U.S. Department of the Interior

U.S. Geological Survey

Data Series 740

Prepared in cooperation with the San Antonio Water System

Quality of Surface-Water Runoff in Selected Streams in the 

San Antonio Segment of the Edwards Aquifer Recharge Zone, 

Bexar County, Texas, 1997−2012
Cover photographs:

U.S. Geological Survey hydrographers at Culebra Creek in Government Canyon State Natural Area near Helotes, Texas, October 23, 2000 

(photographs by Michael B. Nyman).
Quality of Surface-Water Runoff in 

Selected Streams in the San Antonio 

Segment of the Edwards Aquifer Recharge 

Zone, Bexar County, Texas, 1997−2012

By Stephen P. Opsahl

Prepared in cooperation with the San Antonio Water System

Data Series 740

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/

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Opsahl, S.P., 2012, Quality of surface-water runoff in selected streams in the San Antonio segment of the Edwards 

aquifer recharge zone, Bexar County, Texas, 1997−2012: U.S. Geological Survey Data Series 740, 20 p., 2 apps.
iii

Contents

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

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

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

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

Chemical Constituents and Selected Contaminants of Interest—Major Ions,

Nutrients, Trace Metals, and Pesticides .............................................................................3

Methods...........................................................................................................................................................7

Collection of Streamflow Data............................................................................................................7

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

Sample Analyses...................................................................................................................................8

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

Water-Quality Data of Stormwater Runoff in the Edwards Aquifer

Recharge Zone (1997–2012)............................................................................................................9

References Cited..........................................................................................................................................16

Appendixes (available at http://pubs.usgs.gov/ds/740/)

1. Analytical results for physical properties, major ions, nutrients,

bacteria, trace metals, pesticides, and bulk hydrocarbons

(oil and grease) for samples collected from U.S. Geological Survey

stations in Bexar County, Texas, 1997–2012

2. Analytical results for major ions, nutrients, trace metals, and pesticides

for field-equipment blank quality-control samples collected from

U.S. Geological Survey stations in Bexar County, Texas, 1997–2012

Figures

1. Location of data-collection stations in the San Antonio segment

of the Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012...........................2

2. Land cover for stream basins contributing flow to U.S. Geological Survey

stations 08178627 Elm Waterhole Creek Tributary at Evans Road, near

San Antonio, Texas, and 08178628 Unnamed Tributary of Elm Waterhole Creek

at Evans Road at San Antonio, Tex., during A, 2001 and B, 2006 ..........................................5

3. Land cover for stream basins contributing flow to U.S. Geological Survey

stations 08180945 Leon Creek at Scenic Loop Road near Leon Springs,

Texas, 08181400 Helotes Creek at Helotes, Tex., and 08180941 Culebra Creek

in Government Canyon State Natural Area near Helotes, Tex., during A, 2001

and B, 2006.....................................................................................................................................6

4. Graphs of selected trace-metal concentrations for selected streams

in the San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, 1997–2012...............................................................................................14
iv

Tables

1. U.S. Geological Survey water-quality sampling and streamflow

data-collection stations in the San Antonio segment of the

Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012 ......................................4

2. Summary of land-cover percentages for selected stream basins

in the San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, for 2001 and 2006 ....................................................................................4

3. Summary of physical properties measured in samples collected

at U.S. Geological Survey stations from selected streams crossing

the San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, 1997–2012...............................................................................................10

4. Summary of major-ion concentrations measured in samples

collected at U.S. Geological Survey stations from selected streams

crossing the San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, 1997–2012...............................................................................................11

5. Summary of nutrient concentrations measured in samples collected

at U.S. Geological Survey stations from selected streams crossing

the San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, 1997–2012...............................................................................................12

6. Summary of pesticide detections in samples collected at

U.S. Geological Survey stations from selected streams crossing the

San Antonio segment of the Edwards aquifer recharge zone,

Bexar County, Texas, 2001–12...................................................................................................16

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

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).
Quality of Surface-Water Runoff in Selected Streams 

in the San Antonio Segment of the Edwards Aquifer 

Recharge Zone, Bexar County, Texas, 1997−2012

By Stephen P. Opsahl

Abstract

During 1997–2012, the U.S. Geological Survey, in 

cooperation with the San Antonio Water System, collected 

and analyzed water-quality constituents in surface-water 

runoff from five ephemeral stream sites near San Antonio 

in northern Bexar County, Texas. The data were collected 

to assess the quality of surface water that recharges the 

Edwards aquifer. Samples were collected from four stream 

basins that had small amounts of developed land at the 

onset of the study but were predicted to undergo substantial 

development over a period of several decades. Water-quality 

samples also were collected from a fifth stream basin located 

on land protected from development to provide reference 

data by representing undeveloped land cover. Water-quality 

data included pH, specific conductance, chemical oxygen 

demand, dissolved solids (filtered residue on evaporation in 

milligrams per liter, dried at 180 degrees Celsius), suspended 

solids, major ions, nutrients, trace metals, and pesticides. 

Trace metal concentration data were compared to the Texas 

Commission on Environmental Quality established surface 

water quality standards for human health protection (water and 

fish). Among all constituents in all samples for which criteria 

were available for comparison, only one sample had one 

constituent which exceeded the surface water criteria on one 

occasion. A single lead concentration (2.76 micrograms per 

liter) measured in a filtered water sample exceeded the surface 

water criteria of 1.15 micrograms per liter. The average 

number of pesticide detections per sample in stream basins 

undergoing development ranged from 1.8 to 6.0. In contrast, 

the average number of pesticide detections per sample in 

the reference stream basin was 0.6. Among all constituents 

examined in this study, pesticides, dissolved orthophosphate 

phosphorus, and dissolved total phosphorus demonstrated 

the largest differences between the four stream basins 

undergoing development and the reference stream basin with 

undeveloped land cover.

Introduction

The Edwards aquifer is one of the most productive karst 

aquifers in the Nation and the primary public water supply for 

more than 1.7 million people in San Antonio and surrounding 

communities in Bexar County, Texas, a rapidly urbanizing 

region (Sharp and Banner, 1997; U.S. Environmental 

Protection Agency, 2006a; U.S. Census Bureau, 2012). 

The San Antonio segment of the Edwards aquifer system 

(hereinafter Edwards aquifer) in south-central Texas is 

characterized by numerous sinkholes, caves, and streams 

that supply recharge to the aquifer and by rapid interaction 

between surface water and groundwater (Sharp and Banner, 

1997). The Edwards aquifer provides water for public water 

supply, agriculture, industry, military installations, recreational 

activities, and springs. Some springs discharging from the 

Edwards aquifer supply streamflow to downstream users 

and provide habitat for threatened and endangered species 

(Edwards Aquifer Research and Data Center, 2010).

Most recharge to the Edwards aquifer is from direct 

infiltration of precipitation and streamflow that occurs to 

the west of Bexar County and in northern Bexar County 

(Edwards Aquifer Authority, 2011). Increased residential and 

commercial development in Bexar County on the Edwards 

aquifer recharge zone has the potential to discharge substances 

that would impair the quality of water recharging the Edwards 

aquifer (Edwards Aquifer Authority, 2009; Musgrove and 

others, 2009). Impervious land cover associated with such 

development can result in increased stormwater runoff that 

transports contaminants from the land surface to streams 

or geologic features (caves, fractures) that can facilitate 

infiltration to the Edwards aquifer (Ockerman, 2002). The 

potential effects of increased land-surface development on 

surface-water quality in streams recharging the Edwards 

aquifer in northern Bexar County are not well known. To 

learn more about quality of surface water in streams that 

recharge the San Antonio segment of the Edwards aquifer, 
2 Quality of Surface-Water Runoff in Selected Streams of the Edwards Aquifer Recharge Zone, Bexar County, Texas

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

San Antonio Water System (SAWS), conducted a study from 

1997 through 2012 to examine the quality of surface-water 

runoff in streams in northern Bexar County. 

Purpose and Scope

The purpose of this report is to describe water-quality 

data collected at five stations in the Edwards aquifer 

recharge zone in Bexar County, Tex., from 1997–2012. 

Water-quality data collected included measurements of pH, 

specific conductance, chemical oxygen demand (COD), 

and concentrations of dissolved solids (filtered residue 

on evaporation in milligrams per liter [mg/L], dried at 

180 degrees Celsius [°C]), suspended solids, major ions, 

nutrients, trace metals, and pesticides. Sample-collection 

and analysis methods are described for each physical property 

or constituent, and quality-control results are summarized. 

Trace metal concentration data are compared to the Texas 

Commission on Environmental Quality (TCEQ) established 

surface water quality standards for human health protection 

(water and fish), hereinafter referred to as “TCEQ standards” 

(Texas Commission on Environmental Quality, 2010).

Description of Study Area

The study area is within the Edwards aquifer recharge 

zone and adjacent catchment area in northern Bexar County 

(fig. 1). For this report, catchment areas that are immediately 

adjacent to the Edwards aquifer recharge zone and drain to 

the recharge zone are considered to be in the recharge zone. 

Figure 1. Location of data-collection stations in the San Antonio segment of the Edwards aquifer recharge zone, 

Bexar County, Texas, 1997–2012.

C A T C H M E N T A R E A

R E C H A R G E Z O N E

C O N F I N E D Z O N E

Creek

HC

GC BEXAR COUNTY

BEXAR COUNTY

 Recharge zone

 Catchment area

Confined zone
Introduction 3

The catchment area and Edwards aquifer recharge zone 

consist of a network of stream basins and ephemeral streams 

that lose water to the Edwards aquifer as they cross the 

recharge zone. Streamflow data and runoff water-quality 

samples were collected from five ephemeral stream sites 

during 1997–2012: two small unnamed tributaries to Elm 

Waterhole Creek (USGS station 08178627 Elm Waterhole 

Creek tributary at Evans Road, near San Antonio, Tex. 

[hereinafter Elm Waterhole tributary 1, map identifier 

EW], and USGS station 08178628 Unnamed tributary of 

Elm Waterhole Creek at Evans Road at San Antonio, Tex. 

[hereinafter Elm Waterhole tributary 2, map identifier UT]); 

Leon Creek (USGS station 08180945 Leon Creek at Scenic 

Loop Road near Leon Springs, Tex. [hereinafter Leon Creek, 

map identifier LC]); Helotes Creek (USGS station 08181400 

Helotes Creek at Helotes, Tex. [hereinafter Helotes Creek, 

map identifier HC]); and Culebra Creek (USGS station 

08180941 Culebra Creek in Government Canyon State 

Natural Area near Helotes, Tex. [hereinafter Government 

Canyon, map identifier GC]) (table 1). Water-quality samples 

were collected from Government Canyon as a reference site; 

the land in this basin is protected from development, and data 

from this site represents undeveloped land cover.

The five stream basins of the study area encompass 

about 42.6 square miles (mi2) of undeveloped and developed 

land in the Edwards aquifer recharge zone and catchment area 

(table 1). In this report, undeveloped land includes evergreen 

forest, shrub/scrub, deciduous forest, grassland/herbaceous, 

woody wetlands, barren land, and open water (table 2). 

Developed land includes developed open space, developed 

low intensity, and developed medium intensity. The land 

cover of the stream basins from which data were collected is 

predominantly undeveloped (table 2; fig. 2A, B; fig. 3A, B). 

The vegetation is primarily a mixture of oak, juniper, and 

shrub (University of Texas, Bureau of Economic Geology, 

1985). In 2001, the undeveloped land cover ranged from 

85.7 percent in Elm Waterhole tributary 1 stream basin to 

99.9 percent in the Government Canyon stream basin (table 2; 

fig. 2A, B; fig. 3A, B). In 2006, undeveloped land cover ranged 

from 62.5 percent in the Elm Waterhole tributary 1 stream 

basin to 99.2 percent in Government Canyon stream basin. 

There was a large increase (23.2 percent) in the percentage 

of developed land cover in the Elm Waterhole tributary 1 

stream basin. The decrease in the percentage of undeveloped 

land in the Elm Waterhole tributary 1 stream basin occurred 

primarily because of the conversion of evergreen forest and 

shrub/scrub land covers to developed open space or developed 

low intensity land covers. In contrast, the percentage increase 

in total developed land cover in four of the five stream basins 

(Elm Waterhole tributary 2, Leon Creek, Helotes Creek, 

and Government Canyon) between 2001 and 2006 was 

less than 2 percent. 

Chemical Constituents and Selected 

Contaminants of Interest—Major Ions, 

Nutrients, Trace Metals, and Pesticides

Physical properties (pH, specific conductance, COD, 

dissolved solids, and suspended solids) and concentrations of 

water-quality constituents (major ions, nutrients, trace metals, 

and pesticides) were measured in water samples collected 

periodically at sites when runoff events occurred. Major 

ions make up the majority of dissolved constituents in water. 

Major ions are those commonly present in concentrations 

exceeding 1.0 mg/L (Hem, 1985). Within the study area, 

the constituents in water are mostly the ions calcium, 

magnesium, and bicarbonate because these constituents are 

readily weathered from carbonate minerals such as calcite 

and dolomite (Hem, 1985). Noncarbonate constituents 

such as the ions sodium, chloride, and sulfate have several 

possible sources in carbonate terrains, including overlying 

silicate soils; noncarbonate minerals incorporated in the 

bedrock, such as evaporite and clay minerals; and man-made 

compounds such as fertilizers, road salt, treated drinking 

water, septic tanks, and wastewater effluent (Hem, 1985). 

Nutrients are chemicals that organisms need to live and 

grow. Excessive concentrations of nutrients such as nitrogen 

(N) and phosphorus (P) in aquatic environments, however, 

can cause eutrophication, the proliferation of algal blooms and 

other aquatic plants (Lopez and others, 2008; Dubrovsky and 

others, 2010). Common N compounds found in water include 

ammonium, nitrate, nitrite, and organic nitrogen. There are 

many sources of N compounds to water including fertilizers 

(manure, organic fertilizers, and synthetic fertilizers); 

human and animal waste (animal feces, treated wastewater 

effluent, and septic-system drainage); decaying plant debris; 

runoff from fertilized residential lawns, golf courses, 

and construction sites; vehicle exhaust; and precipitation 

(U.S. Environmental Protection Agency, 2012). National 

background concentrations of nitrate in streams have been 

estimated at 0.24 mg/L (Dubrovsky and others, 2010).

Common P compounds found in water include 

orthophosphate, organic P, and sorbed inorganic and organic 

P (Wetzel, 2001). Sources of P to water include fertilizers, 

sewage, detergents, livestock waste, and wildlife waste 

(U.S. Environmental Protection Agency, 2012). Background 

concentrations of orthophosphate in streams in the U.S. have 

been estimated at 0.010 mg/L (Dubrovsky and others, 2010). 

Availability of P is a critical factor controlling plant growth, 

as it frequently is the nutrient in most limited supply in 

aquatic systems (Hem, 1985). 

Residential and commercial development is a potential 

source of trace metals to streams as impervious surfaces 

increase with such development and potential runoff of 

substances from human sources grows. Therefore, relatively 

large concentrations of trace metals in streams that recharge 

the Edwards aquifer potentially may affect groundwater 

quality in the aquifer. A few of the possible sources for 
Table 1. U.S. Geological Survey water-quality sampling and streamflow data-collection stations in the San Antonio segment of the Edwards aquifer County, Texas, 1997–2012. [USGS, U.S. Geological Survey; dd, degrees; mm minutes; ss seconds; mi2, square miles; SW, continuous streamflow gaging station; QW, water quality] Map identifier (fig. 1) USGS station number USGS station name Short name Type of data Latitude (dd mm ss) Longitude (dd mm ss) Drai

EW 08178627 Elm Waterhole Creek Tributary at Evans Road near San Antonio, Tex. Elm Waterhole tributary 1 QW 29 38 48 98 24 23 UT 08178628 Unnamed Tributary of Elm Waterhole Creek at Evans Road at San Antonio, Tex. Elm Waterhole tributary 2 QW 29 38 39.74 98 23 47.57 LC 08180945 Leon Creek at Scenic Loop Road near Leon Springs, Tex. Leon Creek QW 29 40 25.43 98 40 32.04 HC 08181400 Helotes Creek at Helotes, Tex. Helotes Creek SW, QW 29 34 42 98 41 29 1GC 08180941 Culebra Creek in Government Canyon State Natural Area near Helotes, Tex. Government Canyon QW 29 32 21 98 45 05 1

Table 2. Summary of land-cover percentages for selected stream basins in the San Antonio segment of the Edwards aquifer recharge zone, Bexar Cou[USGS, U.S. Geological Survey] Map identifier (fig. 1) USGS station number Short name Evergreen forest (percent) Shrub/ scrub (percent) Decidu- ous forest (percent) Grassland/ herbaceous (percent) Woody wetlands (percent) Barren land (percent) Open water (percent) Developed, open space (percent) 

2001 National Land Cover data (Homer and others, 2004) EW 08178627 Elm Waterhole tributary 1 73.47 8.67 3.57 0.00 0.00 0.00 0.00 7.65 

UT 08178628 Elm Waterhole tributary 2 75.40 10.80 11.50 1.80 0.13 0.00 0.00 0.37 LC 08180945 Leon Creek 47.23 33.64 7.96 9.85 0.04 0.00 0.09 1.15 HC 08181400 Helotes Creek 63.41 19.62 6.16 6.06 0.09 0.01 0.01 4.29 GC 08180941 Government Canyon 87.00 8.64 2.63 1.60 0.00 0.00 0.00 0.00 

2006 National Land Cover data (Fry and others, 2011) EW 08178627 Elm Waterhole tributary 1 56.12 3.83 2.55 0.00 0.00 0.00 0.00 23.21 

UT 08178628 Elm Waterhole tributary 2 68.19 17.12 10.40 1.47 0.13 0.46 0.00 1.67 LC 08180945 Leon Creek 46.71 33.53 7.81 9.73 0.04 0.05 0.09 2.00 HC 08181400 Helotes Creek 62.62 19.71 6.06 5.89 0.09 0.13 0.01 5.14 GC 08180941 Government 

Canyon

86.84 8.27 2.55 1.51 0.00 0.00 0.00 0.69 
BEXAR COUNTY

Elm 

Creek Waterhole Elm Waterhole Creek Tributary 

Figure 2. Land cover for stream basins contributing flow to U.S. Geological Survey stations 08178627 Elm Waterhole Creek Tributary at Evans Road, n(map identifier, EW), and 08178628 Unnamed Tributary of Elm Waterhole Creek at Evans Road at San Antonio, Tex. (map identifier, UT), during A, 2001 a
GC

Figure 3. Land cover for stream basins contributing flow to U.S. Geological Survey stations 08180945 Leon Creek at Scenic Loop Road near Leon SprLC), 08181400 Helotes Creek at Helotes, Tex. (map identifier HC), and 08180941 Culebra Creek in Government Canyon State Natural Area near Helotes, during A, 2001 and B, 2006.
Methods 7

selected trace metals described in this report are as follows: 

barium, discharge of drilling wastes and natural erosion; 

beryllium, coal burning and industrial discharge; cadmium, 

corrosion of pipes and leaching of chemical waste; chromium, 

industrial discharge and natural erosion; copper, corrosion of 

plumbing and natural erosion; lead, corrosion of pipes and 

natural erosion; mercury, industrial discharge and natural 

erosion; antimony, fire retardants, ceramics, and electronic 

waste (U.S. Environmental Protection Agency, 2009). The 

TCEQ standard for mercury in unfiltered water samples 

is 2 micrograms per liter (μg/L). The TCEQ standards for 

barium, cadmium, lead, and antimony in filtered water samples 

are 2,000 μg/L, 5 μg/L, 1.15 μg/L, and 6 μg/L, respectively. 

Pesticides are another group of compounds that were 

analyzed in this investigation; most pesticides had solely 

human origins. Pesticides are toxic compounds designed to 

kill or control plants (herbicides), insects (insecticides), fungi 

(fungicides), or other organisms. Many pesticides commonly 

used before the 1970s were relatively persistent and insoluble 

(for example, organochlorine pesticides). Since the 1970s, 

less persistent and more easily degradable pesticides have 

been developed. Although most recently developed pesticides 

degrade more quickly than organochlorine pesticides, these 

newer pesticides are more soluble and, therefore, frequently 

detected in water (Gilliom and others, 2006). Three 

herbicides—atrazine, prometon, and simazine, as well as 

the degradate of atrazine, 2-Chloro-4 isopropylamino-6-

amino-s-triazine (deethylatrazine)—are among the most 

common compounds detected in urban streams (Ryberg 

and others, 2010). Other herbicides frequently detected 

in urban streams include benfluralin, chlorthal dimethyl, 

metachlor, and pendimethalin. Two of the insecticides most 

commonly detected in urban streams of the U.S. are carbaryl 

and diazinon (Gilliom and others, 2006). Other insecticides 

commonly found in urban streams include fipronil and four 

of its degradates (desulfinylfipronil amide, desulfinylfipronil, 

fipronil sulfide, and fipronil sulfone), disulfoton, and 

malathion (Ryberg and others, 2010).

Methods

Collection of Streamflow Data

Continuous streamflow data for the study period are 

available for one of the five stream sites, the Helotes Creek 

site. The streamflow data collected at the Helotes Creek site 

were published in the USGS National Water Information 

System (NWIS) (U.S. Geological Survey, 2012). At the 

Helotes Creek site, stage is continuously monitored by using a 

water-stage recorder, and streamflow is continuously recorded 

by a USGS-operated streamflow-gaging station. Discrete 

measurements of streamflow were made in accordance with 

USGS methods (Rantz and others, 1982; Turnipseed and 

Sauer, 2010). When conditions allowed (that is, when water 

depths were generally less than 3 feet), streams were waded, 

and velocity measurements were made by using rod-mounted 

acoustic meters (Xylem Analytics, 2012). In all other 

instances, boat-mounted acoustic Doppler current profilers 

were used to measure streamflow (Oberg and others, 2005). 

A stage-discharge relation (rating) was developed for the 

site based on the discrete discharge measurements at the site 

(Kennedy, 1984; Turnipseed and Sauer, 2010). Adjustments 

were made to the rating when discrete discharge measurements 

indicated changing channel conditions. From the stagedischarge relation, stage data were used to compute discharge.

Collection and Processing of

Water-Quality Samples

Water-quality samples were collected during selected 

runoff events. Sample collection and processing techniques 

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

variously dated). Samples for bacteriological analyses 

(enterococcus, Escherichia coli [E. coli], fecal streptococcus, 

and total coliforms) were collected in sterile 250-milliliter 

(mL) glass bottles. Bottles were attached to a weighted 

sampling rod, which was lowered from a bridge or from the 

bank to collect a sample from as close to the center of the 

channel as possible. The remaining water-quality samples 

were collected by using a point-integrated sampling method. 

Each sample was drawn through a fixed intake mounted 

at the mid-point in the stream channel by using a suctionlift type automatic sampler. The automatic sampler was 

programmed to begin sampling at a predetermined stage 

that was selected to ensure streamflow was occurring. The 

autosampler was programmed to fill as many as four 8-liter 

bottles at 30-minute intervals during the runoff event. The 

samples were retrieved at the end of each runoff event or as 

soon as practical after all four bottles were filled. The samples 

were then chilled and transported to the USGS South Texas 

Program Office Laboratory for processing. The four 8-liter 

samples collected by the automatic sampler were composited 

into a single water-quality sample to represent the event mean 

concentration. Composited samples were mixed in a Teflonlined stainless-steel churn, and subsamples were drawn off 

for analysis by the USGS National Water Quality Laboratory 

(NWQL) in Lakewood, Colorado. On many occasions, 

samples representing runoff event mean concentrations could 

not be collected because of variations in precipitation causing 

flows of extended duration, occurrence of multiple discharge 

peaks, mechanical problems with automatic samplers, or 

lack of discharge data. In those instances, multiple grab 

samples were collected from the edge of the stream (sites 

Elm Waterhole tributary 1, Elm Waterhole tributary 2, and 

Government Canyon) or bridge (sites Helotes Creek and Leon 

Creek) and composited into a single water-quality sample. 

Constituent concentrations from multiple grab samples 

represent instantaneous event concentrations. For the purposes 
8 Quality of Surface-Water Runoff in Selected Streams of the Edwards Aquifer Recharge Zone, Bexar County, Texas

of this report, all samples should be considered to represent 

instantaneous event concentrations and should not be used 

to compute longer-term loadings of individual constituents. 

All data from the discrete environmental samples, including 

results for quality-control samples, were published in NWIS 

(U.S. Geological Survey, 2012).

Sample Analyses

The NWQL analyzed the water samples for pH, specific 

conductance, COD, and concentrations of dissolved solids, 

suspended solids, major ions, nutrients, metals, and pesticides 

(appendix 1). Dissolved solids, suspended solids, and COD 

were measured by using methods described by Fishman and 

Friedman (1989). Major ion concentrations were measured 

by using ion-exchange chromatography and inductively 

coupled plasma-atomic emissions spectrometry (ICP-AES), 

respectively, as described by Fishman and Friedman (1989) 

and Fishman (1993). Nutrient concentrations were measured 

by using approved methods as described by Fishman (1993), 

Patton and Kryskalla (2011), and Patton and Truitt (2000). 

Total metals were measured by using methods described by 

Fishman and Friedman (1989), Hoffman and others (1996), 

Garbarino and Damrau (2001), Garbarino and Struzeski 

(1998), and Garbarino and others (2006). Between 1997 and 

2001, organochlorine pesticides and bulk hydrocarbons (oil 

and grease) were measured by using a modification of the 

method described in Wershaw and others (1987). Between 

2002 and 2012, pesticides were measured according to Zaugg 

and others (1995), Lindley and others (1996), and Madsen 

and others (2003). Samples for bacteriological analyses 

were transported to the USGS South Texas Program Office 

Laboratory for processing and analysis. Between 2000 and 

2004, E. coli was measured by using the mTEC MF method 

(U.S. Environmental Protection Agency, 2006b). After 2004, 

E. coli was measured by using defined substrate methods 

described by Myers and others (2007). Fecal coliform 

and fecal streptococci bacteria samples were analyzed by 

using methods described by Britton and Greeson (1987). 

Enterococci were measured by using U.S. Environmental 

Protection Agency method 1600 (U.S. Environmental 

Protection Agency, 2002).

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 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. For each constituent, the 

numerical values of the LT–MDL and LRL can vary over time 

(Childress and others, 1999). 

For inorganic analyses, a constituent concentration 

measured prior to October 2010 was considered estimated by 

the laboratory when results were greater than the LT–MDL 

and less than the LRL; that is, a detection was considered 

likely, but quantification was considered questionable. The 

remark code of “E” (estimated) was assigned by the laboratory 

for these results. Beginning in October 2010, the NWQL 

changed the reporting convention for all inorganic analyses 

that use the LRL convention. For these constituents, the 

reporting level was set at the LT–MDL concentration. Since 

October 2010, concentrations that are less than the LT–MDL 

are reported as “less than” the LT–MDL concentration (<LT–

MDL), and a remark code of “E” is no longer assigned to 

results greater than the LT–MDL and less than the LRL (U.S. 

Geological Survey, 2010). For this report, inorganic analyses 

are reported by using conventions in place at that time the 

sample was collected and analyzed. 

Results for organic analyses (pesticides) were qualified as 

(“E”) estimated by the NWQL following laboratory reporting 

conventions in place at the time the sample was collected and 

analyzed. A remark code of “E” was used when compounds 

were identified, but concentrations could not be accurately 

quantified. Estimated pesticide values (either less than the 

LRL or less than the LT–MDL) were considered as detections.

Quality Assurance

The USGS protocols described in the National Field 

Manual for the Collection of Water-Quality Data (U.S. 

Geological Survey, variously dated) were followed for the 

collection and processing of the quality-assurance samples. 

Sampling methods were designed to minimize potential 

sample contamination and preserve sample integrity. To 

minimize potential contamination of the environmental 

samples, autosamplers and collection bottles were cleaned 

between periods of sample collection. To clean the 

autosamplers, sample tubing was flushed with a soap solution, 

rinsed with deionized water, flushed with a 5-percent solution 

of hydrochloric acid, and re-rinsed with deionized water. 

Sample-collection bottles were washed in the laboratory 

following the same process. 

To document possible contamination, field-equipment 

blank samples (hereinafter field-blank samples) were collected 

before and after equipment rinsing to simulate the collection 

of an environmental sample and to evaluate the effectiveness 

of the equipment-cleaning procedure (appendix 2). Fieldblank samples were collected from each site before and 

after equipment rinsing for a total of 10 field-blank samples. 

These field-blank samples were collected by using inorganic 

and organic blank water (certified ASTM Type I deionized 

water) provided by the NWQL. The organic blank water was 

pumped through the auto-sampler tubing and into the 8-liter 

sample collection jars in the auto-sampler. The field-blank 

samples were processed following the same procedures as 

the environmental samples and analyzed by the NWQL for 

major ions, nutrients, concentrations of metals in unfiltered 

and filtered water samples, and pesticides. The majority of 

reported concentrations of these constituents were below 

LRLs (appendix 2). For cases in which concentrations 

measured in the field-blank samples were greater than the 
Water-Quality Data of Stormwater Runoff in the Edwards Aquifer Recharge Zone (1997–2012) 9

reporting levels, the concentrations were generally less than 

environmental-sample concentrations. For those instances, 

the reported concentrations of environmental samples need 

to be considered as potentially elevated because the sampling 

equipment may have introduced small amounts of the given 

constituent to the environmental samples.

Water-Quality Data of Stormwater 

Runoff in the Edwards Aquifer 

Recharge Zone (1997–2012)

Physical properties (pH, specific conductance, COD, 

dissolved solids, and suspended solids) varied among sites 

(table 3; appendix 1). The ranges of physical properties 

were as follows: pH (6.8 to 8.3), specific conductance 

(85 to 894 microsiemens per centimeter at 25 °C [μS/cm]), 

COD (<10 to 240 mg/L), dissolved solids concentration 

(50 to 593 mg/L), and suspended solids concentration 

(<1 to 1,780 mg/L). Values below the LRL were reported for 

8 of the 59 COD samples and 6 of the 54 suspended solid 

samples. Higher COD values often correspond to wastewater 

discharge into streams; however, there are no wastewater 

discharges into any of the streams sampled in this study. 

Higher COD values also may result from an increase in the 

concentration of natural organic matter, which is common 

during runoff events such as those sampled in this study.

The concentration ranges of major ions (calcium, 

magnesium, potassium, sodium, chloride, silica, and sulfate) 

were as follows: calcium (12.3 to 120 mg/L), magnesium 

(0.573 to 14.8 mg/L), potassium (1.00 to 5.66 mg/L), sodium 

(0.43 to 15.4 mg/L), chloride (0.34 to 241 mg/L), silica 

(4.45 to 17.2 mg/L), and sulfate (0.72 to 50.2 mg/L) (table 4). 

The largest concentration reported for chloride (241 mg/L) 

was measured at the Elm Waterhole tributary 1 site and was 

much larger than all other environmental samples (the next 

highest was 32.8 mg/L at Helotes Creek site). High chloride 

values are sometimes associated with wastewater discharges, 

but there is no source of wastewater discharge to the Elm 

Waterhole tributary 1 site.

Concentrations of N and P constituents varied among 

sites (table 5). The constituents that had one or more samples 

below the LRLs were as follows: dissolved ammonia had 38 

of 60 samples below the LRL, dissolved nitrate had 3 of 40 

samples below the LRL, dissolved and total organic N had 

38 of 57 samples below the LRL, orthophosphate had 9 of 

46 samples below the LRL, dissolved total P had 11 of 57 

samples below the LRL, total P had 3 of 57 samples below the 

LRL, total dissolved N had 5 of 57 samples below the LRL, 

and total N had 4 of 57 samples below the LRL. Among all 

sites, the Government Canyon site had the highest percentage 

of samples (39 percent [44 of 112 samples]) with nutrient 

concentrations that were below the LRL. The concentration 

ranges of nutrient samples that were above the LRLs were as 

follows: dissolved ammonia (0.005 to 0.450 mg/L), dissolved 

nitrate (0.097 to 1.89 mg/L), dissolved nitrite (0.001 to 0.233 

mg/L), dissolved organic N (0.18 to 0.97 mg/L), total organic 

N (0.43 to 9.7 mg/L), dissolved orthophosphate (0.004 to 

0.280 mg/L), dissolved total P (0.003 to 0.320 mg/L), total P 

(0.008 to 1.22 mg/L), total dissolved N (0.26 to 2.7 mg/L), and 

total N (0.42 to 10.0 mg/L). Among all nutrient constituents, 

dissolved orthophosphate P and dissolved total P demonstrated 

the largest differences between the four stream basins 

undergoing development and the reference stream basin with 

undeveloped land cover.

Trace-metal concentrations were measured in filtered 

water samples, unfiltered water samples, or both. Comparisons 

of sample concentrations to TCEQ standards (Texas 

Commission on Environmental Quality, 2010) are provided 

when available (fig. 4A, C, F, G, H). Barium concentrations 

in filtered water samples ranged from 2.54 μg/L to 37.6 μg/L 

(fig. 4A). All barium concentrations in filtered water samples 

were substantially below the TCEQ standard for filtered 

barium (2,000 μg/L). Beryllium concentrations in filtered 

water samples ranged from <0.007 μg/L to 0.085 μg/L (fig. 

4B). Cadmium concentrations ranged from <0.016 μg/L to 

0.08 μg/L in filtered water samples and from <0.018 μg/L to 

0.473 μg/L in unfiltered water samples (fig. 4C). The largest 

cadmium concentration (0.08 μg/L) in filtered water samples 

occurred at Elm Waterhole tributary 1 and that concentration 

was much lower than the TCEQ criterion for filtered cadmium 

(5 μg/L). Chromium in filtered water samples ranged from 

0.06 μg/L to 2.1 μg/L (fig. 4D). Concentrations of copper in 

filtered water samples (<0.4 μg/L to 3.9μg/L) were generally 

smaller compared to copper concentrations in unfiltered 

water samples (<1.0 μg/L to 17.6 μg/L) (fig. 4E). Lead 

concentrations in filtered water samples were relatively 

small, ranging from 0.025μg/L to 2.76 μg/L (fig. 4F). In 

one filtered water sample from site Helotes Creek, the lead 

concentration (2.76 μg/L) exceeded the TCEQ criterion for 

filtered lead of 1.15 μg/L. Lead concentrations in unfiltered 

water samples ranged from 0.33 μg/L to 122 μg/L. Mercury 

concentrations in unfiltered water samples ranged from 

0.005 μg/L to 0.085 μg/L (fig. 4G), all of which were less than 

the TCEQ criterion for unfiltered mercury (2 μg/L). Antimony 

concentrations in filtered water samples ranged from 0.04 

μg/L to 0.203 μg/L (fig. 4H), all of which were less than 

the TCEQ criterion for filtered antimony (6 μg/L). Among 

all constituents for all samples for which TCEQ criteria 

were available, there was only one exceedance, which was 

for filtered lead.

Between 1997 and 2001, 24 organochlorine pesticides 

were measured at four of the five sites: once at the Elm 

Waterhole tributary 1 site, once at the Leon Creek site, 

12 times at the Helotes Creek site, and 5 times at the 

Government Canyon site. Among the 19 samples that were 

analyzed for 24 organochlorine pesticides, there was 1 

constituent detected in 1 sample. The pesticide p, p’-DDD, 

a degradate of p, p’-DDT, was measured at an estimated 
10 Quality of Surface-Water Runoff in Selected Streams of the Edwards Aquifer Recharge Zone, Bexar County, Texas

Table 3. Summary of physical properties measured in samples collected at U.S. Geological Survey stations from selected streams 

crossing the San Antonio segment of the Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012.

[LRL, laboratory reporting level; USGS, U.S. Geological Survey; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees 

Celsius; values for chemical oxygen demand below the laboratory reporting levels are reported as <10 mg/L (appendix 1); values for suspended solids below 

the laboratory reporting levels are reported as <1 mg/L (appendix 1); —, not calculated because of the large number of values less than the laboratory reporting 

level]

Constituent Number of 

Samples

Number of 

concentrations 

equal to or greater 

than the LRL

Minimum

concentration 

(mg/L)

Maximum

concentration 

(mg/L)

Median

concentration 

(mg/L)

USGS station 08178627 Elm Waterhole Creek Tributary at Evans Road, near San Antonio, Tex.

pH 9 9 7.0 8.2 8.0

Specific conductivity (μS/cm

at 25 °C)

10 10 103 894 172

Chemical oxygen demand (mg/L) 11 11 30 130 50

Dissolved solids (mg/L) 10 10 65 593 129

Suspended solids (mg/L) 10 10 26 1,080 66

USGS station 08178628 Unnamed Tributary of Elm Waterhole Creek at Evans Road at San Antonio, Tex.

pH 3 3 7.7 8.0 8.0

Specific conductivity (μS/cm

at 25 °C)

3 3 127 187 186

Chemical oxygen demand (mg/L) 3 2 120 30 —

Dissolved solids (mg/L) 3 3 104 136 119

Suspended solids (mg/L) 3 2 131 65 —

USGS station 08180945 Leon Creek at Scenic Loop Road near Leon Springs, Tex.

pH 9 9 7.8 8.3 8.1

Specific conductivity (μS/cm

at 25 °C)

9 9 96 481 303

Chemical oxygen demand (mg/L) 9 8 110 60 —

Dissolved solids (mg/L) 8 8 80 212 174

Suspended solids (mg/L) 8 8 29 400 211

USGS station 08181400 Helotes Creek at Helotes, Tex.

pH 23 23 6.8 8.3 8.0

Specific conductivity (μS/cm 

at 25 °C)

23 23 85 538 221

Chemical oxygen demand (mg/L) 24 19 110 240 —

Dissolved solids (mg/L) 22 22 50 344 133

Suspended solids (mg/L) 21 17 11 1,780 —

USGS station 08180941 Culebra Creek in Government Canyon State Natural Area near Helotes, Tex.

pH 11 11 7.6 8.1 8.0

Specific conductivity (μS/cm

at 25 °C)

11 11 135 452 195

Chemical oxygen demand (mg/L) 12 11 110 240 40

Dissolved solids (mg/L) 13 13 102 286 140

Suspended solids (mg/L) 12 11 12 455 59

1

Does not include concentrations less than the LRL.
Table 4. Summary of major-ion concentrations measured in samples collected at U.S. Geological Survey stations from selected streams crossing ththe Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012.[USGS, U.S. Geological Survey; n, number of samples analyzed; Min, minimum concentration; Max, maximum concentration; Med, median concentration; mg/L, milligrams p

Constituent

USGS station 08178627

Elm Waterhole Creek

Tributary at Evans Road,

near San Antonio, Tex. (n=10)

USGS station 08178628 

Unnamed Tributary

of Elm Waterhole Creek

at Evans Road at

San Antonio, Tex. (n=3)

USGS station 08180945 Leon 

Creek at Scenic Loop Road 

near Leon Springs, Tex. (n=8)

USGS station 08181400 Helotes 

Creek at Helotes, Tex. (n=22)

Min 

(mg/L)

Max 

(mg/L)

Med 

(mg/L)

Min 

(mg/L)

Max 

(mg/L)

Med 

(mg/L)

Min 

(mg/L)

Max 

(mg/L)

Med 

(mg/L)

Min 

(mg/L)

Max 

(mg/L)

Med 

(mg/L)

Calcium 14.0 120 25.9 20.2 30.4 29.5 14.2 59.6 51.4 12.3 91.3 31.7 Magnesium 0.573 1.70 1.31 1.04 1.74 1.69 0.779 5.28 3.50 0.668 14.8 3.86 Potassium 2.70 5.66 4.25 3.71 4.35 3.77 2.04 3.84 2.51 1.00 3.68 1.99 Sodium 1.10 4.21 2.42 1.37 2.29 2.19 0.93 5.13 2.66 0.43 15.4 2.54 Chloride 1.45 241 3.30 1.18 3.16 2.31 2.50 10.4 3.80 0.34 32.8 4.09 Silica 5.12 17.2 9.13 6.57 14.2 9.37 5.06 10.1 8.99 4.45 11.8 5.25 Sulfate 1.85 11.8 4.43 3.23 13.6 8.30 2.25 13.5 6.14 0.72 50.2 7.25 
12 Quality of Surface-Water Runoff in Selected Streams of the Edwards Aquifer Recharge Zone, Bexar County, Texas

Table 5. Summary of nutrient concentrations measured in samples collected at U.S. Geological Survey stations from selected streams 

crossing the San Antonio segment of the Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012.

[USGS, U.S. Geological Survey; mg/L, milligrams per liter; E, estimated; —, not calculated because of the large number of values less than the laboratory 

reporting level]

Constituent Number of 

samples

Number of

concentrations

equal to or 

greater than the 

LRL

Minimum 

concentration

(mg/L)

Maximum 

concentration

(mg/L)

Median

concentration

(mg/L)

USGS station 08178627 Elm Waterhole Creek Tributary at Evans Road, near San Antonio, Tex.

Dissolved ammonia nitrogen (mg/L) 11 15 0.011 0.450 —

Dissolved nitrate nitrogen (mg/L) 11 11 E0.156 1.89 0.549

Dissolved nitrite nitrogen (mg/L) 11 11 E0.005 0.050 0.014

Dissolved organic nitrogen (mg/L) 11 14 0.58 0.97 —

Total organic nitrogen (mg/L) 11 14 1.10 3.00 —

Dissolved orthophosphate phosphorus 

(mg/L)

11 10 0.010 0.119 0.046

Dissolved total phosphorus (mg/L) 10 10 0.022 0.132 0.051

Total phosphorus (mg/L) 10 10 0.087 0.938 0.163

Total dissolved nitrogen (mg/L) 10 10 0.56 2.7 1.2

Total nitrogen (mg/L) 10 10 1.00 4.60 1.95

USGS station 08178628 Unnamed Tributary of Elm Waterhole Creek at Evans Road at San Antonio, Tex.

Dissolved ammonia nitrogen (mg/L) 3 11 0.076 0.076 —

Dissolved nitrate nitrogen (mg/L) 3 3 0.283 1.57 0.593

Dissolved nitrite nitrogen (mg/L) 3 3 0.007 0.017 0.010

Dissolved organic nitrogen (mg/L) 3 11 0.38 0.38 —

Total organic nitrogen (mg/L) 3 11 0.64 0.64 —

Dissolved orthophosphate

phosphorus (mg/L)

3 3 0.014 0.074 0.031

Dissolved total phosphorus (mg/L) 3 3 0.027 0.081 0.038

Total phosphorus (mg/L) 3 3 0.074 0.146 0.111

Total dissolved nitrogen (mg/L) 3 3 0.81 2.1 1.1

Total nitrogen (mg/L) 3 3 1.20 2.26 1.30

USGS station 08180945 Leon Creek at Scenic Loop Road near Leon Springs, Tex.

Dissolved ammonia nitrogen (mg/L) 9 13 0.005 0.100 —

Dissolved nitrate nitrogen (mg/L) 9 9 0.097 1.07 0.286

Dissolved nitrite nitrogen (mg/L) 9 18 0.001 0.233 —

Dissolved organic nitrogen (mg/L) 8 12 0.42 0.45 —

Total organic nitrogen (mg/L) 8 12 1.20 2.10 —

Dissolved orthophosphate

phosphorus (mg/L)

9 9 0.005 0.160 0.016

Dissolved total phosphorus (mg/L) 8 8 0.011 0.170 0.029

Total phosphorus (mg/L) 8 8 0.050 0.460 0.200

Total dissolved nitrogen (mg/L) 8 8 0.27 1.6 0.65

Total nitrogen (mg/L) 8 8 0.83 2.7 1.7

USGS station 08181400 Helotes Creek at Helotes, Tex.

Dissolved ammonia nitrogen (mg/L) 24 19 0.015 0.130 —

Dissolved nitrate nitrogen (mg/L) 16 16 0.210 0.890 0.494
Water-Quality Data of Stormwater Runoff in the Edwards Aquifer Recharge Zone (1997–2012) 13

Constituent Number of 

samples

Number of

concentrations

equal to or 

greater than the 

LRL

Minimum 

concentration

(mg/L)

Maximum 

concentration

(mg/L)

Median

concentration

(mg/L)

USGS station 08181400 Helotes Creek at Helotes, Tex.—Continued

Dissolved nitrite nitrogen (mg/L) 16 112 0.001 0.020 —

Dissolved organic nitrogen (mg/L) 22 18 0.18 0.64 —

Total organic nitrogen (mg/L) 22 18 0.43 9.7 —

Dissolved orthophosphate

phosphorus (mg/L)

16 112 0.004 0.280 —

Dissolved total phosphorus (mg/L) 22 118 0.003 0.320 —

Total phosphorus (mg/L) 22 22 0.008 1.22 0.120

Total dissolved nitrogen (mg/L) 22 120 0.26 1.5 0.67

Total nitrogen (mg/L) 22 121 0.42 10.0 1.10

USGS station 08180941 Culebra Creek in Government Canyon State Natural Area near Helotes, Tex.

Dissolved ammonia nitrogen (mg/L) 13 14 0.016 0.084 —

Dissolved nitrate nitrogen (mg/L) 7 7 0.256 1.07 0.342

Dissolved nitrite nitrogen (mg/L) 7 16 0.004 0.016 —

Dissolved organic nitrogen (mg/L) 13 14 0.47 0.91 —

Total organic nitrogen (mg/L) 13 14 0.71 4.50 —

Dissolved orthophosphate

phosphorus (mg/L)

7 13 0.005 0.020 —

Dissolved total phosphorus (mg/L) 13 17 0.009 0.028 —

Total phosphorus (mg/L) 13 111 0.040 0.460 —

Total dissolved nitrogen (mg/L) 13 111 0.68 2.0 —

Total nitrogen (mg/L) 13 111 1.00 5.60 —

1

Does not include concentrations less than the LRL.

concentration of 0.0017 μg/L at Helotes Creek (USGS 

station 08181400) on November 23, 2000 (appendix 1).

Between 2002 and 2012, a different pesticide analysis 

that measured 52 pesticides and pesticide degradates was 

used (table 6). During this phase of the study, 35 of the 52 

pesticide compounds were not detected in any of the 40 

samples. A total of 17 of the 52 pesticides were detected one 

or more times over the course of the study, with a total of 102 

detections in 40 samples (table 6). Among all samples, atrazine 

and its degradate, deethylatrazine, were the most commonly 

detected pesticides. For atrazine and deethylatrazine, the fewest 

detections occurred at the Government Canyon site. Simizine, 

an herbicide that is similar to atrazine in chemical composition 

and environmental application, was detected in 9 of the 40 

samples. The insecticide fipronil and four of its degradates were 

also frequently detected, with most detections being measured at 

Elm Waterhole tributary 1 and Elm Waterhole tributary 2. Other 

herbicides that were detected between 2002 and 2012 included 

benfluralin, chlorthal dimethyl, metachlor, pendimethalin, and 

prometon. Other insecticides that were detected between 2002 

and 2012 included carbaryl, diazinon, disulfoton, and malathion. 

Detections among constituents (calculated by dividing the total 

number of detections by the number of samples) showed that 

atrazine was the most frequently detected pesticide during the 

investigation. A comparison of the average number of detections 

per sample among sites (calculated by dividing the total number 

of detections by the number of samples collected at each site) 

showed that sites undergoing development had higher average 

number of detections per sample (1.8 to 6.0 percent) and samples 

collected from the Government Canyon reference site had the 

smallest average number of detections per sample (0.6 percent). 

Table 5. Summary of nutrient concentrations measured in samples collected at U.S. Geological Survey stations from selected streams 

crossing the San Antonio segment of the Edwards aquifer recharge zone, Bexar County, Texas, 1997–2012.—Continued

[USGS, U.S. Geological Survey; mg/L, milligrams per liter; E, estimated; —, not calculated because of the large number of values less than the laboratory 

reporting level]
14 Quality of Surface-Water Runoff in Selected Streams of the Edwards Aquifer Recharge Zone, Bexar County, Texas

0

5

10

15

20

25

30

35

40

Concentration, in micrograms per liter

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Concentration, in micrograms per liter

Filtered

B. Beryllium

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Concentration, in micrograms per liter

Filtered Unfiltered

C. Cadmium

TCEQ standard = 5 μg/L

0.00

0.50

1.00

1.50

2.00

2.50

Concentration, in micrograms per liter

Filtered

D. Chromiun

Filtered

A. Barium

TCEQ standard = 2,000 μg/L

EW UT LC HC GC

EW UT LC HC GC EW UT LC HC GC

EW UT LC HC GC

EW UT LC HC GC

EW Elm Waterhole tributary 1

UT Elm Waterhole tributary 2

LC Leon Creek

HC Helotes Creek

GC Government Canyon

TCEQ Texas Commission on Environmental Quality

Measured concentrations

Sample concentrations—less than the laboratory reporting limit

 in place at the time the sample was collected and analyzed

EXPLANATION

 Figure 4. Graphs of selected trace-metal concentrations for selected streams in the San Antonio segment of the Edwards aquifer 

recharge zone, Bexar County, Texas, 1997–2012.
Water-Quality Data of Stormwater Runoff in the Edwards Aquifer Recharge Zone (1997–2012) 15

0

2

4

6

8

10

12

14

16

18

Concentration, in micrograms per liter

Filtered Unfiltered

E. Copper

0

20

40

60

80

100

120

140

Concentration, in micrograms per liter

Filtered Unfiltered

F. Lead

TCEQ standard = 1.15 μg/L

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Concentration, in micrograms per liter

Unfiltered

G. Mercury

TCEQ standard= 2 μg/L

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Concentration, in micrograms per liter

Filtered

H. Antimony

TCEQ standard= 6 μg/L

EW UT LC HC GC EW UT LC HC GC

EW UT LC HC GC EW UT LC HC GC EW UT LC HC GC EW UT LC HC GC

EW Elm Waterhole tributary 1

UT Elm Waterhole tributary 2

LC Leon Creek

HC Helotes Creek

GC Government Canyon

TCEQ Texas Commission on Environmental Quality

Measured concentrations

Sample concentrations—less than the laboratory reporting limit

 in place at the time the sample was collected and analyzed

EXPLANATION

Figure 4Graphs showing selected tracemetal concentrations for selected streams in the San Antonio segment

 Figure 4. Graphs of selected trace-metal concentrations for selected streams in the San Antonio segment of the Edwards aquifer 

recharge zone, Bexar County, Texas, 1997–2012.—Continued
Table 6. Summary of pesticide detections in samples collected at U.S. Geological Survey stations from selected streams crossing the San Antonio saquifer recharge zone, Bexar County, Texas, 2001–12. [USGS, U.S. Geological Survey; n, number of samples analyzed; H, herbicide; I, insecticide; -, not detected1; —, not applicable. Constituents shown in italics are pesticide degra

Type of pesticide USGS station Elm Waterhole Creek Tributary at Evans Road near San Antonio, Tex. (n=10) USGS station Unnamed Tributary of Elm Waterhole Creek at Evans Road at San Antonio, Tex. (n=3) USGS station Leon Creek at Scenic Loop Road near Leon Springs, Tex. (n=8) USGS station Helotes Creek at Helotes, Tex. (n=12) USGS stati

Culebra Cree

Government C

State Natural 

near Helotes

(n=7) 

2-Chloro-4-isopropylamino- 6-amino-s-triazine H 4 2 2 3 1 Atrazine H 8 3 7 9 2 Benfluralin H - - 1 - - Carbaryl I 2 1 - 5 - Chlorthal dimethyl H 2 1 1 1 - Desulfinylfipronil amide I 1 1 - - - Desulfinylfipronil I 4 2 - 2 - Diazinon I 1 - 1 5 - Disulfoton I - - - 1 - Fipronil sulfide I - 2 - - - Fipronil sulfone I 1 - - - - Fipronil I 2 2 - - - Malathion I 1 1 1 3 - Metachlor H 1 - 1 - 1 Pendimethalin H - - - 1 - Prometon H 3 - - - - Simazine H 3 3 - 3 - Total number of detections per station1 — 33 18 14 33 4 Average number of detections per sample1 — 3.3 6.0 1.8 2.8 0.6 

1Estimated pesticide values (either less than the laboratory reporting level or less than the long-term method dection level) were considered as detections.
References Cited 17

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