BASELINE GROUNDWATER QUALITY TESTING NEEDS IN THE EAGLE FORD SHALE REGION

By

Virginia E. Palacios

Dr. Robert B. Jackson, Advisor

April 2012

2012

Masters project submitted in partial fulfillment of the requirements for the Master of Environmental 

Management degree in the Nicholas School of the Environment of Duke University 
i

ABSTRACT

As the pace of drilling in the Eagle Ford shale increases, so does the potential for groundwater contamination 

incidents. The goals of this analysis are (1) to determine whether existing baseline groundwater quality data 

in the Eagle Ford shale region of southern Texas is adequate to provide a comparison to potential future 

contamination from oil and gas development and (2) to define an appropriate and cost-effective list of 

parameters that will aid in strategic planning of baseline groundwater quality testing in the Eagle Ford shale 

region for the same goal. 

First, a list of potential testing parameters is defined using case studies of proposed groundwater 

contamination. Second, formation water chemistry in the Eagle Ford shale region is compared to 

groundwater chemistry in the counties of the Eagle Ford shale region to determine which chemical indicators 

demonstrate potential to consistently detect contamination. Third, statistical power analysis is used as a 

guideline to decide whether more samples are needed for each testing parameter in each county in the Eagle 

Ford shale region. Next, known health effects of each testing parameter are described in order to highlight 

potential pollutants that should be prioritized in a sampling initiative. Finally, testing costs are reported to 

introduce a perspective about microeconomic choices affecting which stakeholders take responsibility for 

baseline groundwater quality testing.

These tasks led to the findings that some of the most dangerous potential pollutants, including methane, 

total petroleum hydrocarbons, nitrate, volatile organic compounds, polycyclic aromatic hydrocarbons, alpha 

particles, beta particles, and gamma radiation, are poorly characterized in the region, if at all. Furthermore, 

testing these parameters is more expensive than testing less hazardous ones. Water well owners may be 

unable to afford the expense of testing these parameters. Therefore, a testing initiative facilitated by 

agencies, industry, or other organizations may be more efficient at establishing a regional baseline for these 

high priority, expensive tests. As such, the framework and analysis presented here can be used by 

groundwater managers in the Eagle Ford shale region to develop baseline sampling strategies tailored to 

specific counties in the region.
ii

Acknowledgements

The idea for this project came from concerns expressed by residents of a small community in Webb County, 

Texas called “Los Botines.” After hearing about cases of potential groundwater contamination linked to 

hydraulic fracturing, the residents approached a non-profit organization I was working for, the Rio Grande 

International Study Center (RGISC), with questions about how to get baseline water quality tests taken for 

water wells in Los Botines. After some research, my colleagues and I at RGISC found that comprehensive

baseline groundwater quality testing would cost more than the non-profit could afford, and we speculated 

that the expense would deter residents from pursuing testing on their own. With this in mind, the analysis 

presented here seeks to enable water well users, oil and gas companies, and agencies to better understand 

groundwater resources in the Eagle Ford shale region of South Texas. 

Much gratitude is owed to Tricia Cortez, Executive Director of RGISC, who has not only been an outstanding

mentor, but also provided me with opportunities to work with stakeholders on the ground. Additionally, my 

thanks go out to my fellow students at the Nicholas School of the Environment, especially those who are 

members of the Drilling, Environment, and Economics Network; and to members of the Laredo, Texas-based 

Safe Fracking Coalition, who engaged in open-minded discussions about oil and gas development and were 

always willing to share their expertise on relevant topics. Finally, I greatly appreciate the time and feedback 

offered by my advisor, Dr. Robert B. Jackson, who offered careful and thoughtful guidance throughout the

process of writing this analysis.
iii

Contents

Acknowledgements .............................................................................................................................................. ii

Figures................................................................................................................................................................... v

Tables................................................................................................................................................................... vi

I. Introduction ..................................................................................................................................................1

Geographic Scope .............................................................................................................................................3

Pathways and detection ...................................................................................................................................4

Categories.........................................................................................................................................................5

Case Studies......................................................................................................................................................6

i. Emergency Administrative Order: Range Resources (EPA 2010).........................................................6

ii. Investigation of Ground Water Contamination near Pavillion, WY (EPA 2011)...................................7

iii. Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing 

(Osborn et al. 2011)......................................................................................................................................8

iv. Plan to Study the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources (EPA 

2011)...........................................................................................................................................................10

v. Reported Groundwater Contamination in the Eagle Ford shale region ............................................12

II. Data and Methods......................................................................................................................................14

Produced Water Data .....................................................................................................................................15

Groundwater Data ..........................................................................................................................................18

Geospatial Analysis.........................................................................................................................................20

III. Results....................................................................................................................................................21

Testing priority codes .....................................................................................................................................21

A. Gas Hydrocarbons..............................................................................................................................21

B. Liquid Hydrocarbons..........................................................................................................................25

C. Salts....................................................................................................................................................28

D. Metals ................................................................................................................................................31

E. Naturally-Occurring Radioactive Materials (NORM)..........................................................................32

F. Volatile Organic Compounds (VOCs)..................................................................................................35

G. Polycyclic Aromatic Hydrocarbons (PAHs).........................................................................................37

IV. Discussion...............................................................................................................................................40

References..........................................................................................................................................................41
iv

Appendix I – Counties.........................................................................................................................................45

Appendix II – Carbon isotope values ..................................................................................................................46

Appendix III – Starting list of potential testing parameters................................................................................47

Appendix III – Pavillion, WY test results..............................................................................................................49

Appendix IV – RRC groundwater contamination report.....................................................................................51

Appendix V – Data preparation ..........................................................................................................................53

USGS Produced Water Database ....................................................................................................................53

Kearns (2010)..................................................................................................................................................53

TWDB Groundwater Database........................................................................................................................53

Geospatial Analysis.........................................................................................................................................54

Literature Review............................................................................................................................................54

Appendix VI – Stratigraphic column ...................................................................................................................55

Appendix VII – Reliability codes for groundwater quality samples in Texas Water Development Board 

database. ............................................................................................................................................................56

Appendix VIII – TWDB Groundwater Database Listed Parameters ....................................................................57

Appendix IX – All Infrequent Constituents Identified in the TWDB Groundwater Database .............................58

Appendix X - Carbon isotope ratios by depth.....................................................................................................62

Appendix XI – Salts..............................................................................................................................................65

Produced water comparison with groundwater ............................................................................................65

Salts data availability ......................................................................................................................................67

Regulated salts data availability .....................................................................................................................71

Appendix XII – Metals: number of samples........................................................................................................72

Appendix XIII – Radionuclides: number of samples............................................................................................77

Appendix XIV – PAH samples..............................................................................................................................79
v

Figures

Figure 1 Acceleration of drilling activity in the Eagle Ford shale (Railroad Commission of Texas 2012).............1

Figure 2 Windows of oil, condensate, and gas in the Eagle Ford shale................................................................2

Figure 3 Counties included in this analysis. ..........................................................................................................3

Figure 4 Isopachs in the Eagle Ford shale. (adapted from Sellard et al. 1932 and Surles, 1987, Cited in Lai 

1997).....................................................................................................................................................................4

Figure 5 Number of pending cases of groundwater contamination attributed to oil and gas activities in 

counties of the Eagle Ford shale region in 2010.................................................................................................13

Figure 6 Total organic carbon concentrations in the counties of interest..........................................................22

Figure 7 Geographic distribution of concentrations and samples of 13C/

12C stable isotope ratios....................23

Figure 8 Groundwater concentrations of Total Petroleum Hydrocarbons in the counties of interest. Two 

datapoints are found in the database, but their locations overlap on the map. ...............................................26

Figure 9 Alpha particle concentrations in the counties of interest. Red points represent values greater than 

the EPA MCL........................................................................................................................................................34

Figure 10 Benzene concentrations in the counties of interest. Points marked in red indicate concentrations 

that exceed the EPA MCL for benzene. ..............................................................................................................36

Figure 11 Benzo(a)pyrene concentrations in the counties of interest. ..............................................................38

Figure 12 Activity status for pending cases of groundwater contamination attributed to oil and gas activities 

in 2010 (Texas Groundwater Protection Committee 2011). ..............................................................................51

Figure 13 Map of the number of pending groundwater contamination cases caused by oil and gas activities 

reported in 2010 (Texas Groundwater Protection Committee 2011). ...............................................................52

Figure 14 Stratigraphic column of the Eagle Ford formation (Condon and dyman 2006, cited in Mullen 2010).

............................................................................................................................................................................55

Figure 15 13C/12C stable isotope ratios in groundwater by depth in the counties of interest...........................62

Figure 16 Counties where 13C/12C stable isotope ratio data for groundwater or core samples are available. ..62

Figure 17 Distribution of carbon isotope ranges found in groundwater in Eagle Ford shale counties. .............63

Figure 18 Number, percent of total water wells, and groundwater testing needs for gas hydrocarbons with in 

Eagle Ford shale counties. Testing codes: a = as needed, r = recommended, t = testing needed .....................64

Figure 19 Total dissolved solids (TDS) concentrations in the Eagle Ford shale region.......................................70
vi

Tables

Table 1 EPA isotopic fingerprint analysis of allegedly contaminated domestic well in Parker County, Texas 

compared to commingled gas from the Butler and Teal gas wells.......................................................................6

Table 2 Analytical tests used by Osborn, et al. (2011) to detect potential groundwater contamination caused 

by shale gas extraction. ........................................................................................................................................9

Table 3 EPA Tier 2 Initial testing: sample types and testing parameters (Office of Research and Development 

2011)...................................................................................................................................................................11

Table 4 Contaminants reported in groundwater contamination cases attributed to oil and gas activities in 

2010 (Texas Groundwater Protection Committee 2011) ...................................................................................13

Table 5 Number of samples available in the TWDB groundwater database for general water quality 

parameters in each county of the eagle ford shale region.................................................................................19

Table 6 Values of 13C/12C isotope ratios found in potential Eagle Ford Source formations Comet (1993)......24

Table 7 Cost of testing for carbon isotope ratios and total organic carbon with labs certified to test drinking 

water in the state of Texas. ................................................................................................................................25

Table 8 Cost of testing for liquid hydrocarbons with labs certified to test drinking water in the state of Texas.

............................................................................................................................................................................26

Table 9 EPA MCLs and Secondary Standards for salts and pH in groundwater (Environmental Protection 

Agency 2012). .....................................................................................................................................................29

Table 10 Cost of testing, in USD, for various salts with labs certified to test drinking water in the state of 

Texas. ..................................................................................................................................................................30

Table 11 EPA Maximum Contaminant Levels (MCL) and Secondary Standards for metals in drinking water and 

associated health effects. ...................................................................................................................................31

Table 12 Cost of testing, in USD, for metals with labs certified to test drinking water in the state of Texas. ...32

Table 13 Types of radioactive emissions known to occur from various............................................................33

Table 14 EPA Maximum Contaminant Levels for radioactive elements/particles in drinking water.................34

Table 15 Cost of testing, in USD, for Radionuclides with labs certified to test drinking water in the state of 

Texas. ..................................................................................................................................................................35

Table 16 Number and percent of wells with reliable samples for which concentrations of BTEX chemicals are 

found in the TWDB groundwater database........................................................................................................36

Table 17 Health effects of volatile organic compounds commonly found in oil and gas flowback 

(Environmental Protection Agency 2012)...........................................................................................................37

Table 18 Cost of testing, in USD, for VOC with labs certified to test drinking water in the state of Texas. .......37
vii

Table 19 Cost of testing, in USD, for PAH with labs certified to test drinking water in the state of Texas. .......38

Table 20 Categories of potential baseline groundwater chemistry parameters. ..............................................47

Table 21 Contaminants found in monitoring wells drilled by the EPA in Pavillion, WY.....................................49

Table 22 Consolidated categories of RRC reported contaminants.....................................................................51

Table 23 Carbon isotope ranges for groundwater in the counties of interest. ..................................................63

Table 24 Wood County t-test results for produced water compared to groundwater; All concentrations 

measured in mg/l, except for pH........................................................................................................................65

Table 25 Wood County summary statistics. WW = produced water, GW = groundwater; All concentrations 

measured in mg/l, except for pH........................................................................................................................65

Table 26 All Eagle Ford shale Counties: t-test results for produced water compared to groundwater. TDS = 

Total Dissolved Solids; All concentrations measured in mg/l, except for pH. ....................................................66

Table 27 All Counties in the Eagle Ford shale region: summary statistics. WW = produced water, GW = 

groundwater. All concentrations measured in mg/l, except for pH..................................................................66

Table 28 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Atascosa, Austin, Bastrop, Bee, and Brazos Counties. Testing codes: A = As needed, R = 

Recommended, T = Testing needed ...................................................................................................................67

Table 29 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Burleson, Caldwell, Colorado, Dewitt, and Dimmit Counties. Testing codes: a = as needed, r = 

recommended, t = testing needed .....................................................................................................................67

Table 30 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Duval, Edwards, Fayette, Frio, and Goliad Counties. Testing codes: a = as needed, r = recommended, 

t = testing needed...............................................................................................................................................68

Table 31 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Gonzales, Grimes, Guadalupe, Houston, Karnes Counties. Testing codes: a = as needed, r = 

recommended, t = testing needed .....................................................................................................................68

Table 32 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in La Salle, Lavaca, Lee, Leon, and Live Oak Counties. Testing codes: a = as needed, r = recommended, t 

= testing needed .................................................................................................................................................68

Table 33 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Madison, Maverick, McMullen, Milam, and Robertson Counties. Testing codes: a = as needed, r = 

recommended, t = testing needed .....................................................................................................................69

Table 34 Number, percent of total water wells, and groundwater testing needs for salts found in produced 

water in Washington, Webb, Wilson, Wood, and Zavala Counties. Testing codes: a = as needed, r = 

recommended, t = testing needed .....................................................................................................................69
viii

Table 35 Number, percent of total water wells, and groundwater testing needs for salts with primary 

drinking water standards in Eagle Ford shale counties. Testing codes: a = as needed, r = recommended, t = 

testing needed....................................................................................................................................................71

Table 36 Number, percent of total water wells, and groundwater testing needs for metals in Atascosa, Austin, 

Bastrop, Bee, and Brazos Counties. A = As needed, R = Recommended, T = Testing needed ...........................72

Table 37 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................72

Table 38 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................73

Table 39 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................73

Table 40 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................74

Table 41 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................75

Table 42 Number, percent of total water wells, and groundwater testing needs for metals in Counties. A = As 

needed, R = Recommended, T = Testing needed ...............................................................................................75

Table 43 Number of radionuclide samples available in the TWDB groundwater database for each county.....77

Table 44 Number, percent of total water wells, and groundwater testing needs for radionuclide particle 

emissions in Eagle Ford shale counties. A = As needed, R = Recommended, T = Testing needed.....................78

Table 45 Number and percent of samples out of the total number of water wells in each county for PAH 

concentrations in groundwater. .........................................................................................................................79
1

I. Introduction

Rapid increases in the pace of drilling activity in the Eagle Ford shale play (Figure 1) have created dual 

pressures for landowners who also own mineral rights in the region. These landowners theoretically have a 

desire to collect income from leasing their mineral rights, but also have a preference for protecting their 

water quality. Recent developments in the technologies of horizontal drilling and hydraulic fracturing have 

allowed oil and gas extraction from low permeability formations, such as shale, to become more 

economically viable (Hughes 2011). In shale plays1across the United States, drilling rates have increased

(Lippman Consulting and U.S. Energy Information Administration 2010). This increase in drilling activity could 

be problematic however, because chemicals that flow back2 out of oil and gas wells during extraction could

potentially cause groundwater to become contaminated with toxic materials (Colborn, Kwiatkowski et al. 

2011).

FIGURE 1 ACCELERATION OF DRILLING ACTIVITY IN THE EAGLE FORD SHALE 

(RAILROAD COMMISSION OF TEXAS 2012)

Recent proposed cases of groundwater contamination occurring after increases in the rate of hydraulic 

fracturing in Texas, Pennsylvania, Wyoming, and other states have remained contentious because of the lack 

of background water quality data available. “Background” water quality refers to the chemical characteristics 

of the water before a change is made to the water body that could affect its chemical characteristics3. On 

the one hand, oil and gas companies (industry) are aware of the opportunity for landowners to falsely claim 

that contamination has occurred when if water quality problems already exist. On the other hand, 

landowners have been unable to find compensation for groundwater remediation when no comparison to 

background water quality is available to indicate the source of contamination.

As a shale play that is rich in liquids such as oil and condensate (Figure 2), drilling activity in the Eagle 

Ford is expected to continue accelerating. This is because liquids are currently more valuable than dry 

gas (U.S. Energy Information Administration 2011; U.S. Energy Information Administration 2012). In 

 

1

In oil and gas exploration, a “play” is a region where significant quantities of oil and gas are expected to occur. 

Schlumberger. (2012). "Oilfield Glossary." Retrieved April 24, 2012, from http://www.glossary.oilfield.slb.com.

2

The term “flowback” is used to describe the mixture of hydraulic fracturing fluids that returns to the surface 

when the well is producing. Flowback may contain produced water, hydraulic fracturing chemicals, and petroleum 

hydrocarbons.

3

In this report, the term “baseline” will be used interchangeably with the term “background.”
2

fact, the Energy Information Administration recently noted the Eagle Ford as a contributor in Texas’ 

significant gains in production for the year of 2011 (U.S. Energy Information Administration 2012). 

In light of the potential for groundwater contamination to occur in the Eagle Ford shale region, it is 

important to assess the strength of existing background water quality datasets in accurately predicting a 

regional baseline of water quality. Additionally, it is important to determine geographic locations in the 

region where more information is needed and what kind of chemical data is needed. The analysis 

presented in this report finds that current available baseline groundwater quality data is not adequate 

to assess potential groundwater contamination from oil and gas exploration in the Eagle Ford shale

region.

FIGURE 2 WINDOWS OF OIL, CONDENSATE, AND GAS IN THE EAGLE FORD SHALE 

(U.S. ENERGY INFORMATION ADMINISTRATION 2010)

Current regulations and programs exist to facilitate baseline groundwater quality characterization, but to my 

knowledge no widespread initiative exists to assist landowners or the oil and gas industry in the Eagle Ford 

shale region in acquiring the appropriate tests needed to assess baseline water quality for the purpose of 

detecting potential future contamination from oil and gas drilling, extraction, and production activities 

(hereafter termed “oil and gas development”). Yet assessing potential contamination of groundwater wells 

caused by oil and gas development relies on groundwater stakeholders, those who consume groundwater 

and those who have the capacity to pollute groundwater, having access to appropriate and reliable

background water quality data (Texas Administrative Code 2006).

Studies assessing baseline groundwater quality have ranged from testing few parameters to testing extensive 

lists of potential contaminants and indicators. Baseline groundwater quality testing should include a 
3

comprehensive list of parameters in order to be effective at detecting a wide range of potential contaminants 

after oil and gas drilling takes place. However, the expense of testing a comprehensive list of analytes4serves 

to dissuade some water well owners from volunteering to conduct background water quality testing on their 

water well. Considering this potential reaction, limited resources should be spent on the most essential tests 

that are effective in detecting constituents specific to oil and gas contamination. 

Therefore, the purpose of this analysis is (1) to determine whether existing baseline water quality data in the 

Eagle Ford shale region is adequate to provide a comparison to potential future contamination from oil and 

gas development and (2) to define an appropriate and cost-effective list of parameters that would aid in 

strategic planning of baseline water quality testing in the Eagle Ford shale region for the same goal. 

Geographic Scope

The data used in this analysis represents all Eagle Ford shale counties listed by three sources: Railroad 

Commission, Energy Information Administration, and the website EagleFordShale.com. The 35 “counties of 

interest” used in this analysis are depicted in Figure 3 and are listed in Appendix I.

FIGURE 3 COUNTIES INCLUDED IN THIS ANALYSIS.

 

4

The term analytes refers to a chemical parameter which must be analyzed, or tested, in a laboratory.
4

One county included in this analysis, Wood County, is not in geographic proximity to the area of current 

active Eagle Ford shale drilling. However, Wood County is included in this analysis because it is the only 

county for which U.S. Geological Survey (USGS) produced water chemistry data for the Eagle Ford shale

formation was available. Produced water chemistry was documented in the USGS produced waters database 

between the years of 1951 and 1975. The age of this data, and Wood County’s distance from the current 

active drilling region of the Eagle Ford shale demands further supporting evidence that the formation 

examined in Wood County is still taxonomically considered to be a part of the Eagle Ford formation.

Figure 4 depicts isopachs (lines of equal thickness) for the Eagle Ford shale. The area of Wood County, 

depicted in Figure 4 East of Dallas, and in Figure 3 outlined in red, includes regions where the Eagle Ford 

shale is known to exist at a thickness between 200 and 800 feet. Therefore, although the USGS produced 

waters data is old, Lai (1997) confirms that Wood County has been considered to contain portions of the 

Eagle Ford formation more recently than 1975. However, considerable uncertainty still exists as to whether 

produced water chemistry of the Eagle Ford formation in Wood County matches produced water chemistry 

of the Eagle Ford formation in other counties.

FIGURE 4 ISOPACHS IN THE EAGLE FORD SHALE. (ADAPTED FROM SELLARD ET AL. 1932 AND SURLES,

1987, CITED IN LAI 1997)

Pathways and detection

This report does not include analysis of the potential pathways through which groundwater contamination 

from oil and gas extraction activities could occur. However, for the sake of understanding a comparison 

between baseline groundwater quality and potentially contaminated groundwater quality, these pathways 

are outlined in this section. A pathway can be defined as a continuous space between a hydrocarbon-bearing 
5

formation and a groundwater aquifer. Four potential pathways have been postulated in previous research 

(U.S. General Accounting Office 1989; Environmental Protection Agency 2011; Osborn, Vengosh et al. 2011)

5

:

1. Natural migration of fluids and gases from the formation to the aquifer

2. Leaky oil or natural gas well casings

3. Induced fractures connecting with or enlarging existing natural fractures

4. Induced fractures connecting with abandoned oil and gas wells

If the pathways are present, fluids and/or hydrocarbon gases that were naturally occurring or which were 

injected into the oil and gas formation could mix with groundwater. In order to detect potential 

contamination, the concentration of potential contaminants flowing from the hydrocarbon-bearing 

formation into the groundwater aquifer must be higher than the concentration of the same pre-existing 

chemicals in the groundwater aquifer. Additionally, the potential contaminant must enter the aquifer at a 

rate that is greater than the groundwater flow rate, or else the contaminant would be diluted to a level that 

is not different from the pre-existing background water quality. Although expected concentrations are 

compared in this report, groundwater flow modeling is outside the scope of the analysis presented here.

Mixing of petroleum-hydrocarbon gases, such as methane, in an aquifer can be detected through 

measurements of total organic carbon. However, carbon-13 (13C) isotopes have been used in numerous 

cases to detect contamination from hydrocarbons. A description of how 13C isotopic signatures are measured 

is provided in Appendix II. The change in concentration of 13C isotopes in an aquifer can help to determine 

whether gas found in the aquifer was sourced from a specific formation that was explored for natural gas. 

Although this test provides a useful indication of whether contamination was caused by oil and gas 

development activities, without a strong dataset of background carbon isotope values it is difficult to prove 

whether the post-drill δ

13C concentrations matching those found in a shale formation were pre-existing or 

not.

Categories

The case studies that are described below list potential groundwater quality parameters that can be used to 

detect potential contamination from oil and gas development. Those parameters have been grouped into

the following seven categories:

1. Gas hydrocarbons

2. Liquid hydrocarbons

3. Salts

4. Metals

5. Radionuclides

6. Volatile organic compounds (VOC)

7. Polycyclic aromatic hydrocarbons (PAH)

The complete list of parameters attributed to each category can be found in Appendix III, Table 20. A 

characterization of the data available for these parameters, their efficacy for the intended purpose, their 

known health effects, and costs of testing each parameter will be described in the results section. All of this 

information is used to generate a list of the most essential baseline groundwater quality parameters for the 

Eagle Ford shale region and to support recommendations for future testing initiatives.

 

5

Cited in Sumi, L. (2005). Our Drinking Water at Risk. Durango, CO, Oil and Gas Accountability Project,.
6

Case Studies

In the past decade, several prominent studies have linked groundwater quality changes to potential 

migration of hydraulic fracturing fluids, produced water, and/or petroleum hydrocarbons into aquifers. The 

results of these studies were questioned, however, because baseline water quality data was not available for 

a comparison with potentially contaminated groundwater samples. The cases described here are used to 

generate a list of potential baseline groundwater quality testing parameters for the Eagle Ford shale region. 

This discussion is not intended to provide a comprehensive analysis of all cases of potential groundwater 

contamination that have been linked to hydraulic fracturing. Rather, these cases point to the complex nature 

of proving contamination through the scientific process, and the need for more baseline ground water quality 

testing of specific chemicals.

i. Emergency Administrative Order: Range Resources (EPA 2010)

In the summer of 2010 the EPA issued an emergency order to Range Resources Company to investigate and 

remediate contamination of two domestic water wells lying within the Barnett shale in Parker County, Texas 

(Environmental Protection Agency 2010). Private tests, RRC tests, and EPA tests of one water well indicated 

that concentrations of volatile organic compounds (VOC) and gas hydrocarbons including benzene6(C6H6), 

toluene (CH3), propane (C3H8), hexane (C6H14), ethane (C2H6), and dissolved methane (CH4) were present 

(Wilson, Sumi et al. 2011). Tests by the RRC showed benzene and toluene levels in excess of federal 

Maximum Contaminant Levels (MCL) for drinking water (Environmental Protection Agency 2012a; 

Environmental Protection Agency 2012b). Subsequent tests by the EPA returned lower values than the RRC 

found, however toluene concentrations were still above the MCL. Additionally, the EPA’s analyses of 

methane isotopes (δ13C–CH4 and δD–CH4) in the water well returned values nearly identical to the isotopic 

signature of methane and hydrogen that was produced from two nearby gas wells, as seen in Table 1

(Environmental Protection Agency 2010).

TABLE 1 EPA ISOTOPIC FINGERPRINT ANALYSIS OF ALLEGEDLY CONTAMINATED DOMESTIC WELL IN 

PARKER COUNTY, TEXAS COMPARED TO COMMINGLED GAS FROM THE BUTLER AND TEAL GAS WELLS.

Domestic water well Natural Gas well

δ

13C–CH4 -47.05 -46.60

δD–CH4 -188.5 -183.9

Critics of the EPA’s actions toward Range Resources state that the gas was pre-existing, and that no lack of 

wellbore integrity was found, implying that there was no possible way for the fluid and gas migration to have 

occurred as a result of Range Resources’ activities (Smitherman and Porter 2012). Background water quality 

data for the hydrocarbons found in the investigation may have allowed a more robust conclusion. As such, 

this analysis includes a characterization of known background concentrations for the following VOC and gas 

hydrocarbons:

 

6

Benzene, toluene, ethylbenzene, and xylene (BTEX) are volatile organic compounds (VOCs) which are generally 

known to occur in oil and gas formations. BTEX are also found in diesel fuel, which is sometimes used in hydraulic 

fracture treatments. Source: Leusch, F. and M. Bartkow (2010). A short primer on benzene, toluene, ethylbenzene 

and xylenes (BTEX) in the environment and in hydraulic fracturing fluids. G. University and S. W. R. Centre.
7

Category Potential baseline chemical analytical parameter

Gas hydrocarbons Methane

Ethane

Propane

Hexane

VOC Benzene

Toluene

ii. Investigation of Ground Water Contamination near Pavillion, WY (EPA 2011)

The EPA recently issued a draft report analyzing results from alleged cases of contamination in Pavillion, WY

(Environmental Protection Agency 2011). In this study, the EPA performed tests for an extensive list of water 

quality parameters. Categories describing these parameters are listed below.

Elevated levels of methane and DRO were found in domestic water wells in the vicinity of natural gas wells in 

Pavillion, WY (Environmental Protection Agency 2011). The EPA installed two monitoring wells into the 

groundwater aquifer in order to determine the presence of potential contaminants in the aquifer. Tests of 

the two monitoring wells found VOC, synthetic compounds, and components of compounds that had been 

used in hydraulic fracturing at the site (Appendix III, Table 20). 

The EPA concluded that leaching from hydraulic fracturing fluid disposal pits and possibly leaks in the 

wellbore cement casing had allowed contaminants to migrate to the aquifer. Critics have asserted that the 

EPA’s conclusions were based on limited data and that installation of the monitoring wells may have been the 

sources of contamination that the EPA found (Railroad Commission of Texas 2012). 

Not surprisingly, the final paragraph of the EPA draft investigation of Pavillion, WY states:

"Collection of baseline data prior to hydraulic fracturing is necessary to reduce investigative costs and 

to verify or refute impacts to groundwater...this investigation supports recommendations made by 

the U.S. DOE Panel on the need for collection of baseline data, greater transparency on chemical 

composition of hydraulic fracturing fluids, and greater emphasis on well construction and integrity 

requirements and testing."

This conclusion arises from speculation that the organic and inorganic contaminants found at the site might 

be expected to occur naturally, considering that the region has been targeted for oil and gas exploration (U.S. 

 Major anions and alkalinity  Diesel Range Organics (DRO)

 Semi-Volatile Organic Compounds (sVOC)  Total petroleum hydrocarbons (TPH)

 Pesticides  Total phase-separated hydrocarbons

 Polychlorinated Biphenyls (PCB)  Alcohols and Volatile Organic 

Compounds (VOC)

 Total Inorganic Carbon (TIC)  Bacteria

 Metals  Low molecular weight acids

 Gasoline Range Organics (GRO)  Fixed gas isotopes
8

Energy Information Administration 2010). Such speculation underscores the importance of baseline testing. 

However, testing for all of the categories described above is not necessary. From the list of testing categories

described above, sVOC, TIC, Pesticides, PCB, bacteria, and low molecular weight acids were not included in 

this analysis, but the other parameters are included. Reasons for omitting these parameters from the 

analysis are explained below.

First, sVOC are an indication of the presence of dispersed oil droplets in water (Neff, Sauer et al. 2011). While 

a test for sVOC would provide an indication of the presence of petroleum hydrocarbons in water, VOC are 

non-essential parameters for baseline groundwater quality testing if petroleum hydrocarbons are tested in 

other ways.

Next, TIC is a category that is used to detect products of chemical interactions between produced water and 

groundwater (Environmental Protection Agency 2011). TIC includes parameters such as bicarbonate (HCO3

-

) 

and calcite (CaCO3). Bicarbonate is characterized in this analysis, but only in the context of an expected 

change in concentration if produced water breaches an aquifer. TIC, including CaCO3, are not considered as 

parameters in this analysis, because a baseline groundwater quality analysis only needs to look for precontamination constituents, rather than examining expected products of chemical interactions between the 

potential contaminants and background water components.

Finally, Pesticides, PCB, bacteria, alcohols and low molecular weight acids are not included in the categories 

that will be characterized in this report, because they either represent potential contamination from sources 

other than oil and gas; or these parameters would provide non-essential supporting evidence of 

contamination that can be determined more simply by a sample’s comparison with the one or more of the 

other essential baseline water quality parameters. 

Considering these factors, this analysis includes a characterization of known background data availability, and 

some background concentrations, for the following parameters which correspond to the seven categories 

previously listed7:

Category Potential baseline chemical analytical parameter

Gas hydrocarbons Fixed gas isotopes (carbon isotopes)

Liquid hydrocarbons Total Petroleum Hydrocarbons (TPH)

Gasoline Range Organics (GRO)

Diesel Range Organics (DRO)

Salts Major anions and alkalinity (Na+, K+, Ca2+, Mg2+, Cl-, SO4

2-

, F-, NO3

-

)

Metals Al, Ag, B, Ba, Be, Ca, Co, Fe, K, Mn, Mo, Na, Sb, Sr, Ti, Zn, S, P, As, 

Cd, Cr, Cu, Hg, Ni, Pb, Se, U

VOC Benzene, toluene, ethylbenzene, xylene (BTEX)

iii. Methane contamination of drinking water accompanying gas-well drilling 

and hydraulic fracturing (Osborn et al. 2011)

One example of the difficulty that is presented in using δ

13C isotope concentrations for determining whether 

groundwater contamination occurred is offered by Osborn et al. (2011) and Molofsky et al. (2011). Osborn 

et al. (2011) found a correlation between the concentrations of dissolved methane found in domestic water 

wells and the proximity of those water wells to hydraulically fractured natural gas wells. The researchers 

 

7

Analytical parameters described here are based on those tested in the Pavillion, WY EPA Investigation.
9

concluded that water wells within 1 km of a hydraulically fractured natural gas well were more likely to have 

high concentrations of methane in their water wells, sometimes dangerously high, and that this 

contamination may have been caused by shale gas extraction (Osborn, Vengosh et al. 2011). Table 2 lists the 

tests that were used to reach this conclusion. Among the tests used, carbon isotopic values for methane 

(δ

13C-CH4) indicated that methane sourced from the Marcellus shale was present in high concentrations in 

water wells that were in close proximity to hydraulically fractured natural gas wells.

TABLE 2 ANALYTICAL TESTS USED BY OSBORN, ET AL. (2011) TO DETECT POTENTIAL GROUNDWATER 

CONTAMINATION CAUSED BY SHALE GAS EXTRACTION.

Test Purpose Conclusions of Osborn et al. 

Methane, ethane, and propane 

isotopes and concentrations

Indicates whether gas 

contamination is present

Isotopes, and ethane/propane 

ratios help to determine the 

source of the gas

Isotopic signature of the methane 

found in the water wells matched 

the isotopic fingerprint of gas from 

the Marcellus formation, 

indicating that contamination had 

occurred; presence of ethane and 

propane in the ratios found 

supports the conclusion that gas 

migrated from the shale.

δ

13C-DIC, δ18O, and δ2

H Indicators of microbial methane No positive correlation between 

δ

13C-DIC and δ13C-CH4, indicating 

that methane was not formed by 

microbes. No correlation between 

δ

2

H found in water and δ

2

H found 

in methane at the site, indicating

that the methane was not formed 

in the shallow aquifer; δ

18O

composition was of modern 

meteoric origin, indicating that it 

did not come from an older, 

deeper formation.

δ

11B, δ226Ra

Major cations (Na+, Ca+2, Mg+2)

Major anions (Cl-, Br-, NO3

-

SO4

-2

)

Trace metals

Alkalinity as HCO3

-

Additional indicators of potential 

mixing with deep formation waters

Samples matched background

data, therefore no contamination 

from formation fluids could be 

verified.

Molofsky et al. (2011) have questioned the conclusions of Osborn et al. (2011) that higher concentrations of 

methane in water wells near natural gas wells indicated that contamination had occurred after drilling. 

Rather, Molofsky et al. (2011) contended that water wells positioned in valleys tend to have higher 

concentrations of methane than water wells in uplands. Therefore, they posit that this relationship may have 

skewed the results of Osborn et al. (2011). Additionally, they questioned the interpretation that the isotopic 

signatures of methane found in the water wells were sourced from the Marcellus shale, and offered an 

interpretation that suggested the methane was from a separate nearby formation. Nonetheless, Molofsky et 

al. (2011) cite the need for background isotopic data in order to determine if potential contamination has 

occurred as a result of gas extraction and production (Molofsky, Connor et al. 2011).
10

Taking this into consideration, baseline groundwater testing of carbon isotopes is an essential tool for 

assessing whether oil and gas drilling activities have affected an aquifer. Contrastingly, parameters that are 

non-essential for baseline groundwater quality testing are water isotopes (δ

18O and δ2

H) and boron isotopes

(δ

11B). Water and boron isotopes are used with concentrations of δ13C isotopes as additional supporting 

evidence that defines which formation the hydrocarbons were sourced from (Osborn, Vengosh et al. 2011); 

therefore, knowing the concentration of water and boron isotopes is non-essential if δ13C concentrations 

from a background water quality test are available for comparison to a potentially contaminated sample of 

water. As such, the following parameters are considered in this analysis:

Category Potential baseline chemical analytical parameter

Gas hydrocarbons Methane

Ethane

Propane

Carbon isotopes

Salts Major cations and anions (Na+, Ca+2, Mg+2, Cl-, Br-, 

NO3

-

SO4

-2

)

Alkalinity as HCO3

-

Metals Trace metals, although not listed in Osborn et al. 

(2011) or supplemental information, this category 

corroborates that metals should be tested.

iv. Plan to Study the Potential Impacts of Hydraulic Fracturing on Drinking 

Water Resources (EPA 2011)

The EPA is currently conducting a study on the potential impacts of hydraulic fracturing on drinking water 

resources (Office of Research and Development 2011). In an attempt to be comprehensive, the final study 

plan contains a list of chemicals found in hydraulic fracturing fluids and flowback water that spans 25 pages. 

The list is long primarily due to the extensive number of potential chemicals that have been or will be used in 

hydraulic fracturing at the sites considered in the study. 

Due to the expense of testing so many parameters, the EPA has selected five retrospective case studies out of 

a list of 45 nominated cases, and two prospective case studies out of a list of seven possible candidates8. The 

retrospective case studies will examine cases where hydraulic fracturing was alleged to have caused 

groundwater contamination, and in the prospective case studies the EPA will examine sites where drilling has 

not yet occurred. The prospective case studies will allow the EPA to gain understanding of all possible 

pathways of contamination and how to detect when contamination has occurred if that is the case. 

The EPA plan to study the potential impacts of hydraulic fracturing on drinking water resources points out

that interferences can complicate analytical approaches used to characterize samples associated with 

hydraulic fracturing:

“Some gases and organic compounds can partition out of the aqueous phase into a non-aqueous 

phase (already present or newly formed), depending on their chemical and physical properties. With 

the numbers and complex nature of additives used in hydraulic fracturing fluids, the chemical 

composition of each phase depends on partitioning relationships and may depend on the overall 

composition of the mixture. The unknown partitioning of chemicals to different phases makes it 

 

8

The rationale behind the EPA’s selection of case studies is available in their report titled “Plan to Study the 

Potential Impacts of Hydraulic Fracturing on Drinking Water Resources” and will not be discussed here.
11

difficult to accurately determine the quantities of target analytes. In order to address this issue, EPA 

has asked for chemical and physical properties of hydraulic fracturing fluid additives in the request for 

information sent to the nine hydraulic fracturing service providers.” (EPA 2011, p.166)

These complications are illustrative of the challenge that landowners and agencies are faced with in 

conducting post-drill water quality testing for the purpose of detecting potential contamination from oil and 

gas drilling, extraction, production, and waste storage activities. Although the EPA has access to the list of

chemical and physical properties of hydraulic fracturing fluid additives from these nine service providers, this 

kind of information is not publically available for all wells in the United States. Even Texas’ landmark 

disclosure bill passed in the summer of 2011, H.B. 3328, allows oil and gas companies not to disclose 

chemicals considered to be trade secrets9(H.B. 3328, 2011). Additionally, a well operator is not required to 

disclose the list of chemicals used in the hydraulic fracturing treatment until after the well has been 

completed. Therefore, if a hypothetical landowner in Texas desires to test her water well for hydraulic 

fracturing fluid components before drilling takes place, she must choose her baseline groundwater quality 

tests using a general list of hydraulic fracturing fluids known to have been used in her region. Testing for 

region-based hydraulic fracturing fluids would not only be expensive, but it might also be irrelevant since 

hydraulic fracturing fluid mixtures vary from well to well and because chemical formulas for hydraulic 

fracturing fluids are constantly changing. Based on this reasoning, baseline groundwater quality testing 

would be greatly enhanced if oil and gas well drillers conducted the tests before drilling using a list of 

parameters tailored to the hydraulic fracturing fluid mixture that is used at each site.

The list of intended testing parameters, not including hydraulic fracturing fluids, for the EPA’s prospective 

case studies is provided in Table 3. 

TABLE 3 EPA TIER 2 INITIAL TESTING: SAMPLE TYPES AND TESTING PARAMETERS (OFFICE OF RESEARCH 

AND DEVELOPMENT 2011)

Out of the parameters listed for groundwater in Table 3, redox potential and dissolved oxygen will not be 

 

9

Landowners, neighbors and agencies are allowed to challenge a trade secret provision within two years from the 

date the well completion report is filed.
12

considered in this analysis because neither would provide definitive evidence of contamination if 

concentrations with a potentially contaminated sample were compared with background water quality. 

Rather, both redox potential and dissolved oxygen are non-essential parameters that serve to support 

conclusions drawn from other analyses. Additionally, sVOC are not considered for the same reason. Based 

on these decisions, the parameters listed below will be considered in this analysis.

Category Potential baseline chemical analytical parameter

Salts TDS

Cations and anions

Barium

Strontium

Chloride

Boron

Metals Arsenic

Barium

Selenium

Radionuclides Radium

VOC BTEX

PAH Not reported

v. Reported Groundwater Contamination in the Eagle Ford shale region

The Railroad Commission of Texas (RRC) is the agency that has jurisdiction over contamination of 

groundwater associated with exploration, development, or production, including storage or transportation, 

of oil and gas (Texas Groundwater Protection Committee 2011). As a part of the Texas Groundwater 

Protection Council (TGPC), the RRC submits an annual compilation of all pending or completed cases of 

contamination10 in the state of Texas (Texas Groundwater Protection Committee 2011). This is useful for 

determining which contaminants are expected and should be tested for in a baseline groundwater quality 

test.

Here, all of the cases reported by the RRC for the 35 counties of interest in the Eagle Ford shale region were 

compiled. However, cases of contamination were listed for only 21 out of the 35 counties (Figure 5). The 

counties with the most cases of contamination were Austin, Webb, Wood, Bee, Duval, and Lavaca. The 

counties in which no contamination was reported are depicted in Appendix IV, Figure 20. 

 

10 These cases represent incidents where “the detrimental alteration of the naturally occurring physical, thermal, 

chemical, or biological quality of groundwater” has occurred according to TAC, Title 31, Part 18, Chapter 601, 

Subchapter A, Rule §601.2. 
13

FIGURE 5 NUMBER OF PENDING CASES OF GROUNDWATER CONTAMINATION ATTRIBUTED TO OIL AND GAS 

ACTIVITIES IN COUNTIES OF THE EAGLE FORD SHALE REGION IN 2010

Contaminants found in these cases are listed in Table 4, below. In compiling the reported cases for analysis, 

some reported contaminants were consolidated into one category that most accurately represented that 

contaminant (Appendix IV, Table 22). Out of the 64 cases of reported contamination, only four were new 

cases in the year 2010. The status of the case investigations can be found in Appendix IV, Figure 15. 

TABLE 4 CONTAMINANTS REPORTED IN GROUNDWATER CONTAMINATION CASES ATTRIBUTED TO OIL AND 

GAS ACTIVITIES IN 2010 (TEXAS GROUNDWATER PROTECTION COMMITTEE 2011)

*National Secondary Drinking Water Standard

BTEX chemicals and liquid hydrocarbons were the most frequent contaminants reported in 2010 (Table 4). 

Included in Table 4 are the EPA MCLs and Secondary Standards for each of the contaminants found in the 

counties of interest in the 2010 report11. However, it should be noted that no concentrations of the chemical 

contaminants are provided in the joint report. Therefore, no comparison can be made between an expected 

concentration that would be found in the shale formation and the expected concentration that would be 

found in groundwater. Nonetheless, out of the list of potential contaminants in Table 4, all are included in 

this analysis, and are represented by the following parameters:

Category Potential baseline chemical analytical parameter

 

11 Potential hazards and human health effects attributed to each of these contaminants are discussed in the results 

section.

3

10

1

5

2

1

3 3

5

1 1 1

2

1 1

2

3

2

5

6 6

0

2

4

6

8

10

12

Number of cases in process

County

Contaminant Count EPA MCL

Liquid hydrocarbons 27 NA

BTEX 25 0 - 10 mg/l

Chlorides 7 250 mg/l*

Gas hydrocarbons 2 NA

Arsenic 1 0 mg/l

Barium 1 2 mg/l

Metals (not defined) 1 NA

Total 64
14

Gas hydrocarbons Methane

Liquid hydrocarbons Total petroleum hydrocarbons12

Salts Chloride

Sodium

Metals Arsenic

Barium

Radionuclides Radium

VOC BTEX

PAH Not reported

II. Data and Methods

As stated in the introduction, the objectives of this analysis are (1) to determine whether existing baseline 

groundwater quality data in the Eagle Ford shale region is adequate to provide a baseline for comparing to 

potential future contamination from oil and gas drilling, and (2) to define an appropriate and cost-effective 

list of parameters that would aid in strategic planning of baseline groundwater quality testing in the Eagle 

Ford shale region for the same goal. To achieve these objectives, a list of essential parameters was 

established first, using the case studies described in the previous section. Second, using the list of essential 

parameters, the available groundwater data for those contaminants in the counties of interest is described; 

when possible, this data is compared to concentrations of the same parameters known to occur in the Eagle 

Ford shale formation. Finally, recommendations for testing efforts are made based on data needs, whether 

health effects are known to occur for each parameter, and the cost of each test.

In characterizing the baseline data available for each county, the number of samples available in each county 

is given for each parameter, and general statistical principles are used to determine whether enough samples 

are present. If an EPA Maximum Contaminant Level (MCL) or Secondary Standard13 for drinking water has 

been set for the parameter of interest, a map was drawn depicting the known concentration at each sample 

location. Additionally, maps are depicted for selected parameters of interest that have not been assigned an 

MCL or Secondary Standard by the EPA, but which are particularly useful in detecting potential 

contamination.

In order to define which general chemistry parameters would be the best indicators of an influx of flowback 

or hydrocarbons into a groundwater well, chemical concentrations from the USGS Produced Waters database 

were statistically compared to groundwater quality for the counties of interest. This allowed for some 

chemical tests to be removed from the list of recommended tests because the parameters are neither 

 

12 TPH encompasses all of the contaminants listed in Appendix IV, Table 29.

13 Maximum Contaminant Levels (MCLs) are “National Primary Drinking Water Regulations (NPDWRs or 

primary standards) are legally enforceable standards that apply to public water systems.” and “National 

Secondary Drinking Water Regulations (NSDWRs or “secondary standards”) are non-enforceable 

guidelines regulating contaminants that may cause cosmetic effects (such as skin or tooth discoloration) 

or aesthetic effects (such as taste, odor, or color) in drinking water. EPA recommends secondary 

standards to water systems but does not require systems to comply.”

Environmental Protection Agency. (2012, March 6, 2012). "List of Contaminants & their MCLs." Drinking 

Water Contaminants Retrieved March 13, 2012, 2012, from 

http://water.epa.gov/drink/contaminants/index.cfm.
15

dangerous to human health nor are they expected to cause a substantial change in concentration when 

mixed with groundwater.

In addition to comparisons made between the TWDB groundwater database and the USGS produced waters 

database, a systematic literature review was conducted to determine whether additional potential 

contaminants that might be found in the Eagle Ford shale. Additionally, this served to establish expected 

concentrations for known contaminants expected to occur in the Eagle Ford formation. Although the results 

of this attempt were generally not substantial enough to use in a statistical comparison with groundwater, 

some references that were found in this process are used throughout the sections to support various 

findings. The lack of findings in this attempt demonstrates that more publically available data is needed 

about the produced water chemistry of the Eagle Ford shale formation.

Finally, quotes were requested from all laboratories listed by the Texas Commission on Environmental 

Quality (TCEQ) as certified to test samples in the drinking water category in the state of Texas. These 83 

labs14 are certified under the National Environmental Laboratory Accreditation Program (NELAP). Twenty of 

the 83 labs replied to the request. These data are used qualitatively to make general recommendations 

about whether organizations or landowners should be expected to conduct a sampling effort. For more 

expensive tests, it might be less likely that landowners will actively pursue a water analysis for those tests. 

For this reason, a strategic sampling effort in regions that have less publically available information on these 

more expensive analytes might best be taken on by an agency, industry, or other organizations.

Produced Water Data

Produced water can be defined as the water that is naturally occurring in the formation where oil and gas 

exploration occurs. During the production phase of an oil and gas well, produced waters flow out of the well, 

along with hydraulic fracturing fluids that were used in the well, and any hydrocarbons present in the 

formation. Produced water contains a variety of chemicals that occur naturally in a formation. Since 

produced water is known to occur in the Eagle Ford shale formation at moderate to high volumes(Lai 1997; 

Hsu and Nelson 2002; Mullen 2010; Mullen, Lowry et al. 2010)

15, it is conceivable that if a pathway between a 

hydrocarbon-bearing formation and an aquifer exists, then produced water chemistry would cause a change 

in chemical concentrations found in the aquifer. A comparison between the chemistry of produced water 

and the chemistry of groundwater in the counties of interest is used to identify which chemical parameters 

are expected to be the most different, and will therefore serve as good indicators of contamination. 

The USGS provides a database of concentrations of general chemistry parameters for produced waters from 

multiple formations in each state (Breit, Skinner et al. 2006). Procedures used to prepare the dataset for 

analysis are listed in Appendix V. The information contained in the database was offered voluntarily by 

various companies (U.S. Geological Survey 2002). Only 36 records were available for the Eagle Ford 

formation, although 1606 records were available for other formations in the counties of interest. The 

 

14 Some labs were not considered if they did not test any of the chemicals of interest for this report (e.g. if the lab 

only tested for total coliforms).

15 According to Lai (1997), Hsu and Nelson (2002), Mullen 2010, and Mullen, Lowry et al. (2010), the average water 

content of the Eagle Ford shale formation is 16 to 19 percent. Additionally, Mullen, Lowry et al. (2010) reported a 

case where high water volumes in the Eagle Ford shale at depths of 15,230 feet prevented flow of gas from the 

well.
16

samples for the Eagle Ford formation were taken between the years of 1951 and 1975, while samples for 

other formations in the counties of interest were taken between 1929 and 1980. 

Water quality parameters that are provided in both the produced waters database and the groundwater 

database include:

 pH  Chloride  Sodium

 Bicarbonate  Magnesium  Sulfate

 Calcium  Potassium  TDS

Nonetheless, using this dataset for the Eagle Ford shale formation may not be an accurate representation of 

Eagle Ford shale produced water chemistry in other counties, because all of the data representing the Eagle 

Ford formation are from wells drilled in Wood County16. This is problematic, since Wood County is not 

currently in the active drilling area of the Eagle Ford shale. This fact raises doubts about whether the data 

that was labeled as “Eagle Ford” in the USGS produced waters dataset is still considered to be from a 

formation that is within the boundaries of the Eagle Ford formation. 

As it turns out, the samples taken for Wood County represent depths ranging from 4200 feet – 5800 feet. 

Although oil and gas wells have recently been drilled in Wood County at those depths, the wells are 

categorized in the Woodbine and Sub-Clarksville formations17. The Sub-Clarksville formation underlies the 

Eagle Ford (Holbrook 1985), but the Woodbine formations and Eagle Ford are undivided in portions of 

northeast Texas (U. S. Geological Survey ; Condon and Dyman 2006)

18

. Therefore, although the produced 

waters data for the Eagle Ford are in fact representing the Eagle Ford formation, the fact that portions of the 

Eagle Ford and Woodbine are undivided could mean that Eagle Ford produced water samples are chemically 

different in Wood County compared to counties in the active drilling area. 

Although a statistical comparison of produced waters and groundwater in Wood County is described in this 

analysis, it should be understood that a high level of uncertainty is embodied in the analysis. More produced 

water data is needed for the active drilling area in order to draw a relevant and accurate comparison 

between Eagle Ford shale produced water and groundwater in the counties of interest.

In order to understand, in a general sense, how produced water chemistry differs from groundwater 

chemistry, data from other formations in the counties of interest was compared to groundwater chemistry in 

the counties of interest. Such an analysis is useful for guiding future research, but more chemical data from 

Eagle Ford produced water is still needed to learn which parameters will be most useful for baseline testing 

in the active drilling region.

In addition, other sources provide some indication of expected elements in the Eagle Ford formation, but the 

only available data is for core samples. Kearns (2011) provides geochemical analysis of Eagle Ford core 

samples for major elements, trace elements, and isotopes. This data is described further in appendix V. 

 

16 One record in the Eagle Ford shale was documented in Houston County, but this record was not used to allow 

for a simpler comparison of Wood County produced water and groundwater data.

17 The RRC approved 55 drilling permits for Wood County wells from 01/01/2009 to 01/01/2012, and six of those 

wells were within the depth range of 4200 – 5800 feet. Records on the Railroad Commission W-1 Permit 

Application database indicate that these wells were drilled into a Sub-Clarksville and Woodbine formation.

18 Eagle Ford stratigraphy is shown in Appendix VI, Figure 20.
17

Other sources indicate the presence of sulfur (S), nickel (Ni), vanadium (V), aluminum(Al), iron (Fe), calcium 

(Ca), silicon (Si), and zircon(Zi) in core samples(Comet, Rafalska et al. 1993; Mullen 2010). Although this 

information could guide future research, it is not as useful for comparing to liquid concentrations since the 

solubility of the elements in the core samples is unknown. However, values of δ

13C isotopes in the Eagle Ford 

provided by Kearns (2010) and Comet (1993) could be useful for a comparison with groundwater δ

13C 

isotopes. This comparison is described in the results section.
18

Groundwater Data

Data from the Texas Water Development Board’s (TWDB) groundwater quality database19 was used to 

determine if enough samples are available to establish a reliable distribution of potential baseline 

concentrations, where comparisons can be made between baseline groundwater quality and groundwater 

samples that have potentially been contaminated by oil and gas activities. Additionally, as mentioned above,

the groundwater quality data was compared to produced water concentrations when possible to determine 

which chemicals in each county exhibit the greatest differences and are therefore more likely to serve as a

reliable indicator of contamination.

Table 5 depicts the number of samples available for each county in the Eagle Ford shale region. The samples 

were taken between the years of 1988 and 2011. Additionally, the number of samples taken with reliable 

methods, the number of individual wells that are represented in the pools of samples, and the number of 

wells with more than one sample are provided. Not depicted in the table are the total numbers of 

groundwater wells recorded by the TWDB for each county, but this information is included in each data 

availability table as needed (Table 16 and Appendices X through XIV). The total number of groundwater wells 

for each county of interest is a count of state well numbers listed in the TWDB “Well Location Shapefile” 

(TWDB GISa; TWDB GISb). 

Parameters such as those listed in Table 5 can be assessed with statistical principles to determine if the 

number of wells that have been sampled is adequate to define reliable mean concentrations for each 

parameter in the region. As such, power analysis was used for each parameter in each county of interest to 

determine the statistical power of the sample size with a pre-defined effect size (ES) and significance level 

(α)

20

. A larger statistical power indicates greater accuracy (Cohen 1992), and a power of 0.80 is 

recommended21. Cohen (1992) uses a calculation that recommends between 26 and 38 samples for a power 

of 0.80, allowing a large effect size and α = 0.05 or 0.01 for the difference in means, respectively. 

Notably, Caldwell and Guadalupe Counties have the fewest number of samples. Additionally, counties with 

less than 26 wells that have been sampled (out of those with reliable samples) include Guadalupe, Caldwell, 

Madison, Dewitt, McMullen, Washington, Fayette, and Bee. Adding to that list, those counties with fewer 

than 38 samples include Goliad, Austin, Karnes, Grimes, Lee, Leon, Houston, and Wood. . Contrastingly, the 

counties with the strongest statistical power, containing more than 26 individual water wells that also have 

more than one sample attributed to the well are Atascosa, Duval, Frio, Gonzalez, LaSalle, Webb, and Zavala.

A list of each general water chemistry parameter found in the database is provided in Appendix VIII. 

Additionally, the TWDB database contains EPA STORET data with concentrations of infrequent constituents. 

Over 270 different infrequent constituents are listed in the data base (see Appendix IX). Selected 

constituents are described in forthcoming sections of this analysis, as they relate to detecting contamination 

from oil and gas activities.

 

19 TWDB Groundwater Database available at: http://www.twdb.state.tx.us/groundwater/data/gwdbrpt.asp

20 In this case, ES is zero when there is no difference between a given sample and the distribution mean for the 

sample size being considered. ES ranges infinitely upward from zero when the residuals are not equal to zero 

Cohen (1992). Therefore, a small ES renders a more precise statistical value. Additionally, alpha (α) refers to the 

probability of falsely rejecting the null hypothesis.

21 Cohen (1992) chose the value 0.80 as the optimal power, because a greater power would require an infeasible 

sampling effort, and a lower number would increase the chance of a false prediction too greatly.
19

TABLE 5 NUMBER OF SAMPLES AVAILABLE IN THE TWDB GROUNDWATER DATABASE FOR GENERAL WATER 

QUALITY PARAMETERS IN EACH COUNTY OF THE EAGLE FORD SHALE REGION.

County Records in 

database

Individual 

wells

Records 

with reliable 

sampling 

methods

Individual wells 

with reliable 

samples

Wells with 

multiple reliable 

samples

Youngest 

record in 

reliable 

samples

Oldest record in 

reliable 

samples

Atascosa 784 370 182 87 49 2010 1990

Austin 214 104 68 27 16 2009 1992

Bastrop 563 266 84 38 18 2010 1988

Bee 387 243 44 25 11 2009 1990

Brazos 395 222 73 45 18 2010 1992

Burleson 455 297 85 60 16 2011 1992

Caldwell 410 273 26 14 7 2011 1992

Colorado 394 155 108 44 22 2009 1988

Dewitt 357 243 40 17 8 2009 1988

Dimmit 414 198 100 54 24 2010 1990

Duval 452 247 129 75 32 2009 1990

Edwards 166 70 81 42 20 2011 1993

Fayette 615 337 60 24 15 2010 1988

Frio 486 239 143 81 34 2010 1990

Goliad 263 185 42 27 9 2009 1990

Gonzales 588 244 277 68 40 2011 1988

Grimes 292 225 41 30 7 2009 1992

Guadalupe 239 170 18 10 3 2010 1992

Houston 204 95 82 33 21 2010 1993

Karnes 496 356 43 28 10 2009 1988

LaSalle 276 119 85 43 27 2010 1990

Lavaca 204 115 103 49 17 2009 1992

Lee 257 149 74 32 10 2011 1989

Leon 175 74 75 32 21 2010 1992

Live Oak 212 115 65 39 18 2009 1990

Madison 109 41 38 17 11 2011 1993

Maverick 92 72 66 56 9 2002 1992

McMullen 188 70 48 19 13 2010 1992

Milam 379 192 79 39 19 2010 1988

Robertson 529 352 97 48 22 2011 1989

Washington 324 246 38 22 10 1992 2009

Webb 254 115 127 67 35 2010 1989

Wilson 438 185 100 50 24 2010 1990

Wood 326 203 55 35 13 2010 1989

Zavala 563 230 115 62 31 2011 1988

Totals 10329 5555 2503 1240 569
20

Geospatial Analysis

Maps depicting the concentration of selected groundwater quality parameters at each well site were created 

using a graduated color scheme for each concentration range (see Appendix V for procedures used). It is 

possible to interpolate water concentrations across the region for spaces where no water quality data was 

available. However, this kind of analysis was not attempted because even simple water chemistry 

parameters such as total dissolved solids (TDS) exhibited cases of regional heterogeneity across short 

distances of the landscape (Appendix XI, Figure 19). Therefore, a regional interpolation may imply that 

concentrations can be predicted, but true concentrations could still vary significantly from the mean. For this 

reason, it may also not be reasonable to assume that counties with a quantity of samples exhibiting strong 

statistical power are able to predict the concentration of various chemical parameters in their region. This 

point serves to encourage additional sampling when possible, considering that the mass of samples already 

compiled only serves to establish a potential range of concentrations, whereas concentrations that vary 

significantly from the mean could occur in any water well.
21

III. Results

The purpose of characterizing the baseline is to determine which chemical parameters are already well 

construed across the landscape, and to identify geographic regions and chemical parameters where testing 

efforts should be focused. Earlier, certain counties were highlighted as having particularly few groundwater 

samples recorded in the TWDB database, while others were highlighted as having a statistically acceptable 

number of samples. Although records may exist in the TWDB database for the specified number of wells, not 

every well owner has conducted all of the baseline groundwater quality tests that are necessary for 

comparison with a potentially contaminated sample. Therefore, although some counties may seem to have 

enough samples recorded in the TWDB database, closer inspection may reveal deficiencies in baseline 

groundwater quality data.

Additional analysis described below uses the USGS produced waters database and geochemical 

concentrations found in the literature for the Eagle Ford shale, to compare concentrations of various 

chemical parameters to groundwater chemical concentrations. This analysis helped to determine which 

parameters are likely to show changes in groundwater concentrations if an influx of produced water occurs. 

As discussed earlier, high levels of uncertainty are embedded in the produced waters dataset, but a broad 

comparison of produced water and groundwater is presented in section C, below. More data is needed to 

develop an accurate comparison, but the data presented here serve as a useful guide for future research.

Testing priority codes

For most of the categories that follow, codes are used to describe whether increasing the number of samples 

for a particular parameter should be a high priority each county. The codes used are based on the number of 

samples required by power analysis to achieve a power of 0.80, as described in the Groundwater Data 

section. For counties with less than 26 samples of a particular parameter, the code “T” is used to 

recommend that testing be a high priority. In this situation, a region-wide testing initiative is strongly 

recommended to enhance the baseline dataset. For counties that have between 26 and 38 samples, the 

code “R” is used to indicate that testing is recommended. In this situation, testing initiatives are 

recommended to enhance the baseline dataset. Finally, for counties that have more than 38 samples, the 

code “A” is used to indicate that testing should take place as needed. In this situation, testing initiatives are 

not necessary, but those interested in the water quality of a specific well that has not yet been tested should 

understand that the groundwater chemistry of the well could vary significantly from the baseline even if 

more than 38 samples exist for a given county; if financially possible, testing is recommended.

A. Gas Hydrocarbons

Baseline characterization

Natural gas is made up mostly of methane (CH4), but may include hydrocarbon gases such as propane (C3H8), 

hexane (C6H14), and ethane (C2H6) (Union gas 2012). No samples for methane, ethane, propane, or hexane 

were available in the Texas Water Development Board groundwater quality database. However, all of these 

cases contain organic carbon. Therefore, a test for total organic carbon (TOC) would encompass 

measurements for these gases in a baseline water quality test. 
22

Out of the 35 counties examined, eight counties have samples for total organic carbon (TOC), and 13 counties

have samples for 13C/12C stable isotope ratios. Appendix X, Table 23 depicts the total number of samples of 

each of these constituents that are found in the sixteen counties for which samples of these parameters are 

available.

Although the total number of samples known to exist in the entire Eagle Ford shale region conforms to the 

power analysis guidelines presented by Cohen (1992), a map depicting the sample locations (Figure 9) 

indicates that most of the region lacks geographically relevant samples for TOC. More samples taken over a 

broader geographic region are recommended to help verify whether hydrocarbons are pre-existing in certain 

water wells. 

FIGURE 6 TOTAL ORGANIC CARBON CONCENTRATIONS IN THE COUNTIES OF INTEREST.

Testing for TOC over a broader geographic region is recommended, and would provide data necessary for 

comparing baseline water samples to those that are potentially contaminated from oil and gas. However, 

establishing this dataset will take time, and since heterogeneity in TOC concentrations might exist across the 

landscape a reading at any one well has a high probability of Type I or II error22. Simply put, the dataset is 

not yet large enough or well distributed enough to be able to accurately depict when a sample concentration 

is a true representation of what is expected. Therefore, as the dataset for TOC is being established, these 

concentrations should be supported with a carbon isotopic signature. This will enable a more accurate 

 

22 Type I and II error indicate that a sample was mistakenly thought to be different or similar, respectively, when 

compared the average, when in fact the opposite is true. 
23

assessment of whether hydrocarbons occurring in a well without background TOC concentrations were preexisting or were sourced from a petroleum hydrocarbon-bearing formation.

Only 35 samples are available in the TWDB Infrequent Constituents database for 13C/12C stable isotope ratios

in groundwater across the entire Eagle Ford shale region (Appendix X, Table 23). However, these samples 

were more geographically distributed across the region23 than the TOC sample sites. Interestingly, the δ

13C

concentrations found in groundwater for the counties of interest were less negative than δ

13C concentrations 

found in the Eagle Ford shale, indicating that the methane found in these groundwater samples could also be

thermogenic (e.g. sourced from a deep formation which potentially bears petroleum hydrocarbons) (Schoell 

1980; Barker and Fritz 1981).

FIGURE 7 GEOGRAPHIC DISTRIBUTION OF CONCENTRATIONS AND SAMPLES OF 13C/12C STABLE ISOTOPE 

RATIOS.

The range of 13C/12C stable isotope ratios in groundwater is between -7.2 ‰ and -18.1 ‰ (Appendix X, Table 

23)

24

. In a single sample t-test, the mean of 13C/12C stable isotope ratios is -10.57391, with a p-value of less 

than 0.01, and 95 % confidence intervals of -11.687710 and -9.462116. These results indicate that regional 

variation exists in the 13C/12C stable isotope ratio of groundwater in the counties of interest. This result 

 

23 Exploration of the dataset reveals that these samples were taken between the years 2002 and 2009. Some of 

the samples that appear in lines across the landscape were taken within the same month, implying a coordinated 

sampling effort.

24 A boxplot shown in Appendix X, Figure 20 indicates that the 13C/12C stable isotope ratio of -18.1 ‰ could be an 

outlier. 
24

implies that the probability of Type I or II error is high when comparing groundwater samples to potentially 

contaminated samples, and underscores the importance of additional testing.

In order to compare these groundwater samples to expected δ

13C concentrations found in the Eagle Ford 

shale formation, a strategic literature search was conducted, and is described in Appendix V. Out of the 

references returned, only two provided values for δ

13C in the Eagle Ford shale formation. Kearns (2011) 

reports δ

13C values between a range of -24.094 ‰ and -26.023 ‰ for five core samples taken in DeWitt 

County in the Eagle Ford Formation, while 40 Eagle Ford samples taken in Gonzales County at varying depths 

exhibited a δ

13C range of -27.07 ‰ to -26.78 ‰. Additionally, Comet (1993) documents δ13C values in whole 

oil, postulated to have been sourced from the Eagle Ford (Table 6) which overlap with the range found by 

Kearns (2011). However, the locations of Comet’s 13C/12C samples are not provided. Therefore, it is not 

possible to discern whether the samples given represent a range of 13C/12C ratios relevant to the geographic 

scope of this paper.

TABLE 6 VALUES OF 13C/12C ISOTOPE RATIOS FOUND IN POTENTIAL EAGLE FORD SOURCE FORMATIONS 

COMET (1993)

Formation Mean ‰ (range ‰) number of samples 

Mississippi Delta Oils -26.7 (-27.9 to -25.4) n=50

Louisiana Wilcox -26.8 (27.4 to -26.2) n=24

Austin Chalk* -28.0 (-29.1 to -25.4) n=29

The carbon isotope values for Dimmit and Guadalupe Counties differ by up to 3 ‰. When Comet’s data is 

included, the difference is up to 5 ‰. These data offer a more well-defined carbon isotope signature for the 

Eagle Ford formation than can be said for groundwater across the landscape, where the range is up to 10 ‰

different25. One reason for the broader range of 13C/12C isotope ratios in groundwater could be that more 

than one aquifer is included in the analysis. To test this hypothesis, 13C/12C stable isotope ratios were plotted 

against depth, but no trend was discovered (see Appendix X, Figure 20). The wide distribution of values in 

groundwater, and the lack of trend with depth further support the notion that more baseline data is needed

on 

13C/12C isotope ratios. In order to assess whether C13/C12 stable isotope ratiosin produced water of the 

Eagle Ford shale have a unique enough signature from those in aquifers of the region, a paired t-test 

comparing core sample C13/C12 stable isotope ratios from Kearns (2011) to groundwater ratios was 

conducted. The t-test indicates that the mean of differences was 16.2 ‰ with a p-value less than 0.01. This 

indicates that groundwater C

13/C12 stable isotope ratios are distinct enough from C13/C12 stable isotope ratios

values found in the Eagle Ford shale formation to serve as an indicator of contamination from gas leaks into 

aquifers. However, the groundwater samples of 13C/12C isotope ratios do not represent data from Gonzales 

or Dewitt Counties, where the core samples for the Eagle Ford formation were taken (see Appendix XI, Figure 

19 and Table 23). Additionally, for 23 counties in the area of interest, no carbon isotope data for 

groundwater is available. Considering the limitations of the sample size and the geographic distributions of 

 

25 If the outlier of -18.1 ‰ is removed, the range of 13C/12C stable isotope ratios in groundwater is -15.3 ‰ to -7.2 

‰, with a difference of about 8 ‰.
25

data, a general comparison can be made between expected concentrations in the Eagle Ford and the 

overlying groundwater aquifers, but the chance of Type I or Type II error remain. 

Health effects

The human health effects of methane are poorly researched. Neither the EPA nor the Agency for Toxic 

Substances and Disease Registry list any potential human health effects of methane exposure. However, 

methane gas present in high enough concentrations in groundwater could pose an asphyxiation or explosion 

hazard if allowed to build up in an enclosed space (Osborn, Vengosh et al. 2011).

Cost of testing

Testing for total organic carbon is a much cheaper test than a carbon isotope test (Table 7). Additionally, 

relatively few of the labs surveyed conduct carbon isotope testing. 

TABLE 7 COST OF TESTING FOR CARBON ISOTOPE RATIOS AND TOTAL ORGANIC CARBON WITH LABS 

CERTIFIED TO TEST DRINKING WATER IN THE STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Number of Labs

13C/12C 133 171 240 3

TOC 25 35 42 7

Recommendations

Both TOC and 13C/12C stable isotope ratios should be sampled more thoroughly in groundwater across the 

region of the Eagle Ford shale. Additionally, more samples of 13C/12C stable isotope ratios for hydrocarbons 

in the Eagle Ford formation are needed to establish expected regional variation for 13C/12C ratios across the 

region. These are important tests to undertake because of the hazards that are posed when water is 

contaminated by gas hydrocarbons.

TOC sampling is a high priority for baseline water quality testing because it can serve as a primary indicator of 

potential contamination from petroleum hydrocarbons, because it is poorly construed across the landscape 

for the counties of interest, and because no one county has enough TOC samples to be considered 

statistically powerful. Also, testing for TOC is relatively affordable, so establishing a more robust baseline of 

TOC in groundwater across the Eagle Ford shale region is achievable.

That being said, TOC sampling will be ineffective to use as a regional baseline if anomalies in TOC 

concentrations exist across the landscape. In order to support TOC concentration findings, more samples of 

13C/12C stable isotope ratios are needed.

B. Liquid Hydrocarbons

Baseline characterization

Gasoline range organics (GRO) and diesel range organics (DRO) represent liquid petroleum products that 

have a specific number of carbons chained together; C6 – C10, and C10 – C28, respectively (Environmental 

Protection Agency 1999). No samples exist in the TWDB database for either GRO or DRO. However, a test 

for total petroleum hydrocarbons (TPH) would include hydrocarbons in the range of both GRO and DRO. In 

the TWDB database, only two samples exist for the counties of interest. Those samples were taken in 
26

Dimmitt County, and both samples bear the same concentration (see Figure 12). It is unclear whether the 

values provided in the database represent background water quality data or a case of contamination that was 

reported through the EPA STORET database26 (Rein and Hopkins 2008). 

FIGURE 8 GROUNDWATER CONCENTRATIONS OF TOTAL PETROLEUM HYDROCARBONS IN THE COUNTIES OF 

INTEREST. TWO DATAPOINTS ARE FOUND IN THE DATABASE, BUT THEIR LOCATIONS OVERLAP ON THE MAP.

Health effects

Health effects associated with some TPH compounds include headaches and dizziness, numbness in the feet 

and legs. Additionally effects on the blood, immune system, lungs, skin, and eyes are known to occur for 

some TPH compounds(Agency for Toxic Substances and Disease Registry 1999). 

Cost of testing

The cost of testing for TPH is less expensive than testing for both GRO and DRO at the same time (Table 8). 

The test is moderately expensive, but more labs test for TPH than GRO or DRO.

TABLE 8 COST OF TESTING FOR LIQUID HYDROCARBONS WITH LABS CERTIFIED TO TEST DRINKING WATER IN 

THE STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Number of Labs

Diesel Range Organics 68 82 97 3

Gasoline Range Organics 72 84 91 3

 

26 TPH concentrations were reported in the infrequent constituents portion of the TWDB groundwater quality 

database. Most data in the infrequent constituents database came from the EPA STORET database. 
27

TPH 50 89 138 4

Recommendations

Considering these potential health impacts and the lack of current baseline groundwater quality data for 

liquid hydrocarbons, it is clear that more background water quality data is needed to determine whether 

cases of existing liquid hydrocarbon leaks into groundwater exist in counties that make up the Eagle Ford 

shale region. Although liquid hydrocarbons are not expected to occur in groundwater, additional background 

data would aid in preventing false claims that contamination was either present or not present before oil and 

gas extraction activities began. Since virtually no baseline groundwater quality data is available for TPH, and 

testing is relatively expensive, it is recommended that an agency, industry, or other organization take on a 

groundwater sampling initiative for TPH that is geographically representative of the Eagle Ford shale region.
28

C. Salts

Relevant parameters in produced water

The concentration of like constituents in groundwater and produced water can be compared in order to 

determine whether the mean difference in each is statistically significant. If produced water concentrations 

of any particular salt are always much higher than the concentrations in the groundwater, that particular salt 

can be considered an effective parameter to use for detecting a change in groundwater chemistry caused by 

oil and gas activities.

In this analysis, Eagle Ford shale formation produced water samples are compared to groundwater samples 

for Wood County, for the parameters that are available in the produced waters database. However, given 

the uncertainties associated with how closely Wood County data resembles the geochemistry of the Eagle

Ford in the current active drilling area, additional produced water samples from other formations were 

compared to groundwater samples to establish a generally applicable range of how greatly produced water 

differs from groundwater. These additional produced water samples were compiled from only the counties 

of interest, and represent all formations that are listed for those counties.

A paired t-test was used to determine whether the differences in produced water quality of the Eagle Ford 

were statistically significant when compared to groundwater. First, data for Wood County indicates that all 

analyte concentrations were significantly greater in produced water except for pH and sulfate (Appendix XI, 

Table 24). Total dissolved solids (TDS), chloride, and sodium return the greatest mean of differences. 

Additionally, for all three analytes, the maximum values found in groundwater do not overlap with the 

minimum values found in produced water (Appendix XI, Table 25). 

Although only Wood County data was available for the Eagle Ford shale in the produced waters database, 

produced water chemistry for all the counties of interest was compared to groundwater chemistry for all the 

counties. Slight differences from the Wood County data are observed when these additional data points are 

considered. 

First, pH and sulfate are statistically significant when results for all counties are considered (Appendix XI, 

Table 26). Notably, pH in groundwater exhibits a broader range of values than produced water (Appendix XI,

Table 27), and would be a poor indicator of post-drill contamination without a baseline water quality test for 

the actual well at which potential contamination is experienced. Therefore, a baseline of pH in groundwater 

would not be expected to be higher or lower than produced water pH unless both produced water and 

baseline groundwater quality for the site were known prior to the potential influx of contaminants to the 

aquifer.

Next, data for potassium were not available in the Wood County dataset; whereas 143 produced water 

samples recorded potassium concentrations for other formations in the counties of interest (Appendix XI, 

Table 27). Nonetheless, potassium concentrations are not likely to be a useful indicator of contamination, 

considering that the maximum groundwater concentration of potassium is close to the value of the third 

quantile of the potassium concentration for produced water. Also, the maximum concentration of potassium 

in produced water is an order of magnitude higher than the third quantile, indicating that the dataset may 

contain outliers that skew the distribution of potassium concentrations (Appendix XI, Table 27).

According to the produced waters data for all counties of interest, sulfate concentrations in produced water 

are higher than in groundwater by a statistically significant amount (Appendix XI, Table 26). However, the 
29

range of concentrations in groundwater indicates that the maximum concentration of sulfate in groundwater 

is greater than the third quantile in produced water (Appendix XI, Table 27). Similar to potassium, the high 

sulfate concentrations found in the upper quantile of the produced waters may be skewed by outliers.

Baseline characterization

Only Caldwell and Guadalupe counties have poor statistical power for all of the general water chemistry 

parameters that are listed in the produced waters dataset, although testing is recommended for some of 

these parameters in Madison (Appendix XI, Figure 27) and Washington (Appendix XI, Figure 28) counties. 

Every other county has strong statistical power based on the number of samples it has.

Other salts that have not yet been discussed, but which were indicated as important in the case studies, 

include bromide, nitrate, and selenium. Number, percent, and groundwater testing recommendations for 

these parameters are given in Appendix IX, Figure 29. Power analysis indicates that most counties have an 

acceptable number of samples for bromide and selenium, but many counties need more samples for nitrate 

in order to adequately characterize the baseline for these parameters.

Health effects

Although a change in chemistry could occur from an influx of produced water into an aquifer, the 

constituents found in the produced waters database are not necessarily harmful. No Maximum Contaminant 

Levels (MCL) exist for the chemicals listed in the produced waters database. However, the EPA has defined

National Secondary Drinking Water Standards (Secondary Standards) for pH, chloride, sulfate, and Total 

Dissolved Solids (TDS) (Table 9). Alternatively, MCLs are defined for bromate27, nitrate, and selenium. The 

known health effects of these parameters are listed in Table 9.

TABLE 9 EPA MCLS AND SECONDARY STANDARDS FOR SALTS AND PH IN GROUNDWATER 

(ENVIRONMENTAL PROTECTION AGENCY 2012).

Chemical MCL (mg/l) Health effects

pH 6.5 – 8.5*

Chloride 250*

Sulfate 250*

TDS 500*

Bromate 0.01 Increased risk of cancer

Nitrate 10 Infants below the age of six months who drink water containing nitrate in 

excess of the MCL could become seriously ill and, if untreated, may die. 

Symptoms include shortness of breath and blue-baby syndrome.

Selenium 0.05 Hair or fingernail loss; numbness in fingers or toes; circulatory problems

*EPA Secondary Standard for drinking water

 

27 Bromate is a common byproduct of bromide interactions with disinfectants in surface water. Source: New York 

State Department of Health. (2011, April 2011). "Bromate in Drinking Water - Information Fact Sheet." Drinking 

Water Protection Program.
30

Cost of testing

Magnesium, potassium, and bromide are the most expensive parameters to test out of the salt category. 

However, on average, salts are less expensive to test than other parameters. There are also a greater 

number of laboratories that test salts than there are that test some of the other parameters discussed in this 

analysis (Table 10).

TABLE 10 COST OF TESTING, IN USD, FOR VARIOUS SALTS WITH LABS CERTIFIED TO TEST DRINKING WATER 

IN THE STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Number of Labs

Bromide 13 30 60 7

Calcium 9 11 15 3

Chloride 13 22 43 7

Magnesium 9 35 60 2

Nitrate 13 22 43 9

Potassium 9 35 60 2

Selenium 10 17 20 5

Sodium 9 11 15 3

Sulfate 13 23 58 11

TDS 12 21 33 11

Recommendations

If contamination does occur, the baseline for TDS, chlorides, and sodium has already established across most 

of the Eagle Ford shale region28, and concentrations in groundwater as high as what would be found in 

produced water would imply contamination in a post-drilling test. Therefore, rather than spend limited 

resources to test all of the potential analytes in the produced waters database, only the produced water 

parameters that deviate substantially from the mean found in groundwater should be a high priority for 

testing. Also, considering that there is no secondary standard listed for sodium, TDS and chloride should take 

higher priority than sodium in a list of analytes for baseline groundwater quality testing. Likewise,

parameters such as bromide, selenium, and nitrate should be a high priority for testing in counties where 

poor statistical power is available. Considering that the cost of testing these parameters is relatively low, and 

that the baseline is partially established in all counties of the Eagle Ford shale region, testing for salts could 

feasibly be taken on by water well owners.

 

28 Locations of where TDS, chlorides, and sodium have been tested are depicted in Appendix XI, Figure 19.
31

D. Metals

Baseline characterization

Every case study mentioned in this analysis considers trace metals in either baseline groundwater quality 

testing or in testing for potential post-drill contamination. Concentrations of trace metals in produced waters 

of the Eagle Ford shale formation were either not found in the literature, or did not represent enough 

samples to warrant a comparison with groundwater concentrations. Therefore, it is unclear how necessary 

trace metal samples are to background water quality testing in the Eagle Ford shale region in particular. 

Nonetheless, a list of trace metals was compiled from the case studies discussed in the introduction. With 

the exception of phosphorus, for each trace metal identified every county has at least one sample29. The 

number of samples varies by county and parameter, but testing recommendations based on power analysis 

can be found for each parameter and county in Appendix II, Tables 36 to 42.

Health effects

Table 11 lists the potential health effects of metals, metalloids, and additional salts that are known to occur 

in oil and gas flowback fluids. No MCL or health effects were listed for Boron, Cobalt, Molybdenum, Nickel, 

Phosphorus, Silicon, or Strontium.

TABLE 11 EPA MAXIMUM CONTAMINANT LEVELS (MCL) AND SECONDARY STANDARDS FOR METALS IN 

DRINKING WATER AND ASSOCIATED HEALTH EFFECTS.

Element MCL (mg/l) Health effects

Aluminum *0.05 to 

0.2

Antimony 0.006 Increase in blood cholesterol; decrease in blood sugar

Arsenic 0.01 Skin damage or problems with circulatory systems, and may have 

increased risk of getting cancer

Barium 2 Increase in blood pressure

Beryllium 0.004 Intestinal lesions

Cadmium 0.005 Kidney damage

Chromium 0.1 Allergic dermatitis

Copper 1

Iron *0.3 mg/L

Lead 0.015 Infants and children: Delays in physical or mental development; children 

could show slight deficits in attention span and learning abilities; Adults: 

Kidney problems; high blood pressure

Manganese 0.05*

Mercury 0.002 Kidney damage

Selenium 0.05 Hair or fingernail loss; numbness in fingers or toes; circulatory problems

Silver *0.10 mg/L

*EPA Secondary Standard

 

29 Silicon was not included in the database.
32

Cost of testing

Testing metals is generally not expensive (Table 12). Additionally, many metals can be tested together in a 

batch, making the cost of testing multiple parameters more affordable.

TABLE 12 COST OF TESTING, IN USD, FOR METALS WITH LABS CERTIFIED TO TEST DRINKING WATER IN THE 

STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Number of Labs

Metals package 175 175 175 1

Metals single 10 20 35 5

Recommendations

More data is needed on the concentrations of trace metals that are expected to be found in produced water 

of the Eagle Ford shale region. Without this information, defining a narrow list of baseline groundwater 

testing for metals is difficult. Although the baseline is well characterized for some metals, the dataset is less 

robust for others. When it comes to metals, baseline testing must be considered on a county-by-county basis 

for each parameter. At a minimum, it is safe to say that metals which are known to cause health effects 

should be considered as a higher testing priority than other metals.

E. Naturally-Occurring Radioactive Materials (NORM)

Baseline characterization

Various kinds of radionuclides are known to occur naturally in oil and gas formations including uranium-238, 

uranium-235, thorium-232, and their daughter products: radium-226, radium-228, lead-210, and polonium210 (NORSEDECOM 2003)

30. Generally, the concentrations of radionuclides found in produced water are not 

expected to be very high, but waste handling methods can lead to higher concentrations of long-living 

radionuclides being stored at the surface(Valeur and Ramboll Oil and Gas 2010) .

In the Eagle Ford shale formation, very high in-situ gamma radiation from potassium-40, thorium, and 

uranium have been observed (Fertl 1979). At the surface however, the RRC reports that one Eagle Ford 

drilling field in Wood County documented radiation greater than 500 microroentgen/hour (μR/hr)31 (Railroad 

Commission of Texas 2000). Aside from the case in Wood County, groundwater contamination from 

uranium and radon-226 has been documented in Live Oak, Karnes, and Gonzales Counties (Wukasch and 

Cook 1971 or later; McConnell, Ramanujam et al. 1998). These cases of contamination are related to 

historical uranium mining and waste disposal, activities which have also taken place in Duval, Karnes, Webb, 

Live Oak, Bee, and Fayette Counties (Pirson 1970; Annamalai and McGarvey 1980; Fishman and Huang 1980; 

Jones 1981; Otto Jr. 1984; McConnell, Ramanujam et al. 1997). The possibility of encountering historical 

radionuclide contamination underscores the importance of baseline testing for radioactive elements or 

 

30 Cited in Valeur (2010)

31 According to the RRC website, a microroentgen is a measurement of exposure to x-ray or gamma ray radiation in 

the air, whereas picocuries (pCi) measure the amount of a substance that decays over time RRC (2000). 500 μR/hr 

is one order of magnitude greater than the regulatory standard (50 μR/hr) for release of materials for uses other 

than oil and gas activities.
33

particle emissions in the Eagle Ford shale region to avoid false assumptions about the presence or absence of 

radioactive materials before oil and gas drilling begins.

Knowing which parameters are the most important ones to test for, however, is more complicated. Out of 

the radionuclides discussed here, radium-226 and -228 are the most common, and the most long lived (Fertl 

1979; Lawrie, Desmond et al. 2000; Valeur and Ramboll Oil and Gas 2010). Given this evidence, it at first 

seems appropriate to focus baseline water quality testing on radium-226 and radium-228; however, given the 

fact that potassium, thorium, and uranium are known to exist in high concentrations in the Eagle Ford 

formation, only testing for radium would be insufficient. 

Yet, testing for all of these parameters is not necessary, because radionuclides each give off a certain kind of 

radiation that is sometimes the same for other radionuclides, and the number of baseline analytical tests can 

be significantly reduced if only the appropriate kinds of radiation are tested. Three kinds of radioactive 

emissions exist: alpha, beta, and gamma radiation. All of the radionuclides discussed give off alpha or beta 

particle emissions, except for potassium-40 which emits gamma radiation (Table 13).

TABLE 13 TYPES OF RADIOACTIVE EMISSIONS KNOWN TO OCCUR FROM VARIOUS 

RADIONUCLIDES (FERTL 1979)

Radionuclide Type of emission

Uranium-238 Alpha

Thorium-232 Alpha

Radium-226 Alpha

Radium-228 Beta

Radon-222 Alpha

Potassium-40 Gamma

Lead-210 Beta

Polonium-210 Alpha

The numbers of wells with reliable samples that have tested radionuclides in the counties of interest are 

listed in Appendix XIII, Table 43. Radium-226 and Radium-228 have been tested in most counties, but radon, 

thorium and potassium-40 have been tested in fewer than 10 counties. Additionally, lead-210, polonium210, and gamma radiation are not recorded anywhere in the database. 

Every county has tested for alpha or beta radiation (Appendix XIII, Table 34). However, most of the counties 

have poor statistical power due to the low number of samples that have been reported, and additional 

sampling is needed. Additionally, some records indicate concentrations that are greater than the EPA MCL 

(see Figure 15). It is unclear whether these data points were taken in response to a case of contamination, or 

whether they represent unaffected background data.
34

Health effects

Although alpha and beta particles are the most easily shielded from skin contact, ingestion of alpha and beta 

particles is known to carry greater risk than external exposure to these particles (Smith and Argonne National 

Laboratory 1992). Gamma radiation, however, is able to penetrate through thick protective layers, and is 

generally considered to be the most harmful (Environmental Protection Agency 2012). The EPA lists MCLs for 

uranium, radium, alpha particles, and beta particles, and all are associated with an increased risk of cancer 

(Table 14) (Environmental Protection Agency 2012). Potassium-40 and thorium are also known to cause 

cancer (Argonne National Laboratory 2005; Environmental Protection Agency 2012) , but are not regulated in 

drinking water. 

TABLE 14 EPA MAXIMUM CONTAMINANT LEVELS FOR RADIOACTIVE ELEMENTS/PARTICLES IN DRINKING

WATER

Radioactive element MCL

Uranium 30 μg/l

Radium 5 pCi/l

Alpha particles 15 pCi/l

Beta particles 4 millirems/year

FIGURE 9 ALPHA PARTICLE CONCENTRATIONS IN THE COUNTIES OF INTEREST. RED POINTS REPRESENT 

VALUES GREATER THAN THE EPA MCL.
35

Cost of testing

On average, the cost of testing alpha and beta particle emissions together is less expensive than testing these 

parameters separately. Additionally, it is clear that testing for specific radionuclides is much more expensive. 

Similarly, testing gamma emissions is less expensive than testing for potassium-40, the only gamma radiation 

emitter on the list.

TABLE 15 COST OF TESTING, IN USD, FOR RADIONUCLIDES WITH LABS CERTIFIED TO TEST DRINKING 

WATER IN THE STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Number of Labs

Alpha 16 37 50 3

Alpha/Beta 50 54 65 4

Beta 16 37 50 3

Gamma 84 84 84 1

Potassium-40 250 250 250 1

Radium 226 65 92 125 4

Radium 226 and 228 150 168 185 4

Radium 228 85 102 125 4

Radon 222 35 96 200 4

Uranium 20 107 255 8

Recommendations

Considering the health effects associated with radionuclides, regional heterogeneity of alpha concentrations

(Figure 13), and the low statistical power of the available sample sizes more testing is recommended. Testing 

for alpha, beta, and gamma radiation is recommended, since these parameters encompass the effects of 

multiple radionuclides, and because these tests cost less than test for individual radionuclides.

F. Volatile Organic Compounds (VOCs)

The volatile organic compounds (VOCs) most commonly associated with oil and gas are the BTEX chemicals: 

benzene, toluene, ethylbenzene, and xylene. These chemicals are not generally thought to occur in 

groundwater, but determining background concentrations is necessary to verify this speculation. A

comprehensive national study seeking to determine baseline concentrations of VOCs in groundwater found 

toluene in at least 1 % of samples, although concentrations greater than the MCL were not found (Wukasch 

and Cook 1971 or later). The study area, however, did not include samples within counties of the Eagle Ford 

shale region, and ethylbenzene and xylene were not considered in the research. 

Baseline characterization

For this analysis, concentrations of BTEX chemicals are only provided for six counties in the TWDB 

groundwater quality database. All counties have less than 26 samples, indicating insufficient statistical power

(Table 16). Additionally, geospatial analysis indicates that the available samples are not evenly spaced 
36

enough to establish a reliable baseline across the region (Figure 13), and some samples exceeded the MCL. 

In three counties, no samples were taken for xylene; whereas benzene, ethylbenzene, and toluene were 

tested at the same time in all the other samples. It is unclear whether these samples were taken in response 

to a case of potential contamination, or if they were intended to represent baseline groundwater quality. 

TABLE 16 NUMBER AND PERCENT OF WELLS WITH RELIABLE SAMPLES FOR WHICH CONCENTRATIONS OF 

BTEX CHEMICALS ARE FOUND IN THE TWDB GROUNDWATER DATABASE.

FIGURE 10 BENZENE CONCENTRATIONS IN THE COUNTIES OF INTEREST. POINTS MARKED IN RED INDICATE 

CONCENTRATIONS THAT EXCEED THE EPA MCL FOR BENZENE.

County Total 

wells

Benzene Ethylbenzene Toluene Xylene

Bastrop 791 3 0.4% 3 0.4% 3 0.4% NA

Colorado 574 1 0.2% 1 0.2% 1 0.2% NA

Dimmit 527 24 4.6% 24 4.6% 24 4.6% 24 4.6%

Fayette 863 4 0.5% 4 0.5% 4 0.5% NA

La Salle 362 4 1.1% 4 1.1% 4 1.1% 4 1.1%

Zavala 748 3 0.4% 3 0.4% 3 0.4% 3 0.4%

Grand Total 3865 39 1.0% 39 1.0% 39 1.0% 31 0.8%
37

Health effects

All four VOCs listed here are regulated by the EPA in drinking water. All of the BTEX chemicals are considered 

to present serious health effects (Table 21).

TABLE 17 HEALTH EFFECTS OF VOLATILE ORGANIC COMPOUNDS COMMONLY FOUND IN OIL AND GAS 

FLOWBACK (ENVIRONMENTAL PROTECTION AGENCY 2012).

Chemical 

parameter

MCL (mg/l) Health Effects

Benzene 0.005 Anemia; decrease in blood platelets; 

increased risk of cancer

Toluene 1 Nervous system, kidney, or liver problems

Ethylbenzene 0.7 Liver or kidneys problems

Xylene 10 Nervous system damage

Cost of testing

The cost of testing BTEX chemicals is high Table 18). This cost is conceivably not affordable for some water 

well owners.

TABLE 18 COST OF TESTING, IN USD, FOR VOC WITH LABS CERTIFIED TO TEST DRINKING WATER IN THE 

STATE OF TEXAS.

Test Min of Cost Mean of Cost Max of Cost Count of Lab

BTEX 40 87 125 5

VOC 50 149 200 8

Recommendations

Considering the potential health impacts of BTEX chemicals, the fact that they are frequently listed as 

contaminants from oil and gas activities in the counties of interest, that they could occur naturally, and 

because so few samples have been taken across the landscape it is highly recommended that a strategic 

baseline groundwater sampling initiative for VOC, including BTEX, take place by an agency, industry, or other 

organization.

G. Polycyclic Aromatic Hydrocarbons (PAHs)

Baseline characterization

PAHs are known to occur in oil and gas flowback water (Bojes, Pope et al. 2007; Joa, Panova et al. 2009). All 

counties of interest have tested PAHs fewer than 26 times, indicating poor statistical power (Appendix XIV, 

Table 45). Geospatial analysis indicates wide geographical gaps in sampling (Figure 14). All of these tests 

were taken at the same time, and seven of the 49 samples exceeded the EPA MCL for benzo(a)pyrene (Tables 

25 and 26). Therefore, it is unclear whether these samples represent background concentrations or if they 

were taken in response to potential contamination. 
38

Health effects

Animal testing has shown that PAHs cause reproductive and developmental health effects. Additionally, 

detrimental effects to the skin, body fluids, and immune system have been observed in animals (Agency for 

Toxic Substances and Disease Registry 1996). The EPA has listed 16 priority-pollutant PAHs (Appendix 14, 

Table 45), but only Benzo(a)pyrene is regulated in drinking water (Bojes, Pope et al. 2007; Joa, Panova et al. 

2009; Environmental Protection Agency 2012). Seven of the 16 priority-pollutant PAHs have been listed as 

probable carcinogens by the EPA (NTP 2005)

32. Points marked in red in Figure 14 exceed the EPA MCL of 10

μg/l of Benzo(a)pyrene.

FIGURE 11 BENZO(A)PYRENE CONCENTRATIONS IN THE COUNTIES OF INTEREST. 

Cost of testing

All 16 priority PAH can be tested at the same time in a package. In fact, laboratories do not offer individual 

tests of these parameters. Nonetheless, the cost of testing for PAH is expensive, and may be unaffordable to 

some well owners.

TABLE 19 COST OF TESTING, IN USD, FOR PAH WITH LABS CERTIFIED TO TEST DRINKING WATER IN THE 

STATE OF TEXAS.

Test Min of Cost Average of Cost Max of Cost Number of Labs

PAH 120 148 176 2

 

32 Cited inBojes, H. K., P. G. Pope, et al. (2007). "Characterization of EPA’s 16 priority pollutant polycyclic aromatic 

hydrocarbons (PAHs) in tank bottom solids and associated contaminated soils at oil exploration and production 

sites in Texas." Regulatory Toxicology and Pharmacology 47(3): 288-295.
39

Recommendations

Considering the serious health effects associated with PAHs, their association with oil and gas flowback, and 

the lack of available data in the Eagle Ford shale region, a strategic sampling effort for PAHs is recommended 

for the counties of interest. Because the cost of testing is so high, such an initiative might best be taken on 

by an agency, industry or other organization.
40

IV. Discussion

This analysis employed five different strategies to determine which parameters should be tested prior to oil 

and gas drilling to establish a functional baseline for comparison to potentially contaminated samples. First, 

a list of potential parameters was defined using prominent case studies of oil and gas contamination cases. 

Second, known concentrations of parameters found in produced water were compared to known 

concentrations of those same parameters for the same geographic region, in order to determine if 

differences could reasonably be expected. Third, statistical power analysis was used to determine whether 

the minimum number of samples needed to establish confidence in a range of concentrations for any given 

parameter had already been acquired for each county and parameter of interest. Next, health effects were 

considered to highlight potential pollutants that should be prioritized in a sampling initiative. Finally, cost of 

testing was considered to introduce a perspective about whether a well owner might be expected to 

voluntarily participate in baseline groundwater quality sampling, or whether an organized and geographically 

strategic sampling initiative should be led by an agency, industry, or other organization.

The results of employing these strategies reveal that some of the most dangerous chemicals are not 

adequately characterized in baseline water quality, if at all. These high priority baseline tests include TOC, 

13C/12C, TPH, nitrate, alpha particles, beta particles, gamma radiation, VOC, and PAH. The average cost of 

testing all of these parameters is approximately $1000. 

Contrastingly, with the exception of Caldwell and Guadalupe counties, most salts are well characterized 

across the region. In counties where more tests are needed, TDS, chlorides, bromide, nitrate, and selenium 

are the most essential testing parameters. On average, the cost of testing all of these parameters would 

total less than $250.

For the most part, metals are also well characterized, but each county has a slightly different number of tests 

for each parameter, indicating that testing initiatives for metals will be more localized and tailored to each 

county than testing for high priority baseline parameters mentioned above. Another aspect of metals is that 

some of the metals analyzed are regulated in drinking water, because of health effects that are associated 

with their presence at or above threshold concentrations. For counties and well owners faced with many 

baseline water quality needs, these regulated metals are higher priority parameters than other metals or 

salts. The cost of testing these 14 metals listed in table 12 would be about $300 if the average price is true, 

or it could be $140 if the lowest price is true; in which case, the package price would be more expensive. 

Counties seeking to enhance current baseline datasets can use the testing priority codes listed in Appendix XII 

to determine which metals need to be tested the most in that particular county. 

In conclusion, background water quality data is an essential component of assessing potential groundwater 

pollution. Considering that cases of potential groundwater contamination linked to oil and gas activities have 

been contentious due to lack of background water quality data, groundwater stakeholders in the newly and 

rapidly developing Eagle Ford shale region would be wise to quickly establish regional initiatives to conduct 

baseline water quality testing for the parameters recommended here.
41

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45

Appendix I – Counties

The data used in this analysis represents all Eagle Ford shale counties listed by three sources: Railroad 

Commission [R], Energy Information Administration [E], and the website EagleFordShale.com [S]. 

 Atascosa [R, E, S] Colorado [S] Goliad [E, S] Lavaca [R, E, S] Milam [R, E, S]

 Austin [E, S] Dewitt [R, E, S] Gonzales [R, E, S] Lee [R, E, S] Robertson [S]

 Bastrop [E, S] Dimmit [R, E, S] Grimes [R, E, S] Leon [R, S] Washington [E, S]

 Bee [R, E, S] Duval [E, S] Guadalupe [R] Live Oak [R, E, S] Webb [R, E, S]

 Brazos [E, S] Edwards [R] Houston *  Madison [S] Wilson [R, E]

 Burleson [R, E, S] Fayette [E, S] Karnes [R, E, S] Maverick [R, E, S] Wood *

 Caldwell [R] Frio [R, E, S] LaSalle [R, E, S] McMullen [R, E, S] Zavala [R, E, S]

* Houston and Wood Counties were mentioned in EagleFordShale.com as having been listed by the RRC in a list of Eagle Ford shale counties. 

Although they are no longer listed on the RRC’s webpage for the Eagle Ford shale, there is reason to suggest that Eagle Ford drilling is not 

entirely ruled out for Houston and Wood counties.

The Railroad Commission of Texas (RRC) reports that drilling in the Eagle Ford shale is taking place in 16 fields 

in 24 counties (Railroad Commission of Texas 2012a). However, this analysis includes 35 counties in the Eagle 

Ford shale region. The reason for including more counties than the RRC lists as existing in the Eagle Ford 

shale is that both the Energy Information Administration (EIA) (Figure 1) and the website 

EagleFordShale.com33 have included some counties in the Eagle Ford shale region that are not listed by the 

RRC. Additionally, the RRC includes counties in the Eagle Ford shale that have not been listed by these other 

sources (Figure 4) (U.S. Energy Information Administration 2010; Dukes, DuBose et al. 2012). Considering 

that both the EIA and EagleFordShale.com are websites that seek to predict trends in drilling activity, it is 

possible that some counties not listed by the RRC, but which are listed by these other sources, will experience 

drilling in the foreseeable future. Therefore, the decision to include counties not listed by the RRC is 

intended to ensure that this analysis is relevant to future audiences.

 

33 EagleFordShale.com is currently hosting a website for the Eagle Ford Task Force, a group formed by current 

Railroad Commissioner David Porter to help overcome economic and logistical obstacles of developing the Eagle 

Ford shale play. This website provides detailed and referenced information about Eagle Ford shale counties. Since 

this website is about development of the shale, it makes sense that the list of counties the website offers might 

include counties that the business community expects to see drilling at in the near future.
46

Appendix II – Carbon isotope values

Carbon-13 (13C) isotope concentrations are calculated by comparing the ratio of 13C to 12C isotopes of carbon 

in a given sample of gas, to a predefined standard, Pee Dee Belemnite (PDB). The following equation

(Farquhar, O'Leary et al. 1982) describes this comparison: 

δ

13C (‰) = ((Rsample/Rstandard) – 1) x 1000 Equation 1

In equation 1, Rsample represents the ratio of 13C/12C found in the sample, while Rstandard represents the ratio of 

13C/12C found in the PDB standard (Farquhar, O'Leary et al. 1982). The PDB standard has a 13C isotope ratio 

that is less than one (Slater, Preston et al. 2001). Therefore, if the concentration of 13C in a sample is less 

than the concentration of 13C in the standard, the resulting concentration of δ13C (‰) would be more 

negative than it would have been if the 13C sample concentration were higher than the standard.

Methane has a specific δ13C concentration (corresponding to when and how it was formed), which is also 

referred to as an isotopic signature or “fingerprint.” Naturally occurring methane in a groundwater aquifer 

could come from recent microbial degradation of organic matter, termed biogenic methane. However, 

methane in groundwater could also come from an aquifer’s connection with a deeper source of methane that 

was formed from high temperatures and pressures, also called “thermogenic” methane. Thermogenic gas 

tends to have a less negative δ13C concentration than biogenic methane (Schoell 1980; Barker and Fritz 

1981)

34

. 

When thermogenic methane from a shale formation reaches an aquifer that contains more biogenic gas than 

thermogenic gas, it would likely make the overall concentration of δ 13C in the aquifer less negative. If only 

small amounts of biogenic gas were present before the thermogenic methane leaked into the aquifer, then 

the concentration of methane gas found in the aquifer would very closely resemble the isotopic ratio found 

in gas that is produced from the natural gas formation. The extent to which the groundwater δ13C 

concentrations change depends on the quantity and type of thermogenic gas that enters the aquifer.

 

34 Schoell 1980 Cited in Osborn et al. (2011)
47

Appendix III – Starting list of potential testing parameters

This list represents the initial list of potential baseline groundwater quality analytes. From this list, data 

availability for each parameter are assessed for each of the counties of interest, and recommendations for 

future testing are made based on data availability, health impacts, and cost associated with testing each 

parameter.

TABLE 20 CATEGORIES OF POTENTIAL BASELINE GROUNDWATER CHEMISTRY PARAMETERS.

Category Potential baseline chemical analytical parameters

1. Gas hydrocarbons Methane

Ethane

Propane

Hexane

Total organic carbon

Carbon isotopes

2. Liquid hydrocarbons Total Petroleum Hydrocarbons

Gasoline range organics

Diesel range organics

3. Salts Bicarbonate

Bromide

Calcium

Chloride

Magnesium

Nitrate

Potassium

Selenium

Sodium

Sulfate

Total Dissolved Solids

4. Metals Aluminum

Antimony

Arsenic

Barium

Beryllium

Boron

Cadmium

Chromium

Cobalt

Copper

Iron

Lead

Manganese

Mercury

Molybdenum
48

Nickel

Phosphorus

Potassium

Silver

Strontium

Sulfur

Titanium

Uranium

5. Radioactive elements/parameters Alpha

Beta

Gamma

Uranium

Thorium

Radium 226, 228

Radon 222

Potassium-40

6. Volatile organic compounds (VOCs) Benzene

Toluene

Ethylbenzene

Xylene

7. Polycyclic aromatic hydrocarbons (PAHs) Acenaphthene 

Acenaphthylene

Anthracene 

Benzo(a) anthracene 

Benzo-(a)-pyrene 

Benzo(b)fluoranthene 

Benzo(ghi)perylene 

Benzo(k)fluoranthene 

Chrysene 

Dibenzo (a,h) anthracene 

Fluoranthene 

Fluorene 

Indeno (1,2,3-cd) pyrene 

Naphthalene 

Phenanthrene 

Pyrene 
49

Appendix III – Pavillion, WY test results

TABLE 21 CONTAMINANTS FOUND IN MONITORING WELLS DRILLED BY THE EPA IN PAVILLION, WY

Component found in groundwater Hydraulic fracturing ingredient the 

component was used in at the site

 Acetate and benzoic acid  “Natural breakdown products of 

organic contaminants like BTEX and 

glycols”

 Benzene, Toluene, Ethylbenzene, 

and Xylene (BTEX)

 Trimethylbenzenes

 “Aromatic solvent (usually a BTEX 

solution) used in a breaker”

 “Toluene and xylene were used in 

flow enhancers and a breaker”

 Chloride  Potassium chloride used in carbon 

dioxide foam hydraulic fracturing

 Ammonium chloride used in a 

crosslinker

 Diesel range organics (DRO)  “Diesel oil (mixture of saturated 

and aromatic hydrocarbons 

including naphthalenes and 

alkylbenzenes) was used in a guar 

polymer slurry/liquid gel 

concentrate and in a solvent.”

 “Petroleum raffinates (mixture of 

paraffinic, cycloparaffinic, 

olefinic, and aromatic 

hydrocarbons) were used in a 

breaker.”

 “Heavy aromatic petroleum 

naphtha (mixture of paraffinic, 

cycloparaffinic and aromatic 

hydrocarbons) was used in 

surfactants and in a solvent.”

 guar polymer 

 solvent

 Diethylene glycol  foaming agent 

 solvent

 Isopropanol  biocide

 surfactant

 breakers

 foaming agents
50

 Potassium hydroxide (KOH)

 Potassium metaborate

 crosslinker

 solvent

 Triethylene glycol  solvent

 Tert-butyl alcohol  “Breakdown product of methyl 

tert-butyl ether (a fuel additive), 

and tert-butyl hydroperoxide (a 

gel breaker used in hydraulic 

fracturing)”
51

Appendix IV – RRC groundwater contamination report

In the joint groundwater monitoring and contamination report, the RRC indicates that BTEX stands for 

Benzene, Toluene, Ethylbenzene, and Xylene; NG stands for natural gas; but COND is not described. Here it is 

assumed that COND stands for Condensate. These codes were consolidated into their respective categories 

for analysis as described in Table 4.

TABLE 22 CONSOLIDATED CATEGORIES OF RRC REPORTED CONTAMINANTS

Consolidated category Reported contaminant

BTEX Benzene, dissolved benzene, BTEX

Liquid hydrocarbons hydrocarb, hydrocarb seep, PSH, total petroleum hydrocarbons (TPH), COND, 

condensate, crude oil

Gas hydrocarbons NG, methane

Chlorides Chloride, sodium chloride

FIGURE 12 ACTIVITY STATUS FOR PENDING CASES OF GROUNDWATER CONTAMINATION ATTRIBUTED TO OIL 

AND GAS ACTIVITIES IN 2010 (TEXAS GROUNDWATER PROTECTION COMMITTEE 2011).

2

3

3

6

9

18

23

0 5 10 15 20 25

Remediation completed. No further action or inspection

required.

Violations and contamination due to current activities

found.

Contamination confirmed but not fully investigated

No violations and historical contamination.

Corrective action pending review and approval.

Corrective action periodically observed for effectiveness.

Corrective action underway.

Number of Cases
52

FIGURE 13 MAP OF THE NUMBER OF PENDING GROUNDWATER CONTAMINATION CASES CAUSED BY OIL 

AND GAS ACTIVITIES REPORTED IN 2010 (TEXAS GROUNDWATER PROTECTION COMMITTEE 2011).
53

Appendix V – Data preparation

USGS Produced Water Database

The database was filtered for “EAGLEFORD” under the category “RevSAMPFORM.” According to the user 

manual, REVSAMPFORM stands for “Revised sample formation name,” and indicates the following: 

“Geologic units from which the samples was [sic] produced. Corrections to SAMPFORM made to be 

consistent with stratigraphic columns in the USGS 1995 National Oil and Gas Assessment and the USGS 

Geologic Names Lexicon http://ngmdb.usgs.gov/Geolex/geolex_home.html” Additionally, some records in 

the database were codes for “null” entries, where no concentration was available. In preparing the dataset 

for analysis, values of zero in the USGS produced waters database were interpreted as null entries, as were 

values of -3, -2, and -1, because the USGS produced waters database description reads “values of -3 replace 

original character entries of "ND", "NA" or null entries; -2 replace original character entries of "none", 

"negative", or "nil", -1 replaces original character entries of "trace" and "minor". (Null entries: 0).” (U.S. 

Geological Survey 2002)

Kearns (2010)

Geochemical analysis of Eagle Ford core samples taken by Kearns (2010) include major elements, trace 

elements, and isotopes listed below. The data were taken at two sites: one in Gonzales County, and one in 

De Witt County. The samples in Gonzales County (n=72) represent depths from 8090.5 to 8173.5 feet, while 

the samples taken in DeWitt (n=42) County represent depths of 13700.5 to 13828.5 feet (Kearns 2011).

 N %  S %  Ca %  Ni ppm  Sr ppm

 TOC %  Mg %  Ti %  Cu ppm  Zr ppm

 C/N  Al %  Mn %  Zn ppm  Mo ppm

 δ15N  Si %  Fe %  Th ppm

 δ13C  P %  V ppm  Rb ppm

 TIC %  K %  Cr ppm  U ppm

TWDB Groundwater Database

The following procedures were used to extract data from the TWDB groundwater quality database:

1. Data for each county was extracted using the “Water Quality Publication Report Query” and each 

county code.

2. The data was filtered for reliability codes 08 – 14 and 16 using the column labeled “reliability rem.” 

(see Appendix VII – Reliability codes)

3. Flags indicating quantity boundaries, such as greater than “>” or less than “<,” were ignored when 

preceding a concentration.

4. Blanks were revised as “NA” for functionality with statistical and geospatial analysis software.

5. Zeros were set to “NA” if they deviated substantially from the range of other values in the dataset.

6. Zero values for carbonate were changed to NA values when they made up most of the samples in a

county. For example, no data for carbonate was provided for Wood County.
54

Geospatial Analysis

ESRI was the data source for Texas county boundaries data (ESRI 2000). Location data for water wells in 

Texas was acquired from the TWDB’s GIS Data webpage (TWDB GISa; TWDB GISb). The groundwater quality 

data tables were added to the map and then joined by state well number with the TWDB “Well Location 

Shapefile”. Finally, the data were projected in NAD 1983 with no UTM Zone. For displaying graduated colors, 

in some cases the natural breaks (Jenks) method was used to display the range of concentrations. However, 

if significant thresholds, such as an MCL, were known for a specific chemical, the displayed concentration 

ranges were altered to depict this threshold.

Literature Review

Literature was systematically queried using two databases: ISI Web of Knowledge, and OnePetro. Additional 

literature may be included that the author found through other sources. Search terms for the literature 

searches were as follows:

 “Eagle Ford” AND formation

 “Eagle Ford” AND shale

 “County Name County” Texas
55

Appendix VI – Stratigraphic column

FIGURE 14 STRATIGRAPHIC COLUMN OF THE EAGLE FORD FORMATION (CONDON AND DYMAN 2006, CITED 

IN MULLEN 2010).
56

Appendix VII – Reliability codes for groundwater quality 

samples in Texas Water Development Board database.

Code Description

01 Not indicative of aquifer quality. Data should be used carefully.

02 Sample collected from well not sufficiently pumped; and not filtered or preserved. 

Data should still be used carefully.

03 Sample collected from well sufficiently pumped but not filtered or preserved. Holding 

time probably not honored.

04 Chemical analysis taken from a report. Sample collection and preservation 

procedures unknown.

08 Sample filtered in the field. Temperature, conductivity, and pH measured in the field. 

Alkalinity measured in the lab. Nutrient sample not preserved and included in anion 

sub-sample. Sample kept cold until delivered to the lab. 

09 Same as #08, but not filtered in the field.

10 Sampled in accordance with the TWDB's A Field Manual for Ground-Water Sampling, 

2003. Samples are collected when temperature, conductivity, and pH have stablized. 

Sample was filtered and field tested for alkalinity. Samples are preserved as 

applicable, kept chilled, and delivered to the lab. Holding times are honored. Organic 

sub-samples are not filtered.

11 Same as #10 but not filtered.

12 Sample collected by TCEQ staff following prescribed QA-QC procedures.

13 Cation sample preserved and run thru TDH lab. Anion and nutrient sub-samples sent 

to lab at Texas Tech lab and run within 24 hours. Field test completed as described in 

#10.

14 Similar to #10 but sample results determined by using a Hach DR-2000 lab.

15 Collection procedures not documented, results obtained by use of a Groundwater 

Conservation District's Hach equipment. 

16 Sample collected by USGS for NAWQA program using "Clean Sample" technique.

99 Reliability unknown, not available, or not yet entered into database. Sample collected 

from tank, distribution, or bailed from well.
57

Appendix VIII – TWDB Groundwater Database Listed 

Parameters

Concentrations for the following chemicals and parameters are provided in the Texas Water Development 

Board groundwater database:

pH Boron Lead Silicon

Temperature Bromide Lithium Silver

Conductivity Cadmium Magnesium Sodium

Sodium Adsorption Ratio Calcium Manganese Strontium

Total Dissolved Solids Calcium Carbonate Mercury Sufate

Alkalinity Carbonate Molybdenum Tellurium

Aluminum Chloride Nitrate Uranium

Antimony Chromium Nitrite plus Nitrate Vanadium

Arsenic Cobalt Oxygen Zinc

Barium Copper Phosphorus

Beryllium Fluoride Potassium

Bicarbonate Iron Selenium
58

Appendix IX – All Infrequent Constituents Identified in the 

TWDB Groundwater Database

1,1,1-TRICHLOROETHANE, TOTAL, UG/L 

1,1,2,2-TETRACHLOROETHANE, TOTAL, UG/L 

1,1,2-TRICHLOROETHANE, TOTAL, UG/L 

1,1-DICHLOROETHANE, TOTAL, UG/L 

1,1-DICHLOROETHYLENE, TOTAL, UG/L 

1,2,4-TRICHLOROBENZENE, TOTAL, UG/L 

1,2-DICHLOROBENZENE, TOTAL, UG/L 

1,2-DICHLOROETHANE, TOTAL, UG/L 

1,2-DICHLOROPROPANE, TOTAL, UG/L 

1,2-DIPHENYLHYDRAZINE, TOTAL, UG/L 

1,3-DICHLOROBENZENE, TOTAL, UG/L 

1,4-DICHLOROBENZENE, TOTAL, UG/L 

2, 4, 5-T, WATER, DISSOLVED, UG/L 

2, 4-D, WATER, DISSOLVED, UG/L 

2,4,5-T, TOTAL, UG/L 

2,4,5-TP INCLUDES ACIDS & SALTS IN WATER, UG/L 

2,4,6-TRICHLOROPHENOL, TOTAL, UG/L 

2,4-D, TOTAL, UG/L 

2,4-DICHLOROPHENOL, TOTAL, UG/L 

2,4-DIMETHYLPHENOL, TOTAL, UG/L 

2,4-DINITROPHENOL, TOTAL, UG/L 

2,4-DINITROTOLUENE, TOTAL, UG/L 

2,6-DINITRO-2-CRESOL, TOTAL, UG/L 

2,6-DINITROTOLUENE, TOTAL, UG/L 

2-CHLOROETHYL VINYL ETHER, TOTAL, UG/L 

2-CHLORONAPHTHALENE, TOTAL, UG/L 

2-NITROPHENOL, TOTAL, UG/L 

3,3'-DICHLOROBENZIDINE, TOTAL, UG/L 

4,4'-DDD, TOTAL, UG/L 

4,4'-DDE, TOTAL, UG/L 

4,4'-DDT, TOTAL, UG/L 

4-BROMOPHENYL PHENYL ETHER, TOTAL, UG/L 

4-CHLORO-3-CRESOL, TOTAL, UG/L 

4-CHLOROPHENYL PHENYL ETHER, TOTAL, UG/L 

4-NITROPHENOL, TOTAL, UG/L 

A-BHC-ALPHA, TOTAL, UG/L 

ACENAPHTHENE, TOTAL, UG/L 

ACENAPHTHYLENE, TOTAL, UG/L 

ACROLEIN, DISSOLVED, UG/L 

ACRYLONITRILE, DISSOLVED, UG/L 

ALACHLOR, TOTAL, UG/L 

ALDRIN, TOTAL, UG/L 

ALKALINITY PHENOLPHTHALEIN FIELD DATA (MG/L) 

ALKALINITY, FIELD, DISSOLVED AS CACO3 

ALKALINITY, TOTAL (MG/L AS CACO3) 

ALPHA, DISSOLVED (PC/L) 

ALPHA, TOTAL (PC/L) 

ALUMINUM, DISSOLVED (UG/L AS AL) 

ALUMINUM, TOTAL (UG/L AS AL) 

ANION/CATION CHG BAL, PERCENT 

ANTHRACENE, TOTAL, UG/L 

ANTIMONY, DISSOLVED (UG/L AS SB) 

ANTIMONY, TOTAL (UG/L AS SB) 

ARSENIC, DISSOLVED (UG/L AS AS) 

ARSENIC, TOTAL (UG/L AS AS) 

BANVEL (DICAMBA), TOTAL, UG/L 

BARIUM, DISSOLVED (UG/L AS BA) 

BARIUM, TOTAL (UG/L AS BA) 

B-BHC-BETA, TOTAL, UG/L 

BENZENE, VOLATILE ANALYSIS, TOTAL, UG/L 

BENZIDINE, TOTAL, UG/L 

BENZO(A) ANTHRACENE, TOTAL, UG/L 

BENZO-(A)-PYRENE, TOTAL, UG/L 

BENZO(B)FLUORANTHENE, TOTAL, UG/L 

BENZO(GHI)PERYLENE, TOTAL, UG/L 

BENZO(K)FLUORANTHENE, TOTAL, UG/L 

BERYLLIUM, DISSOLVED (UG/L AS BE) 

BERYLLIUM, TOTAL (UG/L AS BE) 

BETA, DISSOLVED (PC/L) 

BETA, TOTAL (PC/L) 

BIS (2-CHLOROETHOXY) METHANE, TOTAL, UG/L 

BIS (2-CHLOROETHYL) ETHER, TOTAL, UG/L 

BIS (2-CHLOROISOPROPYL) ETHER, TOTAL, UG/L 

BIS(2-ETHYLHEXYL) PHTHALATE, TOTAL, UG/L 

BISPHENOL, TOTAL, UG/L 

BORON, DISSOLVED (UG/L AS B) 

BROMIDE, DISSOLVED, (MG/L AS BR) 

BROMIDE, DISSOLVED, (UG/L AS BR) 

BROMODICHLOROMETHANE, TOTAL, UG/L 

BROMOFORM, TOTAL, UG/L 
59

BROMOMETHANE, TOTAL, UG/L 

BUTYLBENZYL PHTHALATE, TOTAL, UG/L 

CADMIUM, DISSOLVED (UG/L AS CD) 

CADMIUM, TOTAL (UG/L) 

CALCIUM, DISSOLVED (MG/L AS CA) 

CARBON DIOXIDE (MG/L AS CO2) 

CARBON TETRACHLORIDE, TOTAL, UG/L 

CARBON, TOTAL (MG/L AS C) 

CARBON, TOTAL INORGANIC (MG/L AS C) 

CARBON, TOTAL ORGANIC (MG/L AS C) 

CARBON-13 / CARBON-12 STABLE ISOTOPE RATIO PER 

MIL

CARBON-14 DISS APPARENT AGE (YEARS BP) 

CARBON-14 FRACTION MODERN 

CHLORDANE, TOTAL, UG/L 

CHLORIDE, TOTAL (MG/L AS CL) 

CHLOROBENZENE, TOTAL, UG/L 

CHLORODIBROMOMETHANE, TOTAL, UG/L 

CHLOROETHANE, TOTAL, UG/L 

CHLOROFORM, TOTAL, UG/L 

CHLOROMETHANE, TOTAL, UG/L 

CHLOROPHENOL, TOTAL, UG/L 

CHROMIUM, DISSOLVED (UG/L AS CR) 

CHROMIUM, FIELD ACIDIFIED W/HNO3, FILTERED, 

UG/L 

CHROMIUM, TOTAL (UG/L AS CR) 

CHRYSENE, TOTAL, UG/L 

CIS-1,3-DICHLOROPROPENE, TOTAL, UG/L 

CIS-1,3-DICHLOROPROPYLENE, TOTAL, UG/L 

COBALT, DISSOLVED (UG/L AS CO) 

COPPER, DISSOLVED (UG/L AS CU) 

COPPER, TOTAL (UG/L AS CU) 

CYCLOHEXANONE, TOTAL, UG/L 

DACTHAL (DCPA), TOTAL, UG/L 

DDD, TOTAL, UG/L 

DDE, TOTAL, UG/L 

DDT, TOTAL, UG/L 

DELTA-BHC, TOTAL, UG/L 

DEUTERIUM, EXPRESSED AS PERMIL VSMOW 

DIAZINON, TOTAL, UG/L 

DIBENZO (A,H) ANTHRACENE, TOTAL, UG/L 

DIBROMOCHLOROMETHANE, TOTAL, UG/L 

DIELDRIN, TOTAL, UG/L 

DIETHYL PTHALATE, TOTAL, UG/L 

DIMETHYL PTHALATE, TOTAL, UG/L 

DI-N-BUTYL PHTHALATE, TOTAL, UG/L 

DI-N-OCTYL PHTHALATE, TOTAL, UG/L 

DIPHENYLAMINE, TOTAL, UG/L 

DURSBAN (CHLOROPYRIFOS), TOTAL, UG/L 

ENDOSULFAN - ALPHA, TOTAL, UG/L 

ENDOSULFAN - BETA, TOTAL, UG/L 

ENDOSULFAN SULFATE, TOTAL, UG/L 

ENDOSULFAN, TOTAL, UG/L 

ENDRIN ALDEHYDE, TOTAL, UG/L 

ENDRIN, TOTAL, UG/L 

ETHYL PARATHION, TOTAL, UG/L 

ETHYLBENZENE IN WATER, UG/L 

FLUORANTHENE, TOTAL, UG/L 

FLUORENE, TOTAL, UG/L 

FLUORIDE, TOTAL (MG/L AS F) 

GAMMA-BHC (LINDANE), TOTAL, UG/L 

GROSS ALPHA RADIATION,TOTAL, PRODUCED 

WATER(pCi/L)

GROSS BETA RADIATION, TOTAL, PRODUCED 

WATER(pCi/L)

HALOGEN, TOTAL ORGANIC, UG/L 

HARDNESS, NON-CARBONATE (MG/L AS CACO3) 

HARDNESS, TOTAL (MG/L AS CACO3) 

HEPTACHLOR EPOXIDE, TOTAL, UG/L 

HEPTACHLOR, TOTAL, UG/L 

HEXACHLOROBENZENE (HCB), TOTAL, UG/L 

HEXACHLOROBUTADIENE, TOTAL, UG/L 

HEXACHLOROCYCLOPENTADIENE, TOTAL, UG/L 

HEXACHLOROETHANE, TOTAL, UG/L 

HYDROGEN ION CONCENTRATION, TOTAL (MG/L) 

HYDROGEN SULFIDE, MG/L 

HYDROXIDE ION (MG/L AS OH) 

INDENO (1,2,3-CD) PYRENE 

IODIDE (MG/L AS I) 

IRON, DISSOLVED (UG/L AS FE) 

IRON, TOTAL (UG/L AS FE) 

ISOPHORONE, TOTAL, UG/L 

LEAD, DISSOLVED (UG/L AS PB) 

LEAD, FIELD FILTERED, ACIDIFIED W/HNO3, UG/L 

LEAD, TOTAL (UG/L AS PB) 

LINDANE, TOTAL, UG/L 
60

LITHIUM, DISSOLVED (UG/L AS LI) 

LITHIUM, TOTAL (UG/L AS LI) 

MAGNESIUM, DISSOLVED (MG/L AS MG) 

MAGNESIUM, TOTAL (MG/L AS MG) 

MALATHION, TOTAL, UG/L 

MANGANESE, DISSOLVED (UG/L AS MN) 

MANGANESE, TOTAL (UG/L AS MN) 

MERCURY, DISSOLVED (UG/L AS HG) 

MERCURY, TOTAL (UG/L AS HG) 

METHOXYCHLOR, TOTAL, UG/L 

METHYL PARATHION, TOTAL, UG/L 

METHYLENE CHLORIDE, TOTAL, UG/L 

MOLYBDENUM, DISSOLVED, UG/L 

NAPHTHALENE, TOTAL, UG/L 

NICKEL, DISSOLVED (UG/L AS NI) 

NICKEL, TOTAL (UG/L AS NI) 

NITRATE NITROGEN, DISSOLVED (MG/L AS N) 

NITRATE NITROGEN, TOTAL (MG/L AS N) 

NITRITE NITROGEN, DISSOLVED (MG/L AS N) 

NITRITE NITROGEN, TOTAL (MG/L AS N) 

NITRITE NITROGEN, TOTAL (MG/L AS NO2) 

NITRITE PLUS NITRATE, DISSOLVED (MG/L AS N) 

NITRITE PLUS NITRATE, TOTAL (MG/L AS N) 

NITROBENZENE, TOTAL, UG/L 

NITROGEN, AMMONIA, DISSOLVED (MG/L AS N) 

NITROGEN, AMMONIA, TOTAL (MG/L AS N) 

NITROGEN, KJELDAHL, DISSOLVED (MG/L AS N) 

NITROGEN, ORGANIC, TOTAL (MG/L AS N) 

N-NITROSODIMETHLAMINE, TOTAL, UG/L 

N-NITROSO-DI-N-PROPYLAMINE, TOTAL, UG/L 

N-NITROSODIPHENYLAMINE, TOTAL, UG/L 

OXIDATION REDUCTION POTENTIAL (ORP), MILLIVOLTS 

OXYGEN, DISSOLVED (MG/L) 

OXYGEN, DISSOLVED, ANALYSIS BY PROBE (MG/L) 

OXYGEN-18, EXPRESSED AS PERMIL VSMOW 

OXYGEN-18/OXYGEN-16 OF SULFATE (RATIO PER MIL) 

PCB - 1221, TOTAL, UG/L 

PCB - 1232, TOTAL, UG/L 

PCB - 1242, TOTAL, UG/L 

PCB - 1248, TOTAL, UG/L 

PCB - 1254, TOTAL, UG/L 

PCB - 1260, TOTAL, UG/L 

PCB - 1262 (ARACLOR), TOTAL, UG/L 

PCB- 1016, TOTAL, UG/L 

PENTACHLOROPHENOL (PCP), TOTAL, UG/L 

PH (STANDARD UNITS) LAB 

PH (STANDARD UNITS), FIELD 

PHENANTHRENE, TOTAL, UG/L 

PHENOL, TOTAL, UG/L 

PHOSPHORUS, DISSOLVED (MG/L AS P) 

PHOSPHORUS, DISSOLVED ORTHOPHOSPHATE (MG/L 

AS P) 

PHOSPHORUS, IN TOTAL ORTHOPHOSPHATE (MG/L AS 

P) 

PHOSPHORUS, TOTAL (MG/L AS P) 

PHOSPHORUS, TOTAL AS PO4 (MG/L) 

PICLORAM, TOTAL, UG/L 

POTASSIUM 40 (K-40), DISSOLVED, PC/L 

POTASSIUM, DISSOLVED (MG/L AS K) 

PURGEABLE ORGANIC CARBON, UG/L 

PYRENE, TOTAL, UG/L 

RADIUM 226 + RADIUM 228, DISSOLVED, PC/L 

RADIUM 226 + RADIUM 228, TOTAL, PC/L 

RADIUM 226, DISSOLVED, PC/L 

RADIUM 226, DISSOLVED, RADON METHOD, PC/L 

RADIUM 226, TOTAL, PC/L 

RADIUM 228, DISSOLVED (PC/L AS RA-228) 

RADIUM 228, TOTAL, PC/L 

RADON 222, DISSOLVED, PC/L 

RESIDUE, TOTAL FILTERABLE (DRIED AT 180C), MG/L 

SELENIUM, DISSOLVED (UG/L AS SE) 

SELENIUM, TOTAL (UG/L) 

SILVER, DISSOLVED (UG/L AS AG) 

SILVER, FIELD FILTERED, ACIDIFIED W/HNO3, UG/L 

SILVER, TOTAL (UG/L AS AG) 

SILVEX, TOTAL, UG/L 

SODIUM CHLORIDE (NACL), MG/L 

SODIUM PLUS POTASSIUM (MG/L) 

SODIUM, DISSOLVED (MG/L AS NA) 

SOLIDS, SUSPENDED, RESIDUE ON EVAP AT 180C, 

MG/L 

SPECIFIC CONDUCTANCE, FIELD (UMHOS/CM AT 25C) 

STRONTIUM, DISSOLVED (UG/L AS SR) 

STRONTIUM, ISOTOPE OF MASS 86 AND 87 RATIO 

SULFATE, TOTAL (MG/L AS SO4) 

SULFIDE, DISSOLVED (MG/L AS S) 

SULFUR-34/32 OF SULFATE, DISSOLVED, PER MIL 
61

TEMPERATURE, WATER (CELSIUS) 

TETRACHLOROETHYLENE, DISSOLVED, UG/L 

THALLIUM, DISSOLVED (UG/L AS TL) 

THALLIUM, TOTAL (UG/L AS TL) 

THORIUM, NATURAL, DISSOLVED PC/L 

TIN, TOTAL (UG/L AS SN) 

TOLUENE, VOLATILE ANALYSIS, TOTAL, UG/L 

TOTAL PETROLEUM HYDROCARNON mg/L 

TOXAPHENE, TOTAL, UG/L 

TRANS-1,2-DICHLOROETHYLENE, TOTAL, UG/L 

TRANS-1,3-DICHLOROPROPENE, TOTAL, UG/L 

TRANS-1,3-DICHLOROPROPYLENE, TOTAL, UG/L 

TRICHLOROETHYLENE, TOTAL, UG/L 

TRICHLOROFLUOROMETHANE, TOTAL, UG/L 

TRITIUM COUNTING ERROR 

TRITIUM IN WATER (TRITIUM UNITS) 

TRITIUM, DISSOLVED (TRITIUM UNITS) 

TRITIUM, TOTAL (TRITIUM UNITS) 

TURBIDITY, LAB, NEPHELOMETRIC TURBIDITY UNITS, 

NTU

URANIUM, NATURAL, DISSOLVED, UG/L 

URANIUM, NATURAL, TOTAL (PC/L AS U) 

VANADIUM, DISSOLVED (UG/L AS V) 

VINYL CHLORIDE, TOTAL, UG/L 

XYLENE, TOTAL, UG/L 

ZINC, DISSOLVED (UG/L AS ZN) 

ZINC, TOTAL (UG/L AS ZN) 
62

Appendix X - Carbon isotope ratios by depth

FIGURE 15 

13C/12C STABLE ISOTOPE RATIOS IN GROUNDWATER BY DEPTH IN THE COUNTIES OF INTEREST.

FIGURE 16 COUNTIES WHERE 13C/12C STABLE ISOTOPE RATIO DATA FOR GROUNDWATER OR CORE SAMPLES 

ARE AVAILABLE.

0

500

1000

1500

2000

2500

3000

3500

-20 -15 -10 -5 0

Depth (ft)

13C/12C stable isotope ratio (‰)
63

TABLE 23 CARBON ISOTOPE RANGES FOR GROUNDWATER IN THE COUNTIES OF INTEREST.

County Number of samples 13C/12C

Brazos 1 -10.1

Burleson 1 -14.5

Colorado 2 -11.3 to -7.9

Dimmit 2 -18.1 to -12.1

Duval 4 -13.2 to -8.2

Karnes 2 -10.6 to -7.2

La Salle 1 -9.6

Lavaca 6 -13.1 to -7.3

Live Oak 4 -10.9 to -8.3

Webb 1 -10.7

Zavala 2 -11.8 to -9.9

Total 26

FIGURE 17 DISTRIBUTION OF CARBON ISOTOPE RANGES FOUND IN GROUNDWATER IN EAGLE FORD SHALE

COUNTIES.
64

FIGURE 18 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR GAS 

HYDROCARBONS WITH IN EAGLE FORD SHALE COUNTIES. TESTING CODES: A = AS NEEDED, R =

RECOMMENDED, T = TESTING NEEDED

County Total wells Total organic carbon 13C/12C

Bastrop 791 13 1.6% T T

Bee 631 1 0.2% T T

Brazos 687 T 1 0.1% T

Burleson 847 T 1 0.1% T

Colorado 574 11 1.9% T 2 0.3% T

Dimmit 527 T 2 0.4% T

Duval 683 1 0.1% T 4 0.6% T

Edwards 604 0.0% T 3 0.5% T

Fayette 863 12 1.4% T T

Goliad 507 1 0.2% T 5 1.0% T

Karnes 410 T 2 0.5% T

La Salle 362 T 1 0.3% T

Lavaca 386 T 6 1.6% T

Live Oak 540 5 0.9% T 5 0.9% T

Webb 339 9 2.7% T 1 0.3% T

Zavala 748 T 2 0.3% T
65

Appendix XI – Salts

Produced water comparison with groundwater

TABLE 24 WOOD COUNTY T-TEST RESULTS FOR PRODUCED WATER COMPARED TO GROUNDWATER; ALL 

CONCENTRATIONS MEASURED IN MG/L, EXCEPT FOR PH. 35

Parameter

Mean of the 

differences p-value t df Lower 95% CI Upper 95% CI

pH -0.3834286 0.1175 -1.6063 34 -0.8685358 0.1016787

Bicarbonate 225.9209 3.86E-12 10.7383 32 183.0664 268.7754

Calcium 1978.569 0 24.5712 33 1814.742 2142.396

Chloride 52614.51 0 46.3473 34 50307.46 54921.56

Magnesium 444.2947 0 24.845 33 407.9122 480.6772

Sodium 31611.01 0 52.758 32 30390.54 32831.47

Sulfate -7.293103 0.1655 -1.4241 28 -17.783536 3.197329

Total Dissolved 

Solids 86411.64 0 43.3803 32 82354.16 90469.12

TABLE 25 WOOD COUNTY SUMMARY STATISTICS. WW = PRODUCED WATER, GW = GROUNDWATER; ALL 

CONCENTRATIONS MEASURED IN MG/L, EXCEPT FOR PH.36

Parameter n Mean Min. 1st Qu. Median 3rd Qu. Max

WW pH 35 7.096 6.4 7.025 7.15 7.25 7.45

GW pH 55 7.472 4.22 6.525 7.91 8.47 8.89

WW 

Bicarbonate 35 414.7 137 366.5 430 459 570

GW Bicarbonate 53 176.63 24.41 125.69 170.85 218.44 452.75

WW Calcium 35 1990 1156 1631 1894 2432 2761

GW Calcium 54 10.14 0.76 1.975 3.425 8.23 61

WW Chloride 35 52651 26562 52497 54285 55156 60965

GW Chloride 55 34.142 1 8.055 14.3 38.4 285

WW 

Magnesium 35 445.5 205 399 415 518 638

GW Magnesium 53 2.066 0.2 0.5 1 1.4 11.8

WW Sodium 34 31665 13590 31356 32110 32802 35963

GW Sodium 53 82.19 2.09 36.8 81 106 344

WW Sulfate 29 17.28 3 12 17 22 48

GW Sulfate 54 23.36 1 10.22 18.3 34.85 95

WW TDS 35 86702 43435 86487 89366 90671 100321

GW TDS 52 266.3 56 204.8 251.5 308.5 887

 

35 For p-values of zero, the p-value returned in the statistical software used, “R,” is “<2.2e-16” which is coded as 

“***” and indicates a p-value of 0.
66

TABLE 26 ALL EAGLE FORD SHALE COUNTIES: T-TEST RESULTS FOR PRODUCED WATER COMPARED TO 

GROUNDWATER. TDS = TOTAL DISSOLVED SOLIDS; ALL CONCENTRATIONS MEASURED IN MG/L, EXCEPT 

FOR PH.

Parameter

Mean of the 

differences p-value t df Lower 95% CI

Upper 95% 

CI

pH -0.220177 0 2827 1524 -0.2693818 -0.1709723

Bicarbonate 536.3342 0 26.6662 1583 496.8834 575.7849

Calcium 2355.311 0 16.8098 1593 2080.48 2630.142

Chloride 26434.53 0 28.7776 1591 24632.77 28236.28

Magnesium 305.6848 0 16.9282 1570 270.2651 341.1045

Sodium 14162.53 0 32.3223 1562 13303.07 15021.98

Sulfate 202.6485 1.62E-09 6.0823 1121 137.2767 268.0203

TDS 44018.91 0 29.5107 1569 41093.12 46944.7

Potassium 147.9325 7.27E-08 5.6843 141 96.48327 199.3818

TABLE 27 ALL COUNTIES IN THE EAGLE FORD SHALE REGION: SUMMARY STATISTICS. WW = PRODUCED 

WATER, GW = GROUNDWATER. ALL CONCENTRATIONS MEASURED IN MG/L, EXCEPT FOR PH.

Parameter n Mean Min 1st Qu. Median 3rd Qu. Max

WW pH 1555 7.254 5 6.65 7.4 7.9 8.9

GW pH 2827 7.459 4.14 7 7.37 7.96 9.86

WW Bicarbonate 2881 898.2 3 344.5 701 1185.5 4731

GW Bicarbonate 2864 333.4 0 214.8 290.4 366.1 2374.8

WW Calcium 1600 2412.3 1 37 189.5 1521.2 40992

GW Calcium 2877 55.96 0.1 6.84 40.7 83 909

WW Chloride 1605 26751 490 3589 11876 32517 245367

GW Chloride 2869 187.6 1 30.7 75.3 194 9579

WW Magnesium 1577 317.8 1 17 51 338 17205

GW Magnesium 2876 12.501 0 1.518 6.55 15.1 327

WW Sodium 1589 14398 148 2725 7357 18061 123775

GW Sodium 2835 213.3 1.1 51.6 119 248 7230

WW Sulfate 1128 336.3 1 13 43 210 14100

GW Sulfate 2868 117.4 0 18.68 46.5 109 2830

WW TDS 1603 45032 1033 7568 21275 54148 398024

GW TDS 1543 795.6 36 333 538 909 17606

WW Potassium 143 155.6 2 10 15 138.5 1635

GW Potassium 2832 6.37 0 2.498 4.64 7.482 134
67

Salts data availability

TABLE 28 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN ATASCOSA, AUSTIN, BASTROP, BEE, AND BRAZOS COUNTIES. TESTING 

CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Atascosa Austin Bastrop Bee Brazos 

Total wells 1117 321 791 631 687

TDS 180 16% A 67 21% A 84 11% A 43 7% A 72 10% A

Chloride 182 16% A 68 21% A 84 11% A 43 7% A 72 10% A

Sodium 180 16% A 68 21% A 84 11% A 44 7% A 73 11% A

Sulfate 182 16% A 68 21% A 84 11% A 43 7% A 72 10% A

Calcium 182 16% A 68 21% A 84 11% A 44 7% A 73 11% A

Magnesium 182 16% A 68 21% A 84 11% A 44 7% A 73 11% A

Bicarbonate 182 16% A 67 21% A 84 11% A 44 7% A 73 11% A

Potassium 176 16% A 68 21% A 84 11% A 42 7% A 73 11% A

TABLE 29 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN BURLESON, CALDWELL, COLORADO, DEWITT, AND DIMMIT COUNTIES.

TESTING CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Burleson Caldwell Colorado De Witt Dimmit 

Total wells 847 599 574 513 527

TDS 85 10% A 25 4% T 104 18% A 40 8% A 97 18% A

Chloride 85 10% A 25 4% T 105 18% A 40 8% A 99 19% A

Sodium 85 10% A 25 4% T 104 18% A 40 8% A 98 19% A

Sulfate 85 10% A 25 4% T 105 18% A 40 8% A 99 19% A

Calcium 85 10% A 25 4% T 105 18% A 40 8% A 100 19% A

Magnesium 85 10% A 25 4% T 105 18% A 40 8% A 100 19% A

Bicarbonate 85 10% A 25 4% T 105 18% A 40 8% A 99 19% A

Potassium 85 10% A 24 4% T 104 18% A 40 8% A 98 19% A
68

TABLE 30 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN DUVAL, EDWARDS, FAYETTE, FRIO, AND GOLIAD COUNTIES. TESTING 

CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Duval Edwards Fayette Frio Goliad 

Total wells 683 604 863 756 507

TDS 129 19% A 75 12% A 60 7% A 140 19% A 42 8% A

Chloride 129 19% A 79 105% A 60 7% A 143 19% A 42 8% A

Sodium 129 19% A 75 95% A 60 7% A 140 19% A 42 8% A

Sulfate 129 19% A 79 105% A 60 7% A 143 19% A 42 8% A

Calcium 129 19% A 79 100% A 60 7% A 143 19% A 42 8% A

Magnesium 129 19% A 79 100% A 60 7% A 143 19% A 42 8% A

Bicarbonate 129 19% A 76 96% A 60 7% A 142 19% A 42 8% A

Potassium 129 19% A 79 104% A 60 7% A 140 19% A 42 8% A

TABLE 31 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN GONZALES, GRIMES, GUADALUPE, HOUSTON, KARNES COUNTIES. TESTING 

CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Gonzales Grimes Guadalupe Houston Karnes 

Total wells 582 357 638 400 410

TDS 277 48% A 40 11% A 18 3% T 81 20% A 43 10% A

Chloride 277 48% A 41 11% A 18 3% T 82 21% A 43 10% A

Sodium 277 48% A 40 11% A 18 3% T 81 20% A 43 10% A

Sulfate 277 48% A 41 11% A 18 3% T 82 21% A 43 10% A

Calcium 277 48% A 41 11% A 18 3% T 82 21% A 43 10% A

Magnesium 277 48% A 41 11% A 18 3% T 82 21% A 43 10% A

Bicarbonate 277 48% A 41 11% A 18 3% T 82 21% A 43 10% A

Potassium 277 48% A 41 11% A 17 3% T 82 21% A 41 10% A

TABLE 32 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN LA SALLE, LAVACA, LEE, LEON, AND LIVE OAK COUNTIES. TESTING CODES:

A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County La Salle Lavaca Lee Leon Live Oak 

Total wells 362 386 399 323 540

TDS 84 23% A 96 25% A 65 16% A 73 23% A 65 12% A

Chloride 85 23% A 97 25% A 73 18% A 73 23% A 65 12% A

Sodium 84 23% A 96 25% A 66 17% A 75 23% A 65 12% A

Sulfate 85 23% A 97 25% A 73 18% A 73 23% A 65 12% A

Calcium 85 23% A 97 25% A 74 19% A 75 23% A 65 12% A

Magnesium 85 23% A 97 25% A 74 19% A 75 23% A 65 12% A
69

Bicarbonate 85 23% A 97 25% A 71 18% A 75 23% A 65 12% A

Potassium 83 23% A 97 25% A 66 17% A 74 23% A 65 12% A

TABLE 33 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN MADISON, MAVERICK, MCMULLEN, MILAM, AND ROBERTSON COUNTIES.

TESTING CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Madison Maverick McMullen Milam Robertson 

Total wells 365 92 184 314 824

TDS 36 10% R 64 70% A 47 26% A 70 22% A 95 12% A

Chloride 37 10% R 66 72% A 47 26% A 78 25% A 96 12% A

Sodium 37 10% R 64 70% A 47 26% A 71 23% A 96 12% A

Sulfate 37 10% R 65 71% A 48 26% A 78 25% A 96 12% A

Calcium 37 10% R 66 72% A 48 26% A 79 25% A 97 12% A

Magnesium 37 10% R 66 72% A 48 26% A 79 25% A 97 12% A

Bicarbonate 37 10% R 66 72% A 48 26% A 78 25% A 97 12% A

Potassium 37 10% R 65 71% A 48 26% A 71 23% A 96 12% A

TABLE 34 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS 

FOUND IN PRODUCED WATER IN WASHINGTON, WEBB, WILSON, WOOD, AND ZAVALA COUNTIES. TESTING 

CODES: A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

County Washington Webb Wilson Wood Zavala 

Total wells 478 339 512 442 748

TDS 37 8% R 126 37% A 95 19% A 52 12% A 114 15% A

Chloride 38 8% A 127 37% A 100 20% A 55 12% A 115 15% A

Sodium 37 8% R 127 37% A 97 19% A 53 12% A 114 15% A

Sulfate 38 8% A 127 37% A 100 20% A 54 12% A 115 15% A

Calcium 38 8% A 127 37% A 100 20% A 54 12% A 115 15% A

Magnesium 38 8% A 127 37% A 100 20% A 53 12% A 115 15% A

Bicarbonate 38 8% A 126 37% A 98 19% A 54 12% A 115 15% A

Potassium 38 8% A 127 37% A 96 19% A 53 12% A 114 15% A
70

FIGURE 19 TOTAL DISSOLVED SOLIDS (TDS) CONCENTRATIONS IN THE EAGLE FORD SHALE REGION.
71

Regulated salts data availability

TABLE 35 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR SALTS

WITH PRIMARY DRINKING WATER STANDARDS IN EAGLE FORD SHALE COUNTIES. TESTING CODES: A = AS 

NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Total wells Bromide Nitrate Selenium 

Atascosa 1117 168 15% A 41 4% A 174 16% A

Austin 321 67 21% A 13 4% T 67 21% A

Bastrop 791 62 8% A 15 2% T 78 10% A

Bee 631 40 6% A 13 2% T 42 7% A

Brazos 687 62 9% A 15 2% T 72 10% A

Burleson 847 70 8% A 22 3% T 76 9% A

Caldwell 599 13 2% T 4 1% T 17 3% T

Colorado 574 87 15% A 22 4% T 95 17% A

De Witt 513 38 7% A 10 2% T 39 8% A

Dimmit 527 103 20% A 35 7% R 104 20% A

Duval 683 139 20% A 17 2% T 141 21% A

Edwards 604 66 11% A 15 2% T 66 11% A

Fayette 863 45 5% A 18 2% T 56 6% A

Frio 756 130 17% A 39 5% A 137 18% A

Goliad 507 41 8% A 15 3% T 42 8% A

Gonzales 582 197 34% A 14 2% T 207 36% A

Grimes 357 39 11% A 17 5% T 40 11% A

Guadalupe 638 14 2% T 3 0% T 17 3% T

Houston 400 66 17% A 17 4% T 79 20% A

Karnes 410 37 9% R 11 3% T 40 10% A

La Salle 362 72 20% A 19 5% T 88 24% A

Lavaca 386 95 25% A 19 5% T 95 25% A

Lee 399 50 13% A 15 4% T 55 14% A

Leon 323 56 17% A 20 6% T 72 22% A

Live Oak 540 72 13% A 26 5% R 75 14% A

Madison 365 28 8% R 9 2% T 34 9% R

Maverick 92 8 9% T 56 61% A 36 39% R

McMullen 184 35 19% R 13 7% T 43 23% A

Milam 314 52 17% A 17 5% T 53 17% A

Robertson 824 75 9% A 14 2% T 83 10% A

Washington 478 42 9% A 19 4% T 42 9% A

Webb 339 114 34% A 37 11% R 119 35% A

Wilson 512 92 18% A 23 4% T 93 18% A

Wood 442 33 7% R 20 5% T 52 12% A

Zavala 748 104 14% A 22 3% T 114 15% A
72

Appendix XII – Metals: number of samples

TABLE 36 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS 

IN ATASCOSA, AUSTIN, BASTROP, BEE, AND BRAZOS COUNTIES. A = AS NEEDED, R = RECOMMENDED, T =

TESTING NEEDED

Atascosa Austin Bastrop Bee Brazos 

Total wells 1117 321 791 631 687

Aluminum 167 15% A 54 17% A 60 8% A 30 5% R 66 10% A

Antimony 139 12% A 53 17% A 56 7% A 28 4% R 60 9% A

Arsenic 179 16% A 67 21% A 78 10% A 42 7% A 74 11% A

Barium 179 16% A 67 21% A 80 10% A 42 7% A 75 11% A

Beryllium 139 12% A 54 17% A 56 7% A 29 5% R 60 9% A

Boron 229 21% A 75 23% A 106 13% A 34 5% R 118 17% A

Cadmium 174 16% A 54 17% A 76 10% A 30 5% R 69 10% A

Chromium 174 16% A 54 17% A 67 8% A 30 5% R 69 10% A

Cobalt 140 13% A 54 17% A 56 7% A 29 5% R 60 9% A

Copper 179 16% A 67 21% A 78 10% A 43 7% A 74 11% A

Iron 204 18% A 71 22% A 146 18% A 44 7% A 75 11% A

Lead 179 16% A 67 21% A 69 9% A 42 7% A 74 11% A

Manganese 180 16% A 67 21% A 81 10% A 44 7% A 74 11% A

Mercury 60 5% A 26 8% R 33 4% R 17 3% T 21 3% T

Molybdenum 167 15% A 54 17% A 58 7% A 35 6% R 66 10% A

Nickel 86 8% A 27 8% R 28 4% R 21 3% T 41 6% A

Phosphorus 60 5% A 13 4% T 29 4% R 1 0% T 14 2% T

Silver 60 5% A 26 8% R 24 3% T 17 3% T 20 3% T

Strontium 139 12% A 54 17% A 56 7% A 29 5% R 60 9% A

TABLE 37 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Burleson Caldwell Colorado De Witt Dimmit 

Total wells 847 599 574 513 527

Aluminum 76 9% A 13 2% T 78 14% A 28 5% R 104 20% A

Antimony 69 91% A 13 2% T 78 14% A 28 5% R 67 13% A

Arsenic 81 117% A 17 3% T 94 16% A 39 5% A 104 20% A

Barium 81 100% A 17 3% T 98 17% A 39 5% A 104 20% A

Beryllium 69 85% A 13 2% T 78 14% A 28 5% R 69 13% A

Boron 137 199% A 36 6% R 135 24% A 49 5% A 125 24% A

Cadmium 73 53% A 17 3% T 74 13% A 29 5% R 97 18% A

Chromium 73 100% A 17 3% T 68 12% A 29 5% R 97 18% A

Cobalt 69 95% A 13 2% T 78 14% A 28 5% R 69 13% A

Copper 81 117% A 17 3% T 94 16% A 39 5% A 104 20% A
73

Iron 87 107% A 45 8% A 104 18% A 61 5% A 105 20% A

Lead 81 93% A 17 3% T 88 15% A 39 5% A 104 20% A

Manganese 81 100% A 17 3% T 91 16% A 39 5% A 104 20% A

Mercury 20 25% T 7 1% T 35 6% R 16 5% T 51 10% A

Molybdenum 76 380% A 13 2% T 78 14% A 28 5% R 104 20% A

Nickel 55 72% A 8 1% T 39 7% A 18 5% T 40 8% A

Phosphorus 23 42% T 14 2% T 6 1% T 5% T 26 5% R

Silver 20 26% T 7 1% T 29 5% R 16 5% T 51 10% A

Strontium 64 320% A 13 2% T 78 14% A 28 5% R 69 13% A

TABLE 38 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Duval Edwards Fayette Frio Goliad 

Total wells 683 604 863 756 507

Aluminum 128 19% A 64 11% A 42 5% A 130 17% A 30 6% R

Antimony 124 18% A 63 10% A 40 5% A 106 14% A 28 6% R

Arsenic 141 21% A 75 12% A 56 6% A 141 19% A 42 8% A

Barium 141 21% A 75 12% A 61 7% A 144 19% A 42 8% A

Beryllium 124 18% A 63 10% A 40 5% A 106 14% A 28 6% R

Boron 147 22% A 69 11% A 62 7% A 181 24% A 31 6% R

Cadmium 115 17% A 66 11% A 50 6% A 137 18% A 32 6% R

Chromium 115 17% A 66 11% A 42 5% A 137 18% A 32 6% R

Cobalt 124 18% A 63 10% A 40 5% A 106 14% A 28 6% R

Copper 141 21% A 75 12% A 57 7% A 141 19% A 42 8% A

Iron 151 22% A 79 13% A 56 6% A 172 23% A 42 8% A

Lead 141 21% A 75 12% A 48 6% A 141 19% A 42 8% A

Manganese 141 21% A 75 12% A 48 6% A 141 19% A 42 8% A

Mercury 51 7% A 12 2% T 21 2% T 52 7% A 18 4% T

Molybdenum 135 20% A 63 10% A 42 5% A 130 17% A 42 8% A

Nickel 50 7% A 38 6% A 27 3% R 63 8% A 17 3% T

Phosphorus 10 1% T 29 5% R 6 1% T 54 7% A 4 1% T

Silver 51 7% A 12 2% T 13 2% T 52 7% A 18 4% T

Strontium 124 18% A 63 10% A 40 5% A 106 14% A 28 6% R

TABLE 39 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Gonzales Grimes Guadalupe Houston Karnes 

Total wells 582 357 638 400 410

Aluminum 193 33% A 24 7% T 14 2% T 66 17% A 31 8% R

Antimony 192 33% A 24 7% T 14 2% T 63 16% A 27 7% R
74

Arsenic 209 36% A 40 11% A 17 3% T 82 21% A 40 10% A

Barium 209 36% A 41 11% A 17 3% T 82 21% A 40 10% A

Beryllium 192 33% A 24 7% T 14 2% T 64 16% A 27 7% R

Boron 293 50% A 48 13% A 30 5% R 68 17% A 81 20% A

Cadmium 206 35% A 33 9% R 17 3% T 79 20% A 32 8% R

Chromium 206 35% A 33 9% R 17 3% T 79 20% A 32 8% R

Cobalt 192 33% A 24 7% T 14 2% T 64 16% A 27 7% R

Copper 209 36% A 40 11% A 17 3% T 82 21% A 40 10% A

Iron 252 43% A 42 12% A 57 9% A 83 21% A 41 10% A

Lead 209 36% A 40 11% A 17 3% T 82 21% A 40 10% A

Manganese 217 37% A 40 11% A 17 3% T 82 21% A 40 10% A

Mercury 29 5% R 18 5% T 5 1% T 26 7% R 16 4% T

Molybdenum 193 33% A 24 7% T 14 2% T 66 17% A 37 9% R

Nickel 152 26% A 19 5% T 10 2% T 43 11% A 18 4% T

Phosphorus 116 20% A 2 1% T 5 1% T 32 8% R 5 1% T

Silver 27 5% R 18 5% T 5 1% T 20 5% T 16 4% T

Strontium 192 33% A 24 7% T 14 2% T 64 16% A 27 7% R

TABLE 40 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

La Salle Lavaca Lee Leon Live Oak 

Total wells 362 386 399 323 540

Aluminum 75 21% A 78 20% A 55 14% A 58 18% A 54 10% A

Antimony 70 19% A 78 20% A 49 12% A 54 17% A 49 9% A

Arsenic 89 25% A 95 25% A 57 14% A 73 23% A 75 14% A

Barium 89 25% A 95 25% A 57 14% A 73 23% A 75 14% A

Beryllium 70 19% A 78 20% A 49 12% A 54 17% A 49 9% A

Boron 116 32% A 91 24% A 129 32% A 56 17% A 72 13% A

Cadmium 88 24% A 76 20% A 55 14% A 72 22% A 62 11% A

Chromium 88 24% A 76 20% A 55 14% A 72 22% A 62 11% A

Cobalt 69 19% A 78 20% A 49 12% A 54 17% A 49 9% A

Copper 89 25% A 95 25% A 57 14% A 73 23% A 75 14% A

Iron 117 32% A 98 25% A 100 25% A 97 30% A 80 15% A

Lead 90 25% A 95 25% A 57 14% A 73 23% A 75 14% A

Manganese 89 25% A 95 25% A 68 17% A 74 23% A 76 14% A

Mercury 28 8% R 37 10% R 13 3% T 26 8% R 36 7% R

Molybdenum 74 20% A 78 20% A 55 14% A 56 17% A 54 10% A

Nickel 54 15% A 38 10% A 34 9% R 40 12% A 24 4% T

Phosphorus 33 9% R 1 0% T 22 6% T 29 9% R 0% T

Silver 28 8% R 37 10% R 13 3% T 25 8% T 36 7% R

Strontium 69 19% A 78 20% A 49 12% A 54 17% A 49 9% A
75

TABLE 41 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Madison Maverick McMullen Milam Robertson 

Total wells 365 92 184 314 824

Aluminum 28 8% R 34 37% R 32 17% R 52 17% A 83 10% A

Antimony 28 8% R 7 8% T 32 17% R 52 17% A 72 9% A

Arsenic 37 10% R 35 38% R 45 24% A 53 17% A 85 10% A

Barium 37 10% R 36 39% R 45 24% A 53 17% A 85 10% A

Beryllium 28 8% R 7 8% T 32 17% R 52 17% A 72 9% A

Boron 33 9% R 17 18% T 48 26% A 80 25% A 182 22% A

Cadmium 34 9% R 35 38% R 41 22% A 53 17% A 78 9% A

Chromium 34 9% R 35 38% R 41 22% A 52 17% A 78 9% A

Cobalt 28 8% R 7 8% T 32 17% R 52 17% A 72 9% A

Copper 37 10% R 36 39% R 45 24% A 53 17% A 85 10% A

Iron 41 11% A 40 43% A 62 34% A 88 28% A 160 19% A

Lead 37 10% R 36 39% R 45 24% A 53 17% A 85 10% A

Manganese 37 10% R 36 39% R 45 24% A 65 21% A 96 12% A

Mercury 10 3% T 29 32% R 14 8% T 14 4% T 30 4% R

Molybdenum 28 8% R 34 37% R 32 17% R 52 17% A 83 10% A

Nickel 20 5% T 7 8% T 25 14% T 30 10% R 36 4% R

Phosphorus 13 4% T 5 5% T 12 7% T 25 8% T 14 2% T

Silver 5 1% T 29 32% R 14 8% T 14 4% T 30 4% R

Strontium 28 8% R 7 8% T 32 17% R 50 16% A 72 9% A

TABLE 42 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR METALS

IN COUNTIES. A = AS NEEDED, R = RECOMMENDED, T = TESTING NEEDED

Washington Webb Wilson Wood Zavala 

Total wells 478 339 512 442 748

Aluminum 24 5% T 106 31% A 90 18% A 51 12% A 104 14% A

Antimony 24 5% T 77 23% A 73 14% A 33 7% R 92 12% A

Arsenic 42 9% A 120 35% A 96 19% A 54 12% A 114 15% A

Barium 42 9% A 117 35% A 96 19% A 55 12% A 114 15% A

Beryllium 24 5% T 77 23% A 73 14% A 35 8% R 92 12% A

Boron 41 9% A 105 31% A 131 26% A 52 12% A 153 20% A

Cadmium 33 7% R 102 30% A 93 18% A 49 11% A 110 15% A

Chromium 33 7% R 103 30% A 93 18% A 51 12% A 110 15% A

Cobalt 24 5% T 77 23% A 73 14% A 33 7% R 92 12% A

Copper 42 9% A 118 35% A 96 19% A 54 12% A 114 15% A

Iron 62 13% A 117 35% A 96 19% A 134 30% A 121 16% A
76

Lead 42 9% A 117 35% A 96 19% A 52 12% A 114 15% A

Manganese 44 9% A 114 34% A 96 19% A 56 13% A 114 15% A

Mercury 21 4% T 47 14% A 37 7% R 25 6% T 49 7% A

Molybdenum 24 5% T 107 32% A 90 18% A 49 11% A 104 14% A

Nickel 18 4% T 63 19% A 38 7% A 21 5% T 39 5% A

Phosphorus 0% T 27 8% R 33 6% R 18 4% T 48 6% A

Silver 21 4% T 47 14% A 37 7% R 28 6% R 49 7% A

Strontium 24 5% T 77 23% A 73 14% A 33 7% R 92 12% A
77

Appendix XIII – Radionuclides: number of samples

TABLE 43 NUMBER OF RADIONUCLIDE SAMPLES AVAILABLE IN THE TWDB GROUNDWATER DATABASE FOR 

EACH COUNTY.

County Total Wells

Radium-226 and 

Radium-228

Radon 

222 Uranium Thorium Potassium-40

Atascosa 1117 20 25

Austin 321 13 13

Bastrop 791 13

Bee 631 5 1 5 1 1

Brazos 687 6 8

Burleson 847 3 8

Caldwell 599 3

Colorado 574 20 22

De Witt 513 6 5

Dimmit 527 9 16

Duval 683 38 4 39 4 4

Edwards 604 9 14

Fayette 863 4 6

Frio 756 17 21

Goliad 507 6 1 8 2 2

Gonzales 582 2 12

Grimes 357 2 2

Guadalupe 638 2

Houston 400 3 11

Karnes 410 9 4 6 3 3

La Salle 362 2 9

Lavaca 386 20 20

Lee 399 7

Leon 323 1 8

Live Oak 540 14 3 13 3 3

Madison 365 1 4

Maverick 92 11

McMullen 184 3

Milam 314 1 11

Robertson 824 18 26

Washington 478 3 3

Webb 339 21 13 25 16 16

Wilson 512 3 1 17

Wood 442 9

Zavala 748 14 27

Grand Total 18715 281 27 421
78

TABLE 44 NUMBER, PERCENT OF TOTAL WATER WELLS, AND GROUNDWATER TESTING NEEDS FOR 

RADIONUCLIDE PARTICLE EMISSIONS IN EAGLE FORD SHALE COUNTIES. A = AS NEEDED, R =

RECOMMENDED, T = TESTING NEEDED

County Total Wells Alpha Beta

Atascosa 1117 41 4% A 40 4% A

Austin 321 26 8% R 13 4% T

Bastrop 791 21 3% T 13 2% T

Bee 631 16 3% T 12 2% T

Brazos 687 21 3% T 15 2% T

Burleson 847 20 2% T 17 2% T

Caldwell 599 4 1% T 4 1% T

Colorado 574 38 7% A 9 2% T

De Witt 513 17 3% T 12 2% T

Dimmit 527 36 7% R 33 6% R

Duval 683 66 10% A 18 3% T

Edwards 604 26 4% R 12 2% T

Fayette 863 20 2% T 18 2% T

Frio 756 34 4% R 34 4% R

Goliad 507 17 3% T 12 2% T

Gonzales 582 18 3% T 15 3% T

Grimes 357 18 5% T 16 4% T

Guadalupe 638 3 0% T 3 0% T

Houston 400 21 5% T 18 5% T

Karnes 410 18 4% T 13 3% T

La Salle 362 19 5% T 19 5% T

Lavaca 386 38 10% A 18 5% T

Lee 399 8 2% T 8 2% T

Leon 323 20 6% T 19 6% T

Live Oak 540 38 7% A 23 4% T

Madison 365 10 3% T 9 2% T

Maverick 92 28 30% R 28 30% R

McMullen 184 13 7% T 13 7% T

Milam 314 3 1% T 3 1% T

Robertson 824 40 5% A 22 3% T

Washington 478 21 4% T 18 4% T

Webb 339 25 7% T 22 6% T

Wilson 512 24 5% T 26 5% R

Wood 442 18 4% T 19 4% T

Zavala 748 28 4% R 22 3% T

Grand Total 18715 814 4% A 596 3% A
79

Appendix XIV – PAH samples

TABLE 45 NUMBER AND PERCENT OF SAMPLES OUT OF THE TOTAL NUMBER OF WATER WELLS IN EACH 

COUNTY FOR PAH CONCENTRATIONS IN GROUNDWATER. Atascosa Bastrop Colorado 

Dimmit Fayette Karnes La Salle Wilson Zavala 

Total wells 1117 791 574 527 863 410 362 512 748

Percent 0.6% 0.4% 0.2% 4.7% 0.3% 0.2% 1.4% 0.2% 0.4%

Acenaphthene 7 3 1 25 3 1 5 1 3

Acenaphthylene 7 3 1 25 3 1 5 1 3

Anthracene 7 3 1 25 3 1 5 1 3

Benzo(a) anthracene 7 3 1 25 3 1 5 1 3

Benzo-(a)-pyrene 7 3 1 25 3 1 5 1 3

Benzo(b)fluoranthene 7 3 1 25 3 1 5 1 3

Benzo(ghi)perylene 7 3 1 25 3 1 5 1 3

Benzo(k)fluoranthene 7 3 1 25 3 1 5 1 3

Chrysene 7 3 1 25 3 1 5 1 3

Dibenzo (a,h) anthracene 7 25 1 5 1 3

Fluoranthene 7 3 1 25 3 1 5 1 3

Fluorene 7 3 1 25 3 1 5 1 3

Indeno (1,2,3-cd) pyrene 7 3 1 25 3 1 5 1 3

Naphthalene 7 3 1 25 3 1 5 1 3

Phenanthrene 7 3 1 25 3 1 5 1 3

Pyrene 7 3 1 25 3 1 5 1 3