The Edwards aquifer is one of the most 

productive carbonate aquifers in the 

Nation. The dissolution-modified, faulted 

limestone aquifer is the sole source of public water supply for San Antonio, Texas 

(fig. 1) and is the major source of water for 

Bexar County. In addition to providing 

public water supply to more than 1 million 

people, the Edwards aquifer supplies large 

quantities of water for agriculture, industry, and military installations. Major 

springs discharging from the aquifer support recreational activities and businesses, 

provide water to downstream users, and 

provide habitat for several threatened or 

endangered species.

The Edwards aquifer recharge zone is 

approximately coincident with the stratigraphic units that constitute the Edwards 

aquifer outcrop; however, the recharge 

zone includes outcrops of other stratigraphic units in proximity to the units of 

the Edwards aquifer outcrop where caves, 

sinkholes, faults, fractures, or other permeable features create a potential for 

recharge. For this report, areas that are 

immediately adjacent to the Edwards aquifer outcrop and that drain to the outcrop 

are considered to be in the recharge zone. 

In Bexar County, the area of the 

Edwards aquifer outcrop is about 119 

square miles (mi2). Most of the land (about 

72 percent) on the outcrop is undeveloped 

(fig. 2). Residential use is about 10 percent 

of the outcrop land use. 

Residential and commercial development on the Edwards aquifer recharge 

zone in Bexar County is increasing. Urban 

development can have an appreciable 

influence on water quality. Impervious 

cover in developed areas can result in 

increased stormwater runoff, conveying 

contaminants from nonpoint sources to 

local streams or geologic features such as 

Stormwater Runoff for Selected Watersheds in the Edwards 

Aquifer Recharge Zone, Bexar County, Texas, 1996–98

In cooperation with the San Antonio Water System

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–172–98

March 1999

Figure 1. Location of selected watersheds and streamflow-gaging stations in the Edwards aquifer recharge zone, Bexar County, Texas.

98o45'

98o

37'30"

98o

30' 98o

22'30"

29o

37'30"

29o

45'

Base modified from U.S. Geological Survey

1:24,000 quadrangles

Universal Transverse Mercator

Zone 14

024 6 MILES

5 1 2

4

3

1

EXPLANATION

Watershed

Edwards aquifer outcrop—Modified from Stein

 and Ozuna (1995, pl. 1)

Catchment area—Drainage basins of streams

 that recharge the Edwards aquifer as they

 cross the outcrop

U.S. Geological Survey streamflow-gaging

 station—Number refers to watershed

State

16

I–10

US 281

Loop 1604

Loop 1604

Medina

 County

Comal

 County

Bexar

 County

San

Antonio

Study area

TEXAS

LOCATION MAP

Medina

New

 Braunfels
2

caves and fractures where infiltration into 

the aquifer can occur.

In 1996, the U.S. Geological Survey 

(USGS), in cooperation with the San 

Antonio Water System, began a study to 

monitor the quality and quantity of stormwater runoff of five selected watersheds in 

the Edwards aquifer recharge zone. The 

purpose of the study is to further the understanding of relations between stormwater 

and land use and to help resource managers assess the effects of development on 

the Edwards aquifer recharge zone. Land 

use is relatively commercial in two of the 

watersheds and predominantly residential 

in two of the watersheds; one watershed is 

largely undeveloped (table 1). This fact 

sheet provides an overview of the datacollection methods and selected results of 

analysis.

Data Collection and Analysis

Rainfall and runoff data and stormwater 

samples were collected during June 1996–

October 1998 from streamflow-gaging 

stations at the outlets of the five selected 

watersheds in the Edwards aquifer 

recharge zone. At each station, rainfall 

and gage height (stream water level) were 

measured during storm-runoff events. A 

computer recorded instrument readings, 

calculated stream discharges (and runoff), 

and activated automatic water samplers 

on the basis of calculated runoff. During 

each storm-runoff event, the autosamplers 

were programmed to collect numerous 

Figure 2. Land use/land cover in the Edwards aquifer recharge zone, Bexar County, Texas, 1996.

Table 1. Characteristics of selected watersheds in the Edwards aquifer recharge zone, Bexar County, Texas, 1996–98 

[<, less than] 

Watershed

(fig. 1)

Drainage

area

(acres)

Impervious

cover

(percent)

Land use/

land cover Remarks

1 19 50 Commercial No stormwater-control structures.

2 315 45–65 Commercial Watershed includes section of I–10; stormwater-control structures required for 

businesses; ongoing construction during study period.

3 75 40–50 Residential Medium density, single-family homes.

4 250 <5 Residential 3- to 5-acre lots; septic tanks.

5 6,880 <2 Undeveloped Upper 2,450 acres of watershed is upstream of recharge zone; runoff events are rare.

98o45'

98o

37'30" 98o

30' 98o

22'30"

29o

37'30"

29o

45'

LAND USE/LAND COVER

Base modified from U.S. Geological Survey

1:24,000 quadrangles

Universal Transverse Mercator

Zone 14

Land-use data from Alamo Area

Council of Governments

digital data, 1996

024 6 MILES

Watershed boundary

State

16

I–10

US 281

Loop 1604

Loop 1604

EXPLANATION

Undeveloped

Residential

Military

Commercial

Industrial

Institutional/cultural/recreational

Quarries

5 1 2

4

3

1 U.S. Geological Survey streamflow-

 gaging station—Number refers to

 watershed

Edwards aquifer outcrop—Modified 

 from Stein and Ozuna (1995, pl. 1)
3

2400

flow-weighted samples. At the end of a 

storm-runoff event, the samples were 

composited, packaged, and shipped to 

the USGS National Water Quality Laboratory in Arvada, Colo., for analysis. The 

composite samples were analyzed for 

common inorganic constituents, chemical 

oxygen demand (COD), biochemical 

oxygen demand (BOD), nutrients, trace 

elements, selected organic compounds, 

and pesticides. 

Analyses of the composite samples 

yielded event-mean concentrations 

(EMC), which represent average waterquality conditions during a runoff event. 

EMCs are useful for comparing concentrations from different sites or from different 

runoff events at the same site, and for estimating the mass of a chemical constituent 

exiting the watershed during an event. The 

mass of a constituent exiting the watershed 

in runoff can be estimated by multiplying 

the EMC by the runoff (and appropriate 

conversion factor). 

Discrete grab samples also were collected during each storm-runoff event and 

analyzed for specific conductance, pH, 

fecal coliform and fecal streptococcus bacteria, cyanide, phenols, oil and grease, and 

volatile organic compounds (VOC). Generally, grab samples are collected during 

the early stages of a runoff event and indicate the presence of constituents in the 

“first flush” of a runoff. As the constituent 

concentrations might not represent the 

average concentrations during a runoff 

event, estimates of mass of these constituents are uncertain. Rainfall, discharge, and 

times of flow-weighted and grab sample 

collection for an example storm-runoff 

event at watershed 4 on March 16, 1998, 

are graphed in figure 3.

Selected Water-Quality Property and 

Constituent Concentrations

Thirty-two composite storm-runoff 

samples (representing 32 storm-runoff 

events) were collected at the five selected 

watersheds during June 1996–October 

1998. The watersheds with the larger percentages of impervious cover (1 and 2, 

table 1) yielded more runoff per acre than 

the less developed watersheds (4 and 5, 

table 1). More samples were collected 

from the watersheds with more impervious 

cover (1 and 2) because runoff occurred 

with as little as 0.05 inch (in.) of rainfall in 

the watershed, whereas runoff in watersheds 4 and 5 occurred only during intense 

rainfall with totals of at least 1.5 in. The 

median EMCs of selected constituents for 

each watershed are listed in table 2.

Boxplots were constructed to show 

example comparisons of concentrations 

Figure 3. Rainfall, discharge, and times of flow-weighted and grab sample collection during a storm-runoff event at watershed 4, Bexar 

County, Texas, March 16, 1998. 0100 0200

03000400050006000700080009001000110012001300140015001600

TIME

0

1.8

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

CUMULATIVE RAINFALL, IN INCHES

0

28

2

4

6

8

10

12

14

16

18

20

22

24

26

DISCHARGE, IN CUBIC FEET PER SECOND

Rainfall

Discharge

Grab sample

Flow-weighted sample
4

of selected trace elements (chromium, 

copper, lead, and zinc) by land use (fig. 4). 

Water-quality data from watersheds 

with similar land uses (1 and 2; 3 and 4) 

(table 1) were grouped together. Boxplots 

with data less than the laboratory minimum reporting levels were truncated at the 

reporting level. The numbers of samples 

with concentrations less than the reporting 

levels are shown in brackets in figure 4. In 

most cases, less than 25 percent of the data 

were below the reporting levels and the 

interquartile range is plotted. Boxplots for 

the undeveloped land-use category were 

not constructed because only four samples 

were collected.

Total chromium, lead, and zinc concentrations were significantly larger, at the 

95-percent confidence level, for commercial land use than for residential land use 

on the basis of one-sided Mann-Whitney 

tests (Helsel and Hirsch, 1992). The statistical test for total copper concentrations 

did not show significant difference 

between commercial and residential land 

use. Statistical comparisons of concentrations in samples from the undeveloped 

watershed were not included because of 

the small data set (four samples).

Example of Mass Yields

The effect of land use and percent 

impervious cover of a watershed in generating stormwater-runoff pollutants is evident when examining the annual mass 

Table 2. Median event-mean concentrations of selected properties and constituents by watershed, Bexar County, Texas, 1996–98

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; cols./100 mL, colonies per 100 milliliters; CaCO3, 

calcium carbonate; <, less than; μg/L, micrograms per liter; --, insufficient data] 

Property or constituent

Watershed

(fig. 1)

1 2 3 45

Number of composite storm-runoff samples collected 10 2 12 4 4

Specific conductance (μS/cm) 91 106 88 114 200

Chemical oxygen demand (mg/L) 38 39 31 27 30

Fecal coliforms (cols./100 mL) 10,000 27,000 13,000 4,900 4,300

Fecal streptococci (cols./100 mL) 5,600 64,000 38,000 67,000 5,180

Calcium, dissolved (mg/L) 12 13 11 16 35

Magnesium, dissolved (mg/L) .4 4.0 .4 .8 2.1

Sodium, dissolved (mg/L) 2.2 1.3 1.3 1.2 1.7

Potassium, dissolved (mg/L) 1.6 2.6 1.6 3.0 2.4

Alkalinity (mg/L as CaCO3) 45 54 48 57 102

Sulfate, dissolved (mg/L) 4.9 4.0 2.3 2.2 4.0

Chloride, dissolved (mg/L) 1.5 1.0 1.8 1.6 2.5

Dissolved solids, sum of constituents (mg/L) 52 60 52 98 119

Suspended solids (mg/L) 114 111 54 42 48

Nitrogen, nitrite plus nitrate, dissolved (mg/L) .41 .20 .28 .20 .44

Nitrogen, ammonia, dissolved (mg/L) .10 .10 .08 .03 <.02

Nitrogen, ammonia plus organic, total (mg/L) .60 .80 1.0 .80 1.02

Nitrogen, ammonia plus organic, dissolved (mg/L) .30 .30 .40 .40 .39

Phosphorus, total (mg/L) .20 .18 .19 .14 .07

Arsenic, total (μg/L) 2 2 12 3 --

Cadmium, total (μg/L) <1 <1 <1 <1 <1

Chromium, total (μg/L) 4 6 1 2 <1

Copper, total (μg/L) 7 6 6 3 2.1

Lead, total (μg/L) 9 10 3 10 2.2

Mercury, total (μg/L) <.1 <.1 <.1 <.1 <.1

Nickel, total (μg/L) 3 4 2 2 --

Zinc, total (μg/L) 100 45 37 16 <10

Organic carbon, total (mg/L) 14 13 10 12 10

Cyanide, total (mg/L) <.01 <.01 <.01 <.01 <.01

Phenols, total (μg/L) 4 -- <1 4 <1

Oil and grease, total (mg/L) 3 -- 2 <1 <1

DDD, total (μg/L) <.1 <.1 <.1 <.1 <.1

DDE, total (μg/L) <.04 <.04 <.04 <.04 <.04

DDT, total (μg/L) <.1 <.1 <.1 <.1 <.1

Diazinon, total (μg/L) <.05 <.05 .5 <.05 <.05
5

150

100

50

0

TOTAL ZINC,

IN MICROGRAMS PER LITER

(10)

(15)

(4)

[0] [0] [2]

COMMERCIAL RESIDENTIAL UNDEVELOPED

LAND USE

20

15

10

5

0

TOTAL LEAD,

IN MICROGRAMS PER LITER

(12)

(16)

(4)

[0] [1] [1]

COMMERCIAL RESIDENTIAL UNDEVELOPED

LAND USE

12

10

8

6

4

2

0

TOTAL COPPER,

IN MICROGRAMS PER LITER

(16)

(4)

[0] [0] [1]

10

8

6

4

2

0

TOTAL CHROMIUM, 

IN MICROGRAMS PER LITER

(12)

(16)

(4)

[1] [4] [2]

(12)

EXPLANATION

(12) Number of samples

Data value 1.5 to 3.0 times the IQR 

outside the box

Largest data value within 1.5 times 

the IQR above the box

75th percentile

Median (50th percentile)

25th percentile

Minimum reporting level

Data value instead of boxplot

Number of samples less than

minimum reporting level

[2]

Smallest data value within 1.5 times 

the IQR below the box

Interquartile

range (IQR)

yield of total lead, for example, for the 

watersheds. Annual mass yields for 1997 

were computed for total lead for each 

watershed using the equation:

, 

where

Y = annual mass yield in pounds per year 

per acre; 

Q = annual runoff in acre-feet per year;

C = flow-weighted average of EMCs in 

micrograms per liter;

A = watershed area in acres; and 

cf = conversion factor (0.00272).

Annual runoff, flow-weighted averages 

of EMCs, and mass yields of total lead for 

the five watersheds are listed in table 3. 

All samples collected from June 1996 

through October 1998 (some watersheds 

had limited runoff during 1997) were used 

to compute the flow-weighted average of 

total lead EMCs, except for watershed 5. 

In 1997, all of the runoff for watershed 5 

occurred during a single event. Samples 

collected for watershed 5 during the event 

yielded total lead concentrations that were 

less than the laboratory minimum reporting level, so a value equal to the reporting 

level was used as the average EMC to 

calculate a maximum for the annual yield. 

The magnitude of differences in annual 

mass yields among the watersheds is illustrated in figure 5.

Y QC

A = -------- • cf

Figure 4. Distributions of total chromium, copper, lead, and zinc concentrations by land use, Bexar County, Texas, 1996–98.
6

References

Helsel, D.R., and Hirsch, R.M., 1992, 

Studies in environmental science 

49—Statistical methods in water 

resources: Amsterdam, Elsevier, 

522 p.

Stein, W.G., and Ozuna, G.B., 1995, 

Geologic framework and hydrogeologic characteristics of the Edwards 

aquifer recharge zone, Bexar County, 

Texas: U.S. Geological Survey WaterResources Investigations Report 

95–4030, 8 p., 1 pl.

—D.J. Ockerman, B.L. Petri, and 

R.N. Slattery

Any use of trade, product, or firm names 

is for descriptive purposes only and 

does not imply endorsement by the 

U.S. Government.

For more information, please contact:

San Antonio Subdistrict Chief

U.S. Geological Survey

435 Isom Road, Suite 234

San Antonio, TX 78216

Phone: (210) 321–5200

FAX: (210) 530–6008

E-mail: gbozuna@usgs.gov

World Wide Web: http://tx.usgs.gov

0.028

0.015

0.008

0.001

<0.001

12345

WATERSHED

0

0.030

0.005

0.010

0.015

0.020

0.025

MASS YIELD FOR TOTAL LEAD, IN POUNDS PER YEAR PER ACRE

Figure 5. Annual mass yields for total lead by watershed, Bexar County, Texas, 1997.

Table 3. Runoff, flow-weighted averages of event-mean concentrations of lead, and mass 

yields by watershed, Bexar County, Texas, 1997

[μg/L, micrograms per liter; <, less than] 

Watershed

Runoff

(acre-feet per year)

Flow-weighted average of

event-mean concentrations

of total lead

(μg/L)

Mass yield of

total lead

(pounds per

year per acre)

1 24.5 8 0.028

2 149 12 .015

3 41.5 5 .008

4 39.6 3 .001

5 2,650 <1 <.001