U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5278

Prepared in cooperation with the San Antonio Water System

Groundwater Levels and Water-Quality Observations 

Pertaining to the Austin Group, Bexar County, Texas, 2009–11
Front cover: McAllister Park near U.S. Geological Survey station 293305098265301, San Antonio, Texas, 

March 21, 2012. 

Back cover: U.S. Geological Survey station 293305098265301, San Antonio, Texas, March 30, 2011. 
Groundwater Levels and Water-Quality 

Observations Pertaining to the Austin 

Group, Bexar County, Texas, 2009–11

By J. Ryan Banta and Allan K. Clark

Prepared in cooperation with the San Antonio Water System

Scientific Investigations Report 2012–5278

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: 2012

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

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1-888-ASK-USGS

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U.S. Government.

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:

Banta, J.R., and Clark, A.K., 2012, Groundwater levels and water-quality observations pertaining to the Austin Group, 

Bexar County, Texas, 2009–11: U.S. Geological Survey Scientific Investigations Report 2012–5278, 18 p., 2 apps. 
iii

Contents

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

Introduction ....................................................................................................................................................1

Purpose and Scope ......................................................................................................................................2

Description of the Study Area ....................................................................................................................2

Geologic Framework ...........................................................................................................................2

Hydrogeologic Characteristics ..........................................................................................................2

Data Collection Methods .............................................................................................................................4

Groundwater-Level Measurements ..................................................................................................4

Water Quality ........................................................................................................................................7

Collection and Processing of Water-Quality Samples .........................................................7

Laboratory Analysis ....................................................................................................................7

Quality Assurance .......................................................................................................................8

Groundwater Levels .....................................................................................................................................8

Water-Quality Data .....................................................................................................................................10

Summary ......................................................................................................................................................14

References ...................................................................................................................................................15

Appendixes (available at http://pubs.usgs.gov/sir/2012/5278/)

1. Daily mean groundwater-level altitudes, in feet above North American Vertical 

Datum of 1988, at sites in the study area, Bexar County, Texas, 2009–11

2. Water-quality samples and associated quality-control samples collected in the 

study area, Bexar County, Texas

Figures

1. Map showing location of data-collection sites in the study area, Bexar County, 

Texas, 2009–11 ..............................................................................................................................3

2. Map showing surficial geologic units, Edwards aquifer outcrop, and datacollection sites in the study area, Bexar County, Texas ........................................................5

3. Graphs showing A, Hydrographs of daily mean groundwater-level altitudes 

during 2009–11 of sites in the study area, Bexar County, Texas, and B, weekly 

precipitation measured at National Weather Service station number 417945 

(San Antonio International Airport) ..........................................................................................9

4. Trilinear diagrams of chemical composition for samples collected in the 

study area, Bexar County, Texas, April 2011 .........................................................................11

5. Graphs showing relations between molar ratios and strontium-87/strontium-86 

isotopic ratios for samples collected in the study area, Bexar County, Texas, 

April 2010 .....................................................................................................................................13

6. Graph showing relation of hydrogen and oxygen isotopic ratios for samples 

collected in the study area, Bexar County, Texas, April 2010 .............................................14
iv

Tables

1. Data-collection sites in the study area, Bexar County, Texas, 2009–11 .............................4

2. Summary of geologic framework and hydrogeologic characteristics of 

stratigraphic units in central Bexar County, Texas ................................................................6

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

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

(NAVD 88).

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or in micrograms per liter (μg/L).
Groundwater Levels and Water-Quality Observations 

Pertaining to the Austin Group, Bexar County, Texas, 

2009–11

By J. Ryan Banta and Allan K. Clark

Abstract 

The U.S. Geological Survey, in cooperation with 

the San Antonio Water System, examined groundwaterlevel altitudes (groundwater levels) and water-quality data 

pertaining to the Austin Group in Bexar County, Texas, 

during 2009–11. Hydrologic data collected included daily 

mean groundwater levels collected at seven sites in the study 

area. Water-quality samples were collected at six sites in 

the study area and analyzed for major ions, nutrients, trace 

elements, organic carbon, and stable isotopes. The resulting 

datasets were examined for similarities between sites as well 

as similarities to data from the Edwards aquifer in Bexar 

County, Tex. Similarities in the groundwater levels between 

sites completed in the Austin Group and site J (State well 

AY-68-37-203; hereafter referred to as the “Bexar County 

index well”) which is completed in the Edwards aquifer might 

be indicative of groundwater interactions between the two 

hydrologic units as a result of nearby faulting or conduit flow. 

The groundwater levels measured at the sites in the study area 

exhibited varying degrees of similarity to the Bexar County 

index well. Groundwater levels at site A (State well AY-68-36-

136) exhibited similar patterns as those at the Bexar County 

index well, but the hydrographs of groundwater levels were 

different in shape and magnitude in response to precipitation 

and groundwater pumping, and at times slightly offset in time. 

The groundwater level patterns measured at sites C, D, and 

E (State wells AY-68-29-513, AY-68-29-514, and AY-68-29-

512, respectively) were not similar to those measured at the 

Bexar County index well. Groundwater levels at site F (State 

well AY-68-29-819) exhibited general similarities as those 

observed at the Bexar County index well; however, there were 

several periods of notable groundwater-level drawdowns at 

site F that were not evident at the Bexar County index well. 

These drawdowns were likely because of pumping from 

the well at site F. The groundwater levels at sites H and I 

(State wells AY-68-37-205 and AY-68-29-932, respectively) 

exhibited similar patterns as those at the Bexar County 

index well (coefficient of determination [R2] of 0.99 at both 

wells), indicating there might be some degree of hydrologic 

connectivity to the Edwards aquifer. 

In general, the water-quality data indicated that the 

samples were representative of a calcium carbonate 

dominated system. The major ion chemistry and relations 

between magnesium to calcium molar ratios and 87Sr/86Sr 

isotopic ratios of samples collected from sites H and I 

indicated that the groundwater from these sites was most 

geochemically similar to groundwater collected from site B 

(State well AY-68-36-134), which is representative of 

groundwater in the Edwards aquifer. Of the sites sampled in 

this study, there appears to be varying hydrologic connectivity 

between groundwater from wells completed in the Austin 

Group and the Edwards aquifer.

Introduction

San Antonio, in Bexar County, Texas, was the seventh 

largest city in the United States in 2010, and its population 

increased 16 percent between 2000 and 2010 (U.S. Census 

Bureau, 2010). The Edwards aquifer is considered one of 

the most productive karst aquifers in the nation (Sharp and 

Banner, 1997), and it is the primary source of drinking 

water for San Antonio and the surrounding communities. 

Because of increases in population and water demands, water 

resource managers seek to gain a better understanding of the 

hydrogeologic characteristics of water-bearing units in the 

greater San Antonio area and of possible relations these units 

might have to the Edwards aquifer. One such hydrogeologic 

unit is the Austin Group, commonly referred to as the “Austin 

Chalk.” 

The Austin Group is considered a confining unit of the 

Edwards aquifer (Maclay and Small, 1984) and is geologically 

separate from the Edwards Group (which comprises the 

Edwards aquifer). There is historical and anecdotal evidence 

that the Austin Group might have some hydrogeologic 

connectivity to the Edwards aquifer through faults, fractures, 

or caves (Small, 1986; Clark, 2003). In some locations, wells 

completed in the Austin Group can produce water at rates of 

more than 500 gallons per minute, whereas at other locations, 

wells completed in the Austin Group produce only enough 

water for domestic and livestock needs (Arnow, 1963). Garza 
2 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

(1962, p. 9) reported that many of the wells completed in the 

Austin Group yielded only small quantities of water with 

hydrogen sulfide gas in “objectionable quantities,” but also 

noted that in some areas, wells completed in the Austin Group 

may produce moderate quantities of water “similar in chemical 

quality” to groundwater in the Edwards aquifer. At the time 

of this study, there were few data available with which to 

characterize the hydrogeologic properties of the Austin Group 

in Bexar County, Tex. The U.S. Geological Survey (USGS), 

in cooperation with the San Antonio Water System, completed 

a reconnaissance-level study to measure groundwater-level 

altitudes and collect water-quality samples pertaining to the 

Austin Group in Bexar County, Tex., during 2009–11.

Purpose and Scope

This report documents a reconnaissance-level assessment 

during 2009–11 of daily mean groundwater-level altitudes and 

selected water-quality data pertaining to the Austin Group, in 

Bexar County, Tex. Water-quality data included measurements 

of physical properties, major ions, nutrients, trace elements, 

organic carbon, and stable isotopes. The groundwater level 

and water quality data were examined for similarities between 

sites as well as to data collected from the Edwards aquifer in 

Bexar County, Tex., to assess possible relations between the 

Austin Group and the Edwards aquifer. 

Description of the Study Area

The study area covers approximately 377 square miles 

within the Balcones fault zone in central Bexar County, Tex. 

(fig. 1; table 1). The study area ranges from the approximate 

southern extent of the surficial expression of the Edwards 

Group (outcrop of the Edwards aquifer; fig. 2) in the north, to 

the borders of Bexar County in the west and east, and to U.S. 

Highway 90 and Interstate 10 in the south (fig. 1). The climate 

in the study area is subtropical subhumid (Larkin and Bomar, 

1983). The average annual rainfall during 1971–2000 at the 

San Antonio International Airport (National Weather Service 

station 417945) was approximately 32.9 inches per year (in/

yr) (National Oceanic and Atmospheric Administration, 2012). 

Rainfall was approximately 30.7, 37.4, and 17.6 in/yr during 

2009, 2010, and 2011, respectively (National Oceanic and 

Atmospheric Administration, 2012).

Geologic Framework

The Austin Group is an Upper Cretaceous Age marine 

sediment consisting primarily of chalk and interbedded marls 

(Koger, 1981; Small, 1986; Hanson and Small, 1994; Maclay, 

1995), which unconformably overlies the Eagle Ford Group 

(Martinez, 1982), and is one of a series of geologic formations 

that make up the confining units of the Edwards aquifer in 

central Bexar County, Tex. (figs. 1, 2; table 2). The confining 

units of the Edwards aquifer include the Georgetown 

Formation, Del Rio Clay, Buda Limestone, Eagle Ford Group, 

Austin Group, Taylor Group (Anacacho Limestone and Pecan 

Gap Chalk), and Navarro Group (table 2).

The Austin Group is subject to structural controls 

associated with the Balcones fault zone (Hill and Vaughn, 

1898; Arnow, 1963; Maclay and Land, 1988). The Balcones 

fault zone is an en echelon, normal fault system (George, 

1952), with fault blocks typically downthrown to the southeast 

toward the coast. The Balcones fault zone trends southwest 

to northeast, but a smaller set of secondary or cross-faults 

trend southeast to northwest (Clark, 2005). Many of the 

faults in the Balcones fault zone are not single, sharp breaks 

as shown on geologic maps but are actually shatter zones 

(Arnow, 1959). Secondary fractures, which resulted from the 

faulting, are generally at strikes between 45 and 145 degrees 

from the primary fault (Clark, 2005). Additionally, some of 

the faults might not result in topographic relief, partly because 

the rocks on both sides of the fault have similar weathering 

characteristics, and possibly because the rate of movement 

was approximately the same as the rate of erosion (Stein 

and Ozuna, 1995). Dissolution associated with preferential 

groundwater flow paths might enhance the permeability of 

fault and fracture systems, including solution enlargement 

at scales from individual fractures to cave networks (Ferrill, 

2004). Where these openings reach the surface or intersect 

other pathways such as faults or sinkholes that extend to the 

surface, they provide paths for surface water infiltration, or 

exchange of water between formations (Maclay, 1995). An 

example of solution-enlarged fractures in the Austin Group 

within the study area is Robber Baron Cave (fig. 1), which 

has over 4,000 feet (ft) of mapped cave networks (Veni, 

1988). In another example, Ewing (1996) reported numerous 

fractures and joints were observed at the Alamo Cement 

Quarry (fig. 1). The fractures and joints, spaced 0.5 to 8-ft 

apart and up to 40-ft long, were determined to be the result of 

geologic factors and not of blasting during quarry processes 

(Ewing, 1996).

Hydrogeologic Characteristics

Water entering the Austin Group in Bexar County, Tex., 

is likely derived from multiple sources, including direct 

infiltration of precipitation and infiltration along stream 

channels where the Austin Group outcrops. Because the Austin 

Group is generally categorized as a confining unit of the 

Edwards aquifer and not as a separate water-bearing aquifer, 

the term “Austin Group” will hereinafter be used to describe 

both the geologic framework and hydrologic characteristics 

of the formation. The Edwards aquifer, which is comprised 

of the Edwards Group and underlies the Austin Group in 

Bexar County, Tex., is similarly recharged from surface water 

infiltrating where the Edwards Group outcrops, including 

outcrops west of the study area (Ashworth and Hopkins, 

1995). 
Description of the Study Area 3

Figure 1. Location of data-collection sites in the study area, Bexar County, Texas, 2009–11 (modified from Pedraza and Shah, 2010).

 San Antonio

International

Airport

Mitchell

Lake

Calaveras

 Lake

Braunig

Lake

¦10

¦410

¦10

¦37

¦35

¦35

¦37

1604

1604

£¤281

£¤90

£¤181

£¤87

San 

Antonio

San Antonio River

Medina River

Cibolo

Creek

Calaveras Creek

BEXAR COUNTY

ATASCOSA COUNTY

MEDINA COUNTY

WILSON COUNTY

KENDALL

COUNTY

BANDERA

COUNTY

GUADALUPE

COUNTY

COMAL

COUNTY

 Alamo Cement

Quarry

 Robber Baron 

Cave

 McAllister Park

98°10'

98°20'

98°30' 98°40'

29°40'

29°30'

29°20'

29°10'

0 2.5 5 10 MILES

0 2.5 5 10 KILOMETERS

7.5

7.5

Edwards 

aquifer

outcrop

BEXAR

COUNTY

Study area

TEXAS

BEXAR COUNTY

BALCONES 

FAULT ZONE

Edwards 

aquifer

outcrop

Base modified from U.S. Geological Survey (USGS) digital data, 1:24,000

USGS National Map Hillshade

Universal Transverse Mercator projection, zone 14

North American Datum of 1983

! !

!

!

!

!

!

a

!!

!

F

C

D

K

I

B

E

J

G

H

A

EXPLANATION

!

City of San Antonio

Study area boundary

Well and site identifier—Table 1

National Weather Service station 

 and site identifier—Table 1

A

K a
4 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

Data Collection Methods

Though many wells in the study area penetrate the 

Austin Group, the majority of wells were completed in a 

deeper formation, such as the Edwards Group; hence, the 

wells used in this study represent wells completed in the 

Austin Group (that is, did not penetrate deeper formations) 

that were available for sampling. Daily mean groundwaterlevel altitudes were reported by the USGS in seven wells 

completed in the Austin Group (State wells AY-68-36-136 [site 

A], AY-68-29-513 [site C], AY-68-29-514 [site D], AY-68-

29-512 [site E], AY-68-29-819 [site F], AY-68-37-205 [site 

H], and AY-68-29-932 [site I]) (fig. 1; table 1; appendix 1). 

Groundwater levels reported by the Edwards Aquifer 

Authority (2012) for the Bexar County index well (State well 

AY-68-37-203 [site J]) were used for comparison purposes 

(fig. 1; table 1; appendix 1). The Bexar County index well is 

also commonly referred to as the “J-17 index well” (Edwards 

Aquifer Authority, 2012). Water-quality samples collected 

from six sites in the study area (State well AY-68-36-134 [site 

B], State well AY-68-37-133 [site G], and sites E, F, H, and I) 

(fig. 1, table 1; appendix 2) were analyzed for major ions, 

nutrients, trace elements, and stable isotopes. Note that sites 

E, F, G, H, and I were completed in the Austin Group, whereas 

site B is completed in the Edwards aquifer. All measured 

constituents are presented in appendix 2 for completeness. 

Precipitation data were compiled from the National Weather 

Service cooperative station number 417945 San Antonio 

International Airport during 2009–11 (fig. 1; table 1). 

Groundwater-Level Measurements

Groundwater-level measurements, reported as depth 

to water (in feet) below the land surface, were made every 

15 minutes with a pressure transducer and transferred to the 

USGS National Water Information System (NWIS) database 

(U.S. Geological Survey, 2012). The groundwater-level 

data were verified and corrected as necessary by periodic 

measurements by using a graduated electric tape (Freeman 

and others, 2004; Cunningham and Schalk, 2011). Daily 

mean groundwater-level altitudes (hereinafter referred to as 

“groundwater levels”) were calculated by subtracting the 

daily mean depth to water below the land surface from the 

land-surface datum (the elevation of the land surface above 

North American Datum 1988 [NAVD88]). Sites F, H, and I 

had a previously installed pump, whereas sites A, C, D, and 

E did not. Groundwater pumping at sites F, H, and I were for 

domestic or commercial purposes. Periods of pumping were 

not known, so corrections for pumping were not made to the 

groundwater-level records. 

Table 1. Data-collection sites in the study area, Bexar County, Texas, 2009–11.

[dd, degrees; mm, minutes; ss, seconds; WL, groundwater-level altitude; QW, water-quality sample; --, not applicable]

Site 

identifier

(fig. 1)

U.S. Geological 

Survey

station number

State well 

number

Latitude

(dd mm ss)1

Longitude

(dd mm ss)1

Elevation2

(feet)

Geologic 

unit of well 

completion

Type of 

data

Period of 

water-level record 

used in report

A 292938098370901 AY-68-36-136 29° 29′ 38′′ 98° 37′ 09′′ 816 Austin Group WL Aug. 2009–Dec. 2011

B 292943098354403 AY-68-36-134 29° 29′ 43′′ 98° 35′ 44′′ 848 Edwards Group QW --

C 293314098271801 AY-68-29-513 29° 33′ 14′′ 98° 27′ 18′′ 747 Austin Group WL July 2009–Dec. 2011

D 293332098265301 AY-68-29-514 29° 33′ 32′′ 98° 26′ 53′′ 746 Austin Group WL July 2009–Dec. 2011

E 293305098265301 AY-68-29-512 29° 33′ 05′′ 98° 26′ 53′′ 766 Austin Group WL, QW July 2009–Dec. 2011

F 293040098271301 AY-68-29-819 29° 30′ 40′′ 98° 27′ 13′′ 799 Austin Group WL, QW July 2009–Dec. 2011

G 292959098285002 AY-68-37-133 29° 29′ 59′′ 98° 28′ 50′′ 712 Austin Group QW --

H 292910098272201 AY-68-37-205 29° 29′ 10′′ 98° 27′ 22′′ 804 Austin Group WL, QW Aug. 2009–Dec. 2011

I 293228098233301 AY-68-29-932 29° 32′ 28′′ 98° 23′ 33′′ 763 Austin Group WL, QW Aug. 2009–Dec. 2011

J -- AY-68-37-2033 29° 28′ 44′′ 98° 25′ 57′′ 731 Edwards Group WL July 2009–Dec. 2011

Site

identifier

National Weather 

Service station 

number

National Weather 

Service station name

Latitude

(dd mm ss)

Longitude

(dd mm ss)

Elevation

(feet)

Type of 

data

Period of record 

used in report

K 417945 San Antonio International Airport 29° 32′ 38′′ 98° 28′ 59′′ 789 Rainfall July 2009–Dec. 2011

1

Latitude and longitude datum is the North American Datum of 1983.

2

Elevation datum is the North American Vertical Datum of 1988.

3

Bexar County index well. 
Qal

Qt

Qle

T-Qu

Kknm Kpg

Kau

Kef

Kbu

Kdr

Ked

Qt

Qle

Qt

Qt

Qt

Qle

T-Qu

T-Qu

Kknm

Kknm

Kac

Kpg

Kpg

Kpg

Kpg

Kau

Kau

Kau

Kau

Kbu

Kef

S S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S

S S

S

S

S

S S

S

S

S S

S

S

S

MEDINA COUNTY

BEXAR COUNTY

COMAL COUNTY GUADALUPE CO

98°20' 98°30'

98°40'

29°37'30"

29°32'

29°26'30"

0 2 4 

0 2 4 8 KILOMETERS

6

6

Base modified from U.S. Geological Survey digital data, 1:24,000Universal Transverse Mercator projection, zone 14North American Datum of 1983

Surficial geology modified from 

EXPLANATION

Study area boNormal faultUnspecified fWell and site National Wea and site id

Qal S

Qt

Qle

T-Qu

Kknm

Kac

Kpg

Kau

Kef

Kbu

Kdr

Ked

Pecan Gap Group (Taylor Group)Austin GroupEagle Ford GroupBuda LimestoneDel Rio ClayEdwards Group

Surficial geologic unit

Alluvium

 Fluviatile terrace deposits Leona Formation Uvalde Gravel Navarro Group  Anacacho Limestone (Taylor Group)

! A

K a

!

!

!

!

!

!

!

!!

!

F

C

A

D

B

E

K

I

J

G H

a

Edwards 

aquifer

outcrop

BEXAR

COUNTY

Study area TEXAS

BEXAR COUNTY

BALCONES 

FAULT ZONE

Edwards 

aquifer

outcrop

Figure 2. Surficial geologic units, Edwards aquifer outcrop, and data-collection sites in the study area, Bexar County, Texas (modified from Pedraza 
6 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

Table 2. Summary of geologic framework and hydrogeologic characteristics of stratigraphic units in central Bexar County, Texas.

[System modified from Maclay and Small (1984); Group or formation modified from Sellards and others (1923) and Brown and others (1983); thickness 

modified from Livingston (1947), Stein and Ozuna (1995), and Blome and others (2005); lithology modified from Dunham (1962); hydrologic function and 

water-yielding characteristics modified from Maclay and Small (1984), and Livingston (1947); --, not available; AQ, aquifer; na, not applicable; CU, confining 

unit; table modified from Pedraza and Shah (2010)]

Geologic framework Hydrogeologic characteristics

System Group or 

formation

Thickness 

(feet) Lithology Hydrologic function

Water-yielding 

characteristics

Quaternary

Alluvium -- Silt, sand, gravel. AQ Yields small quantities of water.

Fluviatile terrace deposits -- Coarse, gravel, sand, 

and silt.

Not saturated Yields small quantities of water.

Leona Formation 10–90 Sand, gravel, silt, 

and clay.

AQ Yields small quantities of water mostly to 

wells for rural-domestic and livestock use.

Quaternary 

or Tertiary

Uvalde Gravel 10–20 Gravel and sand with 

some silt and clay.

AQ Yields no water to wells.

Upper Cretaceous

Navarro Group 1300 Marl, clay, and sand 

in upper part; 

chalky limestone 

and marl in lower 

part.

CU Yields no water to wells.

Taylor 

Group

Pecan Gap

Chalk

 Anacacho

 Limestone

1

200 CU Not known to yield water to wells.

Austin Group 2130–350 Chalk, marl, and hard 

limestone. Chalk is 

largely a carbonate 

mudstone.

CU; AQ where 

connected to 

faults/fractures 

and caves

Locally yields small to large quantities of 

water to wells.

Eagle Ford Group 330–50 Brown, flaggy shale 

and argillaceous 

limestone.

CU Yields no water to wells.

Buda Limestone 340–50 Buff, light-gray, dense 

mudstone.

CU Yields no water to wells.

Del Rio Clay 340–50 Blue-green to yellowbrown clay.CU Yields no water to wells.

Lower Cretaceous

Georgetown Formation4 32–20 Reddish-brown, gray 

to light-tan marly 

limestone.

CU Is not commonly water bearing but yields 

water to some wells.

Edwards 

Group

Person 

Formation4

3

80–90 Mudstone to 

packstone; miliolid 

grain-stone; chert.

AQ Chief water-bearing formation in area; source 

of large springs; yields water to many 

pumped and flowing wells.

1

 Livingston, 1947.

2

 Blome and others, 2005.

3

 Stein and Ozuna, 1995.

4

 Not shown in study area.
Data Collection Methods 7

Water Quality

Collection and Processing of Water-Quality 

Samples

Sample collection and processing techniques followed 

standard USGS protocols (U.S. Geological Survey, variously 

dated). Water-quality samples were collected from raw-water 

spigots at or near the wellhead, prior to any pressure tanks 

or water treatment, for wells that had previously installed 

pumps (sites F, G, H, and I; fig. 1). If the well did not have 

a previously installed pump (sites B and E; fig. 1), the 

sample was collected by lowering a submersible piston pump 

equipped with 3/8-inch Teflon-lined tubing into the well 

(U.S. Geological Survey, variously dated). Before samples 

were collected and processed, all wells except for site F were 

purged to remove at least three casing volumes of water or 

until physical properties measured in the field (dissolved 

oxygen, pH, specific conductance, water temperature, and 

turbidity) stabilized (U.S. Geological Survey, 2006). Purging 

the well was done to ensure that the samples collected were 

representative of water from the aquifer. Physical properties 

were measured onsite by inserting a multiparameter probe 

into a sample removed from the well at the time of sample 

collection. Physical property measurements were considered 

stable when five consecutive measurements, which were 

made every 5 minutes, were within the stabilization criteria 

for the water-quality indicators (Wilde, variously dated). Only 

one casing volume of water was removed from site F prior 

to sampling because the well stopped producing water at the 

rate at which water was being pumped. After the one casing 

volume of water was removed from site F, the pump was 

turned off, and groundwater refilled the well. The sample was 

collected when enough water was present in the well to fill 

the sample bottles (U.S. Geological Survey, 2006). Samples 

were transported to the USGS south-central Texas program 

office and were then processed (U.S. Geological Survey, 

variously dated). Samples were filtered through a 0.45-micron 

filter, acidified to pH less than 2, and then shipped at 4 

degrees Celsius (°C) to the National Water Quality Laboratory 

(NWQL) in Denver, Colorado, for laboratory analysis. 

Laboratory Analysis

Water-quality samples were analyzed for major ions, 

nutrients, trace elements, and organic carbon concentrations by 

the NWQL (appendix 2). Major inorganic ions were analyzed 

by using methods described by Fishman and Friedman 

(1989) and Fishman (1993). Nutrients were analyzed by 

using methods documented by Fishman (1993) and Patton 

and Kryskalla (2011). Trace elements were analyzed by 

using methods described by Fishman and Friedman (1989), 

Garbarino (1999), and Garbarino and others (2006). Total 

organic carbon was analyzed by using methods described by 

Clesceri and others (1998). Dissolved organic carbon was 

analyzed by using methods described by Brenton and Arnett 

(1993). 

Concentrations for major ion, nutrients, and trace 

elements were reported by the laboratory as described by 

Oden and others (2011, p. 9):

The analytical quantification procedure used by 

the NWQL for reporting results is based on the 

long-term method detection level (LT–MDL) and 

laboratory reporting level (LRL). The LT–MDL concentrations are defined as a censoring limit for most 

analytical methods at the NWQL, and its purpose is 

to limit the false positive rate to less than or equal 

to 1 percent. An LT–MDL is a modification of the 

[U.S. Environmental Protection Agency] 40 Code 

of Federal Regulations Part 136 definition of the 

method detection limit (MDL). The LRL is defined 

as twice the LT–MDL and is established to limit the 

occurrence of false negative detections to less than 

or equal to 1 percent (Childress and others, 1999). 

A constituent concentration is considered estimated 

by the laboratory when results are greater than the 

LT–MDL and less than the LRL; that is, a detection 

is considered likely, but quantification is considered 

questionable. The remark code of “E” (estimated) is 

assigned by the laboratory for these results.

When the measured values were greater than the 

calibration range of the instrument or when the field 

measurement stabilization criteria were not met, the data were 

reported with an “E” remark code (Childress and others, 1999; 

Wilde, variously dated).

Stable isotopic ratios of carbon (δ13C) were measured by 

the National Ocean Sciences Accelerator Mass Spectrometry 

Facility at the Woods Hole Oceanographic Institution in 

Woods Hole, Massachusetts, by using methods described 

by Vogel and others (1987), Donahue and others (1990), 

McNichol and others (1992), Gagnon and Jones (1993), 

McNichol and others (1994), and Schneider and others (1994). 

The results were reported by the laboratory in units of parts 

per thousand (per mil). Isotopic ratios of hydrogen (δD) and 

oxygen (δ18O) were measured by the USGS Stable Isotope 

Laboratory in Reston, Virginia (RSIL), by using methods 

described by Révész and Coplen (2008a, 2008b). The results 

were reported by the laboratory in per mil relative to Vienna 

Standard Mean Ocean Water (VSMOW). The isotopic 

ratios of strontium (strontium-87/strontium-86 [87Sr/86Sr]) 

were measured by the USGS National Research Program 

Laboratory in Menlo Park, California, by using methods 

described by Bullen and others (1996) and reported as a 

dimensionless ratio. The laboratory reported 87Sr/86Sr ratio 

for standard reference material (SRM) 987 from the National 

Institute of Standards and Technology as 0.71024 (0.00002 

2-sigma, 95 percent confidence level). 
8 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

Quality Assurance

Quality-control samples (an equipment blank and 

a replicate sample) were collected to evaluate potential 

contamination of the water-quality samples and data 

variability. In order to evaluate potential contamination as 

a result of the sampling equipment and sample processing, 

the equipment blank sample was collected prior to the start 

of sampling from a well where a preexisting pump was 

not installed. The equipment blank was identified with 

State well number AY-68-36-135 (appendix 2), though an 

environmental sample was not collected from this site. The 

equipment blank sample was intended to measure potential 

contamination from the sampling equipment, including 

the submersible pump sampling equipment which was not 

used when the sample was collected from the spigot at the 

wellhead. To be conservative, the concentrations measured 

in all of the environmental samples (including those not 

collected with a submersible pump) were compared to the 

concentrations measured in the equipment blank sample. 

The equipment blank was analyzed for major ions, nutrients, 

and trace elements. Calcium, chloride, copper, manganese, 

molybdenum, and nickel were detected in the equipment blank 

sample. All other analyzed constituent concentrations were 

less than the LRL. The concentrations of calcium, chloride, 

and molybdenum were less than concentrations measured in 

the environmental samples, indicating possible contamination 

likely did not affect the concentrations in the environmental 

samples. For samples in which measured concentrations of 

copper and manganese in the equipment blank sample were 

greater than those measured in the environmental samples, 

the environmental sample data were rejected because 

contamination might have affected the environmental results; 

rejected are not reported herein. 

The replicate sample was collected from site I to evaluate 

the potential variability introduced during sample collection, 

processing, or laboratory analysis (appendix 2). The replicate 

sample was compared to the associated environmental sample 

by calculating the relative percent difference (RPD) for each 

pair of detected constituents (eq. 1; appendix 2). The RPD was 

computed with the equation

RPD = [|X1 – X2|]/[(X1 + X2)/2] x 100, (1) 

where

X1 is the concentration of the constituent 

measured in the environmental sample; and 

X2 is the concentration of the constituent 

measured in the sequential-replicate 

sample.

The RPD was not computed for a constituent pair if 

either of the values was reported as less than the LRL. RPDs 

of 10 percent or less were used to indicate good agreement 

between analytical results if the concentrations were 

sufficiently large compared to the LRL. RPDs of 10 percent 

or less were measured for most constituents, indicating that 

there was acceptable agreement and reproducibility between 

the environmental sample values and the sequential replicate 

value. The calculated RPDs that exceeded 10 percent were 

iron (34 percent), lead (22 percent), nickel (17 percent), and 

antimony (58 percent). The RPDs for iron and antimony might 

have been above 10 percent because the reported values were 

relatively small (less than four times the LRL). 

Groundwater Levels

In karst groundwater systems, changes in groundwater 

levels are often associated with changes in hydrologic 

conditions such as precipitation or drought conditions. The 

changes in groundwater levels measured at the Bexar County 

index well (site J), which is completed in the Edwards 

aquifer, were assumed to be a general indicator of changes in 

groundwater levels for the Edwards aquifer on a regional scale 

in Bexar County, and have been linked with regional climate 

and pumping events (Lindgren and others, 2004). Similarities 

between the groundwater levels of wells completed in the 

Austin Group in the study area and the Bexar County index 

well might be indicative of groundwater interactions between 

the Edwards aquifer and Austin Group through structural or 

hydrologic processes such as nearby faulting or conduit flow, 

though the degree of connectivity (if any) is unknown. 

The groundwater levels reported at the sites in the study 

area exhibited varying degrees of similarity to groundwater 

levels reported for the Bexar County index well (fig. 3). The 

coefficient of determination (R2) can be used to indicate the 

relation between two variables of interest (Helsel and Hirsh, 

2002; Navidi, 2008). Groundwater levels at site A exhibited 

similar patterns as those at the Bexar County index well (R2

of 0.91), but the hydrographs of groundwater levels were 

different in shape and magnitude in response to precipitation 

and groundwater pumping, and at times slightly offset in 

time. The similar behavior in groundwater levels at site A 

indicated that some degree of hydrologic connectivity between 

this site and the Edwards aquifer is possible, although a 

high coefficient of determination value does not necessarily 

indicate causality (Navidi, 2008). Additionally, groundwater 

levels at site A were higher in altitude by approximately 

50–70 ft than the groundwater levels at the Bexar County 

index well. This difference might exist because groundwater 

levels in Edwards aquifer are generally higher in the northwest 

part of the study area than groundwater levels at the Bexar 

County index well (Lindgren and others, 2004). Site A might 

also be influenced by local processes, such as flows in nearby 

creeks acting as sources of local recharge. For example, the 

difference in groundwater levels at site A compared to those 

at the Bexar County index well increased from approximately 

50 ft during fall 2009 to up to approximately 70 ft during 

summer 2010 (following an extended wet period), but 

returned to approximately 50 ft about the beginning of 2011 
Groundwater Levels 9

EXPLANATION

State well number

 (site identifier)

 AY-68-36-136 (site A)

 AY-68-29-513 (site C)

 AY-68-29-514 (site D)

 AY-68-29-512 (site E)

 AY-68-29-819 (site F)

 AY-68-37-205 (site H)

 AY-68-29-932 (site I)

 AY-68-37-203 (site J)—

 Bexar County index well

A

B

July Dec. Jan. July Dec. Jan. July Dec.

2009 2010 2011

600

760

740

720

700

680

660

640

620

Daily mean groundwater-level altitude, in feet Weekly precipitation, in inches

0

1

2

3

4

5

6

7

Figure 3. A, Hydrographs of daily mean groundwater-level altitudes during 2009–11 of sites in the study area, Bexar County, Texas, and 

B, weekly precipitation measured at National Weather Service station number 417945 (San Antonio International Airport). 
10 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

(the beginning of an extended dry period) (fig. 3). Also, a 

downhole video camera deployed at site A showed a clear 

conduit of water flowing into the well at approximately 90 

ft below land surface (Chris Villarreal, U.S. Environmental 

Protection Agency, written commun., 2010); hence, the 

groundwater levels observed at site A might be related in part 

to potential hydrologic connectivity to the Edwards aquifer, 

such as by faulting, in conjunction with influences from local 

hydrologic processes. 

The pattern of groundwater levels from the sites in 

McAllister Park (sites C, D, and E) were the least similar 

to the pattern of groundwater levels measured at the Bexar 

County index well. The smaller coefficients of determination 

between groundwater levels at the Bexar County index well 

and site C (R2 of 0.13), D (R2 of 0.66), and E (R2 of 0.30) 

indicated less connectivity to the Edwards aquifer as compared 

with sites H and I (both R2 of 0.99). The groundwater levels 

measured from the sites in McAllister Park appeared to have 

a relatively stable baseline at about 680 ft above NAVD88; 

however, changes in groundwater levels at these sites occurred 

rapidly in response to precipitation events (fig. 3). The 

baseline at about 680 ft above NAVD88 is present during 

wetter periods when groundwater levels at the Bexar County 

index well are relatively high (for example, during the spring 

2010), as well as during drier periods (for example, during 

summer 2011). Groundwater levels at site C responded 

most appreciably, with sharp rises and falls in response to 

precipitation events. Groundwater levels at sites D and E also 

responded to precipitation events, but the rising and falling 

limbs of the hydrograph were more gradual than observed at 

site C. 

The groundwater levels from site F were generally 

similar to those observed at the Bexar County index well; 

however, there were several periods of notable groundwaterlevel drawdowns at site F that were not observed at the Bexar 

County index well. These drawdowns were likely because of 

pumping from the well at site F, notably during the summer of 

2011 when San Antonio experienced a prolonged dry period 

(Texas Commission on Environmental Quality, 2012). The 

hydrographs of groundwater levels measured at sites H and 

I were the most similar in shape and magnitude to those at 

the Bexar County index well indicating there might be some 

degree of hydrologic connectivity to the Edwards aquifer. The 

R2 between groundwater levels at the Bexar County index well 

and sites H and I were 0.99. 

Water-Quality Data

Water-quality samples were collected from six sites 

in the study area (sites B, and E–I). Water-quality samples 

were collected during a period of relatively stable hydrologic 

conditions (April, 2011) in order to minimize local effects 

such as precipitation recharge on the groundwater chemistry. 

For example, only 0.02 inches of precipitation occurred 

during the month prior to sampling (National Oceanic and 

Atmospheric Administration, 2012). The water-quality data 

were examined to characterize the Austin Group and were 

compared to water-quality data of the Edwards aquifer. 

Trilinear (Piper) diagrams indicated that the groundwater 

samples had similar major ion chemical characteristics and 

generally can be categorized as a calcium-carbonate water 

type (fig. 4) (Piper, 1944). 

A groundwater sample was collected at site B, which is 

completed in the Edwards aquifer. The major ion chemistry in 

the groundwater sample collected at site B plots similarly to 

the major ion chemistry of previously collected groundwater 

samples by the USGS at this site (Musgrove and others, 2011) 

indicating that the chemistry of the groundwater at site B was 

similar during the two sampling events. Musgrove and others 

(2011) found that water-quality samples collected from site B 

were representative of groundwater in the Edwards aquifer; 

hence, for the remainder of this report, site B is assumed to be 

representative of Edwards aquifer groundwater chemistry.

Samples collected from sites E, F, and G plot slightly 

separately from the sample collected from site B on the 

trilinear diagram (fig. 4). This indicates that there may be 

either site-specific unique conditions occurring at these sites, 

mixing with different source water, or a combination of both. 

For example, site G is located near a former cement quarry 

which was in operation from 1908 to 1985 (Rybczyk, 1992; 

Texas State Historical Association, 2012), and that is where 

the previously mentioned fractures were observed. 

The major ion chemistry of samples collected from 

sites H and I are geochemically most similar to groundwater 

collected from site B, representative of groundwater in the 

Edwards aquifer. Because the Austin Group and Edwards 

aquifer are calcium carbonate dominated systems, some 

similarities in the geochemistry in the collected samples are to 

be expected. Although similarities in concentration values in 

samples collected from sites completed in the Austin Group 

and Edwards aquifer are present, it is not known if the water 

sampled from the sites completed in the Austin Group and 

Edwards aquifer are from the same source. Given the small 

sample dataset, it is not possible to differentiate the potential 

sources of groundwater collected from the Austin Group on 

the basis of major ion geochemistry.

Concentrations of the nutrients nitrate, nitrite, and 

orthophosphate measured in the collected groundwater 

samples varied between samples (appendix 2). For example, 

nitrate plus nitrite concentrations range from 0.02 milligrams 

per liter (mg/L) at site G to 2.54 mg/L at site H. The 

concentration of nitrate plus nitrite at site B was 1.97 mg/L, 

which is consistent with previously measured concentrations 

from Edwards aquifer groundwater samples (Musgrove and 

others, 2011). The nitrate plus nitrite concentration at site I 

(1.81 mg/L) was most similar to the concentration measured 

at site B (1.97 mg/L) though concentrations at site E were 

also similar (2.15 mg/L). Further, the concentration of 

orthophosphate measured at site B (0.014 mg/L) was similar to 

concentrations measured at sites H and I (0.014 mg/L for both 

samples). Orthophosphate concentrations in samples collected 
Water-Quality Data 11

Calcium

Magnesium

Sodium plus potassium

Chloride, fluoride, nitrite plus nitrate

Carbonate plus bicarbonate

Sulfate

Calcium plus magnesium

Sulfate plus chloride

PERCENT

PERCENT

PERCENT

 0 

 20 

 40 

 60 

 80 

 100 

 0 

 20 

 60 

 80 

 100 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

 40 

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 100 

 80 

 60 

 40 

 20 

 0 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 60 

 80 

 100 0 

 20 

 40 

 60 

 80 

 100 

EXPLANATION

Sample collected from well, 

 by State well number

 (site identifier)

 AY-68-36-134 (site B)

 AY-68-29-512 (site E)

 AY-68-29-819 (site F)

 AY-68-37-133 (site G)

 AY-68-37-205 (site H)

 AY-68-29-932 (site I)

Figure 4. Trilinear diagrams of chemical composition for samples collected in the study area, Bexar County, Texas, April 2011.
12 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

from sites F and G were slightly less (0.010 and 0.012 mg/L, 

respectively), and the concentrations in samples from site E 

was slightly more (0.024 mg/L). Hence, a consistent pattern 

between sites was not observed in nutrient concentrations; 

however, nutrient concentrations measured in samples from 

sites B, H, and I were often the most similar, indicating a 

degree of hydrogeologic similarity between these sites. 

Similar to nutrient concentrations, the trace element 

concentrations in the groundwater samples exhibited 

variability between samples, though a consistent pattern 

was not observed (appendix 2). Concentrations of barium, 

for example, ranged from 34.8 (site B) to 117 (site E) 

micrograms per liter (μg/L). Barium concentrations in 

samples collected from sites H and I were 60.5 and 41.4 

μg/L, respectively. Concentrations of uranium ranged from 

0.134 (site G) to 3.57 (site E) μg/L with samples from sites 

B, H and I having similar concentrations [ranging from 0.812 

(site I) to 0.884 (site H) μg/L]. The measured concentrations 

of organic carbon (filtered and unfiltered) were generally 

similar between samples collected at each of the sites. The 

measured concentrations of turbidity, many trace elements, 

and organic carbon (filtered and unfiltered) in the sample 

from site F, however, were notably elevated compared to 

the concentrations measured at the other sites (appendix 2). 

These elevated concentrations may be in part because site F 

is an open hole well, has a comparatively low production rate 

(for example, the well stopped producing water at the rate at 

which water was being pumped during sampling), and only 

intermittently is used for domestic purposes. It is possible 

that this well is reflecting more local processes (for example, 

local lithology) than being representative of groundwater 

from the Austin Group; hence, it is included in the subsequent 

discussions and figures, but with so few data, the groundwater 

at site F might not be representative of local groundwater. 

Groundwater can also be characterized through the use 

of relations between ratios of major ions, trace elements, or 

isotopes, which can be used as an indicator of water-rock 

interaction, geochemical evolution, or other geochemical 

processes (Clark and Fritz, 1997; Sharp and Banner, 1997; 

Fairchild and others, 2000; Banner, 2004; Mahler 2008; 

Musgrove and others, 2010; Banta and others, 2012). For 

example, increased magnesium to calcium (Mg/Ca) molar 

ratio and strontium to calcium (Sr/Ca) molar ratio, and 

decreased strontium isotopic ratios might be related to waterrock interactions and geochemical evolution (Fairchild and 

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

others, 2010). 

The relations between Mg/Ca molar ratios and 87Sr/86Sr 

isotopic ratios for samples collected in the study area are 

depicted in figure 5. The concentrations measured in the 

sample collected from site B are consistent with previous 

studies that characterized Edwards aquifer geochemistry 

(Musgrove and others, 2010). On the basis of chemical ratios, 

groundwater from site E appears to be the least geochemically 

evolved water of the samples collected in this study, possibly 

related in part that site E was the most responsive to local 

precipitation events and the least correlated with the Edwards 

aquifer of the sites sampled in this study. Site E had the 

highest measured specific conductivity of the samples 

collected, however, indicating the groundwater at site E may 

have had a longer residence time than the groundwater at 

some of the other sites. Hence, the groundwater geochemistry 

reflects that site E was possibly being recharged, at least in 

part, by local recharge events, which could potentially mean 

that the groundwater at this site might not be regionally 

recharged to the same extent as the Edwards aquifer. 

The plot location of sites F and G on figure 5 indicate 

that the sampled groundwater might be more geochemically 

evolved than the other sites. As previously noted, site F is 

from an open hole and went dry during sample collection. 

Given the low productivity of the well, it is possible that the 

sampled groundwater had longer residence times. It might also 

indicate that local lithology is influencing the geochemistry 

through water-rock interaction. Another possibility is that 

the groundwater from sites F and G is mixing with another 

source of groundwater that has a different geochemical 

composition. The groundwater samples collected from sites 

H and I have similar Mg/Ca and Sr/Ca molar and 87Sr/86Sr 

isotopic ratios as groundwater in the Edwards aquifer (site B) 

(fig. 5). 

The hydrogen and oxygen isotopic ratios can be used to 

evaluate geochemical interactions and possible source waters 

(Clark and Fritz, 1997; Kehew, 2001). The groundwater 

samples collected in this study ranged from -23.90 to -22.75 

per mil for hydrogen (δD), and -4.18 to -4.01 per mil for 

oxygen (δ18O). Comparing the hydrogen and oxygen isotopic 

sample data to the global meteoric water line (Craig, 1961) 

and the local meteoric water line (Pape and others, 2010) 

indicated the groundwater is meteoric in origin (fig. 6). 

Groundwater collected from the Austin Group does not appear 

to have a geochemically unique isotopic enrichment compared 

to groundwater collected from the Edwards aquifer (Musgrove 

and others, 2011).

The different relations among the groundwater-level data 

and the water-quality data indicate that the water quality in 

the different watersheds is likely influenced by differences 

in source areas, geochemical evolution, groundwater flow 

paths and residence time, local lithology, or some combination 

thereof. Determining the source areas and other possible 

contributors based on these data is not possible given the small 

sample size of the water-quality dataset (both in number of 

samples and spatial distribution of samples).

Though a single line of evidence might indicate some 

degree of hydrologic connectivity between the Austin 

Group and the Edwards aquifer (or a lack thereof), using 

multiple lines of evidence together can provide a better 

characterization of groundwater in the Austin Group and its 

potential hydrologic connectivity to the Edwards aquifer. 

Specifically, groundwater-level hydrology as compared to 

the Bexar County index well and geochemistry as compared 
Water-Quality Data 13

with site B indicate that groundwater at well E is not as 

hydrologically similar to the Edwards aquifer groundwater 

as other sites sampled in the this study. Groundwater-level 

data from well F were similar to those from the Bexar County 

index well; however, the relation was not as clear as with 

some of the other wells in the study. Further, the trace element 

concentrations and isotopic ratios were not always consistent 

with those from site B. Lastly, of the sites sampled in this 

study, groundwater-level data, trilinear diagram, trace element 

concentrations, and isotopic ratios all indicate that sites H and 

I are the most hydrologically similar to site B. Of the sites 

sampled in this study, there appears to be varying hydrologic 

connectivity between groundwater from wells completed in 

the Austin Group wells and the Edwards aquifer. 

Figure 5. Relations between molar ratios and strontium-87/strontium-86 isotopic ratios for samples collected in the study area, Bexar 

County, Texas, April 2010. A, magnesium to calcium (Mg/Ca) molar ratio and strontium-87/strontium-86 isotopic ratio. B, magnesium to 

calcium (Mg/Ca) molar ratio and strontium to calcium (Sr/Ca) molar ratio.

Strontium to calcium molar ratio x 103

Strontium-87/strontium-86 isotopic ratio

0.7076 0.7077 0.7078 0.7079 0.708 0.7081 0.7082

0 1 2 3 4 5 6 7

Magnesium to calcium molar ratio Magnesium to calcium molar ratio

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0

A

B

Sample collected from well, 

 by State well number

 (site identifier)

 AY-68-36-134 (site B)

 AY-68-29-512 (site E)

 AY-68-29-819 (site F)

 AY-68-37-133 (site G)

 AY-68-37-205 (site H)

 AY-68-29-932 (site I)

EXPLANATION
14 Groundwater Levels and Water-Quality Observations Pertaining to the Austin Group, Bexar County, Texas, 2009–11

Summary 

The Edwards aquifer is the primary source of drinking 

water for the City of San Antonio, in Bexar County, Texas, 

and surrounding areas and is considered to be one of the most 

productive karst aquifers in the nation. Because of increases 

in population and water demands, water resource managers 

seek to gain a better understanding of the hydrogeologic 

characteristics of water-bearing units in the greater San 

Antonio area and of possible relations these units might have 

to the Edwards aquifer. One such hydrogeologic unit is the 

Austin Group, commonly referred to as the “Austin Chalk.” 

The U.S. Geological Survey (USGS), in cooperation with 

the San Antonio Water System, examined groundwaterlevel altitudes (groundwater levels) and water-quality data 

pertaining to the Austin Group in Bexar County, Texas during 

2009–11. 

This report documents a reconnaissance-level assessment 

during 2009–11 of daily mean groundwater level altitudes 

(groundwater levels) and selected water-quality data pertaining 

to the Austin Group, in Bexar County, Tex. Water-quality data 

included measurements of physical properties, major ions, 

nutrients, trace elements, organic carbon, and stable isotopes. 

The data were examined for similarities between sites, as 

well as to data from site J (State well AY-68-37-203; hereafter 

referred to as the “Bexar County index well”), which is 

completed in the Edwards aquifer. 

Similarities between the groundwater levels of wells 

completed in the Austin Group in the study area and the 

Bexar County Index well might be indicative of groundwater 

interactions between the Edwards aquifer and the Austin 

Group through structural or hydrologic processes such as 

nearby faulting or conduit flow. The groundwater levels 

at the sites in the study area exhibited varying degrees of 

similarity to those recorded at the Bexar County index well. 

Groundwater levels at site A (State well AY-68-36-136) 

exhibited similar patterns as those at the Bexar County 

index well (coefficient of determination [R2] of 0.91), 

but the hydrographs of groundwater levels were different 

in shape and magnitude in response to precipitation and 

groundwater pumping, and at times slightly offset in time. 

The groundwater levels at site A appear to be offset (higher) 

from the groundwater level at the Bexar County index well by 

approximately 50–70 feet. Groundwater levels at site A might 

be influenced by local hydrologic processes, such as flows in 

nearby creeks acting as a source of recharge.

The groundwater levels recorded at sites C, D, and E 

(State wells AY-68-29-513, AY-68-29-514, and AY-68-29-512, 

respectively) were not similar to those measured at the Bexar 

County index well. The lower relations between groundwater 

-5 -4 -3 -2 -1 0

Delta oxygen -18, in per mil

-30

-25

-20

-15

-10

-5

0

Deuterium/protium ratio, in per mil

Local meteoric water line

(Pape and others, 2010)

Global meteoric water line

(Craig, 1961)

EXPLANATION

Sample collected from well, 

 by State well number

 (site identifier)

 AY-68-36-134 (site B)

 AY-68-29-512 (site E)

 AY-68-29-819 (site F)

 AY-68-37-133 (site G)

 AY-68-37-205 (site H)

 AY-68-29-932 (site I)

Figure 6. Relation of hydrogen and oxygen isotopic ratios for samples collected in the study area, Bexar County, Texas, April 2010. 
References 15

levels at the Bexar County index well and site C (R2 of 

0.13), D (R2 of 0.66), and E (R2 of 0.30) indicated limited 

connectivity to the Edwards aquifer. 

The groundwater levels from site F were generally 

similar to those observed at the Bexar County index well; 

however, there were several periods of notable groundwaterlevel drawdowns at site F that were not observed at the Bexar 

County index well. The groundwater levels at sites H and I 

(State wells AY-68-37-205 and AY-68-29-932, respectively) 

exhibited similar patterns compared to groundwater levels 

at the Bexar County index well (R2 of 0.99 at both wells) 

indicating there might be some degree of hydrologic 

connectivity to the Edwards aquifer. 

Water-quality samples were collected from six sites 

in the study area. The water quality data were examined to 

characterize the Austin Group and for relations to the 

Edwards aquifer. In general, the water-quality data indicated 

the samples were representative of a calcium carbonate 

dominated system. Of the samples collected in this study, 

the major ion chemistry of samples collected from sites H 

and I were most similar to groundwater collected from site 

B, representative of groundwater in the Edwards aquifer. A 

consistent pattern in nutrient and trace element concentrations 

was not observed among the sample collected from site B 

as compared to those from the other sites that exhibited 

similar hydrograph characteristics and major ion chemistry; 

however, nutrient and trace element concentrations measured 

in samples from sites B, H, and I were often the most similar. 

Of the samples collected in this study, site E demonstrated the 

lowest geochemically evolved water. The relations between 

Mg/Ca molar ratios and 87Sr/86Sr isotopic ratios for samples 

collected from sites H and I are similar to those measured 

at site B, indicating that the groundwater from these sites 

were most geochemically similar to groundwater in the 

Edwards aquifer. 

Though a single line of evidence might indicate some 

degree of hydrologic connectivity between the Austin Group 

and the Edwards aquifer (or a lack thereof), using multiple 

lines of evidence together can provide a better characterization 

of the Austin Group and its potential hydrologic connectivity. 

Specifically, groundwater-level hydrology as compared to 

the Bexar County index well and geochemistry as compared 

with site B indicate that groundwater at well E is not as 

hydrologically similar to the Edwards aquifer groundwater 

as other sites sampled in this study. Groundwater-level data 

from well F were more similar to those from the Bexar 

County index well; however, the relation was not as clear as 

with some of the other wells in the study. Further, the trace 

element concentrations and isotopic ratios were not always 

consistent with those from site B. Lastly, of the sites sampled 

in this study, groundwater-level data, trilinear diagram, trace 

element concentrations, and isotopic ratios all indicated that 

sites H and I were the most hydrologically similar to site B. 

Of the sites sampled in this study, there appears to be varying 

hydrologic connectivity between groundwater from wells 

completed in the Austin Group wells and the Edwards aquifer. 

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