U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the San Antonio River Authority

Land-Cover Changes Associated With Oil and Natural-Gas 

Production and Concentrations of Selected Constituents in 

Surface-Water and Streambed-Sediment Samples Collected 

Upstream From and Within an Area of Oil and Natural-Gas 

Production, South Texas, 2008–17

Scientific Investigations Report 2018–5119
Front cover, Oil and natural-gas drilling rigs near U.S. Geological Survey sampling site 08186550 Ecleto 

Creek at County Road 326 near Runge, Texas. Photograph by Cassi L. Crow, U.S. Geological Survey.

Back cover:

Top left, Kayak containing streambed-sediment sampling equipment and samples. Photograph by 

Michael L. Willis, U.S. Geological Survey.

Top right, U.S. Geological Survey personnel kayaking to collect streambed-sediment samples. 

Photograph by Michael L. Willis, U.S. Geological Survey.

Middle left, U.S. Geological Survey personnel processing streambed-sediment sample through 

63-micrometer plastic sieve fabric. Photograph by Cassi L. Crow, U.S. Geological Survey.

Middle right, U.S. Geological Survey personnel processing streambed-sediment sample through 

2-millimeter sieve. Photograph by Cassi L. Crow, U.S. Geological Survey.

Bottom, Equipment and various stages of streambed-sediment sample processing. Photograph by 

Cassi L. Crow, U.S. Geological Survey.
Land-Cover Changes Associated With 

Oil and Natural-Gas Production and 

Concentrations of Selected Constituents in 

Surface-Water and Streambed-Sediment 

Samples Collected Upstream From and 

Within an Area of Oil and Natural-Gas 

Production, South Texas, 2008–17

By Cassi L. Crow, Stephen P. Opsahl, Diana E. Pedraza, Emily C. Pease, and 

Ross K. Kushnereit

Prepared in cooperation with the San Antonio River Authority

Scientific Investigations Report 2018–5119

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Crow, C.L., Opsahl, S.P., Pedraza, D.E., Pease, E.C., and Kushnereit, R.K., 2018, Land-cover changes associated with 

oil and natural-gas production and concentrations of selected constituents in surface-water and streambed-sediment 

samples collected upstream from and within an area of oil and natural-gas production, south Texas, 2008–17: U.S. 

Geological Survey Scientific Investigations Report 2018–5119, 52 p., https://doi.org/10.3133/sir20185119.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-4261-3
iii

Contents

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

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

Purpose and Scope .............................................................................................................................5

Description of Study Area ..................................................................................................................5

Methods ..........................................................................................................................................................5

Computation of Land-Cover Changes Associated with Oil and Natural-Gas Production .......5

Aerial Imagery Collection ..........................................................................................................6

Imagery Interpretation and Data Analysis .............................................................................6

Imagery Quality Assurance .......................................................................................................6

Collection of Streamflow Data ...........................................................................................................9

Collection and Processing of Surface-Water and Streambed-Sediment Samples ..................9

Sample Analyses ................................................................................................................................11

Quality Assurance ..............................................................................................................................11

Land-Cover Changes Associated with Oil and Natural-Gas Production ...........................................30

Well Pads ............................................................................................................................................32

Oil and Natural-Gas Production-Related Features ......................................................................32

Roads ...................................................................................................................................................34

Storage Ponds ....................................................................................................................................34

Runoff Ponds .......................................................................................................................................34

Impervious Surface ...........................................................................................................................34

Continued Changes Since 2015 .......................................................................................................34

Concentrations of Selected Constituents ...............................................................................................34

Constituents in Surface-Water Samples .......................................................................................34

Major and Trace Elements ......................................................................................................36

Organic Constituents ................................................................................................................36

Constituents in Streambed-Sediment Samples ............................................................................41

Major and Trace Elements ......................................................................................................41

Organic Constituents ................................................................................................................42

Summary .......................................................................................................................................................46

References Cited .........................................................................................................................................47
iv

Figures

1. Map showing location of surface-water and streambed-sediment sampling sites 

in the lower San Antonio River watershed, Texas, 2015–17 .................................................3

2. Aerial photographs showing examples of land-cover attributes that were 

classified by using 2008 imagery from the National Agricultural Imagery Program 

and 2015 imagery from the Texas Natural Resources Information System .......................7

3. Photograph showing a ponar sampler used to collect streambed-sediment 

samples at sites from selected streams in the lower San Antonio River 

watershed, Texas, 2015–16 .......................................................................................................11

4. Aerial photographs showing examples of land-cover changes associated with 

oil and natural-gas production in the lower San Antonio River watershed based 

on 2008 imagery from the National Agricultural Imagery Program and 2015 

imagery from the Texas Natural Resources Information System ......................................31

5. Aerial photographs showing examples of land-cover changes associated with 

oil and natural-gas production in the lower San Antonio River watershed based 

on 2008 imagery from the National Agricultural Imagery Program and 2015 

imagery from the Texas Natural Resources Information System ......................................31

6. Graphic showing a comparison of well-pad locations for the 2008 imagery from 

the National Agricultural Imagery Program and the 2015 imagery from the Texas 

Natural Resources Information System within the image analysis area of the 

lower San Antonio River watershed oil and natural-gas production ...............................33

7. Graphs showing concentrations of trace elements in streambed-sediment 

samples in relation to sediment quality guidelines, lower San Antonio River 

watershed, 2011 and 2012 and 2016 ........................................................................................43

8. Graph showing total polycyclic aromatic hydrocarbon concentrations in less 

than 2-millimeter size-fraction streambed-sediment samples collected from 

selected sites in the lower San Antonio River watershed, Texas, 2015–16 .....................44

9. Graph showing total polycyclic aromatic hydrocarbon concentrations in less 

than 63-micrometer size-fraction streambed-sediment samples collected from 

selected sites in the lower San Antonio River watershed, Texas, 2015–16 .....................45

10. Graph showing relation of total polycyclic aromatic hydrocarbon concentrations 

in less than 2-millimeter size-fraction samples to percentage of streambedsediment samples smaller than 63 micrometers collected from selected sites in 

the lower San Antonio River watershed, Texas, 2015–16 ...................................................45

Tables

1. Data-collection sites in the lower San Antonio River watershed, Texas, 2015–17 ...........4

2. Summary of stream discharge and physicochemical properties in surface-water 

samples collected at sites from selected streams in the lower San Antonio River 

watershed, Texas, 2015–17 .......................................................................................................10

3. Analyzing laboratories and analytical methods used in the chemical and physical 

analysis of surface-water and streambed-sediment samples collected from 

select streams in the lower San Antonio River watershed, Texas, 2015–17 ...................12
v

4. Statistical summary of relative percent difference between environmental and 

replicate streambed-sediment samples collected from selected sites in the 

lower San Antonio River watershed, Texas, 2015–16 ..........................................................13

5. Relative percent differences between environmental samples and replicate 

samples for trace elements measured in streambed-sediment samples collected 

from selected sites in the lower San Antonio River watershed, Texas, 2016 ..................13

6. Relative percent differences between environmental samples and replicate 

samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, 

Texas, 2015–16 ...........................................................................................................................14

7. Relative percent differences between environmental samples and replicate 

samples for organic carbon concentrations measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, 

Texas, 2015–16 ...........................................................................................................................17

8. Environmental samples and replicate data for size analysis measured in 

streambed-sediment samples collected from three sites in the lower San Antonio 

River watershed, Texas, 2015–16 ............................................................................................20

9. Percent recovery values for matrix spike samples for polycyclic aromatic 

hydrocarbons measured in streambed-sediment samples collected from 

selected sites in the lower San Antonio River watershed, Texas, 2015–16 .....................21

10. Percent recovery values for matrix spike samples for organic carbon measured 

in streambed-sediment samples collected from selected sites in the lower San 

Antonio River watershed, Texas, 2015–16 .............................................................................27

11. Mean recovery by analyte in matrix spike analysis of streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, 

Texas, 2015–16 ............................................................................................................................30

12. Summary statistics for oil- and natural-gas-related attributes delineated using 

2008 imagery from the National Agricultural Imagery Program and 2015 imagery 

from the Texas Natural Resources Information System for the lower San Antonio 

River watershed, Texas .............................................................................................................32

13. Drinking water standards and health advisories and national recommended 

aquatic life criteria that were used to evaluate the contaminant concentrations 

in surface-water samples collected from five sites in the lower San Antonio River 

watershed, Texas, 2015–17 .......................................................................................................35

14. Glycol concentrations in surface-water samples collected from five sites in the 

lower San Antonio River watershed, Texas, 2015–17 ..........................................................37

15. Summary of semivolatile organic compound and volatile organic compound 

detections in surface-water samples collected at five sites in the lower San 

Antonio River watershed, Texas, 2015–17 .............................................................................38

16. Consensus-based sediment quality guidelines of MacDonald and others (2000) 

that were used to evaluate the contaminant concentrations in streambedsediment samples collected from selected sites in the lower San Antonio River 

watershed, Texas, 2015–16 .......................................................................................................42
vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 0.00254 micrometer (μm)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2) 

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

International System of Units to U.S. customary units

Multiply By To obtain

Length

micrometer (μm) 0.00003937 inch (in.)

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Volume

liter (L) 33.81402 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

liter (L) 61.02 cubic inch (in3) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound, avoirdupois (lb)

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.
vii

Datum

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

Supplemental Information

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 in either milligrams per liter (mg/L) 

or micrograms per liter (μg/L). Concentrations of chemical constituents in streambed sediment 

are given in either milligrams per kilogram (mg/kg) or micrograms per kilogram (μg/kg).

Abbreviations

< less than

BTEX benzene, toluene, ethylbenzene, and all isomers of xylene

CFR Code of Federal Regulations

DOC dissolved organic carbon

EPA U.S. Environmental Protection Agency

GIS geographic information system

LRL laboratory reporting limit

LT-MDL long-term method detection level

MBAS methylene blue active substances

MCL maximum contaminant level

MDL method detection limit

NAIP National Agricultural Imagery Program

NWQL National Water Quality Laboratory (USGS)

PAHs polycyclic aromatic hydrocarbons

PEC probable effect concentration

QC quality control

RPD relative percent difference

RTI RTI Laboratories, Inc.

SQG sediment quality guideline

SVOC semivolatile organic compound

TEC threshold effect concentration

TNRIS Texas Natural Resources Information System

TOC total organic carbon

TSS total suspended solids

USGS U.S. Geological Survey

VOC volatile organic compound

Land-Cover Changes Associated With Oil and Natural-Gas 

Production and Concentrations of Selected Constituents 

in Surface-Water and Streambed-Sediment Samples 

Collected Upstream From and Within an Area of Oil and 

Natural-Gas Production, South Texas, 2008–17

By Cassi L. Crow, Stephen P. Opsahl, Diana E. Pedraza, Emily C. Pease, and Ross K. Kushnereit

Abstract

The extensive development of oil and natural-gas 

resources in south Texas during the past 10 years has led 

to questions regarding possible environmental effects of 

processes associated with oil and natural-gas production, 

in particular the process of hydraulic fracturing, on water 

and other natural resources. Part of the lower San Antonio 

River watershed intersects an area of oil and natural-gas 

production from the sedimentary rocks that compose the 

Eagle Ford Group. 

The rapid expansion of infrastructure associated with oil 

and natural-gas production increases potential pathways for 

inorganic and organic contaminants to enter surface-water 

systems. The U.S. Geological Survey, in cooperation with the 

San Antonio River Authority, analyzed geospatial data from 

different years (2008 and 2015) to evaluate changes in land 

cover associated with oil and natural-gas production activities 

in the lower San Antonio River watershed. Impervious surface 

in this study is defined as land cover consisting of well pads, 

oil- and gas-related features, or roads. The areal coverage 

associated with impervious surface increased from 201 acres 

to 5,390 acres (net increase of 5,189 acres) between 2008 

and 2015. The total percentage of the study area accounted 

for by impervious surface resulting from oil and naturalgas production activities increased from 0.034 percent to 

0.912 percent, which is an increase of approximately 27-fold. 

Collectively, 0.878 percent of the study area was converted to 

new impervious surface between 2008 and 2015. If the area 

associated with new storage ponds (0.066 percent) is added to 

the estimate of total land-cover changes as a result of oil and 

natural-gas production, then 0.944 percent of the study area 

was altered.

During 2015–17, surface-water samples collected from 

5 sites and streambed-sediment samples collected from 

17 sites in the lower San Antonio River watershed were 

analyzed for a broad range of constituents that might be 

associated with oil and natural-gas production. All major 

elements, trace elements, semivolatile organic compounds 

(SVOCs), and volatile organic compounds (VOCs) measured 

in surface-water samples were detected at concentrations 

less than any of the U.S. Environmental Protection Agency’s 

water-quality standards. In general, the greatest SVOC and 

VOC concentrations were observed in samples collected from 

sites upstream from the area of active oil and natural-gas 

production and just downstream from urban areas. The lack 

of benzene, toluene, ethylbenzene, and all isomers of xylene 

(hereinafter referred to as BTEX) for most sites within the 

area of active oil and natural-gas production indicates that 

little, if any, local runoff associated with the area of active oil 

and natural-gas production has contaminated the surface water 

with BTEX compounds. Glycols, which are commonly used 

in hydraulic fracturing fluids as scale inhibitors, were detected 

in one surface-water sample from Ecleto Creek within the 

area of oil and natural-gas production; however, the presence 

of glycols does not necessarily indicate contamination from 

hydraulic fracturing fluid. The glycols detected also have other 

potential sources including the use of diethylene and ethylene 

glycols in antifreeze used in vehicles and the use of triethylene 

glycol in antibacterial air sanitizers.

The concentrations of select constituents in the 

streambed-sediment samples were compared to sediment 

quality guidelines (SQGs). The SQGs evaluate the potential 

toxicity of bed sediments to sediment-dwelling organisms. 

Two SQG concentration levels are used: (1) a lower level, 

called the threshold effect concentration (TEC), below which 

harmful effects to benthic biota are not expected, and (2) a 

higher level, the probable effect concentration (PEC), above 

which harmful effects are expected to occur frequently. The 

PEC for arsenic was exceeded in a sample collected from 

one site on Ecleto Creek. The origin of the elevated arsenic 

concentration is unknown; the contamination likely is not 

related to oil and natural-gas production because the site of 

the sample collection is located upstream from the area of 

active oil and natural-gas production. Streambed-sediment 

samples were analyzed for selected polycyclic aromatic 
2 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

hydrocarbons (PAHs) because PAHs can be used as indicators 

of petroleum hydrocarbons associated with produced waters. 

Each streambed-sediment sample was analyzed for two 

size fractions of PAHs: less than (<) 63 micrometers (μm) 

and < 2 millimeters (mm). Total PAH concentrations in all 

samples, regardless of size fraction, were less than the TEC 

for total PAHs of 1,610 micrograms per kilogram. Total PAH 

concentrations generally were greater in the <63-μm sizefraction samples than in the <2-mm size-fraction samples, 

indicating that PAHs could potentially sorb more readily to 

the exclusively silt- and clay-sized particles that compose 

<63-μm size-fraction samples than to the mixture of silt and 

clay and larger sized particles that compose the <2-mm sizefraction samples. Total PAH concentrations typically were 

greater in the samples collected from the sites upstream from 

the area of active oil and natural-gas production compared 

to those collected from sites within the area in both the 

<2-mm and <63-μm size-fraction samples. The smaller PAH 

concentrations measured in samples collected from within the 

area of active oil and natural-gas production in comparison 

to the upstream urbanized areas indicate relatively minor 

additional local contributions of PAHs of uncertain origin to 

the watershed.

Introduction

The Eagle Ford Group is an important source of oil 

and natural gas in south Texas and contributes greatly to 

the energy production of the United States. Oil and naturalgas production from shale formations such as those present 

in the Eagle Ford Group in south Texas represented about 

50 percent of U.S. crude oil production in 2017 (U.S. 

Energy Information Administration, 2018). The extensive 

development of oil and natural-gas resources in south Texas 

during the past 10 years has led to questions regarding 

possible environmental effects of processes associated with 

oil and natural-gas production, in particular the process of 

hydraulic fracturing, on water and other natural resources. 

In addition to the water required for initial well drilling, 

hydraulic fracturing of a typical well completed in shale 

formations in Texas requires an average 4.3 million gallons 

of water (Nicot and Scanlon, 2012). This water, along with a 

proppant (typically sand) and various chemicals, are pumped 

into a well completed in a shale formation under increased 

pressure to fracture the oil reservoir. The fractures, held open 

by the proppant, facilitate oil and natural-gas flow into the 

well (Veatch, 1983). Flowback water, which is water that 

“flows back” from the shale into the well during the hydraulic 

fracturing process, is a mixture of the hydraulic fracturing 

fluid and water that exists naturally in the shale. Once the 

water flowing out of the well no longer contains any traces of 

hydraulic fracturing fluid and contains only natural formation 

water, the water is called produced water. Produced water can 

continue to be generated along with oil and natural gas for 

as long as the well is in operation (Schramm, 2011). 

Water for the initial well drilling and hydraulic 

fracturing typically is sourced locally and can sometimes 

come from the same sources that provide water for 

domestic, agricultural, and industrial use (Nicot and 

Scanlon, 2012). Concerns about water availability, coupled 

with technological improvements, have led to increased 

use of brackish groundwater and recycled flowback and 

produced water in the hydraulic fracturing process in some 

areas (Nicot and Scanlon, 2012). Concerns about possible 

environmental effects from releases of inorganic and 

organic constituents found in flowback and produced waters 

have been expressed by the public leading to previous 

assessments such as Mahler and Van Metre (2001), Kresse 

and others (2012), and U.S. Environmental Protection 

Agency (2018a). 

Oil and natural-gas production from the Eagle Ford 

Group involves moving large amounts of equipment into 

mostly rural areas for the construction of new well pads. 

Oil- and gas-related equipment often involves heavy 

machinery, trucks, pipe, large warehouses, storage tanks, 

fluids, and parking areas to support oil and natural-gas 

production activities. New roads are constructed to access 

and service newly constructed well pads. Storage ponds 

are created to provide water to support drilling and oil and 

natural-gas production activities. Temporary runoff ponds 

are constructed adjacent to well pads during the drilling 

process to capture runoff from the well pad during the 

drilling phase of oil and natural-gas production. Together, 

these activities could result in alterations to regional land 

cover within the area of development.

Part of the lower San Antonio River watershed 

intersects an area of oil and natural-gas production from 

the Eagle Ford Group (Opsahl and Crow, 2015). The 

rapid expansion of infrastructure associated with oil and 

natural-gas production increases potential pathways for 

inorganic and organic contaminants to enter surface-water 

systems (Kappel and others, 2013). The U.S. Geological 

Survey (USGS), in cooperation with the San Antonio 

River Authority, analyzed geospatial data from different 

years (2008 and 2015) to evaluate changes in land cover 

associated with oil and natural-gas production activities 

in the lower San Antonio River watershed. Additionally, 

surface-water samples collected from 5 sites and streambedsediment samples collected from 17 sites in the lower San 

Antonio River watershed during 2015–17 (fig. 1; table 1) 

were analyzed for a broad range of constituents that might 

be associated with oil and natural-gas production activities. 

The surface-water sample collection effort was intended to 

serve as a follow-up to the more extensive surface-water 

sampling that was done in 2011–13 (Opsahl and Crow, 

2015); therefore, only a few samples were collected from 

a subset of sites which may be used to develop a long-term 

(years to decades) monitoring record. 
Base modified from U.S. Geological Surveydigital data 1:24,000 quadranglesUniversal Transverse Mercator projection,North American Datum of 1983, Zone 14

EXPLANATION

Subwatershed of lower San Antonio Ri

San Antonio River upstream from Guad

 Cibolo Creek Ecleto CreekLower San Antonio River watershed boImage analysis boundaryStudy area boundaryU.S. Geological Survey sampling site aU.S. Geological Survey sampling site a

2

3

0 5 10 15 20 MILES

0 10 15 20 KILOMETERS 5

TEXASLocation map

Lower San Antonio

River watershed

GULF OF 

MEXICO Eagle Ford Group

Sites north of dividing linewere upstream from mostoil and natural-gas production

97°30'

98°0'

29°30'

29°0'

28°30'

123456789

1011121314151617

SSSSS

Map 

identifier

1 

2

3

4

5

6

9

12

11

13

14

15

16

17

7 8

10

Manahuilla Creek

San Antonio River

Ecleto Creek

San Antonio River

Martinez Creek

Calaveras Lake

San Antonio

Cibolo Creek

 River

Cibolo Creek

San Antonio 

Cabeza Creek

M

 River

edina

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORI

DEWITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOLIAD

COUNTY

GOLIAD

COUNTY

REFUGIO

COUNTY

BEE COUNTY

LIVE OAK

COUNTY

BEXAR COUNTY

ATASCOSA COUNTY

VICTORI

DEWITT

COUNTY

WILSON COUNTY

KARNES COUNTY

GONZALES

COUNTY

GUADALUPE COUNTY

COMAL

COUNTY

WILSON COUNTY

KARNES COUNTY

GOLIAD

COUNTY

GOLIAD

COUNTY

REFUGIO

COUNTY

£¤87

£¤183

£¤281

£¤87

£¤90

£¤281

£¤90

£¤59

£¤77

£¤281

£¤181

£¤77

£¤181

§ ̈¦37

§ ̈¦35

§ ̈¦35

§ ̈¦10

§ ̈¦35

§ ̈¦410

§ ̈¦35

§ ̈¦37

§ ̈¦10

§ ̈¦410

Poth

Falls 

City

Floresville

Stockdale

Nixon

Cibolo

San

Antonio

Runge

Karnes

City

St Hedwig

Elmendorf

Selma

Kenedy

Sutherland

Springs

Poth

Falls 

City

Floresville

Stockdale

Nixon

Cibolo

San

Antonio

Runge

Karnes

City

St Hedwig

Elmendorf

Selma

Kenedy

Sutherland

Springs

Figure 1. Location of surface-water and streambed-sediment sampling sites in the lower San Antonio River watershed, Texas, 2015–17.
Table 1. Data-collection sites in the lower San Antonio River watershed, Texas, 2015–17. [USGS, U.S. Geological Survey; NAD 83, horizontal coordinate information referenced to North American Datum of 1983; dd, degrees; mm, minutes; ss, seconds; SAR San AnTrib, tributary] Map identifier (fig. 1) Short name USGS station number USGS station name Latitude (NAD 83) (dd mm ss) Longitude (NAD 83) (dd mm ss) S

1 SAR Elmendorf 08181800 San Antonio River near Elmendorf, Tex. 29° 13’ 19" 98° 21’ 20" Wat

2 SAR 541 08183300 San Antonio River at Farm to Market Road 541 near Poth, Tex. 29° 02’ 22" 98° 07’ 31.8" Bed 3 SAR Falls City 08183550 San Antonio River at Highway 181 at Falls City, Tex. 28° 58’ 38.46" 98° 00’ 36.51" Bed 

4 Cibolo St. Hedwig 08185065 Cibolo Creek near Saint Hedwig, Tex. 29° 30’ 05.2" 98° 11’ 10.5" Wat

5 Cibolo Falls City 08186000 Cibolo Creek near Falls City, Tex. 29° 00’ 50" 97° 55’ 48" Wat

6 Ecleto 627 08186450 Ecleto Creek at Farm to Market Road 627 near Gillett, Tex. 29° 03’ 04" 97° 49’ 00" Bed 7 Ecleto Runge 08186500 Ecleto Creek near Runge, Tex. 28° 55’ 12" 97° 46’ 19" Bed 

8 Ecleto 326 08186550 Ecleto Creek at County Road 326 near Runge, Tex. 28° 53’ 28.4" 97° 45’ 03.4" Wat

9 Escondido 08187500 Escondido Creek at Kenedy, Tex. 28° 49’ 11" 97° 51’ 32" Bed 

10 SAR 72 08188060 San Antonio River at State Highway 72 near Runge, Tex. 28° 50’ 55.3" 97° 44’ 13.7" Wat

11 Cibolo 81 285515097462400 Cibolo Creek at Farm to Market Road 81 near Runge, Tex. 28° 55’ 14.87" 97° 46’ 23.81" Bed 

12 Ecleto 295 285920097475800 Ecleto Creek at County Road 295 near Gillett, Tex. 28° 59’ 20.32" 97° 47’ 58.35" Bed 13 Cibolo Trib 3191 290055097563400 Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex. 29° 00’ 54.67" 97° 56’ 33.71" Bed 14 SAR Trib 220 290340098074500 Unnamed tributary to San Antonio River at Farm to Market Road 220 near Poth, Tex. 29° 03’ 40.47" 98° 07’ 44.86" Bed 15 Cibolo 537 291014097594200 Cibolo Creek at Farm to Market Road 537 near Stockdale, Tex. 29° 10’ 13.66" 97° 59’ 42.01" Bed 16 Cibolo Trib 332 291533098011600 Unnamed tributary to Cibolo Creek at Farm to Market 

Road 332 near Sutherland Springs, Tex.

29° 15’ 32.86" 98° 01’ 16.48" Bed 

17 Ecleto 3335 291733097535800 Ecleto Creek at Farm to Market Road 3335 near 

Pandora, Tex.

29° 17’ 33.32" 97° 53’ 57.83" Bed 
Methods 5

Purpose and Scope

This report summarizes changes in land cover associated 

with oil and natural-gas production between 2008 and 2015 

and describes detections and concentrations of inorganic 

and organic constituents measured in surface-water and 

streambed-sediment samples collected during 2015–17 from 

selected stream sites located upstream from and within the 

area evaluated for land-cover change in the lower San Antonio 

River watershed in south Texas. The results from surfacewater and streambed-sediment samples were used to evaluate 

inorganic and organic constituents which might be associated 

with oil and natural-gas production activities. Streambedsediment samples also were collected for a comparison of 

different sediment particle-size fractions to determine if PAHs 

would more readily sorb to silt- and clay-sized particles than 

to larger sand-sized particles. To determine if any constituent 

concentrations had changed since the previous study, surfacewater and sediment analytical results, where possible, 

were compared to results from Opsahl and Crow (2015), 

a study that summarized the detections and concentrations 

of inorganic and organic constituents in surface-water and 

streambed-sediment samples during 2011–13 in the lower 

San Antonio River watershed. All analytical data collected in 

this study are presented in Crow and others (2018). Possible 

effects associated with oil and natural-gas production activities 

on groundwater were not evaluated in this study.

Description of Study Area

The study area encompasses parts of Bexar, Comal, 

Guadalupe, Wilson, Karnes, and DeWitt Counties in south 

Texas and consists of 1,593 square miles (mi2

) in the lower 

San Antonio River watershed (fig. 1). Since 2008, rapid 

expansion of oil and natural-gas production with the use 

of hydraulic fracturing in the Eagle Ford Group has taken 

place in south Texas (Railroad Commission of Texas, 2018). 

Most of the hydraulic fracturing activity for oil and naturalgas production in the study area is taking place in an area 

encompassing parts of three counties—Wilson, DeWitt, and 

Karnes (Railroad Commission of Texas, 2018). 

The upper part of the Eagle Ford Group extends 

approximately 400 miles (mi) from the Maverick Basin in the 

southwest of the State to the East Texas Basin in the northeast 

of the State (Bureau of Economic Geology, 2018). The Eagle 

Ford Group consists primarily of carbonaceous marls and 

interbedded sandstones, silts, and shale, and it is divided into 

three formations bounded by major unconformities (Surles, 

1987). Deposition of the Eagle Ford Group recorded a northnorthwestward transgression of Cretaceous seas along the 

ancestral Gulf of Mexico during periods when the sea level 

rose, followed by regressions caused by the prograding of 

deltaic complexes during periods when the sea level fell 

(Surles, 1987; Dawson, 2000). In general, lower strata in the 

Eagle Ford Group represent low-energy, poorly oxygenated, 

deep-water environments, whereas the upper strata represent 

high-energy, well-oxygenated, shallow marine environments 

(Dawson, 1997, 2000). The Eagle Ford Group occurs in the 

subsurface of south Texas where, in addition to producing oil 

and natural gas through hydraulic fracturing, it is the source 

rock for oil found in the Austin Chalk and the Buda Limestone

(Surles, 1987). 

Three main streams flow through the area of oil and 

natural-gas production in the study area (fig. 1). The longest of 

the three streams is the San Antonio River. The San Antonio 

River flows about 90 mi through the study area, from near 

Elmendorf, Tex., to just beyond the most downstream site in 

the study area at SAR 72. The second longest stream is Cibolo 

Creek which flows into the San Antonio River approximately 

3 mi northeast of Karnes City, Tex. The length of Cibolo 

Creek from Selma, Tex., to the confluence with the San 

Antonio River is about 75 mi. The third stream, Ecleto Creek, 

flows into the San Antonio River approximately 1 mi west of 

Runge, Tex. The length of Ecleto Creek is about 55 mi from 

northern Wilson County to the confluence of Ecleto Creek and 

the San Antonio River in Karnes County. 

The study area is composed of gently sloping, rolling 

terrain, with the upper part of the watershed more dissected 

and rolling compared to the lower part of the watershed 

(Ryder, 1996). The land cover consists mostly of brush and 

grassland (Homer and others, 2015), and average annual 

precipitation ranges from about 30 inches in the northern part 

of the lower San Antonio River watershed to about 40 inches 

in the southern part of the watershed near the coast (National 

Oceanic and Atmospheric Administration, 2018a, b).

Methods

The study entailed a two-part effort to (1) quantify the 

change in oil and natural gas-associated land-use in the lower 

San Antonio River watershed by analyzing aerial imagery 

data and (2) to determine to what degree, if any, land-cover 

changes might have resulted in changes in the concentrations 

of selected constituents in surface-water and streambedsediment samples. The imagery data and the environmental 

data were analyzed independently to determine changes in 

each data type prior to completing analyses that included all of 

the data.

Computation of Land-Cover Changes Associated 

with Oil and Natural-Gas Production

Land-cover changes associated with oil and natural-gas 

production activities were quantified by comparing aerial 

imagery data collected during 2008 and 2015. Oil and naturalgas features were identified on the aerial imagery so that total 

area covered by each feature could be calculated to provide 

an estimate of change in each feature between 2008 and 2015. 
6 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

The study area used for geographic information system (GIS) 

analysis (fig. 1), a subsection of the study area from which 

surface-water and streambed-sediment samples were collected, 

was estimated to be 591,049 acres and was used to estimate 

the percentage change for each land-use category.

Aerial Imagery Collection

Aerial imagery (hereinafter referred to as “imagery”) 

from the National Agricultural Imagery Program (NAIP) (U.S. 

Department of Agriculture, Natural Resources Conservation 

Service, 2011) was used for the 2008 analysis. NAIP 

imagery provides high-resolution data (1-meter [m] by 1-m 

resolution), suitable for detailed analysis of the landscape, 

that are available for download from the U.S. Department 

of Agriculture Geospatial Data Gateway website (U.S. 

Department of Agriculture, Natural Resources Conservation 

Service, 2011). High-resolution imagery from the Texas 

Natural Resources Information System (TNRIS) was used 

for the 2015 analysis and is available for download from 

the TNRIS website (Texas Natural Resources Information 

System, 2018). The imagery from TNRIS is 6-inch by 6-inch 

(0.1524-m by 0.1524-m) resolution and was recorded during 

January and February 2015. The higher spatial resolution 

and appropriate temporal resolution of the TNRIS imagery 

compared to the 2008 NAIP imagery allowed for increased 

detail and precision during imagery interpretation for the 2015 

analysis. The amount of error introduced to the spatial data 

analysis from comparing imagery with different resolutions 

was not quantified but was expected to be negligible 

considering the small number of features available for 

digitization during the 2008 analysis when most of the land in 

the study area was undeveloped compared to the large number 

of man-made features available for digitizing during the 2015 

analysis. The 2008 NAIP imagery and 2015 TNRIS imagery 

will hereinafter be referred to as the 2008 imagery and 2015 

imagery, respectively.

Imagery Interpretation and Data Analysis

The 2008 imagery and 2015 imagery data were visualized 

and analyzed by using ArcMap (Esri, 2018a). To evaluate 

differences in the 2008 and 2015 imagery, a process referred 

to as “heads-up digitizing” was used to assess differences 

in land-cover attributes associated with oil and natural-gas 

features in 2015 compared to 2008. Heads-up digitizing 

involves manually tracing features on a computer monitor 

from another dataset, usually aerial or satellite imagery, and 

saving the traced features to a new shapefile for use in GIS 

applications (Esri, 2018b). Polygons were drawn around 

five types of oil- and gas-related land-cover attributes based 

on visual interpretation of the 2008 and 2015 imagery data, 

and total areas were calculated from the polygons that were 

delineated from the imagery. The five types of land-cover 

attributes classified from the imagery are the following:

1. Well pads – Well pads were digitized if clearly 

identifiable well location or pump jack was onsite 

(fig. 2A). Well pads that were digitized in the 2008 

imagery but were overgrown by vegetation in the 2015 

imagery were not included in the estimate of the total 

number of new well pads constructed between 2008 and 

2015. Well pads that were identified in the 2008 imagery 

but were classified as a different attribute in the 2015 

imagery (for example, a well pad that was converted to a 

storage pond) were included in total area estimate for the 

new attribute identified in 2015.

2. Oil- and gas-related features – Areas with no apparent 

wells or drilling activity that had been cleared and were 

being used for storing equipment, trucks, chemicals, 

or other materials were considered oil- and gas-related 

features (fig. 2B). 

3. New roads – Roads that run directly from new well pads 

to a main road that were not present in 2008 but were 

present in 2015 were considered new roads (fig. 2C). 

Based on visual inspection of the imagery, new roads 

in this region were generally constructed with a caliché 

base material. Highways, county roads, city streets, 

and other roads commonly associated with population 

centers were not included in this analysis. 

4. Storage ponds – Storage ponds were typically observed 

near multiple well pads and usually had a visible lining 

on the bottom (fig. 2D). Pumps and hoses were often 

present near storage ponds.

5. Runoff ponds – Runoff ponds were located immediately 

adjacent to a well pad (fig. 2E) and were constructed 

to capture the well cuttings (broken fragments of rock 

produced during drilling) and any runoff from the well 

pad during drilling.

Imagery Quality Assurance

Visual inspection of the delineations of oil and naturalgas related land-cover features and attributes were done by 

using GIS. The topology tool was used to locate and correct 

potential topological errors such as dangles and overlapping 

lines created during the digitization process. The 2008 imagery 

used for delineations was visually compared to Google Earth 

imagery from the same time period to ensure that no features 

were missed. The 2015 imagery was not compared to Google 

Earth imagery because the 2015 imagery was of higher 

resolution than the Google Earth imagery.
A B

C Well pad

Well pad

Well pad

Well pad

Well pad

Well pad

Well pad

Well pad

Well pad

Figure 2. Examples of land-cover attributes that were classified by using 2008 imagery from the National Agricultural Imagery Program (U.S. Depart

Resources Conservation Service, 2011) and 2015 imagery from the Texas Natural Resources Information System (Texas Natural Resources Informatiopads; B, oil- and gas-related features; C, new roads; D, a storage pond; and E, a runoff pond. All images shown in this figure were obtained from the 20
D E Figure 2. Examples of land-cover attributes that were classified by using 2008 imagery from the National Agricultural Imagery Program (U.S. Departm

Resources Conservation Service, 2011) and 2015 imagery from the Texas Natural Resources Information System (Texas Natural Resources Information pads; B, oil- and gas-related features; C, new roads; D, a storage pond; and E, a runoff pond. All images shown in this figure were obtained from the 20
Methods 9

Collection of Streamflow Data 

The methods described here for streamflow-data 

collection are modified from Opsahl and Crow (2015). 

Continuous streamflow was measured at USGS streamflowgaging stations (sites 1, 4, 5, and 10) by using methods 

described by Rantz and others (1982a, b) and Turnipseed and 

Sauer (2010). A stage-discharge relation (rating curve) was 

developed from the discrete discharge and stage measurements 

made at each streamflow-gaging station (Kennedy, 1984; 

Turnipseed and Sauer, 2010). Adjustments were made to site 

stage-discharge ratings when discrete discharge measurements 

indicated changing channel conditions. From the stagedischarge ratings, stage data were used to compute continuous 

discharge (Kennedy, 1983). Discharge measurements at 

the time of surface-water sample collection (table 2) were 

obtained from the site stage-discharge relations. Stage at each 

streamflow-gaging station was recorded every 15 minutes 

and transmitted hourly by the Geostationary Operational 

Environmental Satellite transmitter to the USGS National 

Water Information System database (U.S. Geological Survey, 

2017). The sampling site Ecleto 326 was not equipped 

with a continuous streamflow gage, and discrete discharge 

measurements were not made at the time that surface-water 

samples were collected from this site. Discrete discharge 

measurements were not made at the time of streambedsediment sample collection at any of the 17 sites.

Collection and Processing of Surface-Water and 

Streambed-Sediment Samples 

Five surface-water samples and 34 streambed-sediment 

samples were collected from 17 sites in the lower San 

Antonio River watershed during 2015–17 (fig. 1; table 1). 

Sampling sites included a subset of seven sites that were 

previously sampled for surface water or sediment during 

2011–2013 (Opsahl and Crow, 2015; sites 1, 3, 4, 5, 7, 8, 

and 10). During the 2015–17 study, streambed-sediment 

samples were collected from all seven of these sites, and 

surface-water samples from five of these seven sites were 

collected during stormflow-runoff conditions for analysis of 

water-quality constituents. 

Surface-water samples were collected and processed 

by following the methods and guidelines described in U.S. 

Geological Survey (variously dated) and Shelton (1997). 

Values of physicochemical properties of surface water 

(dissolved oxygen concentration, pH, specific conductance, 

water temperature, and turbidity) were measured and recorded 

from the centroid of each stream site by using a YSI model 

6920 multiparameter water-quality sonde (Xylem Analytics, 

2018) prior to collection of surface-water samples (table 2). 

At each sampling site, surface-water samples were collected 

from a minimum of 10 locations spaced at equal-width 

increments across the stream by using samplers designed 

to allow water to enter the sampler with no change in speed 

or direction (isokinetic). When stream depths were shallow 

enough to be waded, surface-water samples were collected 

by using a US DH-81 1-liter (L) bottle sampler (Davis, 2005) 

attached to a wading rod. When the stream was too deep to 

be waded, surface-water samples were collected by using 

either a US DH-2 1-L collapsible bag sampler or a US DH-95 

1-L bottle sampler (Davis, 2005) attached to a reel and crane 

system. Surface-water samples collected by using the US 

DH-81 sampler at streamflow velocities less than (<) 1.5 feet 

per second (ft/s) and samples collected by using the US DH-95 

sampler at velocities <2.0 ft/s were labeled as being collected 

by using the multiple vertical method because the samplers 

are unable to collect isokinetic samples at those velocities. 

Surface-water samples were composited into a 14-L Teflon 

(polytetrafluoroethylene) churn, and aliquots of representative 

whole-water (unfiltered) samples were dispensed from the 

churn into the appropriate sample bottles for analysis of total 

suspended solids (TSS), sulfide, glycols, and methylene blue 

active substances (MBAS). Unfiltered samples for analysis of 

semivolatile organic compounds (SVOCs) and volatile organic 

compounds (VOCs) were collected directly from the centroid 

of the stream to avoid aeration of the samples which would 

allow the constituents to volatize. Additional water samples 

for dissolved solids and inorganics were collected by passing 

water from the churn through a 0.45-micrometer (μm) filter 

into the appropriate sample collection bottles. 

Streambed-sediment samples were collected twice 

from all sampling sites (table 1), once during base flow 

(hereinafter referred to as 2015 samples) and again during 

base flow shortly after multiple stormwater-runoff events 

(hereinafter referred to as 2016 samples). Both sets of 

streambed-sediment samples (2015 and 2016 samples) were 

collected either manually with a scoop while wading or by 

using a ponar sampler (fig. 3) from a watercraft by following 

standard USGS methods as described by Shelton and Capel 

(1994) and Edwards and Glysson (1999). At each site, a 

depositional area was identified, and a minimum of three 

separate samples were collected and composited into a single 

volume. Each sample was collected from an area at least 

20 centimeters (cm) by 20 cm, and only the top 2–3 cm of 

streambed sediment was collected. The streambed sediment 

was composited and put through a stainless steel 2-millimeter 

(mm) sieve in preparation for the analyses of polycyclic 

aromatic hydrocarbons (PAHs), total organic carbon (TOC), 

radium-226, and radium-228; instead of the 2-mm sieve, a 

63-μm plastic sieve fabric was used in preparation for the 

analyses of major and trace elements and for additional 

analyses of PAHs and TOC. Streambed-sediment samples 

that pass through a 2-mm sieve contain sand-, silt-, and claysized particles, whereas streambed-sediment samples that 

pass through a 63-μm sieve contain only the smaller silt- and 

clay-sized particles (Guy, 1969). The purpose of analyzing 

PAHs from samples passed through two different sieve sizes 

(2 mm and 63 μm) was to determine if PAHs would more 

readily sorb to silt- and clay-sized particles than to larger 

sand-sized particles. An unsieved portion of each of the 

streambed-sediment samples also was analyzed for particlesize distribution. 
Table 2. Summary of stream discharge and physicochemical properties in surface-water samples collected at sites from selected streams in the lowwatershed, Texas, 2015–17. [USGS, U.S. Geological Survey; ft3/s, cubic feet per second; mg/L, milligrams per liter; μS/cm at 25 °C, microsiemens per centimeter at 25 degrees Celsius; °C, degrees Celsius

units; SAR, San Antonio River; St., Saint; --, not recorded. Dates are formatted as month-day-year] Map identifier (fig. 1) Short name USGS station number USGS station name Date Discharge (ft3/s) Dissolved oxygen (mg/L) pH Specific conductance (μS/cm at 25 °C) Tempera- ture (°C) Turbidity (FNU) 

1 SAR Elmendorf 08181800 San Antonio River near Elmendorf, Tex. 08-07-2017 4,800 6.1 7.9 224 25.1 590 

4 Cibolo St. Hedwig 08185065 Cibolo Creek near Saint Hedwig, Tex. 10-25-2015 48.4 7.5 7.5 305 21.5 35 

5 Cibolo Falls City 08186000 Cibolo Creek near Falls City, Tex. 10-25-2015 100 7.1 7.1 492 20.3 78 

8 Ecleto 326 08186550 Ecleto Creek at County Road 326 near Runge, Tex. 05-20-2016 -- 6.2 7.7 213 20.4 420 

10 SAR 72 08188060 San Antonio River at State Highway 72 near Runge, Tex. 08-03-2017 506 5.8 7.9 346 26.2 1,120 
Methods 11

Figure 3. A ponar sampler used to collect streambed-sediment 

samples at sites from selected streams in the lower San Antonio 

River watershed, Texas, 2015–16.

Sample Analyses

A complete list of the analytical methods and laboratories 

used to analyze the surface-water and streambed-sediment 

samples for chemical constituents and physical properties is 

provided in table 3. Surface-water samples were analyzed 

for concentrations of dissolved solids, TSS, MBAS, major 

and trace elements, SVOCs, and VOCs by the USGS 

National Water Quality Laboratory (NWQL) in Denver, 

Colorado. Glycol and sulfide concentrations were analyzed 

in surface-water samples by RTI Laboratories, Inc. (RTI), 

in Livonia, Michigan. Streambed-sediment samples were 

analyzed for moisture content and concentrations of major 

and trace elements by the NWQL, and for TOC and PAH 

concentrations by RTI. Streambed-sediment samples also were 

analyzed for concentrations of radium-226 and radium-228 

by ALS Environmental Laboratory in Fort Collins, Colorado. 

Radium data are not evaluated in this report because detected 

concentrations were not distinguishable from background 

sources, but the data are available in Crow and others (2018). 

Thirty-four streambed-sediment samples were submitted to the 

USGS Kentucky Water Science Center Sediment Laboratory 

in Louisville for particle-size analysis. Following an analytical 

issue that involved the exceedance of sample holding times 

that occurred at the NWQL, major and trace element data 

associated with streambed-sediment samples collected in 

2015 were thoroughly reviewed. After review, the data were 

rejected because it could not be verified that the holding-time 

exceedances did not affect the results. All major and trace 

element data for samples collected in 2015 are not used in this 

report, but major and trace element data for samples collected 

in 2016 did not have any issues, and those data are included in 

this report. 

Values reported for samples analyzed by the NWQL and 

RTI are censored by using a laboratory reporting limit (LRL) 

established by each analytical laboratory. For trace elements, 

the NWQL sets the LRL equal to the long-term method 

detection level (LT-MDL) (Childress and others, 1999). 

The LRL for organic constituents analyzed by the NWQL is 

calculated as twice the LT-MDL, which is a modification of 

the U.S. Environmental Protection Agency (EPA) definition 

of the method detection limit (MDL) provided in the Code of 

Federal Regulations (CFR) Title 40 Part 136 (EPA, 1992). The 

LRLs for constituents analyzed by RTI are usually established 

at levels corresponding to the lowest calibration standard used 

to determine an MDL using methods described in CFR Title 

40 Part 136. 

For this report, concentrations equal to or greater 

than the LRL hereinafter are referred to as “detections” or 

“detected concentrations.” Concentrations less than the LRL 

that were reported as less than the LRL by the laboratory 

hereinafter are referred to as “nondetections” or were said 

to be “not detected.” Concentrations less than the LRL but 

greater than the LT-MDL were typically reported by the 

NWQL as estimated concentrations and denoted with the “E” 

remark code. For information-rich analyses, the NWQL can 

report concentrations less than the LT-MDL if the analyte 

is determined to be positively identified (Childress and 

others, 1999). Constituents that were verified as present but 

not quantified in a sample (that is, less than the LRL) were 

qualified by the NWQL with an “M” remark code. Likewise, 

RTI considered a value estimated and denoted with the “E” 

remark code any constituent for which the value was greater 

than the MDL but less than the LRL. Total PAH concentrations 

were computed as the sum of the detected values of 

13 individual PAH compounds: 2-methylnaphthalene, 

acenaphthene, acenaphthylene, anthracene, benzo(a)

anthracene, benzo(a)pyrene, chrysene, dibenzo(a,h)anthracene, 

fluoranthene, fluorene, naphthalene, phenanthrene, and pyrene 

(Ingersoll and others, 2001).

Quality Assurance 

Quality-control (QC) samples were collected and 

analyzed to estimate the amount of variability and bias found 

in the environmental sample results. Variability is the extent to 

which data points in a statistical distribution or dataset diverge 

from the average, or mean, value as well as the extent to which 

these data points differ from each other. Bias is the systematic 

error inherent in a method or caused by some artifact of the 
12 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 3. Analyzing laboratories and analytical methods used in the chemical and physical analysis of 

surface-water and streambed-sediment samples collected from select streams in the lower San Antonio 

River watershed, Texas, 2015–17.

[NWQL, U.S. Geological Survey National Water Quality Laboratory; RTI, RTI Laboratories, Inc.; ALS, ALS Environmental 

Laboratory; KY-WSC, Kentucky Water Science Center Sediment Laboratory]

Constituent Analyzing 

laboratory Method reference(s)

Surface-water samples

Dissolved solids NWQL Fishman and Friedman (1989)

Total suspended solids NWQL Fishman and Friedman (1989)

Methylene blue active substances NWQL Burkhardt and others (1995)

Major and trace elements NWQL Fishman and Friedman (1989)

Fishman (1993)

Struzeski and others (1996)

Garbarino (1999)

Garbarino and others (2006)

Semivolatile organic compounds NWQL Fishman (1993)

Volatile organic compounds NWQL Rose and others (2016)

Glycols RTI U.S. Environmental Protection Agency (1996a)

Sulfide RTI American Public Health Association (1998)

Stream-sediment samples

Percent moisture NWQL American Society for Testing and Materials (1992)

Major and trace elements NWQL Fishman and Friedman (1989)

Garbarino and Struzeski (1998)

U.S. Environmental Protection Agency (1998)

Garbarino and others (2006)

Total organic carbon RTI U.S. Environmental Protection Agency (2004)

Polycyclic aromatic hydrocarbons RTI U.S. Environmental Protection Agency (1996b)

Radium-226, radium-228 ALS U.S. Environmental Protection Agency (1980)

Particle-size distribution KY-WSC Guy (1969)

measurement system (Mueller and others, 2015). The QC 

samples submitted to the analytical laboratories included 

streambed-sediment sample replicates and matrix spikes. 

All QC samples were analyzed by using the same analytical 

methods that were used for analysis of the environmental 

samples. In addition to the QC samples submitted from 

the field, the NWQL analyzed internal laboratory blanks, 

replicates, and spikes. 

Replicate samples are used to evaluate variability in 

measurements associated with analytical processes, sample 

processing protocols, or natural variation, and to provide 

information on the reproducibility and precision of sample 

processing and analysis. In this study, split replicate samples 

were used to evaluate sample processing and analytical 

variability. Split replicate samples are duplicate samples that 

are collected from a single sample container by using identical 

methodology (Horowitz and others, 1994). Variability 

can be evaluated by comparing concentrations between 

environmental and replicate analyte pairs and expressed as 

relative percent differences (RPDs). The RPD was calculated 

between each pair of replicate analyses to provide a measure 

of variability by using the following equation: 

RPD = |C1 – C2|/((C1 + C2)/2) × 100, (1)

where 

C1 is the constituent concentration in the 

environmental sample, and 

C2 is the constituent concentration in the 

duplicate sample.

RPDs calculated for each replicate pair of analytes having 

detectable concentrations are summarized in table 4 and listed 

in tables 5–7. RPDs were not computed for analyte pairs 

where either one or both concentrations were nondetected 

values. 
Methods 13

Table 4. Statistical summary of relative percent difference between environmental and replicate streambed-sediment samples 

collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.

[RPD, relative percent difference; <, less than; mm, millimeter; μm, micrometer]

Category

Number 

of 

samples

Total 

number 

of 

analyte 

pairs

Number of 

analyte pairs 

where both 

samples yielded 

nondetections

Number of 

analyte pairs 

yielding one 

nondetection

Number 

of 

computed 

RPDs

Minimum 

RPD 

(percent)

Maximum 

RPD 

(percent)

Mean 

RPD 

(percent)

Median 

RPD 

(percent)

Trace elements 3 45 2 1 42 0.0 54 11 8.4

Polycyclic aromatic 

hydrocarbons

12 204 153 8 43 1.9 120 46 48

 <2-mm size fraction 6 102 79 7 16 1.9 120 41 22

 <63-μm size fraction 6 102 74 1 27 16 75 46 49

Organic carbon 12 12 0 2 10 0.0 55 20 21

 <2-mm size fraction 6 6 0 2 4 21 35 26 23

 <63-μm size fraction 6 6 0 0 6 0.0 55 17 8.0

Table 5. Relative percent differences between environmental samples and replicate samples for trace elements measured in 

streambed-sediment samples collected from selected sites in the lower San Antonio River watershed, Texas, 2016. 

[USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; REP, replicate sample 

concentration; RPD, relative percent difference; <, less than; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and replicate sample information by USGS station number

USGS station 

number 08186500 08188060 290055097563400

USGS station 

name

Ecleto Creek near 

Runge, Tex.

San Antonio River at State 

Highway 72 near Runge, Tex.

Unnamed tributary to Cibolo Creek 

at Farm to Market Road 3191 

near Falls City, Tex.

Short name Ecleto Runge SAR 72 Cibolo Trib 3191

Map identifier 

(fig. 1) 7 10 13

Sample date 6/24/2016 7/26/2016 6/22/2016

Sieve size 

(mm) 0.063 0.063 0.063

Sample type ENV

(mg/kg)

REP

(mg/kg)

RPD

(percent)

ENV

(mg/kg)

REP

(mg/kg)

RPD

(percent)

ENV

(mg/kg)

REP

(mg/kg)

RPD

(percent)

Calcium 42,100 44,100 4.6 100,000 103,000 3.0 84,800 84,600 0.2

Sodium 661 615 7.21 389 416 6.7 2,230 2,300 3.1

Aluminum 15,000 12,000 22.2 8,900 9,600 7.6 11,000 12,000 8.7

Barium 310 300 3.3 97 100 3.0 110 120 8.7

Cadmium 0.221 0.253 13.5 0.189 0.211 11.0 0.324 0.297 8.7

Chromium 12 8.9 29.7 14 15 6.9 13 13 0.0

Iron 13,000 11,000 16.7 10,000 11,000 9.5 12,000 13,000 8.0

Lead 20 18 10.5 12 14 15.4 15 15 0.0

Lithium 9.5 8 17.1 8.5 9.2 7.9 11 12 8.7

Manganese 610 570 6.8 280 340 19.4 370 380 2.7

Molybdenum E0.130 0.226 53.9 <0.100 <0.100 -- 10 7.3 31.2

Nickel 8.5 7.7 9.9 8.8 9.8 10.8 12.3 12 2.5

Zinc 43 36 17.7 33 38 14.1 47 51 8.2

Arsenic 8.3 8.2 1.2 4.3 4.6 6.7 6.8 6.1 10.9

Boron 4.9 <4.8 -- <9.6 <9.6 -- 6.6 7.1 7.3
Table 6. Relative percent differences between environmental samples and replicate samples for polycyclic aromatic hydrocarbons measured in strecollected from selected sites in the lower San Antonio River watershed, Texas, 2015–16. [USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; REP, replicate sample concentration; RPD, relatithan; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year] Environmental and replicate sample information by USGS station number USGS station number 08186500 USGS station name Ecleto Creek near Runge, Tex. Short name Ecleto Runge Map identifier (fig. 1) 7 Sample date 4/3/2015 4/3/2015 6/24/2016 

Sieve size (mm) 2 0.063 2 

Sample type ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg

2-Methylnaphthalene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

9H-Fluorene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Acenaphthene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Acenaphthylene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Anthracene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Benzo[a]anthracene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Benzo[a]pyrene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Benzo[b]fluoranthene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Benzo[ghi]perylene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Benzo[k]fluoranthene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Chrysene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Dibenzo[a,h]anthracene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Fluoranthene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Indeno[1,2,3-cd]pyrene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Naphthalene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Phenanthrene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 

Pyrene <0.79 <0.80 -- <3.3 <3.8 -- <0.84 <0.83 -- <4.0 
Table 6. Relative percent differences between environmental samples and replicate samples for polycyclic aromatic hydrocarbons measured in strecollected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; REP, replicate sample concentration; RPD, relatithan; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and replicate sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 4/6/2015 4/6/2015 7/26/2016 

Sieve size 

(mm) 2 0.063 2 

Sample type ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) EN

(μg/k

2-Methylnaphthalene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

9H-Fluorene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

Acenaphthene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

Acenaphthylene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

Anthracene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

Benzo[a]anthracene <0.79 <0.88 -- 11 16 37 2.5 2.3 8.3 4.

Benzo[a]pyrene <0.79 <0.88 -- 13 17 27 2.9 2.6 11 6.

Benzo[b]fluoranthene 0.97 1.0 3.0 22 31 34 5.8 4.7 21 11 

Benzo[ghi]perylene 0.87 1.1 23 13 20 42 3.0 1.8 50 <1.

Benzo[k]fluoranthene <0.79 <0.88 -- 8.3 9.8 17 1.7 1.9 11 3.

Chrysene <0.79 <0.88 -- 16 19 17 3.2 2.9 9.8 6.

Dibenzo[a,h]anthracene <0.79 <0.88 -- 3.2 7.0 75 <0.94 <0.94 -- <1.

Fluoranthene <0.79 1.0 -- 23 30 26 5.3 5.2 1.9 9.

Indeno[1,2,3-cd]pyrene <0.79 1.0 -- 11 21 63 3.0 2.0 40 <1.

Naphthalene <0.79 <0.88 -- <2.0 <2.3 -- <0.94 <0.94 -- <1.

Phenanthrene <0.79 <0.88 -- 7.3 8.6 16 <0.94 <0.94 -- <1.

Pyrene <0.79 <0.88 -- 19 24 23 4.5 4.3 4.5 8.
Table 6. Relative percent differences between environmental samples and replicate samples for polycyclic aromatic hydrocarbons measured in strecollected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued [USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; REP, replicate sample concentration; RPD, relatithan; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year] Environmental and replicate sample information by USGS station number USGS station number 290055097563400 USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex. Short name Cibolo Trib 3191 Map identifier (fig. 1) 13 Sample date 4/2/2015 4/2/2015 6/22/2016 

Sieve size (mm) 2 0.063 2 

Sample type ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) REP (μg/kg) RPD (percent) ENV (μg/kg) 

2-Methylnaphthalene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

9H-Fluorene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Acenaphthene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Acenaphthylene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Anthracene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Benzo[a]anthracene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 3.3 -- 3.1 

Benzo[a]pyrene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 19 -- 16 

Benzo[b]fluoranthene 1.2 <1.1 -- <1.8 <1.8 -- 3 10 108 8.2 Benzo[ghi]perylene <1.0 <1.1 -- <1.8 <1.8 -- 2 3.9 64 3.9 

Benzo[k]fluoranthene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 1.9 -- 2.9 

Chrysene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 4.8 -- 4.5 

Dibenzo[a,h]anthracene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Fluoranthene <1.0 <1.1 -- 1.8 <1.8 -- 1.7 6.5 117 6.3 

Indeno[1,2,3-cd]pyrene <1.0 <1.1 -- <1.8 <1.8 -- 1.7 3.9 79 3.9 

Naphthalene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 <1.5 -- <2.0 

Phenanthrene <1.0 <1.1 -- <1.8 <1.8 -- <1.2 2.5 -- 2.3 

Pyrene <1.0 <1.1 -- <1.8 <1.8 -- 1.8 6.2 110 5.8 
Table 7. Relative percent differences between environmental samples and replicate samples for organic carbon concentrations measured in streamcollected from selected sites in the lower San Antonio River watershed, Texas, 2015–16. [USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; REP, replicate sample concentration; RPD, relativthan; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and replicate sample information by USGS station number

USGS station number 08186500

USGS station name Ecleto Creek near Runge, Tex.

Short name Ecleto Runge

Map identifier 

(fig. 1) 7

Sample date 4/3/2015 4/3/2015 6/24/2016 

Sieve size (mm) 2 0.063 2 

Sample type ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) E

(mg

Organic carbon 770 <1,600 -- 20,000 21,000 4.9 1,100 860 24 24,

Environmental and replicate sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 4/6/2015 4/6/2015 7/26/2016 

Sieve size (mm) 2 0.063 2 

Sample type ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) EN

(mg

Organic carbon <1,900 730 -- 13,000 13,000 0.0 9,100 7,300 22 19,
Table 7. Relative percent differences between environmental samples and replicate samples for organic carbon concentrations measured in streamcollected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued [USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; REP, replicate sample concentration; RPD, relativthan; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year] Environmental and replicate sample information by USGS station number USGS station number 290055097563400 USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex. Short name Cibolo Trib 3191 Map identifier (fig. 1) 13 Sample date 4/2/2015 4/2/2015 6/22/2016 

Sieve size (mm) 2 0.063 2 

Sample type ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) REP (mg/kg) RPD (percent) ENV (mg/kg) 

Organic carbon 9,800 14,000 35 12,000 21,000 55 13,000 16,000 21 21,000 
Methods 19

Replicate streambed-sediment samples were collected 

from the same three sites (Ecleto Runge, SAR 72, and 

Cibolo Trib 3191) in 2015 and 2016. Replicates consisted 

of 3 samples for trace elements, 12 samples for PAHs (6 for 

the <2-mm size fraction and 6 for the <63-μm size fraction), 

12 samples for organic carbon (6 for the <2-mm size 

fraction and 6 for the <63-μm size fraction), and 6 samples 

for sediment size analysis. RPDs were computed for trace 

elements, PAHs, and organic carbon for 95 of the total 

261 analyte pairs. The remaining 166 analyte pairs included 

results in which one or both values were nondetections. 

Data for sediment size analysis replicates were evaluated 

by preparing scatterplots to graphically assess differences 

between paired values. The QC data for major and trace 

elements in the 2015 samples were affected by the same 

holding-time exceedance at the NWQL as the environmental 

samples and were, therefore, not used to evaluate variability in 

this study. 

The median value for the 42 RPDs computed for trace 

elements was 8.4 percent (table 4); RPDs ranged from 

0.0 percent for chromium and lead in the 2016 sample 

collected from the Cibolo Trib 3191 site to 54 percent for 

molybdenum in the 2016 sample collected from the Ecleto 

Runge site (table 5). The median value for the 43 RPDs 

computed for all PAHs was 48 percent (table 4); RPDs 

ranged from 1.9 percent for fluoranthene in the <2-mm 

size-fraction 2016 sample collected from the SAR 72 site to 

117 percent for fluoranthene in the <2-mm size-fraction 2016 

sample collected from the Cibolo Trib 3191 site (table 6). 

When evaluated by size fraction, more analytical variability 

in PAHs was observed in the <2-mm size fraction compared 

to the <63-μm size fraction. The median value for the <2-mm 

size-fraction RPDs was 22 percent, and the range of these 

RPDs was similar to the range of the RPDs computed for all 

PAHs (table 4). 

Replicate sample results can be affected by sample 

heterogeneity, particularly when sediment is the sample 

medium (Pirkey and Glodt, 1998). Additionally, when 

constituent concentrations are small, minor differences 

in analyte pair concentrations can cause large RPDs. For 

example, the RPD computed for fluoranthene in the <2-mm 

size-fraction 2016 sample collected from the Cibolo Trib 

3191 site was 117 percent, but the actual concentrations only 

differed by 4.8 micrograms per kilogram (μg/kg) (table 6). 

Based on replicate analysis, no adjustment to interpretation 

of environmental major or trace element, PAH, or organic 

carbon data or rejection of individual values was needed due 

to variability.

Twelve environmental and replicate analyte pairs were 

analyzed for organic carbon concentration from samples 

collected at the Ecleto Runge, SAR 72, and Cibolo Trib 

3191 sites in 2015 and 2016 (table 7). The RPDs for analytes 

in the <2-mm size fraction ranged from 21 to 35 percent 

(4 analyte pairs), and the RPDs for analytes in the <63-μm size 

fraction ranged from 0.0 to 55 percent (6 analyte pairs). The 

median RPD was 23 percent in the <2-mm size fraction and 

8.0 percent in the <63-μm size fraction (table 4).

Six environmental and replicate analyte pairs were 

analyzed for streambed-sediment sample size distribution. 

Three analyte pairs were collected during 2015 from the 

Ecleto Runge, SAR 72, and Cibolo Trib 3191 sites, and 

three pairs were collected during 2016 from the same three 

sites (table 8). Overall, the size fractions associated with 

environmental and replicate sample pairs were similar. The 

small amount of variability observed in the replicate data 

did not affect the ability to evaluate the environmental data; 

therefore, no environmental streambed-sediment size data 

were rejected based on RPDs from this study.

Laboratory matrix spikes (hereinafter referred to 

as “matrix spikes”) are used to assess bias from method 

performance or the effects of the sample matrix (Mueller and 

others, 2015). Matrix spikes are environmental samples to 

which a laboratory adds a known volume and concentration 

of an analyte before analysis; analytical recoveries of the 

spiked analytes are expressed as percentages of expected 

concentrations. Computed spike recoveries are compared 

to expected and laboratory recoveries to evaluate matrix 

interferences or degradation of analytes.

Streambed-sediment samples for matrix spike analysis 

were collected from the same three sites (Ecleto Runge, SAR 

72, and Cibolo Trib 3191) in 2015 and 2016 (tables 9 and 10). 

Samples in both years included <2-mm and <63-μm sizefraction samples, resulting in a total of 12 matrix spikes. Each 

sample was spiked with known concentrations of 17 PAHs and 

dissolved organic carbon (DOC) prior to analysis. Bias can 

be evaluated by computing percent recovery using the known 

spike concentration, environmental concentration in each 

environmental sample, and the concentration recovered from 

each matrix spike. The percent recovery was calculated for 

each matrix spike sample by using the following equation: 

R = ((Cspike – Cenv)/Cexpected) × 100, (2)

where 

R is recovery, in percent

Cspike is the concentration of the analyte in the 

spiked matrix sample,

Cenv is the concentration of the analyte in the 

background environmental sample, and

Cexpected is the concentration of the spiked analyte 

expected in the sample.

When the constituent concentration in the environmental 

sample was a nondetected value, instead of a single percent 

recovery value, a possible range of percent recoveries had to 

be computed. The lower end of the range was the “minimum 

nondetection recovery” (NDRMIN in tables 9 and 10), which 

is computed by using the LRL (the greatest concentration 

of the constituent that could have been contributed by the 

environmental sample) as the Cenv value in equation 2. The 

upper end of the range was the “maximum nondetection 

recovery” (NDRMAX in tables 9 and 10), which is computed 

by using zero (meaning the environmental sample contributed 

none of the constituent to the final concentration) as 

the Cenv value. 
Table 8. Environmental samples and replicate data for size analysis measured in streambed-sediment samples collected from three sites in the lowewatershed, Texas, 2015–16. [USGS, U.S. Geological Survey; mm, millimeter; ENV, environmental sample; REP, replicate sample; --, no data; SAR, San Antonio River; Trib, tributary. Sample dates are for

USGS station number USGS station name Short name Map identi- fier (fig. 1) Sample date Sample type Bed sediment, percent smaller than 0.002 mm Bed sediment, percent smaller than 0.004 mm Bed sediment, percent smaller than 0.008 mm Bed sediment, percent smaller than 0.016 mm Bed sedime

percen

smalle

than 0.06

mm 

08186500 Ecleto Creek near Runge, Tex. Ecleto Runge 7 04-03-2015 ENV 1 1 1 1 2 

REP 1 1 1 2 2 

06-24-2016 ENV 0.0 1 1 1 1 

REP 1 1 1 1 1 

08188060 San Antonio River at State Highway 72 near Runge, Tex. SAR 72 10 04-06-2015 ENV 4 5 5 5 9 

REP 3 3 4 4 6 

07-26-2016 ENV 16 18 19 22 47 

REP 19 21 22 25 50 

290055097563400 Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex. Cibolo Trib 3191 13 04-02-2015 ENV 24 28 30 33 41 

REP 25 26 28 31 39 

06-22-2016 ENV 30 38 45 48 59 

REP 38 48 53 61 71 

USGS station number USGS station name Short name Map identi- fier (fig. 1) Sample date Sample type Bed sediment, percent smaller than 0.5 mm Bed sediment, percent smaller than 1 mm Bed sediment, percent smaller than 2 mm Bed sediment, percent smaller than 4 mm 

08186500 Ecleto Creek near Runge, Tex. Ecleto Runge 7 04-03-2015 ENV 61 88 98 100 

REP 68 90 98 100 

06-24-2016 ENV 56 85 93 96 

REP 62 91 98 99 

08188060

San Antonio River at 

State Highway 72 near Runge, Tex.

SAR 72 10

04-06-2015

ENV 81 99 100 -- 

REP 84 98 100 -- 

07-26-2016

ENV 99 99 100 -- REP 100 -- -- -- 

290055097563400

Unnamed tributary to 

Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex.

Cibolo Trib 

3191 13

04-02-2015

ENV 77 88 97 98 

REP 69 82 92 95 

06-22-2016

ENV 83 87 93 96 

REP 93 96 98 100 
Methods 21

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16. 

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08186500

USGS station name Ecleto Creek near Runge, Tex.

Short name Ecleto Runge

Map identifier 

(fig. 1) 7

Sample date 4/3/2015 4/3/2015

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 15.70 62.91

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <0.79 11 -- 66.7 70.1 <3.3 52 -- 78.5 82.7

9H-Fluorene <0.79 13 -- 78.8 82.8 <3.3 47 -- 71.0 74.7

Acenaphthene <0.79 11 -- 66.7 70.1 <3.3 47 -- 71.0 74.7

Acenaphthylene <0.79 13 -- 78.8 82.8 <3.3 57 -- 86.1 90.6

Anthracene <0.79 13 -- 78.8 82.8 <3.3 56 -- 84.6 89.0

Benzo[a]anthracene <0.79 15 -- 91.0 95.5 <3.3 61 -- 92.1 97.0

Benzo[a]pyrene <0.79 13 -- 78.8 82.8 <3.3 61 -- 92.1 97.0

Benzo[b]fluoranthene <0.79 13 -- 78.8 82.8 <3.3 53 -- 80.0 84.2

Benzo[ghi]perylene <0.79 10 -- 60.6 63.7 <3.3 42 -- 63.4 66.8

Benzo[k]fluoranthene <0.79 9.8 -- 59.4 62.4 <3.3 40 -- 60.4 63.6

Chrysene <0.79 11 -- 66.7 70.1 <3.3 49 -- 74.0 77.9

Dibenzo[a,h]anthracene <0.79 13 -- 78.8 82.8 <3.3 53 -- 80.0 84.2

Fluoranthene <0.79 15 -- 91.0 95.5 <3.3 62 -- 93.6 98.6

Indeno[1,2,3-cd]pyrene <0.79 13 -- 78.8 82.8 <3.3 54 -- 81.6 85.8

Naphthalene <0.79 11 -- 66.7 70.1 <3.3 48 -- 72.5 76.3

Phenanthrene <0.79 13 -- 78.8 82.8 <3.3 52 -- 78.5 82.7

Pyrene <0.79 13 -- 78.8 82.8 <3.3 55 -- 83.1 87.4
22 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08186500

USGS station name Ecleto Creek near Runge, Tex.—Continued

Short name Ecleto Runge

Map identifier 

(fig. 1) 7

Sample date 6/24/2016 6/24/2016

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 16.71 78.92

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <0.84 13 -- 74.1 77.8 <4.0 54 -- 65.1 68.4

9H-Fluorene <0.84 13 -- 74.1 77.8 <4.0 53 -- 63.9 67.2

Acenaphthene <0.84 13 -- 74.1 77.8 <4.0 52 -- 62.7 65.9

Acenaphthylene <0.84 13 -- 74.1 77.8 <4.0 52 -- 62.7 65.9

Anthracene <0.84 13 -- 74.1 77.8 <4.0 51 -- 61.5 64.6

Benzo[a]anthracene <0.84 17 -- 96.9 101.7 <4.0 67 -- 80.8 84.9

Benzo[a]pyrene <0.84 13 -- 74.1 77.8 <4.0 52 -- 62.7 65.9

Benzo[b]fluoranthene <0.84 15 -- 85.5 89.8 <4.0 61 -- 73.6 77.3

Benzo[ghi]perylene <0.84 14 -- 79.8 83.8 <4.0 58 -- 69.9 73.5

Benzo[k]fluoranthene <0.84 12 -- 68.4 71.8 <4.0 51 -- 61.5 64.6

Chrysene <0.84 12 -- 68.4 71.8 <4.0 46 -- 55.5 58.3

Dibenzo[a,h]anthracene <0.84 15 -- 85.5 89.8 <4.0 61 -- 73.6 77.3

Fluoranthene <0.84 12 -- 68.4 71.8 <4.0 47 -- 56.7 59.6

Indeno[1,2,3-cd]pyrene <0.84 12 -- 68.4 71.8 <4.0 49 -- 59.1 62.1

Naphthalene <0.84 12 -- 68.4 71.8 <4.0 51 -- 61.5 64.6

Phenanthrene <0.84 15 -- 85.5 89.8 <4.0 60 -- 72.4 76.0

Pyrene <0.84 14 -- 79.8 83.8 <4.0 56 -- 67.5 71.0
Methods 23

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 4/6/2015 4/6/2015

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 15.45 40.25

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <0.79 11 -- 67.7 71.2 <2.0 23 -- 54.4 57.1

9H-Fluorene <0.79 13 -- 80.0 84.1 <2.0 29 -- 68.6 72.0

Acenaphthene <0.79 12 -- 73.9 77.7 <2.0 26 -- 61.5 64.6

Acenaphthylene <0.79 13 -- 80.0 84.1 <2.0 28 -- 66.3 69.6

Anthracene <0.79 13 -- 80.0 84.1 <2.0 25 -- 59.2 62.1

Benzo[a]anthracene <0.79 17 -- 105 110 11 51 99.5 -- --

Benzo[a]pyrene <0.79 13 -- 80.0 84.1 13 44 82.6 -- --

Benzo[b]fluoranthene 0.97 14 85.3 -- -- 22 49 78.7 -- --

Benzo[ghi]perylene 0.87 16 98.0 -- -- 13 37 69.5 -- --

Benzo[k]fluoranthene <0.79 12 -- 73.9 77.7 8.3 40 82.4 -- --

Chrysene <0.79 12 -- 73.9 77.7 16 40 71.1 -- --

Dibenzo[a,h]anthracene <0.79 18 -- 111 117 3.2 33 75.9 -- --

Fluoranthene <0.79 15 -- 92.4 97.1 23 54 85.4 -- --

Indeno[1,2,3-cd]pyrene <0.79 17 -- 105 110 11 39 76.1 -- --

Naphthalene <0.79 11 -- 67.7 71.2 <2.0 25 -- 59.2 62.1

Phenanthrene <0.79 13 -- 80.0 84.1 7.3 42 88.3 -- --

Pyrene <0.79 14 -- 86.2 90.6 19 49 82.7 -- --
24 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.—Continued

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 7/26/2016 7/26/2016

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 18.85 35.65

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <0.94 16 -- 80.8 84.9 <1.8 18 -- 48.1 50.5

9H-Fluorene <0.94 17 -- 85.9 90.2 <1.8 20 -- 54.7 56.1

Acenaphthene <0.94 17 -- 85.9 90.2 <1.8 20 -- 54.7 56.1

Acenaphthylene <0.94 17 -- 85.9 90.2 <1.8 21 -- 57.4 58.9

Anthracene <0.94 20 -- 101 106 <1.8 23 -- 62.9 64.5

Benzo[a]anthracene 2.5 24 112 -- -- 4.9 26 64.1 -- --

Benzo[a]pyrene 2.9 22 101 -- -- 6 27 64.8 -- --

Benzo[b]fluoranthene 5.8 29 118 -- -- 11 37 79.3 -- --

Benzo[ghi]perylene 3 19 87.0 -- -- <1.8 18 -- 49.2 50.5

Benzo[k]fluoranthene 1.7 19 92.5 -- -- 3.4 24 61.5 -- --

Chrysene 3.2 22 99.8 -- -- 6.1 27 64.7 -- --

Dibenzo[a,h]anthracene <0.94 18 -- 91.0 95.5 <1.8 18 -- 49.2 50.5

Fluoranthene 5.3 26 108 -- -- 9.8 31 68.2 -- --

Indeno[1,2,3-cd]pyrene 3 19 87.0 -- -- <1.8 24 -- 65.6 67.3

Naphthalene <0.94 15 -- 75.8 79.6 <1.8 18 -- 49.2 50.5

Phenanthrene <0.94 19 -- 96.0 101 <1.8 22 -- 60.1 61.7

Pyrene 4.5 24 103 -- -- 8.2 29 66.1 -- --
Methods 25

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 290055097563400

USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex.

Short name Cibolo Trib 3191

Map identifier 

(fig. 1) 13

Sample date 4/2/2015 4/2/2015

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 20.68 35.39

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <1.0 16 -- 73.8 77.4 <1.8 28 -- 75.3 79.1

9H-Fluorene <1.0 16 -- 73.8 77.4 <1.8 30 -- 80.7 84.8

Acenaphthene <1.0 15 -- 69.2 72.5 <1.8 26 -- 69.9 73.5

Acenaphthylene <1.0 18 -- 83.0 87.0 <1.8 31 -- 83.4 87.6

Anthracene <1.0 18 -- 83.0 87.0 <1.8 32 -- 86.0 90.4

Benzo[a]anthracene <1.0 19 -- 87.6 91.9 <1.8 39 -- 105 110

Benzo[a]pyrene <1.0 20 -- 92.3 96.7 <1.8 36 -- 96.8 102

Benzo[b]fluoranthene 1.2 18 82.3 -- -- <1.8 30 -- 80.7 84.8

Benzo[ghi]perylene <1.0 14 -- 64.6 67.7 <1.8 24 -- 64.5 67.8

Benzo[k]fluoranthene <1.0 13 -- 60.0 62.9 <1.8 25 -- 67.2 70.6

Chrysene <1.0 15 -- 69.2 72.5 <1.8 28 -- 75.3 79.1

Dibenzo[a,h]anthracene <1.0 17 -- 78.4 82.2 <1.8 30 -- 80.7 84.8

Fluoranthene <1.0 21 -- 96.9 102 1.8 35 94.1 -- --

Indeno[1,2,3-cd]pyrene <1.0 17 -- 78.4 82.2 <1.8 30 -- 80.7 84.8

Naphthalene <1.0 15 -- 69.2 72.5 <1.8 26 -- 69.9 73.5

Phenanthrene <1.0 16 -- 73.8 77.4 <1.8 29 -- 78.0 81.9

Pyrene <1.0 17 -- 78.4 82.2 <1.8 31 -- 83.4 87.6
26 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 9. Percent recovery values for matrix spike samples for polycyclic aromatic hydrocarbons measured in streambed-sediment 

samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued

[USGS, U.S. Geological Survey; mm, millimeter; μg/kg, micrograms per kilogram; ENV, environmental sample concentration; SPK, spike sample 

concentration; NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, 

San Antonio River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 290055097563400

USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex.—Continued

Short name Cibolo Trib 3191

Map identifier 

(fig. 1) 13

Sample date 6/22/2016 6/22/2016

Sieve size

(mm) 2 0.063

Spike concentration 

(μg/kg) 24.86 39.65

Sample type ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(μg/kg)

SPK

(μg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

2-Methylnaphthalene <1.2 18 -- 69.1 72.4 <2.0 21 -- 50.4 53.0

9H-Fluorene <1.2 19 -- 72.9 76.4 <2.0 21 -- 50.4 53.0

Acenaphthene <1.2 18 -- 69.1 72.4 <2.0 22 -- 52.8 55.5

Acenaphthylene <1.2 18 -- 69.1 72.4 <2.0 21 -- 50.4 53.0

Anthracene <1.2 18 -- 69.1 72.4 <2.0 25 -- 60.0 63.1

Benzo[a]anthracene <1.2 25 -- 95.9 101 3.1 27 63.2 -- --

Benzo[a]pyrene <1.2 19 -- 72.9 76.4 16 25 44.9 -- --

Benzo[b]fluoranthene 3 26 93.3 -- -- 8.2 33 69.0 -- --

Benzo[ghi]perylene 2 21 78.2 -- -- 3.9 26 59.7 -- --

Benzo[k]fluoranthene <1.2 16 -- 61.4 64.4 2.9 24 56.4 -- --

Chrysene <1.2 17 -- 65.2 68.4 4.5 26 58.9 -- --

Dibenzo[a,h]anthracene <1.2 20 -- 76.7 80.5 <2.0 24 -- 57.6 60.5

Fluoranthene 1.7 19 71.5 -- -- 6.3 30 65.3 -- --

Indeno[1,2,3-cd]pyrene 1.7 18 67.8 -- -- 3.9 26 59.7 -- --

Naphthalene <1.2 17 -- 65.2 68.4 <2.0 21 -- 50.4 53.0

Phenanthrene <1.2 22 -- 84.4 88.5 2.3 23 54.8 -- --

Pyrene 1.8 21 78.8 -- -- 5.8 29 63.8 -- --
Methods 27

Table 10. Percent recovery values for matrix spike samples for organic carbon measured in streambed-sediment samples collected 

from selected sites in the lower San Antonio River watershed, Texas, 2015–16. 

[USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; SPK, spike sample concentration; 

NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, San Antonio 

River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08186500

USGS station name Ecleto Creek near Runge, Tex.

Short name Ecleto Runge

Map identifier 

(fig. 1) 7

Sample date 4/3/2015 4/3/2015

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 19,740 38,430

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon 770 22,000 107 -- -- 20,000 58,000 99.3 -- --

Environmental and spike sample information by USGS station number

USGS station number 08186500

USGS station name Ecleto Creek near Runge, Tex.—Continued

Short name Ecleto Runge

Map identifier 

(fig. 1) 7

Sample date 6/24/2016 6/24/2016

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 19,860 33,110

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon 1,100 20,000 95.4 -- -- 24,000 58,000 102 -- --
28 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 10. Percent recovery values for matrix spike samples for organic carbon measured in streambed-sediment samples collected 

from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued 

[USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; SPK, spike sample concentration; 

NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, San Antonio 

River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 4/6/2015 4/6/2015

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 19,520 31,930

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon <1,900 20,000 -- 93.4 102 13,000 43,000 95.7 -- --

Environmental and spike sample information by USGS station number

USGS station number 08188060

USGS station name San Antonio River at State Highway 72 near Runge, Tex.—Continued

Short name SAR 72

Map identifier 

(fig. 1) 10

Sample date 7/26/2016 7/26/2016

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 32,320 32,590

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon 9,100 40,000 96.6 -- -- 19,000 47,000 91.1 -- --
Methods 29

Table 10. Percent recovery values for matrix spike samples for organic carbon measured in streambed-sediment samples collected 

from selected sites in the lower San Antonio River watershed, Texas, 2015–16.—Continued 

[USGS, U.S. Geological Survey; mm, millimeter; mg/kg, milligrams per kilogram; ENV, environmental sample concentration; SPK, spike sample concentration; 

NDRMIN, nondetection percent recovery minimum; NDRMAX, nondetection percent recovery maximum; <, less than; --, not calculated; SAR, San Antonio 

River; Trib, tributary. Sample dates are formatted as month/day/year]

Environmental and spike sample information by USGS station number

USGS station number 290055097563400

USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex.

Short name Cibolo Trib 3191

Map identifier 

(fig. 1) 13

Sample date 4/2/2015 4/2/2015

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 37,980 35,340

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon 9,800 49,000 103 -- -- 12,000 47,000 99.3 -- --

Environmental and spike sample information by USGS station number

USGS station number 290055097563400

USGS station name Unnamed tributary to Cibolo Creek at Farm to Market Road 3191 near Falls City, Tex.—Continued

Short name Cibolo Trib 3191

Map identifier 

(fig. 1) 13

Sample date 6/22/2016 6/22/2016

Sieve size 

(mm) 2 0.063

Spike concentration 

(mg/kg) 44,170 39,950

Sample type ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

ENV

(mg/kg)

SPK

(mg/kg)

Recovery

(percent)

NDRMIN

(percent)

NDRMAX

(percent)

Organic carbon 13,000 56,000 98.0 -- -- 21,000 56,000 91.9 -- --
30 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Mean recoveries for each PAH and for DOC were 

computed by averaging all percent recovery values for 

each analyte (table 11). The average of the nondetection 

percent recovery range [(NRDMIN + NDRMAX)/2] was 

used if the environmental sample concentration was a 

nondetection. Based on the range of mean recoveries (66.2 to 

92.7 percent) calculated for each PAH, method accuracy 

appears to be generally consistent to varying degrees for 

all analytes in both streambed-sediment size fractions. 

Given the consistency of percent recovery values, potential 

corrections to the PAH concentration data were considered 

to be minor and unnecessary for the purposes of this study. 

The mean recovery for DOC (98.0 percent) was much closer 

to expected recoveries, indicating little bias in the DOC 

measurements; therefore, no recovery corrections were 

applied to the DOC data.

Land-Cover Changes Associated with 

Oil and Natural-Gas Production 

Changes in oil and natural-gas production-related 

land-cover features during the study period can be seen by 

comparing the 2008 and 2015 imagery. Examples of the landcover features of interest described in this section are labeled 

on figure 2. In one 6.8-mi2 area, more than 20 well pads were 

added between 2008 and 2015 (fig. 4). Imagery of a smaller 

area near the San Antonio River (approximately 1 mi2) shows 

the addition of eight well pads, one storage pond, three runoff 

ponds, one recently removed runoff pond, and multiple new 

roads (fig. 5). Summary statistics derived from the 2008 and 

2015 imagery (table 12) were used to characterize changes 

in land cover resulting from the increase in oil and naturalgas production activities within the image analysis boundary 

(fig. 1). 

Table 11. Mean recovery by analyte in matrix spike analysis of streambed-sediment samples collected from selected sites in the lower 

San Antonio River watershed, Texas, 2015–16.

[<, less than; mm, millimeter; μm, micrometer]

Analyte

Mean recovery with standard deviation (percent)

All samples Standard 

deviation

<2-mm size 

fraction 

Standard 

deviation

<63-μm size 

fraction

Standard 

deviation

2-Methylnaphthalene 68.7 11.1 73.8 5.4 63.6 13.4

9H-Fluorene 73.0 10.9 79.5 5.2 66.4 11.5

Acenaphthene 69.3 9.3 75.0 7.1 63.6 7.9

Acenaphthylene 74.9 12.3 80.4 6.3 69.3 14.7

Anthracene 76.8 13.3 83.0 11.3 70.7 13.1

Benzo[a]anthracene 92.7 15.8 100 8.5 85.3 18.6

Benzo[a]pyrene 80.0 16.5 84.9 10.6 75.1 20.7

Benzo[b]fluoranthene 84.5 12.1 91.2 13.7 77.9 5.1

Benzo[ghi]perylene 71.3 13.1 78.9 13.3 63.7 7.9

Benzo[k]fluoranthene 68.1 10.6 70.6 12.2 65.7 9.1

Chrysene 71.4 11.0 75.3 12.4 67.5 8.6

Dibenzo[a,h]anthracene 80.0 15.9 89.0 13.3 70.9 13.4

Fluoranthene 83.6 16.1 89.4 15.3 77.9 16.1

Indeno[1,2,3-cd]pyrene 76.9 13.3 82.2 14.3 71.5 10.8

Naphthalene 66.2 8.5 70.6 3.8 61.9 10.0

Phenanthrene 79.2 11.9 85.2 7.8 73.1 12.8

Pyrene 80.5 10.6 85.5 9.1 75.4 10.1

Organic carbon 98.0 4.5 99.6 4.5 96.5 4.3
Land-Cover Changes Associated with Oil and Natural-Gas Production 31

A B

Figure 4. Examples of land-cover changes associated with oil and natural-gas production in the lower San Antonio River watershed 

based on A, 2008 imagery from the National Agricultural Imagery Program (U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2011) and B, 2015 imagery from the Texas Natural Resources Information System (Texas Natural Resources 

Information System, 2018). The area in this figure is approximately 7 square miles.

A B

Removed

runoff pond

Figure 5. Examples of land-cover changes associated with oil and natural-gas production in the lower San Antonio River watershed 

based on A, 2008 imagery from the National Agricultural Imagery Program (U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2011) and B, 2015 imagery from the Texas Natural Resources Information System (Texas Natural Resources 

Information System, 2018). The area in this figure is approximately 1 square mile.
32 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 12. Summary statistics for oil- and natural-gas-related attributes delineated using 2008 imagery from the National Agricultural 

Imagery Program (U.S. Department of Agriculture, Natural Resources Conservation Service, 2011) and 2015 imagery from the Texas 

Natural Resources Information System (TNRIS) (Texas Natural Resources Information System, 2018) for the lower San Antonio River 

watershed, Texas.

[NA, not applicable]

Number 

identified

Surface area 

(acres)

Average 

individual size 

(acres)

Percentage of 

land covered in 

the study area1

2008

Well pads 75 128 1.71 0.022

Oil- and natural-gas-related features 12 35 2.90 0.006

Roads 67 38 0.56 0.006

Storage ponds 1 1 0.96 0.000

Runoff ponds 5 3 0.52 0.000

Total impervious surface associated 

with oil- and natural-gas production2

NA 201 NA 0.034

2015

Well pads 1,340 3,920 2.93 0.663

Oil- and natural-gas-related features 247 897 3.63 0.152

Roads 1,182 573 0.48 0.097

Storage ponds 127 393 3.09 0.066

Runoff ponds 112 120 1.07 0.020

Total impervious surface associated 

with oil- and natural-gas production2

NA 5,390 NA 0.912

1

Study area is 591,049 acres.

2

Total impervious surface associated with oil and natural-gas production was calculated as the sum of the areas of all well pads, oil- and natural-gas-related 

features, and roads.

Well Pads

Well pads accounted for the largest land-cover change 

among the categories defined and delineated within the study 

area (fig. 6; table 12). The total number of well pads increased 

from 75 to 1,340 (net increase of 1,265) between 2008 and 

2015. The number of new well pads (1,275) is slightly greater 

than the net increase in well pads (1,265) because 10 of 

the 75 well pads delineated in the 2008 imagery either had 

returned to a vegetated state or were converted to another 

attribute in the 2015 imagery. The surface area associated 

with well pads increased from 128 acres to 3,920 acres (net 

increase of 3,792 acres) between 2008 and 2015. The average 

size of a well pad was smaller in 2008 (1.71 acres) than in 

2015 (2.93 acres). The total percentage of the study area 

accounted for by the increase in well pads changed from 

0.022 percent in 2008 to 0.663 percent in 2015, which is an 

increase of approximately thirtyfold.

Oil and Natural-Gas Production-Related 

Features

Oil and natural-gas production-related features accounted 

for the second largest change (in surface area acres) in 

land cover associated with oil and natural-gas production 

activities (table 12). The total number of oil and naturalgas production-related features increased from 12 to 247 

between 2008 and 2015. The areal coverage associated with 

oil and natural-gas production-related features increased 

from 35 acres to 897 acres. The average size of an oil and 

natural-gas production-related feature was 2.90 acres in 2008 

and 3.63 acres in 2015, both of which were larger than the 

corresponding average well-pad sizes in 2008 (1.71 acres) 

and 2015 (2.93 acres). The total percentage of the study 

area accounted for by oil and natural-gas production-related 

features increased from 0.006 percent in 2008 to 0.152 percent 

in 2015.
Land-Cover Changes Associated with Oil and Natural-Gas Production 33

Cibolo

San

Creek

River

EcletoCreek

San Antonio

Antonio

River

WILSON COUNTY

DEWITT

COUNTY

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COUNTY

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 COUNTY

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COUNTY

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28°50'

Cibolo

San

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San Antonio

Antonio

River

WILSON COUNTY

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98°06'30" 97°51'

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29°05'30"

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0 5 10 15 20 MILES

0 10 20 KILOMETERS 5 15

EXPLANATION

Subwatershed of lower San Antonio River watershed

 San Antonio River upstream from Guadalupe River

 Cibolo Creek

 Ecleto Creek

Lower San Antonio River watershed boundary

Image analysis boundary

2008 Well pad location

2015 Well pad location

!

!

Base modified from U.S. Geological Survey

digital data 1:24,000 quadrangles

Universal Transverse Mercator projection,

North American Datum of 1983, Zone 14

A B

Figure 6. Comparison of well-pad locations for A, the 2008 imagery from the National Agricultural Imagery Program (U.S. Department 

of Agriculture, Natural Resources Conservation Service, 2011) and B, the 2015 imagery from the Texas Natural Resources Information 

System (Texas Natural Resources Information System, 2018) within the image analysis area of the lower San Antonio River watershed 

oil and natural-gas production.
34 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Roads

Roads associated with oil and natural-gas production 

activities also accounted for a substantial change in land cover 

(surface area) during the study period (table 12). The number of 

individual roads associated with oil and natural-gas production 

activities increased from 67 to 1,182 between 2008 and 2015. 

The areal coverage from roads associated with oil and naturalgas production activities increased from 38 acres in 2008 to 

573 acres in 2015, a net increase of 535 acres. The average 

land-surface area represented by delineated roads associated 

with an individual oil and natural-gas production activity was 

0.56 acres in 2008 and 0.48 acres in 2015, indicating little 

difference between years. The total percentage of the study 

area accounted for by roads associated with oil and naturalgas production activities increased more than sixteenfold from 

0.006 percent in 2008 to 0.097 percent in 2015.

Storage Ponds

Patterns of change observed for ponds varied based on 

pond type (table 12). The number of storage ponds changed 

from 1 in 2008 to 127 in 2015. Storage ponds can be used to 

provide water for wells and thus are expected to remain on the 

landscape as long as there is a demand for the water, which 

can be indefinite because of ongoing drilling in the region. The 

average surface area of storage ponds in 2015 was 3.09 acres, 

and storage ponds covered 393 acres, or 0.066 percent of the 

study area. 

Runoff Ponds

The number of runoff ponds changed from 5 in 2008 to 

112 in 2015 (table 12). Runoff ponds are used to capture runoff 

associated with drilling activities; once drilling is completed, 

the runoff pond is typically removed from the well-pad area. 

Therefore, runoff ponds are likely to be largely absent except in 

areas of active drilling operations. 

Impervious Surface

Impervious surface in this study is defined as land cover 

consisting of well pads, oil- and gas-related features, or roads 

(table 12). The areal coverage associated with impervious 

surface increased from 201 acres to 5,390 acres (net increase 

of 5,189) between 2008 and 2015. The total percentage of 

the study area accounted for by impervious surface resulting 

from oil and natural-gas production activities increased 

from 0.034 percent to 0.912 percent, which is an increase 

of approximately 27-fold. Collectively, 0.878 percent of the 

study area was converted to new impervious surface between 

2008 and 2015. If the area associated with new storage ponds 

(0.066 percent) is added to the estimate of total land-cover 

changes as a result of oil and natural-gas production, then 

0.944 percent of the study area was altered between 2008 

and 2015.

Continued Changes Since 2015

Oil and natural-gas production activities in the study 

area have continued to increase since 2015, and the resulting 

land-cover changes are likely to continue to increase 

proportionally. In 2015, there were 7,574 permitted oil wells 

and 3,886 permitted gas wells in the Eagle Ford Group 

throughout Texas, for a total of 11,460 wells at the time the 

2015 imagery was produced (Railroad Commission of Texas, 

2018). As of April 2018, there were 12,219 permitted oil 

wells, 5,866 permitted gas wells, and 2,482 pending permits 

for wells completed in the Eagle Ford Group throughout 

Texas (Railroad Commission of Texas, 2018). If all pending 

permits are completed, then the number of oil and natural-gas 

wells within the Eagle Ford Group as a whole could increase 

to 20,567 wells, representing a 1.8-fold increase as compared 

to 2015. Assuming that changes in impervious cover within 

the study area will increase proportionally to the Eagle Ford 

Group as a whole, then impervious cover is estimated to 

increase from 0.912 percent (table 12) to 1.64 percent of the 

total study area.

Concentrations of Selected 

Constituents

For this report, parts of Wilson, Karnes, and DeWitt 

Counties are considered the area of active oil and natural-gas 

production in the study area (fig. 1). The following sites were 

considered upstream from most active oil and natural-gas 

production: (1) SAR Elmendorf, SAR Trib 220, and SAR 

541 sites in the San Antonio River watershed; (2) Cibolo St. 

Hedwig, Cibolo Trib 332, and Cibolo 537 sites in the Cibolo 

Creek watershed, and (3) the Ecleto 3335 site in the Ecleto 

Creek watershed. The remaining 10 sampling sites (fig. 1; 

table 1) were considered to be within the area of active oil and 

natural-gas production. 

Constituents in Surface-Water Samples

Surface-water samples collected from five of the sites 

were analyzed for concentrations of a broad range of major 

and trace elements and organic constituents that might be 

associated with oil and natural-gas production. To help put 

concentrations of select major and trace elements and organic 

constituents in the surface-water samples in context, detected 

concentrations were compared to the EPA’s drinking water 

standards and health advisories (EPA, 2018b) and national 

recommended aquatic life criteria for freshwater (EPA, 2018c) 

(table 13). EPA bases aquatic life criteria on how much of a 

chemical can be present in surface water before it is likely to 

harm plant and animal life. 
Concentrations of Selected Constituents 35

Table 13. Drinking water standards and health advisories (U.S. Environmental Protection Agency, 2018b) and national recommended 

aquatic life criteria (U.S. Environmental Protection Agency, 2018c) that were used to evaluate the contaminant concentrations in 

surface-water samples collected from five sites in the lower San Antonio River watershed, Texas, 2015–17.—Continued

[MCL, maximum contaminant level; DWEL, drinking water equivalent level; μg/L, micrograms per liter; --, no data; <, less than; E, estimated]

Constituent

Drinking water 

standards Health advisories Aquatic life criteria Concentrations in surface-water samples

MCL

(μg/L)

DWEL 

(μg/L)

Lifetime 

(μg/L)

Chronic 

(μg/L)

Acute 

(μg/L)

Minumum 

(μg/L)

Maximum1

(μg/L)

Major and trace elements

Calcium -- -- -- -- -- 16,600 37,400

Sodium -- -- -- -- -- 13,200 49,900

Bromide -- -- -- -- -- 37 170

Chloride -- -- -- 230,000 860,000 9,700 57,100

Sulfate -- -- -- -- -- 11,600 50,800

Sulfide -- -- -- -- -- 49 190

Aluminum -- -- -- -- -- <20 130

Barium 2,000 7,000 -- -- -- 24,500 51,600

Cadmium 5 20 5 0.72 1.8 <0.6 <0.6

Chromium -- -- -- -- -- <0.6 <0.6

Iron -- -- -- 1,000 -- 31.2 135

Lead 15 -- -- 2.5 65 0.078 0.263

Lithium -- -- -- -- -- 3.56 16.4

Manganese -- 1,600 300 -- -- 0.86 6.38

Molybdenum -- 200 40 -- -- <2.2 1.53

Nickel -- 700 100 52 470 <1.2 2.3

Strontium -- 20,000 4,000 -- -- 88.1 266

Zinc -- 10,000 2,000 120 120 <2.0 3.1

Arsenic 10 10 -- 150 340 1.4 2.7

Boron -- 7,000 6,000 -- -- 51 153

Semivolatile organic compounds

2,4,6-Trichlorophenol -- 10 -- -- -- <0.34 0.03

2,4-Dichlorophenol -- 100 20 -- -- <0.36 0.05

4-Nitrophenol -- 300 60 -- -- <0.52 0.36

Pentachlorophenol 1 200 40 15 19 <0.6 0.8

9H-Fluorene -- 1,000 -- -- -- <0.034 0.02

Acenaphthene -- 2,000 -- -- -- <0.28 0.02

Benzo[a]pyrene 0.2 -- -- -- -- <0.32 0.08

Benzo[b]fluoranthene -- -- -- -- -- <0.30 E0.12

Benzo[ghi]perylene -- -- -- -- -- <0.38 E0.13

Benzo[k]fluoranthene -- -- -- -- -- <0.30 E0.07

Chrysene -- -- -- -- -- <0.32 E0.08

Diethyl phthalate -- 30,000 -- -- -- <0.62 E0.11

Di-n-butyl phthalate -- 4,000 -- -- -- <2.80 E0.27

Di-n-octyl phthalate 6 700 -- -- -- <0.6 0.1

Fluoranthene -- -- -- -- -- <0.30 0.14

Indeno[1,2,3-cd]pyrene -- -- -- -- -- <0.38 E0.13

Isophorone -- 7,000 100 -- -- <0.26 0.07

Table 13. Drinking water standards and health advisories (U.S. Environmental Protection Agency, 2018b) and national recommended 

aquatic life criteria (U.S. Environmental Protection Agency, 2018c) that were used to evaluate the contaminant concentrations in 

surface-water samples collected from five sites in the lower San Antonio River watershed, Texas, 2015–17.

[MCL, maximum contaminant level; DWEL, drinking water equivalent level; μg/L, micrograms per liter; --, no data; <, less than; E, estimated]
36 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 13. Drinking water standards and health advisories (U.S. Environmental Protection Agency, 2018b) and national recommended 

aquatic life criteria (U.S. Environmental Protection Agency, 2018c) that were used to evaluate the contaminant concentrations in 

surface-water samples collected from five sites in the lower San Antonio River watershed, Texas, 2015–17.—Continued

[MCL, maximum contaminant level; DWEL, drinking water equivalent level; μg/L, micrograms per liter; --, no data; <, less than; E, estimated]

Constituent

Drinking water 

standards Health advisories Aquatic life criteria Concentrations in surface-water samples

MCL

(μg/L)

DWEL 

(μg/L)

Lifetime 

(μg/L)

Chronic 

(μg/L)

Acute 

(μg/L)

Minumum 

(μg/L)

Maximum1

(μg/L)

Semivolatile organic compounds—Continued

Naphthalene -- 700 100 -- -- <0.26 0.03

Phenanthrene -- -- -- -- -- <0.32 0.08

Pyrene -- -- -- -- -- <0.36 0.11

Volatile organic compounds

Carbon disulfide -- -- -- -- -- <0.1 E0.1

1,2,4-Trimethylbenzene -- -- -- -- -- <0.032 0.033

Benzene 5 100 3 -- -- <0.026 0.048

Bromodichloromethane 80 100 -- -- -- <0.034 0.050

Ethylbenzene 700 3,000 700 -- -- <0.036 0.022

Hexane -- -- -- -- -- <0.024 E0.011

m-Xylene plus p-xylene 10,000 7,000 -- -- -- <0.08 0.03

Naphthalene -- 700 100 -- -- <0.26 0.03

n-Pentane -- -- -- -- -- <0.022 0.031

o-Xylene 10,000 7,000 -- -- -- <0.032 0.014

Toluene 1,000 3,000 -- -- -- <0.02 0.16

Trichloromethane 80 350 70 -- -- <0.03 0.21

1

Dynamic reporting levels in some constituents can result in a maximum concentration value that appears to be less than the minimum concentration value.

Major and Trace Elements

Eighteen of the 20 major and trace elements measured 

in surface-water samples were detected in the samples 

collected from the 5 sites (Crow and others, 2018). Cadmium 

and chromium were not detected in any of the surface-water 

samples. All major and trace element concentrations were less 

than any of the EPA’s water-quality standards (table 13). 

Sulfide was selected for analysis because of its common 

occurrence in produced waters (Tibbetts and others, 1992). 

Small concentrations of sulfide ranging from verified values 

less than the LRL to values slightly greater than the LRL 

were detected in all five surface-water samples (Crow and 

others, 2018). The LRL for sulfide provided by RTI was 

0.10 milligram per liter (mg/L), and reported concentrations 

ranged from 0.049 mg/L at the Cibolo St. Hedwig site to 

0.190 mg/L at the Ecleto 326 site (Crow and others, 2018). 

The presence of sulfide was verified but not quantified at the 

SAR Elmendorf site. Sulfide was not detected in samples 

collected in the previous study in this area (Opsahl and Crow, 

2015); however, because of improved analytical methods, the 

LRL for sulfide used during the previous study (0.79 mg/L) 

was greater than the LRL used in this study, and sulfide 

concentrations less than 0.79 mg/L may have been present 

in samples collected in the previous study. The observed 

distribution of small sulfide concentrations, including those at 

sites located upstream from the area of active oil and naturalgas production, is consistent with little or no enhanced inputs 

of sulfide from sites within the area of active oil and naturalgas production.

Organic Constituents

Glycols are commonly used in hydraulic fracturing 

fluids as scale inhibitors (FracFocus Chemical Disclosure 

Registry, 2018). Four glycols—diethylene glycol, ethylene 

glycol, propylene glycol, and triethylene glycol—were 

analyzed for in surface-water samples collected for this study. 

The MDLs provided by RTI were 1 mg/L for diethylene 

glycol, propylene glycol, and triethylene glycol and 2 mg/L 

for ethylene glycol. Diethylene glycol, ethylene glycol, and 

triethylene glycol were detected in a water sample collected 

from the Ecleto 326 site at concentrations of 3.1, 11, and 

3.8 mg/L, respectively (table 14). Glycols were not detected in 

the other four water samples collected for this study. Glycols 

were not detected in samples collected for the previous study 
Concentrations of Selected Constituents 37

Table 14. Glycol concentrations in surface-water samples collected from five sites in the lower San Antonio River watershed, Texas, 

2015–17.

[USGS, U.S. Geological Survey; mg/L, milligrams per liter; SAR, San Antonio River; St., Saint; <, less than. Dates are formatted as month-day-year]

Map 

identifier 

(fig. 1)

Short name

USGS 

station 

number

USGS 

station name Date

Diethylene 

glycol 

(mg/L)

Ethylene 

glycol 

(mg/L)

Propylene 

glycol 

(mg/L)

Triethylene 

glycol 

(mg/L)

1 SAR Elmendorf 08181800 San Antonio River near 

Elmendorf, Tex.

08-07-2017 <1 <2 <1 <1

4 Cibolo St. Hedwig 08185065 Cibolo Creek near Saint 

Hedwig, Tex.

10-25-2015 <1 <2 <1 <1

5 Cibolo Falls City 08186000 Cibolo Creek near Falls City, 

Tex.

10-25-2015 <1 <2 <1 <1

8 Ecleto 326 08186550 Ecleto Creek at County Road 

326 near Runge, Tex.

05-20-2016 3.1 11 <1 3.8

10 SAR 72 08188060 San Antonio River at State 

Highway 72 near Runge, Tex.

08-03-2017 <1 <2 <1 <1

in this area (Opsahl and Crow, 2015). Because of improved 

analytical methods, however, the LRLs available during the 

previous study (1.9 to 18.7 mg/L) were greater than those 

used in this study, so it cannot be determined whether the 

increased number of glycol detections indicate a change in 

the environmental concentrations between studies. Although 

concentrations of glycols were greater than the MDLs and 

indicate the presence glycol in the surface water at the 

Ecleto 326 site, the presence of glycols does not necessarily 

indicate that the source is from hydraulic fracturing fluid 

because diethylene glycol, ethylene glycol, and triethylene 

glycol also have other sources including the use of diethylene 

and ethylene glycols in antifreeze used in vehicles (Dow 

Chemical Company, 2018) and the use of triethylene glycol in 

antibacterial air sanitizers (EPA, 2005).

Surface-water samples were analyzed for SVOCs and 

VOCs during this study because of their common occurrence 

as petroleum hydrocarbons in oil and associated produced 

waters (Tibbetts and others, 1992). Of the 56 SVOCs and 

49 VOCs analyzed for during this study, there were 30 lowlevel detections of 20 different SVOCs and 23 low-level 

detections of 12 different VOCs in the 5 water samples 

(table 15). None of the detected SVOCs and VOCs exceeded 

any of the EPA’s water-quality standards (table 13), including 

drinking water maximum contaminant levels (MCLs). The 

benzene concentrations in the surface-water samples also 

were less than the EPA’s health advisory criteria for lifetime 

exposure for benzene (3 μg/L), which is less than the MCL for 

benzene (5 μg/L).

Seventeen SVOCs were detected in the sample collected 

from the SAR Elmendorf site, 4 were detected in the water 

sample collected from the SAR 72 site, and 3 each were 

detected in the water samples collected from the Cibolo St. 

Hedwig, Cibolo Falls City, and Ecleto 326 sites (table 15). 

The concentrations of SVOCs detected in the SAR Elmendorf 

sample were generally greater that those detected in the other 

four water samples and ranged from 0.02 μg/L for 9H-fluorene 

and acenaphthene to 0.36 μg/L for 4-nitrophenol, with a 

median detected concentration of 0.11 μg/L. The SVOC 

concentrations detected in the other four samples ranged 

from an estimated 0.01 μg/L for 2,4-dichlorophenol in the 

Cibolo Falls City sample to 0.8 μg/L for pentachlorophenol 

in the Cibolo St. Hedwig sample, with a median detected 

concentration of 0.03 μg/L. 

The majority of VOCs were detected in samples from 

the main stem of the San Antonio River (table 15). Eleven 

VOCs were detected in the water sample collected from 

the SAR Elmendorf site (upstream from the area of oil and 

natural-gas production), and 9 were detected in the sample 

collected from the SAR 72 site (downstream from the 

area of oil and natural-gas production) (fig. 1). The VOC 

concentrations detected in the samples collected from SAR 

Elmendorf and SAR 72 were similar. VOC concentrations 

in the SAR Elmendorf sample ranged from 0.012 μg/L 

for n-pentane to 0.16 μg/L for toluene, with a median of 

0.03 μg/L. VOC concentrations in the SAR 72 sample ranged 

from 0.01 μg/L for trichloromethane to an estimated 0.1 μg/L 

for carbon disulfide, with a median of 0.03 μg/L. 
Table 15. Summary of semivolatile organic compound and volatile organic compound detections in surface-water samples collected at five sites in twatershed, Texas, 2015–17. [USGS, U.S. Geological Survey; μg/L, micrograms per liter; SAR, San Antonio River; St., Saint; --, not detected; E, estimated; M, presence verified but not quantified. Dates ar

Map identi- fier (fig. 1) Short name USGS station number USGS station name Date Sample start time Semivolatile organic compoun

2,4,6-Trichlo- rophenol, unfiltered (μg/L) 2,4-Dichlo- rophenol, unfiltered (μg/L) 4-Nitro- phenol, unfiltered (μg/L) Pentachlo- rophenol, unfiltered (μg/L) 

Sites upstream from the area of active oil and natural-gas production 1 SAR Elmendorf 08181800 San Antonio River near Elmendorf, Tex. 08-07-2017 1605 -- -- 0.36 0.2 

4 Cibolo St. Hedwig 08185065 Cibolo Creek near Saint Hedwig, Tex. 10-25-2015 1700 0.02 -- -- 0.8 

Sites within the area of active oil and natural-gas production 5 Cibolo Falls City 08186000 Cibolo Creek near Falls City, Tex. 10-25-2015 1400 -- E0.01 -- E0.8 

8 Ecleto 326 08186550 Ecleto Creek at County Road 326 near Runge, Tex. 05-20-2016 1030 0.03 0.05 -- -- 

10 SAR 72 08188060 San Antonio River at State Highway 72 near Runge, Tex. 08-03-2017 1530 -- -- -- 0.1 

Map identi- fier (fig. 1) Short name USGS station number USGS station name Date Sample start time Semivolatile organic compoun

Benzo[a] pyrene, unfiltered (μg/L) Benzo[b] fluoran- thene, unfiltered (μg/L) Benzo[ghi] perylene, unfiltered (μg/L) Benzo[k] fluoran- thene, unfiltered (μg/L) Chrys

unfilt

(μg

Sites upstream from the area of active oil and natural-gas production 1 SAR Elmendorf 08181800 San Antonio River near Elmendorf, Tex. 08-07-2017 1605 0.08 E0.12 E0.13 E0.07 E0

4 Cibolo St. 

Hedwig

08185065 Cibolo Creek near Saint 

Hedwig, Tex.

10-25-2015 1700 -- -- -- -- -

Sites within the area of active oil and natural-gas production

5 Cibolo Falls 

City

08186000 Cibolo Creek near Falls City, 

Tex.

10-25-2015 1400 -- -- -- -- -

8 Ecleto 326 08186550 Ecleto Creek at County Road 

326 near Runge, Tex.

05-20-2016 1030 -- -- -- -- -

10 SAR 72 08188060 San Antonio River at State 

Highway 72 near Runge, Tex.

08-03-2017 1530 -- E0.02 -- -- -
Table 15. Summary of semivolatile organic compound and volatile organic compound detections in surface-water samples collected at five sites in t

watershed, Texas, 2015–17.—Continued

[USGS, U.S. Geological Survey; μg/L, micrograms per liter; SAR, San Antonio River; St., Saint; --, not detected; E, estimated; M, presence verified but not quantified. Dates ar

Map 

identifier 

(fig. 1)

Short name

USGS 

station 

number

USGS 

station name Date

Sample 

start 

time

Semivolatile organic compoun

Di-n-octyl 

phthalate, 

unfiltered 

(μg/L)

Fluoranthene, 

unfiltered 

(μg/L)

Indeno[1,2,3-

cd]pyrene, 

unfiltered 

(μg/L)

Isophorone, 

unfiltered 

(μg/L)

Na

tha

unfi

(μ

Sites upstream from the area of active oil and natural-gas production

1 SAR Elmendorf 08181800 San Antonio River near 

Elmendorf, Tex.

08-07-2017 1605 0.1 0.14 E0.13 -- 0

4 Cibolo St. 

Hedwig

08185065 Cibolo Creek near Saint 

Hedwig, Tex.

10-25-2015 1700 -- -- -- 0.03 

Sites within the area of active oil and natural-gas production

5 Cibolo Falls 

City

08186000 Cibolo Creek near Falls City, 

Tex.

10-25-2015 1400 -- -- -- E0.04 

8 Ecleto 326 08186550 Ecleto Creek at County Road 

326 near Runge, Tex.

05-20-2016 1030 -- -- -- 0.07 

10 SAR 72 08188060 San Antonio River at State 

Highway 72 near Runge, Tex.

08-03-2017 1530 -- 0.02 -- -- 

Map 

identifier 

(fig. 1)

Short name

USGS 

station 

number

USGS 

station name Date

Sample 

start 

time

Volatile organic compounds

Carbon 

disulfide, 

unfiltered 

(μg/L)

1,2,4-Trimethylbenzene, 

unfiltered 

(μg/L)

Benzene, 

unfiltered 

(μg/L)

Bromodichlo

methane, 

unfiltered 

(μg/L)

Sites upstream from the area of active oil and natural-gas production

1 SAR Elmendorf 08181800 San Antonio River near 

Elmendorf, Tex.

08-07-2017 1605 M 0.033 E0.016 0.032 

4 Cibolo St. 

Hedwig

08185065 Cibolo Creek near Saint 

Hedwig, Tex.

10-25-2015 1700 -- -- -- 0.05 

Sites within the area of active oil and natural-gas production

5 Cibolo Falls 

City

08186000 Cibolo Creek near Falls City, 

Tex.

10-25-2015 1400 -- -- -- -- 

8 Ecleto 326 08186550 Ecleto Creek at County Road 

326 near Runge, Tex.

05-20-2016 1030 M -- -- -- 

10 SAR 72 08188060 San Antonio River at State 

Highway 72 near Runge, Tex.

08-03-2017 1530 E0.1 -- 0.048 -- 
Table 15. Summary of semivolatile organic compound and volatile organic compound detections in surface-water samples collected at five sites in t

watershed, Texas, 2015–17.—Continued [USGS, U.S. Geological Survey; μg/L, micrograms per liter; SAR, San Antonio River; St., Saint; --, not detected; E, estimated; M, presence verified but not quantified. Dates ar

Map identi- fier (fig. 1) Short name USGS station number USGS station name Date Sample start time Volatile organic compounds m-Xylene plus p-xylene, unfiltered (μg/L) Naph- thalene, unfiltered (μg/L) n-Pentane, unfiltered (μg/L) o-Xylene, unfiltered (μg/L) 

Sites upstream from the area of active oil and natural-gas production 1 SAR Elmendorf 08181800 San Antonio River near Elmendorf, Tex. 08-07-2017 1605 0.03 0.03 0.012 0.014 

4 Cibolo St. Hedwig 08185065 Cibolo Creek near Saint Hedwig, Tex. 10-25-2015 1700 -- -- -- -- 

Sites within the area of active oil and natural-gas production 5 Cibolo Falls City 08186000 Cibolo Creek near Falls City, Tex. 10-25-2015 1400 -- -- -- -- 

8 Ecleto 326 08186550 Ecleto Creek at County Road 326 near Runge, Tex. 05-20-2016 1030 -- -- -- -- 

10 SAR 72 08188060 San Antonio River at State Highway 72 near Runge, Tex. 08-03-2017 1530 0.03 -- 0.031 0.012 
Concentrations of Selected Constituents 41

The SAR Elmendorf site is the site from which the 

water sample with the greatest number of detections and, 

generally, the greatest concentrations of SVOCs and VOCs 

was collected (table 15). Compared with samples collected 

for the previous study in this area (Opsahl and Crow, 2015), 

in general, a greater number of SVOCs and VOCs were 

detected in samples collected for this study, but at smaller 

concentrations. The greatest SVOC and VOC concentrations 

from the previous study also were observed in samples 

collected from sites upstream from the area of active oil 

and natural-gas production and just downstream from urban 

areas. Many SVOCs and VOCs are commonly found in 

urban settings (Lopes and Dionne, 1998; Bender and others, 

2009), and previous studies (Ging, 1999; Bush and others, 

2000) confirm that SVOCs and VOCs have been detected 

in streams in the San Antonio, Tex., area at concentrations 

similar to those detected in this study. The SAR Elmendorf 

site is upstream from the area of active oil and natural-gas 

production and just downstream from San Antonio (fig. 1). 

The SAR 72 site is further downstream from San Antonio and 

is within the area of active oil and natural-gas production. The 

SVOCs and VOCs that were detected at the SAR 72 site also 

were detected upstream at the SAR Elmendorf site, indicating 

that the likely source of most SVOCs and VOCs in the study 

area is the upstream urbanized part of the lower San Antonio 

River watershed. 

Benzene, toluene, ethylbenzene, and all isomers of 

xylene (hereinafter referred to as BTEX) are a subset of 

VOC compounds that often are useful for tracing petroleum 

hydrocarbon sources in aquatic environments (Khan and 

others, 2016). In this study, all BTEX compounds were 

measured in the SAR Elmendorf and SAR 72 samples at small 

concentrations, and no BTEX compounds were measured 

in samples collected from the other three sites, including 

Cibolo Falls City and Ecleto 326 within the area of active 

oil and natural-gas production. The SAR 72 site is within 

the area of active oil and natural-gas production, which 

raises the possibility of a local source of BTEX compounds 

in this section of the San Antonio River. However, BTEX 

concentrations were small and generally similar to those at 

the SAR Elmendorf site, indicating that BTEX compounds 

detected at the SAR 72 site likely originated upstream. 

Although only five surface-water samples were collected and 

analyzed for BTEX compounds, the lack of BTEX compounds 

for most sites within the area of active oil and natural-gas 

production indicates that little, if any, local runoff associated 

with the area of active oil and natural-gas production has 

contaminated the surface water with BTEX compounds.

Constituents in Streambed-Sediment Samples

Some constituents that could be indicators of the 

presence of hydraulic fracturing fluids or produced waters, 

such as certain trace elements and PAHs, are hydrophobic 

and preferentially sorb to sediment (Harwell and others, 

2003). Sediment-associated contaminants can affect the health 

of aquatic organisms that ingest particulate matter when 

bioaccumulation of trace elements and organic compounds 

occurs (Förstner and Wittmann, 1979; Luoma, 1983). The 

accumulation of some inorganic and organic compounds can 

cause various physiological issues and can ultimately result in 

death of aquatic organisms. Subsequent ingestion of aquatic 

organisms by consumers (organisms that eat other organisms) 

can transfer accumulated fat-soluble (lipophilic) contaminants 

upward through the food chain, resulting in biomagnification 

(Neely, 1980; Borgå and others, 2004).

Concentrations of the contaminants in the streambedsediment samples were compared to the consensus-based 

sediment quality guidelines (SQGs) of MacDonald and others 

(2000) (table 16). The SQGs evaluate the potential toxicity 

of bed sediments to sediment-dwelling organisms. Two SQG 

concentration levels are used: (1) a lower level, called the 

threshold effect concentration (TEC), below which harmful 

effects to benthic biota are not expected, and (2) a higher 

level, the probable effect concentration (PEC), above which 

harmful effects are expected to occur frequently. In samples 

with concentrations between the TEC and PEC, additional 

environmental conditions often factor into the toxicity to 

different biota, so samples are not predicted to be toxic or 

nontoxic to benthic biota. The sediment screening levels 

used by the Texas Commission on Environmental Quality’s 

Ecological Assessment Program (Texas Commission on 

Environmental Quality, 2015) are identical to the PECs of 

MacDonald and others (2000).

Major and Trace Elements

Many of the major and trace elements measured for this 

study are naturally occurring and typically found in sediment 

samples; they also can be derived from anthropogenic sources 

(Levings and others, 1998; Kimball and others, 2008). All 

15 of the major and trace elements analyzed in streambedsediment samples were detected in at least 1 sample (Crow 

and others, 2018). The concentrations of arsenic, cadmium, 

chromium, lead, nickel, and zinc in the streambed-sediment 

samples collected in 2016 were compared to their SQGs 

(table 16). Chromium, lead, nickel, and zinc concentrations in 

all samples were less than their respective TEC concentrations. 

The arsenic and cadmium concentrations in the sample 

collected from the Cibolo Trib 332 site were greater than 

their TECs (fig. 7). The PEC for arsenic was exceeded in the 

sample collected from the Ecleto 3335 site. The origins of the 

elevated arsenic and cadmium concentrations are unknown, 

but the Cibolo Trib 332 and the Ecleto 3335 sites are upstream 

from the area of active oil and natural-gas production; 

therefore, the elevated arsenic and cadmium concentrations 

at these sites are likely not related to oil and natural-gas 

production. Major and trace element concentrations from 

streambed-sediment samples collected from six sites for this 

study (SAR Elmendorf, SAR Falls City, SAR 72, Cibolo St. 

Hedwig, Cibolo Falls City, and Ecleto Runge) were compared 

with samples collected from the same sites during 2011–13 
42 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Table 16. Consensus-based sediment quality guidelines of MacDonald and others (2000) that were used to evaluate the contaminant 

concentrations in streambed-sediment samples collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.

[mg/kg, milligrams per kilogram; <, less than; μg/kg, micrograms per kilogram; PAHs, polycyclic aromatic hydrocarbons]

Constituent Threshold effect 

concentration

Probable effect 

concentration

Concentrations in 

streambed-sediment samples

Minumum Maximum

Trace elements (mg/kg)

Arsenic 9.79 33.0 3.0 43.6

Cadmium 0.99 4.98 <0.1 1.1

Chromium 43.4 111 3.4 22

Lead 35.8 128 9.1 32

Nickel 22.7 48.6 3.0 16.7

Zinc 121 459 19 120

Polycyclic aromatic hydrocarbons (μg/kg)

Anthracene 57.2 845 <0.79 12

Benzo[a]anthracene 108 1,050 <0.79 53

Benzo[a]pyrene 150 1,450 <0.79 98

Chrysene 166 1,290 <0.79 67

Dibenzo[a,h]anthracene 33.0 none <0.79 12

Fluoranthene 423 2,230 <0.79 110

Fluorene 77.4 536 <0.79 <21

Naphthalene 176 561 <0.79 3.9

Phenanthrene 204 1,170 <0.79 24

Pyrene 195 1,520 <0.79 88

Total PAHs 1,610 22,800 <0.79 423

for the study done by Opsahl and Crow (2015). All arsenic, 

cadmium, chromium, lead, nickel, and zinc concentrations 

detected in the previous study were less than their respective 

TECs (fig. 7). No evidence was found in the major and 

trace element data to indicate changes related to oil and 

natural-gas production.

Organic Constituents

Selected PAHs were analyzed for in streambed-sediment 

samples for this study because of their potential usefulness 

as indicators of petroleum hydrocarbons associated with 

produced waters resulting from oil and natural-gas production 

(Hostettler and others, 2013). Other common sources of PAHs 

include coal-tar-based sealcoat; automobile exhaust; used 

motor oil; automobile tire particles; and burning wood, oil, 

coal, or other combustibles for cooking, heating, or energy 

production (Mahler and others, 2005; Van Metre and Mahler, 

2010). PAH concentrations were analyzed in each streambedsediment sample for two different size fractions to determine 

if these organic constituents are more likely to sorb to the 

exclusively silt- and clay-sized particles that compose the 

<63-μm size fraction than to the mixture of silt and clay and 

larger sized particles that compose the <2-mm size fraction.

Total PAH concentrations in all samples, regardless 

of size fraction, were less than the TEC for total PAHs of 

1,610 μg/kg (table 16). Total PAH concentrations in the 

<2-mm size-fraction samples ranged from less than the 

LRL in samples collected from multiple sites in 2015 and 

2016 to 221 μg/kg in the 2016 sample collected from the 

Cibolo St. Hedwig site (fig. 8). Total PAH concentrations in 

the <63-μm size-fraction samples ranged from less than the 

LRL in samples collected from multiple sites in 2015 and 

2016 to 424 μg/kg in the 2015 sample collected from the SAR 

Elmendorf site (fig. 9). Total PAH concentrations generally 

were greater in the <63-μm size-fraction samples than in the 

<2-mm size-fraction samples, indicating that PAHs could 

potentially sorb to the silt- and clay-sized particles that 

compose the <63-μm size-fraction samples more readily 

than to the mixture of silt-, clay-, and sand-sized particles 

that compose the <2-mm size-fraction samples. Based on 

these findings, it might be anticipated that the greater the 

percentage of silt- and clay-sized particles in a sample, the 

greater the total PAH concentration in that sample. However, 

a comparison of the total PAH concentrations in the <2-mm 

size-fraction samples to the percentage of silt- and clay-sized 

particles (<63-μm size fraction) derived from the particle 

size distribution data showed no correlation (r-squared value 
Concentrations of Selected Constituents 43

0

5

10

15

20

25

30

35

40

45

50

Arsenic

Threshold effect concentration

Probable effect concentration

2016 sample - main stem

Within area of oil and 

natural-gas production

2016 sample - tributary

2011 sample (Opsahl and Crow, 2015) - main stem

Upstream from area of oil 

and natural-gas production

2012 sample (Opsahl and Crow, 2015) - main stem

SAR Elmendorf

SAR Trib 220

SAR Falls City

SAR 541

Escondido

SAR 72

Cibolo St. Hedwig

Cibolo Trib 332

Cibolo 537

Cibolo Falls City

Cibolo Trib 3191

Cibolo 81

Ecleto 3335

Ecleto 627

Ecleto 295

Ecleto Runge

Ecleto 326

Short name for sampling site (fig. 1, table 1)

San Antonio River Cibolo Creek Ecleto Creek

0

0.2

0.4

0.6

0.8

1.0

1.2

Cadmium

San Antonio River Cibolo Creek Ecleto Creek

0

5

10

15

20

25

30

35

40

45

50 Chromium

0

5

10

15

20

25

30

35

40

Lead

0

5

10

15

20

25 Nickel

0

20

40

60

80

100

120

140

Zinc

Concentration, in milligrams per kilogram

Concentration, in milligrams per kilogram

2011 sample (Opsahl and Crow, 2015) - main stem

2016 sample - main stem

2012 sample (Opsahl and Crow, 2015) - main stem

Streambed-sediment sample Sediment quality guideline

2016 sample - tributary

SAR Elmendorf

SAR Trib 220

SAR Falls City

SAR 541

Escondido

SAR 72

Cibolo St. Hedwig

Cibolo Trib 332

Cibolo 537

Cibolo Falls City

Cibolo Trib 3191

Cibolo 81

Ecleto 3335

Ecleto 627

Ecleto 295

Ecleto Runge

Ecleto 326

Short name for sampling site (fig. 1, table 1)

EXPLANATION

Figure 7. Concentrations of trace elements in streambed-sediment samples in relation to sediment quality guidelines (MacDonald and 

others, 2000), lower San Antonio River watershed, 2011 and 2012 (from Opsahl and Crow, 2015) and 2016. 
44 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Totalpolycyclic aromatic hydrocarbon concentration, in micrograms per kilogram

San Antonio River Cibolo Creek Ecleto Creek

0

50

100

150

200

250

Short name for sampling site (fig. 1, table 1)

SAR Elmendorf

SAR Trib 220

SAR Falls City

SAR 541

Escondido

SAR 72

Cibolo St. Hedwig

Cibolo Trib 332

Cibolo 537

Cibolo Falls City

Cibolo Trib 3191

Cibolo 81

Ecleto 3335

Ecleto 627

Ecleto 295

Ecleto Runge

Ecleto 326

Main stem—upstream from oil and

natural-gas production area

Tributary—upstream from oil and

natural-gas production area

Main stem—within oil and

natural-gas production area

Tributary—within oil and

natural-gas production area

Main stem—upstream from oil and

natural-gas production area

Tributary—upstream from oil and

natural-gas production area

Main stem—within oil and

natural-gas production area

Tributary—within oil and

natural-gas production area

2015 streambed-sediment sample

2016 streambed-sediment sample

EXPLANATION

Figure 8. Total polycyclic aromatic hydrocarbon concentrations in less than 2-millimeter size-fraction streambed-sediment samples 

collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.

of 0.16) (fig. 10). At a given site, the concentration of PAHs 

is likely to be greater in the silt- and clay-sized particles in 

comparison to sand particles; however, the concentration of 

PAHs is primarily determined by the presence or absence of a 

local source.

In the <63-μm and <2-mm size fractions, total PAH 

concentrations typically were greater in the samples collected 

from the sites upstream from the area of active oil and 

natural-gas production compared to those collected from 

sites within the area (figs. 8 and 9). Because of improved 

analytical methods, the LRLs for individual PAHs available 

during the previous study (13–250 μg/kg) were greater 

than those used in this study (0.80–6.3 μg/kg); therefore, 

PAHs in the previous study could have been present at 

concentrations that were less than the LRLs used in this 

study. The observed distribution of small PAH concentrations 

and the observations of often greater concentrations at sites 

located upstream from the area of active oil and natural-gas 

production are consistent with little or no enhanced inputs of 

PAHs from sites within the area of active oil and natural-gas 

production.

Total PAH concentrations were computed by using 

a different method than was used in the Opsahl and Crow 

(2015) study, so direct comparison between the results of the 

two studies was not possible. The total PAH concentrations 

observed in the previous study display a similar pattern to 

those in the current study in which the greater concentrations 

are observed near urban areas upstream from the area of 

active oil and natural-gas production. PAHs are commonly 

found in urban settings (Moring and Rose, 1997; Van Metre 

and Mahler, 2005), and previous studies (Ging and others, 

1999; Wilson, 2011; Crow and others, 2016) confirm that 

PAHs have been detected in streams in the San Antonio 

area at concentrations that are similar to or greater than the 

concentrations detected in this study. Ging and others (1999) 

examined sediment cores from a lake in a rapidly urbanizing 

area of Bexar County and found that concentrations of PAHs 

were increasing with increasing urbanization. Wilson (2011) 

examined sediments throughout Bexar County where total 

PAH concentrations ranged from not detected to 4,000 μg/kg. 

In a study that assessed possible contamination in streams 

flowing through some of San Antonio’s oldest neighborhoods 

(Crow and others, 2016), total PAH concentrations ranged 

from 750 to 99,000 μg/kg. The smaller PAH concentrations 

measured in samples collected from within the area of active 

oil and natural-gas production in comparison to the upstream 

urbanized areas indicate relatively minor additional local 

contributions of PAHs of uncertain origin to the watershed.
Concentrations of Selected Constituents 45

Totalpolycyclic aromatic hydrocarbon concentration, in micrograms per kilogram

San Antonio River Cibolo Creek

Short name for sampling site (fig. 1, table 1)

SAR Elmendorf

SAR Trib 220

SAR Falls City

SAR 541

Escondido

SAR 72

Cibolo St. Hedwig

Cibolo Trib 332

Cibolo 537

Cibolo Falls City

Cibolo Trib 3191

Cibolo 81

Ecleto 3335

Ecleto 627

Ecleto 295

Ecleto Runge

Ecleto 326

Ecleto Creek

0

50

100

150

200

250

300

350

400

450

Main stem—upstream from oil and

natural-gas production area

Tributary—upstream from oil and

natural-gas production area

Main stem—within oil and

natural-gas production area

Tributary—within oil and

natural-gas production area

Main stem—upstream from oil and

natural-gas production area

Tributary—upstream from oil and

natural-gas production area

Main stem—within oil and

natural-gas production area

Tributary—within oil and

natural-gas production area

2015 streambed-sediment sample

2016 streambed-sediment sample

EXPLANATION

Figure 9. Total polycyclic aromatic hydrocarbon concentrations in less than 63-micrometer size-fraction streambed-sediment samples 

collected from selected sites in the lower San Antonio River watershed, Texas, 2015–16.

R2 = 0.16

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250

Percentage of streambed sediment smallerthan 63 micrometers

Total polycyclic aromatic hydrocarbon concentration, in micrograms per kilogram

Streambed-sediment

sample

Percentage of

streambed-sediment

sample smaller than

63 micrometers

Total polycyclic

aromatic hydrocarbon

concentration

Coefficient of

determination

EXPLANATION

R2

SB=0.19 x PAH + 15.99

SB

PAH

Figure 10. Relation of total polycyclic aromatic hydrocarbon concentrations in less than 2-millimeter size-fraction samples to 

percentage of streambed-sediment samples smaller than 63 micrometers collected from selected sites in the lower San Antonio River 

watershed, Texas, 2015–16.
46 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Summary

Oil and natural-gas production from shale formations 

such as those present in the Eagle Ford Group in south Texas 

represented about 50 percent of U.S. crude oil production 

in 2017. The extensive development of oil and natural-gas 

resources in south Texas during the past 10 years has led 

to questions regarding possible environmental effects of 

processes associated with oil and natural-gas production, in 

particular the process of hydraulic fracturing, on water and 

other natural resources. 

Part of the lower San Antonio River watershed 

intersects an area of oil and natural-gas production from 

the sedimentary rocks that compose the Eagle Ford Group. 

The rapid expansion of infrastructure associated with oil 

and natural-gas production increases potential pathways for 

inorganic and organic contaminants to enter surface-water 

systems. The U.S. Geological Survey, in cooperation with the 

San Antonio River Authority, analyzed geospatial data from 

different years (2008 and 2015) to evaluate changes in land 

cover associated with oil and natural-gas production activities 

in the lower San Antonio River watershed. Additionally, 

surface-water samples collected from 5 sites and streambedsediment samples collected from 17 sites in the lower San 

Antonio River watershed during 2015–17 were analyzed for a 

broad range of constituents that might be associated with oil 

and natural-gas production. 

Land-cover changes associated with oil and naturalgas production activities were quantified by comparing 

aerial imagery data collected during 2008 and 2015. Oil and 

natural-gas features were identified on the aerial imagery so 

that total area covered by each feature could be calculated 

to provide an estimate of change in each feature between 

2008 and 2015. Well pads accounted for the largest landcover change among the categories defined and delineated 

within the study area. The total percentage of the study area 

accounted for by well pads increased from 0.022 percent 

in 2008 to 0.663 percent in 2015, which is an increase of 

approximately thirtyfold. Oil and natural-gas productionrelated features accounted for the second largest change in 

land cover associated with oil and natural-gas production 

activities; the total percentage of the study area accounted for 

by oil and natural-gas production-related features increased 

from 0.006 percent in 2008 to 0.152 percent in 2015. Roads 

associated with oil and natural-gas production activities also 

accounted for a substantial change in land cover during the 

study period; the total percentage of the study area accounted 

for by roads associated with oil and natural-gas production 

increased from 0.006 percent in 2008 to 0.097 percent in 2015. 

Patterns of change observed for ponds varied based on pond 

type. The number of storage ponds increased from 1 in 2008 

to 127 in 2015. The average surface area of storage ponds in 

2015 was 3.09 acres, and storage ponds covered 393 acres, or 

0.066 percent of the study area. The number of runoff ponds 

increased from 5 in 2008 to 112 in 2015. Impervious surface 

in this study is defined as land cover consisting of well pads, 

oil- and gas-related features, or roads. The areal coverage 

associated with impervious surface increased from 201 acres 

to 5,390 acres (net increase of 5,189 acres) between 2008 

and 2015. The total percentage of the study area accounted 

for by impervious surface resulting from oil and naturalgas production activities increased from 0.034 percent to 

0.912 percent, which is an increase of approximately 27-fold. 

Collectively, 0.878 percent of the study area was converted to 

new impervious surface between 2008 and 2015. If the area 

associated with new storage ponds (0.066 percent) is added to 

the estimate of total land-cover changes as a result of oil and 

natural-gas production, then 0.944 percent of the study area 

was altered between 2008 and 2015.

Five surface-water samples and 34 streambed-sediment 

samples were collected from 17 sites in the lower San Antonio 

River watershed during 2015–17. Surface-water samples were 

analyzed for total suspended solids, dissolved solids, major 

and trace elements, glycols, methylene blue active substances, 

semivolatile organic compounds (SVOCs), and volatile 

organic compounds (VOCs). Streambed-sediment samples 

were composited and put through a stainless steel 2-millimeter 

(mm) sieve in preparation for the analyses of polycyclic 

aromatic hydrocarbons (PAHs), total organic carbon (TOC), 

radium-226, and radium-228; instead of the 2-mm sieve, 

a 63-micrometer (μm) plastic sieve fabric was used in 

preparation for the analyses of major and trace elements and 

for additional analyses of PAHs and TOC. 

The surface-water samples collected from five of the 

sites were analyzed for concentrations of a broad range 

of major and trace elements and organic constituents that 

might be associated with oil and natural-gas production. To 

help put concentrations of select trace elements and organic 

constituents in the surface-water samples in context, detected 

concentrations were compared to the U.S. Environmental 

Protection Agency’s (EPA) drinking water standards and 

health advisories and national recommended aquatic life 

criteria for freshwater. Eighteen of the 20 major and trace 

elements measured in surface-water samples were detected 

in the samples. Cadmium and chromium were not detected 

in any of the surface-water samples. All major and trace 

element concentrations were less than any of the EPA’s 

water-quality standards. Glycols, which are commonly 

used in hydraulic fracturing fluids as scale inhibitors, were 

detected in the sample collected from Ecleto Creek within 

the area of oil and natural-gas production. The presence of 

glycols does not necessarily indicate that the source is from 

hydraulic fracturing fluid because the glycols detected in this 

sample also have other potential sources, including the use of 

diethylene and ethylene glycols in antifreeze used in vehicles 

and the use of triethylene glycol in antibacterial air sanitizers. 

Surface-water samples also were analyzed for SVOCs 

and VOCs during this study because of their common 

occurrence as petroleum hydrocarbons in oil and associated 

produced waters. Of the 56 SVOCs and 49 VOCs analyzed 

for during this study, there were 30 low-level detections 
References Cited 47

of 20 different SVOCs and 23 low-level detections of 

12 different VOCs in the 5 water samples. None of the 

detected SVOCs and VOCs exceeded any of the EPA’s 

water-quality standards, including drinking water maximum 

contaminant levels (MCLs). The benzene concentrations 

in the surface-water samples also were less than the EPA’s 

health advisory criteria for lifetime exposure for benzene 

(3 micrograms per liter [μg/L]), which is less than the MCL 

for benzene (5 μg/L). The surface-water sample from the 

SAR Elmendorf site had the greatest number of SVOC and 

VOC detections and, generally, the greatest concentrations. 

The greatest SVOC and VOC concentrations were observed 

in samples collected from sites upstream from the area of 

active oil and natural-gas production and just downstream 

from urban areas. Benzene, toluene, ethylbenzene, and 

all isomers of xylene (hereinafter BTEX) are a subset of 

VOC compounds that often are useful for tracing petroleum 

hydrocarbon sources in aquatic environments. Although only 

five surface-water samples were collected and analyzed for 

BTEX compounds, the lack of BTEX compounds for most 

sites within the area of active oil and natural-gas production 

indicates that little, if any, local runoff associated with the area 

of active oil and natural-gas production has contaminated the 

surface water with BTEX compounds.

Some constituents that could be indicators of the 

presence of hydraulic fracturing fluids or produced waters, 

such as certain trace elements and PAHs, are hydrophobic 

and preferentially sorb to sediment. Concentrations of the 

contaminants in the streambed-sediment samples were 

compared to the consensus-based sediment quality guidelines 

(SQGs). The SQGs evaluate the potential toxicity of bed 

sediments to sediment-dwelling organisms. Two SQG 

concentration levels are used: (1) a lower level, called the 

threshold effect concentration (TEC), below which harmful 

effects to benthic biota are not expected, and (2) a higher level, 

the probable effect concentration (PEC), above which harmful 

effects are expected to occur frequently. 

Many of the major and trace elements measured for this 

study are naturally occurring and typically found in sediment 

samples; they also can be derived from anthropogenic sources. 

All 15 of the major and trace elements analyzed in streambedsediment samples were detected in at least 1 sample. The 

concentrations of arsenic, cadmium, chromium, lead, nickel, 

and zinc in the streambed-sediment samples collected in 

2016 were compared to their SQGs. Chromium, lead, nickel, 

and zinc concentrations in all samples were less than their 

respective TEC concentrations. The arsenic and cadmium 

concentrations in the sample collected from the Cibolo Trib 

332 site were greater than their TECs. The PEC for arsenic 

was exceeded in the sample collected from the Ecleto 

3335 site. The origins of the elevated arsenic and cadmium 

concentrations are unknown, but the Cibolo Trib 332 and the 

Ecleto 3335 sites are upstream from the area of active oil and 

natural-gas production; therefore, the elevated arsenic and 

cadmium concentrations at these sites are likely not related to 

oil and natural-gas production.

Streambed-sediment samples were analyzed for selected 

PAHs during this study because of their potential usefulness 

as indicators of petroleum hydrocarbons associated with 

produced waters resulting from oil and natural-gas production. 

PAH concentrations were analyzed in each streambedsediment sample for two different size fractions to determine 

if these organic constituents are more likely to sorb to the 

exclusively silt- and clay-sized particles that compose the 

<63-μm size fraction than to the mixture of silt and clay 

and larger sized particles that compose the <2-mm size 

fraction. Total PAH concentrations in all samples, regardless 

of size fraction, were less than the TEC for total PAHs of 

1,610 micrograms per kilogram. Total PAH concentrations 

generally were greater in the <63-μm size-fraction samples 

than in the <2-mm size-fraction samples, indicating that 

PAHs could potentially sorb to the silt- and clay-sized 

particles that compose the <63-μm size-fraction samples 

more readily than to the mixture of silt-, clay-, and sand-sized 

particles that compose <2-mm size-fraction samples. Based 

on these findings, it might be anticipated that the greater the 

percentage of silt- and clay-sized particles in a sample, the 

greater the total PAH concentration in that sample. However, 

a comparison of the total PAH concentrations in the <2-mm 

size-fraction samples to the percentage of silt- and clay-sized 

particles (<63-μm size fraction) derived from the particle 

size distribution data showed no correlation (r-squared value 

of 0.16). At a given site, the concentration of PAHs is likely 

to be greater in the silt- and clay-sized particles than in sand 

particles; however, the concentration of PAHs is primarily 

determined by the presence or absence of a local source. In 

both size fractions, total PAH concentrations typically were 

greater in the samples collected from the sites upstream from 

the area of active oil and natural-gas production compared to 

those collected from sites within the area. The smaller PAH 

concentrations measured in samples collected from within the 

area of active oil and natural-gas production in comparison 

to the upstream urbanized areas indicate relatively minor 

additional local contributions of PAHs of uncertain origin to 

the watershed.

References Cited 

American Public Health Association, 1998, Sulfide method 

4500-S2-D, in Standard methods for the examination 

of water and wastewater, 20th ed.: Washington, D.C., 

American Public Health Association, American Water 

Works Association, and Water Environment Federation, 

1325 p.

American Society for Testing and Materials, 1992, Standard 

test method for laboratory determination of water (moisture) 

content of soil and rock: Philadelphia, Pa., Annual Book of 

ASTM Standards, Designation D2216–92, p. 177–180.
48 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Bender, D.A., Delzer, G.C., Price, C.V., and Zogorski, 

J.S., 2009, Occurrence of volatile organic compounds in 

selected urban streams in the United States, 1995–2003: 

U.S. Geological Survey Scientific Investigations Report 

2009–5203, 87 p. [Also available at https://doi.org/10.3133/

sir20095203].

Borgå, Katrine, Fish, A.T., Hoekstra, P.F., and Muir, D.C.G., 

2004, Biological and chemical factors of importance in 

the bioaccumulation and trophic transfer of persistent 

organochlorine contaminants in arctic marine food webs: 

Environmental Toxicology and Chemistry, v. 23, p. 2367–

2385. [Also available at https://doi.org/10.1897/03-518].

Bureau of Economic Geology, 2018, Eagleford shale-gas 

study, accessed May 22, 2018, at http://www.beg.utexas.

edu/research/programs/starr/unconventional-resources/

eagleford.

Burkhardt, M.R., Cinotto, P.J., Frahm, G.W., Woodworth, 

M.T., and Pritt, J.W., 1995, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of methylene blue active substances by 

spectrophotometry: U.S. Geological Survey Open-File 

Report 95–189, 20 p. [Also available at https://doi.

org/10.3133/ofr95189].

Bush, P.W., Ardis, A.F., Fahlquist, Lynne, Ging, P.B., 

Hornig, C.E., and Lanning-Rush, Jennifer, 2000, Water 

quality in south-central Texas, 1996–98: U.S. Geological 

Survey Circular 1212, 32 p. [Also available at https://doi.

org/10.3133/cir1212].

Childress, C.J.O., Foreman, W.T., Connor, B.F., and Maloney, 

T.J., 1999, New reporting procedures based on long-term 

method detection levels and some considerations for 

interpretations of water-quality data provided by the U.S. 

Geological Survey National Water Quality Laboratory: U.S. 

Geological Survey Open-File Report 99–193, 19 p. [Also 

available at https://doi.org/10.3133/ofr99193].

Crow, C.L., Opsahl, S.P., Pedraza, D.E., and Pease, E.C., 

2018, Land-use, water-quality, and sediment-quality data 

from an area upstream from and within an area of oil and 

natural-gas production in the lower San Antonio River 

watershed, south Texas, 2008–17: U.S. Geological Survey 

data release, https://dx.doi.org/10.5066/F74J0DDQ. 

Crow, C.L., Wilson, J.T., and Kunz, J.L., 2016, Occurrence 

and concentrations of selected trace elements, halogenated 

organic compounds, and polycyclic aromatic hydrocarbons 

in streambed sediments and results of water-toxicity 

testing in Westside Creeks and the San Antonio River, San 

Antonio, Texas, 2014: U.S. Geological Survey Scientific 

Investigations Report 2016–5136, 56 p., [Also available at 

http://dx.doi.org/10.3133/sir20165136].

Davis, B.E., 2005, A guide to the proper selection and use of 

federally approved sediment and water-quality samplers: 

U.S. Geological Survey Open-File Report 2005–1087, 20 p. 

[Also available at https://doi.org/10.3133/ofr20051087].

Dawson, W.C., 1997, Limestone microfacies and sequence 

stratigraphy: Eagle Ford Group (Cenomanian-Turonian) 

north-central Texas outcrops: Gulf Coast Association of 

Geological Societies Transactions 47, p. 99–105. 

Dawson, W.C., 2000, Shale microfacies: Eagle Ford 

Group (Cenomanian-Turonian) north-central Texas 

outcrops and subsurface equivalents: Gulf Coast 

Association of Geological Societies Transactions 50, 

p. 607–621.

Dow Chemical Company, 2018, DOW ethylene glycols—

Antifreeze and coolants, accessed July 31, 2018, at 

https://www.dow.com/ethyleneglycol/app/antifreeze.htm.

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. C2, 89 p. [Also available at https://doi.org/10.3133/

twri03C2].

Esri, 2018a, ArcGIS Desktop (ver. 10.4.1): Esri ArcGIS 

Desktop web page, accessed February 22, 2018, at 

http://desktop.arcgis.com/en/.

Esri, 2018b, Esri Technical Support, GIS dictionary, heads-up 

digitizing, accessed August 3, 2018, at https://support.esri.

com/en/other-resources/gis-dictionary/term/0c14e614-30c2-

4bee-a1e4-4ff2c60d4626.

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of inorganic and organic constituents in 

water and fluvial sediments: U.S. Geological Survey OpenFile Report 93–125, 217 p. [Also available at https://doi.

org/10.3133/ofr93125].

Fishman, M.J., and Friedman, L.C., eds., 1989, Methods 

for determination of inorganic substances in water and 

fluvial sediments: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 5, chap. A1, 

545 p. [Also available at https://doi.org/10.3133/ofr93125].

Förstner, U., and Wittmann, G.T.W., eds., 1979, Metal 

pollution in the benthic environment: New York, SpringerVerlag, 486 p.

FracFocus Chemical Disclosure Registry, 2018, What 

chemicals are used?, accessed April 2, 2018, at http://

fracfocus.org/chemical-use/what-chemicals-are-used.

Garbarino, J.R., 1999, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—

Determination of dissolved arsenic, boron, lithium, 

selenium, strontium, thallium, and vanadium using 

inductively coupled plasma-mass spectrometry: U.S. 

Geological Survey Open-File Report 99–093, 31 p. [Also 

available at https://doi.org/10.3133/ofr9993].
References Cited 49

Garbarino, J.R., Kanagy, L.K., and Cree, M.E., 2006, 

Determination of elements in natural-water, biota, sediment 

and soil samples using collision/reaction cell inductively 

coupled plasma–mass spectrometry: U.S. Geological 

Survey Techniques and Methods, book 5, sec. B, chap. 1, 

88 p. [Also available at https://pubs.usgs.gov/tm/2006/

tm5b1/].

Garbarino, J.R., and Struzeski, T.M., 1998, Methods of 

analysis by the U.S. Geological Survey National Water 

Quality Laboratory—Determination of elements in wholewater digests using inductively coupled plasma–optical 

emission spectrometry and inductively coupled plasma–

mass spectrometry: U.S. Geological Survey Open-File 

Report 98–165, 101 p. [Also available at https://doi.

org/10.3133/ofr98165].

Ging, P.B., 1999, Water-quality assessment of southcentral Texas—Descriptions and comparisons of 

nutrients, pesticides, and volatile organic compounds 

at three intensive fixed sites, 1996–98: U.S. Geological 

Survey Water-Resources Investigations Report 99–4155, 

24 p. [Also available at https://pubs.usgs.gov/wri/

wri994155/99-4155.pdf].

Ging, P.B., Van Metre, P.C., and Callender, Edward, 1999, 

Bottom sediments of Lorence Creek Lake, San Antonio, 

Texas, reflect contaminant trends in an urbanizing 

watershed: U.S. Geological Survey Fact Sheet FS–149–99, 

4 p. [Also available at https://doi.org/10.3133/fs14999].

Guy, H.P., 1969, Laboratory theory and methods for sediment 

analysis: U.S. Geological Survey Techniques of WaterResources Investigations, book 5, chap. C1, 58 p. [Also 

available at http://pubs.usgs.gov/twri/twri5c1/].

Harwell, G.R., Van Metre, P.C., Wilson, J.T., and Mahler, 

B.J., 2003, Spatial distribution and trends in trace elements, 

polycyclic aromatic hydrocarbons, organochlorine 

pesticides, and polychlorinated biphenyls in Lake Worth 

sediment, Fort Worth, Texas: U.S. Geological Survey 

Water-Resources Investigations Report 03–4269, 56 p. 

[Also available at https://pubs.usgs.gov/wri/wri034269/pdf/

wri03-4269.pdf].

Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, 

P., Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., 

and Megown, K., 2015, Completion of the 2011 National 

Land Cover Database for the conterminous United 

States—Representing a decade of land cover change 

information: Photogrammetric Engineering and Remote 

Sensing, v. 81, no. 5, p. 345–354, accessed March 29, 2018, 

at https://www.mrlc.gov/nlcd2011.php.

Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., 

and Rickert, D.A., 1994, U.S. Geological Survey protocol 

for the collection and processing of surface-water samples 

for the subsequent determination of inorganic constituents 

in filtered water: U.S. Geological Survey Open-File Report 

94–539, 57 p. [Also available at https://doi.org/10.3133/

ofr94539].

Hostettler, F.D., Lorenson, T.D., and Bekins, B.A., 

2013, Petroleum fingerprinting with organic markers: 

Environmental Forensics, v. 14, p. 262–277.

Ingersoll, C.G., MacDonald, D.D., Wang, N., Crane, J.L., 

Field, L.J., Haverland, P.S., Kemble, N.E., Lindskoog, 

R.A., Severn, C., and Smorong, D.E., 2001, Prediction 

of sediment toxicity using consensus-based freshwater 

sediment quality guidelines: Archives of Environmental 

Contamination and Toxicology, v. 34, no. 4, p. 552–559. 

Kappel, W.M., Williams, J.H., and Szabo, Zoltan, 2013, Water 

resources and shale gas/oil production in the Appalachian 

Basin—Critical issues and evolving developments: U.S. 

Geological Survey Open-File Report 2013–1137, 12 p. 

[Also available at https://doi.org/10.3133/ofr20131137].

Kennedy, E.J., 1983, Computation of continuous records of 

streamflow: U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A13, 53 p. [Also 

available at https://pubs.usgs.gov/twri/twri3-a13/html/pdf.

html].

Kennedy, E.J., 1984, Discharge ratings at gaging stations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A10, 59 p. [Also available at 

https://pubs.usgs.gov/twri/twri3-a10/html/pdf.html].

Khan, N.A., Engle, M., Dungan, B., Holguin, F.O., 

Xu, Pei, and Carroll, K.C., 2016, Volatile-organic 

molecular characterization of shale-oil produced 

water from the Permian Basin: Chemosphere, v. 148, 

p. 126–136. [Also available at https://doi.org/10.1016/j.

chemosphere.2015.12.116].

Kimball, B.A., Runkel, R.L., and Walton-Day, K., 2008, 

Principal locations of major-ion, trace-element, nitrate, 

and Escherichia coli loading to Emigration Creek, Salt 

Lake County, Utah, October 2005: U.S. Geological 

Survey Scientific Investigations Report 2008–5043, 33 p. 

[Also available at https://pubs.er.usgs.gov/publication/

sir20085043].

Kresse, T.M., Warner, N.R., Hays, P.D., Down, A., Vengosh, 

A., and Jackson, R.B., 2012, Shallow groundwater quality 

and geochemistry in the Fayetteville Shale gas-production 

area, north-central Arkansas, 2011: U.S. Geological Survey 

Scientific Investigations Report 2012–5273, 31 p. [Also 

available at https://doi.org/10.3133/sir20125273].
50 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

Levings, G.W., Healy, D.F., Richey, S.F., and Carter, L.F., 

1998, Water quality in the Rio Grande Valley, Colorado, 

New Mexico, and Texas, 1992–95: U.S. Geological 

Survey Circular 1162, 39 p. [Also available at https://doi.

org/10.3133/cir1162].

Lopes, T.J., and Dionne, S.G., 1998, A review of semivolatile 

and volatile organic compounds in highway runoff and 

urban stormwater: U.S. Geological Survey Open-File 

Report 98–409, 67 p. [Also available at https://doi.

org/10.3133/ofr98409].

Luoma, S.N., 1983, Bioavailability of trace metals to aquatic 

organisms—A review: Science of the Total Environment, 

v. 28, no. 1–3, p. 1–22. [Also available at https://doi.

org/10.1016/S0048-9697(83)80004-7].

MacDonald, D.D., Ingersoll, C.G., and Berger, T.A., 2000, 

Development and evaluation of consensus-based sediment 

quality guidelines for freshwater ecosystems: Archives of 

Environmental Contamination and Toxicology, v. 39, p. 

20–31. [Also available at https://www.waterboards.ca.gov/

water_issues/programs/tmdl/docs/303d_policydocs/241.

pdf].

Mahler, B.J., and Van Metre, P.C., 2001, Effects of oil and gas 

production on Lake Meredith sediments, 1964–99: U.S. 

Geological Survey Fact Sheet 072–01, 6 p. [Also available 

at https://doi.org/10.3133/fs07201].

Mahler, B.J., Van Metre, P.C., Bashara, T.J., Wilson, J.T., and 

Johns, D.A., 2005, Parking lot sealcoat—An unrecognized 

source of urban polycyclic aromatic hydrocarbons: 

Environmental Science & Technology, v. 39, no. 15, 

p. 5560–5566. [Also available at https://doi.org/10.1021/

es0501565].

Moring, J.B., and Rose, D.R., 1997, Occurrence and 

concentrations of polycyclic aromatic hydrocarbons in 

semipermeable membrane devices and clams in three urban 

streams of the Dallas-Fort Worth metropolitan area, Texas: 

Chemosphere, v. 34, no. 3, p. 551–566.

Mueller, D.K., Schertz, T.L., Martin, J.D., and Sandstrom, 

M.W., 2015, Design, analysis, and interpretation of field 

quality-control data for water-sampling projects: U.S. 

Geological Survey Techniques and Methods, book 4, chap. 

C4, 54 p. [Also available at https://doi.org/10.3133/tm4C4].

National Oceanic and Atmospheric Administration, 2018a, 

National Centers for Environmental Information: Climate 

Data Online, Station WBAN:12911, accessed August 3, 

2018, at https://www.ncdc.noaa.gov/cdo-web/datasets/LCD/

stations/WBAN:12911/detail.

National Oceanic and Atmospheric Administration, 2018b, 

National Centers for Environmental Information: Climate 

Data Online, Station GHCND:USC00413618, accessed 

August 3, 2018, at https://www.ncdc.noaa.gov/cdo-web/

datasets/GHCND/stations/GHCND:USC00413618/detail.

Neely, W.B., 1980, Chemicals in the environment—

Distribution, transport, fate, analysis: New York, Marcel 

Dekker, 245 p.

Nicot, J.P., and Scanlon, B.R., 2012, Water use for shalegas production in Texas, U.S.: Environmental Science & 

Technology, v. 46, no. 6, p. 3580–3586. [Also available at 

https://doi.org/10.1021/es204602t].

Opsahl, S.P., and Crow, C.L., 2015, Concentrations of selected 

constituents in surface-water and streambed-sediment 

samples collected from streams in and near an area of oil 

and natural-gas development, south-central Texas, 2011–13 

(ver. 1.1, January 2015): U.S. Geological Survey Data 

Series 836, 25 p., https://doi.org/10.3133/ds836.

Pirkey, K.D., and Glodt, S.R., 1998, Quality control at the 

U.S. Geological Survey National Water Quality Laboratory: 

U.S. Geological Survey Fact Sheet FS–026–98, 4 p.

Railroad Commission of Texas, 2018, Eagle Ford Shale 

information, accessed March 28, 2018, at http://www.rrc.

state.tx.us/oil-gas/major-oil-and-gas-formations/eagle-fordshale-information/.

Rantz, S.E., and others, 1982a, Measurement and computation 

of streamflow—Volume 1, measurement of stage and 

discharge: U.S. Geological Survey Water-Supply Paper 

2175, 284 p. [Also available at http://pubs.usgs.gov/wsp/

wsp2175/pdf/WSP2175_vol1a.pdf].

Rantz, S.E., and others, 1982b, Measurement and computation 

of streamflow—Volume 2, computation of discharge: 

U.S. Geological Survey Water-Supply Paper 2175, 631 p. 

[Also available at http://pubs.usgs.gov/wsp/wsp2175/pdf/

WSP2175_vol2a.pdf].

Rose, D.L., Sandstrom, M.W., and Murtagh, L.K., 

2016, Determination of heat purgeable and ambient 

purgeable volatile organic compounds in water by gas 

chromatography/mass spectrometry: U.S. Geological 

Survey Techniques and Methods, book 5, chap. B12, 61 p. 

[Also available at https://doi.org/10.3133/tm5B12].

Ryder, P.D., 1996, Ground water atlas of the United States—

Segment 4, Oklahoma, Texas: U.S. Geological Survey 

Hydrologic Investigations Atlas 730–E, 30 p. [Also 

available at https://pubs.er.usgs.gov/publication/ha730E].

Schramm, Erich, 2011, What is flowback, and how does 

it differ from produced water?: Institute for Energy and 

Environmental Research of Northeastern Pennsylvania 

Clearinghouse, accessed May 22, 2018, at http://energy.

wilkes.edu/pages/205.asp.

Shelton, L.R., 1997, Field guide for collecting samples for 

analysis of volatile organic compounds in stream water 

for the National Water-Quality Assessment Program: U.S. 

Geological Survey Open-File Report 97–401, 44 p. [Also 

available at https://doi.org/10.3133/ofr97401].
References Cited 51

Shelton, L.R., and Capel, P.D., 1994, Guidelines for collecting 

and processing samples of stream bed sediment for analysis 

of trace elements and organic contaminants for the National 

Water-Quality Assessment Program: U.S. Geological 

Survey Open-File Report 94–458, 20 p. [Also available at 

https://doi.org/10.3133/ofr94458].

Struzeski, T.M., DeGiacomo, W.J., and Zayhowski, E.J., 1996, 

Methods of analysis by the U.S. Geological Survey National 

Water Quality Laboratory—Determination of dissolved 

aluminum and boron in water by inductively coupled 

plasma–atomic emission spectrometry: U.S. Geological 

Survey Open-File Report 96–149, 17 p. [Also available at 

https://doi.org/10.3133/ofr96149].

Surles, M.A., Jr., 1987, Stratigraphy of the Eagle Ford Group 

(upper Cretaceous) and its source-rock potential in the 

East Texas basin: Waco, Texas, Baylor Geological Studies 

Bulletin 45, 57 p. [Also available at https://www.baylor.edu/

geology/doc.php/142587.pdf].

Texas Commission on Environmental Quality, 2015, 2014 

Guidance for assessing and reporting surface water quality 

in Texas in compliance with sections 305(b) and 303(d) 

of the Federal Clean Water Act: Surface Water Quality 

Monitoring Program, Monitoring and Assessment Section, 

Water Quality Planning Division, accessed March 6, 2018, 

at https://www.tceq.texas.gov/assets/public/waterquality/

swqm/assess/14txir/2014_guidance.pdf.

Texas Natural Resources Information System, 2018, StratMap 

SARA 2015 6in NC\CIR Orthoimagery, accessed January 1, 

2018, at https://tnris.org/data-catalog/entry/stratmap-sara2015-6in-nc-cir/.

Tibbetts, P.J.C., Buchanan, I.T., Gawel, L.J., and Large, R., 

1992, A comprehensive determination of produced water 

composition, in Ray, J.P., and Engelhardt, F.R., eds., 

Produced waters: New York, Plenum Press, p. 97–112.

Turnipseed, D.P., and Sauer, V.B., 2010, Discharge 

measurements at gaging stations: U.S. Geological Survey 

Techniques and Methods, book 3, chap. A8, 87 p. [Also 

available at https://doi.org/10.3133/tm3A8].

U.S. Department of Agriculture, Natural Resources 

Conservation Service, 2011, Geospatial Data Gateway, 

accessed August 26, 2011, at http://datagateway.nrcs.usda.

gov/.

U.S. Energy Information Administration, 2018, How much 

shale (tight) oil is produced in the United States?, accessed 

March 28, 2018, at https://www.eia.gov/tools/faqs/faq.

php?id=847&t=6.

U.S. Environmental Protection Agency [EPA], 1980, 

Prescribed procedures for measurement of radioactivity 

in drinking water: U.S. Environmental Protection Agency 

report EPA/600/4-80-032 [variously paged].

U.S. Environmental Protection Agency [EPA], 1992, 

Definition and procedure for the determination of the 

method detection limit—Revision 1.11: Code of Federal 

Regulations 40, Protection of the Environment, CFR Part 

136, Appendix B, p. 565–567.

U.S. Environmental Protection Agency [EPA], 1996a, Test 

methods for evaluating solid waste, physical/chemical 

methods, SW-846 (3d ed.), update IV, method 8015C, 

nonhalogenated organics by gas chromatography, rev. 2: 

U.S. Environmental Protection Agency, 36 p.

U.S. Environmental Protection Agency [EPA], 1996b, Test 

methods for evaluating solid waste, physical/chemical 

methods, SW-846 (3d ed.), update IV, method 8270D, 

semivolatile organic compounds by gas chromatography/

mass spectrometry (GC/MS): U.S. Environmental 

Protection Agency, 62 p.

U.S. Environmental Protection Agency [EPA], 1998, 

Microwave assisted acid digestion of sediments, sludges, 

soils, and oils: U.S. Environmental Protection Agency 

Method 3051A, Revision 1, January 1998, 25 p.

U.S. Environmental Protection Agency [EPA], 2004, Test 

methods for evaluating solid waste, physical/chemical 

methods, SW-846 (3d ed.), update IV, method 9060A-1, 

total organic carbon: U.S. Environmental Protection 

Agency, 5 p.

U.S. Environmental Protection Agency [EPA], 2005, 

Reregistration eligibility decision (RED) document 

for triethylene glycol: U.S. Environmental Protection 

Agency report EPA739-R-05-002, 75 p. [Also available at 

https://archive.epa.gov/pesticides/reregistration/web/pdf/

triethylene_glycol_red.pdf].

U.S. Environmental Protection Agency [EPA], 2018a, 

Unconventional oil and natural gas development, accessed 

March 28, 2018, at https://www.epa.gov/uog.

U.S. Environmental Protection Agency [EPA], 2018b, 

2018 Edition of the drinking water standards and health 

advisories tables: U.S. Environmental Protection Agency 

report EPA 822-F-18-001, 20 p., accessed April 2, 2018, 

at https://www.epa.gov/sites/production/files/2018-03/

documents/dwtable2018.pdf.

U.S. Environmental Protection Agency [EPA], 2018c, 

National recommended water quality criteria—Aquatic life 

criteria table, accessed March 30, 2018, at https://www.

epa.gov/wqc/national-recommended-water-quality-criteriaaquatic-life-criteria-table.

U.S. Geological Survey, 2017, USGS water data for the 

Nation: U.S. Geological Survey National Water Information 

System database, accessed September 30, 2017, at http://

dx.doi.org/10.5066/F7P55KJN.
52 Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17

U.S. Geological Survey, variously dated, National field 

manual for the collection of water-quality data: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 9, chaps. A1–A9, https://water.usgs.

gov/owq/FieldManual/. 

Van Metre, P.C., and Mahler, B.J., 2005, Trends in 

hydrophobic organic contaminants in urban and reference 

lake sediments across the United States, 1970–2001: 

Environmental Science & Technology, v. 39, no. 15, 

p. 5567–5574. [Also available at https://doi.org/10.1021/

es0503175].

Van Metre, P.C., and Mahler, B.J., 2010, Contribution of 

PAHs from coal-tar pavement sealcoat and other sources 

to 40 U.S. lakes: Science of the Total Environment, 

v. 409, p. 334–344. [Also available at https://tx.usgs.gov/

coring/pubs/Van%20Metre%20PAH%20sources%20

STOTEN2010.pdf].

Veatch, R.W., Jr., 1983, Overview of current hydraulic 

fracturing design and treatment technology—Part 2: Journal 

of Petroleum Technology, v. 35, no. 5, p. 853–864.

Wilson, J.T., 2011, Assessment of selected contaminants in 

streambed- and suspended-sediment samples collected 

in Bexar County, Texas, 2007–09: U.S. Geological 

Survey Scientific Investigations Report 2011–5097, 57 p. 

[Appendixes available online only at http://pubs.usgs.gov/

sir/2011/5097/.]

Xylem Analytics, 2018, 6920 V2-2 Multi-Parameter Water 

Quality Sonde, accessed April 18, 2018, at https://www.ysi.

com/6920-V2-2.

For more information about this publication, contact

Director, Texas Water Science Center

U.S. Geological Survey

1505 Ferguson Lane

Austin, TX 78754–4501 

For additional information visit https://www.usgs.gov/centers/tx-water.

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Crow and others—Land-Cover Changes Associated With Oil and Natural-Gas Production, South Texas, 2008–17—SIR 2018–5119

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