge to the well was held constant 

for 30 years and then ceased. Figure 5 

shows the response of the public-supply 

well to the simulation with no contaminant degradation and a simulation with 

degradation at a rate of 10 percent per 

year. Results indicate that the concentrations at the public-supply well would 

begin to respond to contaminant loading in the recharge zone within 1 year 

because of short traveltimes through fast 

flow paths. Within 10 years, contaminant 

concentrations in the public-supply well 

would be equal to 90 percent of the input 

concentration for a contaminant (such as 

nitrate) that does not degrade in the oxic 

Land application 

permits

Septic system

permits

1990 2000 2010

Catchment

area

Recharge zone

Catchment

area

Recharge zone

N

N

Figure 4. Permits for septic systems and land application of treated wastewater in the 

Barton Springs catchment area and recharge zone have increased greatly in recent years 

(modified from Mahler and others, 2011).
input

period

 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

100

0 10 20 30 40 50 60 70 80

CONCENTRATION IN SUPPLY WELL AS A 

PERCENTAGE OF INPUT CONCENTRATION

ELAPSED TIME, YEARS

No degradation

Degradation = 10 percent/year

Contacts

Sandra Eberts, Team Leader (614) 430-7740, smeberts@usgs.gov

Richard Lindgren, Groundwater-Flow Modeling (210) 691-9232, 

lindgren@usgs.gov

MaryLynn Musgrove, Geochemistry (512) 927-3522, mmusgrov@usgs.gov

For more information on the NAWQA topical study of contaminant transport to 

public-supply wells (TANC), see

http://water.usgs.gov/nawqa/

Texas Administrative Code, 2011, Title 

30, Part 1, Chapter 213—Edwards 

aquifer, accessed August 24, 2011, at 

www.sos.state.tx.us/tac/index.shtml.

Texas Commission on Environmental 

Quality, 2011, Edwards aquifer 

protection program, accessed August 

24, 2011, at http://www.tceq.texas.gov/

field/eapp.

Texas Commission on Environmental 

Quality, 2006, Potential sources of 

contamination database.

By Martha L. Jagucki, MaryLynn 

Musgrove, Richard J. Lindgren, 

Lynne Fahlquist, and Sandra M. Eberts 

conditions of the Edwards aquifer. For a 

contaminant that would degrade (such as 

atrazine), maximum concentrations at the 

supply well would eventually equal about 

70 percent of the input concentration 

(if degradation occured at the simulated 

rate). The aquifer also would respond 

quickly (less than 1 year to several years) 

to the removal of contaminant loading; 

however, it would take decades to reach 

near-background concentrations.

References 

Edwards Aquifer Authority, 2011, Policies and Rules, accessed August 24, 

2011, at http://www.edwardsaquifer.

org/display_policies_rules.php.

Hamilton, P.A., Miller, T.L., and Myers, 

D.N., 2004, Water quality in the Nation’s 

streams and aquifers—Overview of 

selected findings, 1991–2001: U.S. Geological Survey Circular 1265, 20 p. 

Lindgren, R.J., Houston, N.A., Musgrove, 

M., Fahlquist, L., and Kauffman, L.J., 

2011a, Simulations of groundwater flow 

and particle-tracking analysis in the 

zone of contribution to a public-supply 

well in San Antonio, Texas: U.S. Geological Survey Scientific Investigations 

Report 2011–5149, 93 p.

Lindgren, R.J., Houston, N.A., Musgrove, M., Fahlquist, L., and Kauffman, L.J., 2011b, Hydrogeologic setting and groundwater flow simulations 

of the south-central Texas regional 

study area, Texas, section 3 of Eberts, 

S.E., ed., Hydrogeologic settings 

and groundwater flow simulations 

for regional studies of the transport 

of anthropogenic and natural contaminants to public-supply wells: U.S. 

Geological Survey Professional Paper 

1737–B, 51 p.

Maclay, R.W., 1995, Geology and hydrology of the Edwards aquifer in the San 

Antonio area, Texas: U.S. Geological 

Survey Water-Resources Investigations 

Report 95–4186, 64 p.

Mahler, B.J., Musgrove, M., and Herrington, C., 2011, Nitrate concentrations and potential sources in the Barton Springs segment of the Edwards 

aquifer and its contributing zone, 

central Texas: U.S. Geological Survey 

Fact Sheet 2011–3035, 4 p.

Musgrove, M., Fahlquist, L., Stanton, 

G.P., Houston, N.A., and Lindgren, 

R.J., 2011, Hydrogeology, chemical 

characteristics, and water sources and 

pathways in the zone of contribution of 

a public-supply well in San Antonio, 

Texas: U.S. Geological Survey Scientific Investigations Report 2011–5146, 

194 p. 

Musgrove, M., Fahlquist, L., Houston, 

N.A., Lindgren, R.J., and Ging, P.B., 

2010, 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, 1996–2006: U.S. Geological 

Survey Scientific Investigations Report 

2010–5129, 93 p. 

Figure 5. On the basis of simulated groundwater flow and particle-tracking results, the 

response of the modeled public-supply well to the hypothetical input (over a 30-year period) 

of a contaminant in the area contributing recharge to the well was examined. Within 10 years, 

concentrations in water withdrawn from the well would be equal to about 90 percent of the 

input concentration, if the contaminant were stable in the oxic conditions of the aquifer (as is 

the case for nitrate).