U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5157

Prepared in Cooperation with the Indiana Department of Natural Resources, 

Division of Reclamation

Occurrence and Attempted Mitigation of Carbon Dioxide 

in a Home Constructed on Reclaimed Coal-Mine Spoil, 

Pike County, Indiana

CO2

CO2 CO2

CO2

CO2

CO2 CO2

CO2 CO2

CO2

CO2

CO2 CO2 CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2 CO2

CO2

CO2 CO2

Bedrock surface

Land

surface Carbon dioxide

accumulates

in basement

CO2 CO2

CO2

RAPID DROP IN

BAROMETRIC 

Reclaimed

coal-mine

spoil

Compression of soil gases

Infiltration and percolation

of rainwater

Wetting front

Compressed soil gases

are displaced to areas

of low pressure

(Lisse Effect)

WIND and COLD

cause a low-pressure

"stack effect"

within structures

Wind

RAIN

PRESSURE
Cover image (also figure 7 in report): Conceptual model depicting four of the environmental conditions that may 

lead to carbon dioxide intrusion to structures: rapid drops in barometric pressure, rain, wind, and cold.
Occurrence and Attempted Mitigation of 

Carbon Dioxide in a Home Constructed on 

Reclaimed Coal-Mine Spoil, Pike County, 

Indiana

By Bret A. Robinson

Prepared in Cooperation with the Indiana Department of Natural Resources, 

Division of Reclamation

Scientific Investigations Report 2010–5157

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Robinson, B.A., 2010, Occurrence and attempted mitigation of carbon dioxide in a home constructed on reclaimed 

coal-mine spoil, Pike County, Indiana: U.S. Geological Survey Scientific Investigations Report 2010–5157, 21 p.
iii

Contents

Acknowledgments........................................................................................................................................vi

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

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

Carbon Dioxide and Associated Health Concerns..........................................................................1

Physical Setting and History of the Study Area...............................................................................2

Study Design..........................................................................................................................................3

Purpose and Scope ..............................................................................................................................4

Methods of Study...........................................................................................................................................4

Geological Data Collection..................................................................................................................4

Soil-Gas Flux.................................................................................................................................4

Well Construction and Groundwater-Level Measurements.................................................6

Spoil Sampling and Chemical Analysis....................................................................................7

Meteorological Data Collection .........................................................................................................7

Meteorological Observation Platform......................................................................................7

Precipitation pH ...........................................................................................................................7

Within-Residence Data Collection.....................................................................................................7

Gas Concentrations.....................................................................................................................8

Masonry Sampling and Chemical Analysis.............................................................................8

Occurrence of Within-Residence Carbon Dioxide...................................................................................9

Meteorological Controls....................................................................................................................10

Conceptual Model...............................................................................................................................11

Attempted Mitigation...................................................................................................................................12

Block-Wall Depressurization ............................................................................................................13

Block-Wall and Sub-Slab Depressurization...................................................................................14

Block-Wall and Sub-Slab Pressurization........................................................................................15

Limitations Regarding Extrapolation of Mitigation Results..........................................................16

Summary and Conclusions.........................................................................................................................16

References Cited..........................................................................................................................................17

Appendix 1. Detailed Descriptive and Analytical Data From the Study Site......................................18

Figures

1. Photograph showing carbon dioxide cloud that has settled to floor level .........................2

2. Photograph showing the study-site home in east-central Pike County, Ind.,

where the intrusion of carbon dioxide as a soil gas has posed a health risk........................3

3. Diagram showing location of outdoor data-collection sites relative to the 

study-site house............................................................................................................................5

4. Photograph showing drilling activities for installation of a temporary network 

of monitoring wells at the study site..........................................................................................6
iv

5. Photograph showing meteorological observation platform designed for 

hourly transmission of local meteorological data measured at the 

study site. .......................................................................................................................................8

6. Graphs showing CO2-concentration, barometric-pressure, and precipitation 

data collected from March 25 to April 25, 2009, at the study site.......................................10

 7. Conceptual model depicting four of the environmental conditions that may 

lead to carbon dioxide intrusion to structures: rapid drops in barometric 

pressure, rain, wind, and cold ..................................................................................................11

8. Photograph of syringe in a beaker of water, used to illustrate the effect of 

how a drop in barometric pressure draws soil gases into a structure .............................11

9. Diagram showing new-construction steps that can be taken to reduce 

soil-gas intrusion or facilitate mitigation ................................................................................12

10. Photograph showing mitigation system established to depressurize the 

block walls of the study-site home ..........................................................................................13

11–13. Graphs showing CO2-concentration, barometric-pressure, and precipitation 

data collected at the study site—

11. April 15 to May 10, 2009; block-wall depressurization was established 

on April 28, 2009 .................................................................................................................14

12. September 1 to October 13, 2009; a combination of block-wall and 

sub-slab depressurization was established on May 13, 2009 ....................................15

13. October 5 to November 5, 2009; a combination of block-wall and 

sub-slab pressurization was established on October 21, 2009 ..................................16

Tables

1. Selected characteristics of monitoring wells at the study site, Pike County, 

 Indiana............................................................................................................................................7

2. Determinations of pH for rainwater samples collected at the study site .........................10

1–1. Locational data for spoil samples collected at the carbon dioxide-

 accumulation study site.............................................................................................................18

1–2. Results of major-ion analysis of spoil samples collected at the study site ......................18

1–3. Description of materials and locations where masonry samples were 

collected from the study-site home.........................................................................................19

1–4. Results of major-ion analysis of masonry samples collected from the 

study-site home...........................................................................................................................19

1–5. Daily maximum carbon dioxide percentages and daily minimum oxygen 

percentages within the study-site home, February 18–December 30, 2009 .....................20
v

Conversion Factors, Datums, and Abbreviated Units 

of Measure

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square meter (m2) 10.764 square foot (ft2)

acre 0.4047 hectare (ha)

Volume

cubic feet (ft3) 0.0283 cubic meter (m3)

Pressure

millibar (mbar) 0.1 kilopascal (kPa) 

millibar (mbar) 0.7501 millimeters of mercury 

(mmHg)

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

follows:

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to the World Geodedic System of 

1984 (WGS 84).

Elevation, as used in this report, refers to distance above or below a local arbitrary 

vertical datum established for the study site.

Gas concentrations are given in parts per million (ppm).

Carbon dioxide fluxes are given in micromoles per square meter per second 

(μmol/m2/s).
vi

Other Abbreviations 

Additional abbreviations and acronyms used in this report:

CO2 carbon dioxide

OSHA Occupational Safety and Health Administration

ASHRAE American Society of Heating, Refrigerating, and Air-conditioning Engineers

O2 oxygen

IDNR Indiana Department of Natural Resources

USGS U.S. Geological Survey

PVC polyvinylchloride

NADP National Atmospheric Deposition Program

Acknowledgments

The author gratefully acknowledges those individuals who contributed to this investigation 

and regrets that many will remain unnamed. Steve Herbert, Jim Metzger, Randy Hoffman, 

Laura Montgrain, and Marvin Ellis (all of Indiana Department of Natural Resources, Division 

of Reclamation) provided valuable assistance with data acquisition and are acknowledged for 

their constructive study input. Bill Ehler (U.S. Office of Surface Mining) and the professionals at 

Radon Environmental Inc. provided vital technical support regarding mitigation methods. Finally, 

I would like to express my sincere appreciation to the Hudson family, who graciously permitted 

numerous intrusions of personnel and equipment to their home. By allowing us to collect the 

required data, they have potentially spared countless other families from having to endure the 

inconvenience and health risks that are associated with the CO2-intrusion phenomenon.
Occurrence and Attempted Mitigation of Carbon Dioxide 

in a Home Constructed on Reclaimed Coal-Mine Spoil, 

Pike County, Indiana

By Bret A. Robinson

Abstract

In recent years carbon dioxide (CO2) intrusion has 

become recognized as a potentially serious health threat where 

homes are constructed on or near reclaimed surface coal 

mines. When CO2 invades the living space of a home, it can 

collect near the floor, displace the oxygen there, and produce 

an oxygen-deficient environment. In this investigation, several 

lines of inquiry were pursued to determine the environmental 

factors that most influence CO2-intrusion at a Pike County, 

Ind., home where this phenomenon is known to occur. It was 

found that CO2-intrusion events at the home are most closely 

tied to rapid drops in barometric pressure and rainfall. Other 

researchers have shown that windy conditions and periods 

of cold weather also can contribute to soil-gas intrusion to 

structures. From this, a conceptual model was developed to 

illustrate the influence of these four meteorological conditions. 

Additionally, three mitigation methods—block-wall depressurization, block-wall and sub-slab depressurization, and blockwall and sub-slab pressurization—were applied successively 

to the study-site home, and environmental data were collected 

to evaluate the effectiveness of each mitigation method. In 

each case, it was found that these methods did not ensure a 

safe environment when meteorological conditions were favorable for CO2

 intrusion.

Introduction 

Recently it has been recognized that potentially hazardous concentrations of carbon dioxide (CO2

) may be found in 

structures constructed on or near surface coal mines. Several 

documented cases in West Virginia (Rao and Kreiss, 2004), 

Pennsylvania (Ehler, 2002; Laughrey and others, 2002), and 

Great Britain (Hendrick and Sizer, 1992) show that CO2 may 

be associated with either underground or surface mines and 

illustrate how hazardous these occurrences can be. In addition, 

these case studies provide some guidelines for study and 

mitigation in cases where elevated CO2 concentrations are 

suspected or realized.

Carbon dioxide, as a soil gas, may be generated by the 

dissolution of carbonate material within bedrock, glacial till, 

or coal-mine spoil; by the microbial breakdown of buried 

organic material; or as the direct product of biological respiration. In most documented cases of CO2

 derived from reclaimed 

coal-mine spoil, the underlying factors that allow for the 

potentially hazardous generation and accumulation of CO2 are 

thought to include (1) a carbonate or organic source of carbon 

in the spoil or capping materials, (2) water chemistry appropriate to generate CO2

 from the carbon-rich source material, (3) 

a pressure gradient that promotes CO2 migration toward the 

structure, (4) pathways that allow CO2 to migrate from the 

source area and into a structure, and (5) a structure feature—

basement, crawl space, sump, or other enclosed space—that 

accommodates CO2 accumulation. 

Carbon Dioxide and Associated Health 

Concerns

At standard atmospheric temperature and pressure, CO2 is 

a nonflammable, odorless, and colorless gas. It has a specific 

gravity of 1.524, which means that in a closed container or 

space, without air currents to ensure mixing, CO2 will settle 

toward the floor (fig. 1). Although it is only slightly toxic, the 

primary health hazard associated with high concentrations 

of CO2 stems from the fact that it is an asphyxiant (that is, it 

displaces oxygen). 

Typically the concentration of CO2 in the atmosphere 

ranges from 350 to 600 parts per million (ppm), with an average of approximately 380 ppm. The Occupational Safety and 

Health Administration (OSHA; 2009) established the workplace limit for CO2

 in air at 5,000 ppm. Even lower concentrations are generally desirable for areas frequented by children, 

seniors, or people with special health concerns. 
2 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

The concentration of oxygen (O2) in air is typically 

around 21 percent (210,000 ppm); however, in areas with 

elevated CO2 concentrations, O2 concentrations may be 

reduced to dangerously low levels. OSHA Standard 29 CFR 

1910.134 states that air with an O2 concentration below 19.5 

percent is considered oxygen deficient. In an oxygen-deficient 

environment, an individual may experience a variety of symptoms: with an O2

 concentration of 16 to 19 percent, breathing becomes rapid or labored; below 16 percent, individuals 

may experience headaches and dizziness; below 14 percent, 

thinking and judgment can be impaired; below 10 percent, 

unconsciousness and death may result (National Institute for 

Occupational Safety and Health, 2004). Because CO2 tends to 

first accumulate at floor level and displace the oxygen there, 

special consideration must be given for the impact that the 

CO2-intrusion phenomenon may have on infants, toddlers, and 

pets who breathe at near-floor levels. 

Physical Setting and History of the Study Area

In 2006, the Indiana Department of Natural Resources 

(IDNR), Division of Reclamation became aware of at least 

three homes in southern Indiana that were experiencing 

elevated levels of CO2 and depressed levels of O2. All three 

homes were constructed on or near recently reclaimed surface 

coal mines. At two of the homes, mitigation efforts were able 

to reduce CO2 concentrations to acceptable levels. Unfortunately, at one site (fig. 2) in east-central Pike County, Ind., 

herein referred to as the “study site,” building-foundation 

characteristics did not appear to be compatible with commonly 

applied mitigation approaches: sub-slab depressurization 

systems. Therefore, this site continued to represent a health 

hazard for the homeowners; in fact, when environmental 

conditions were unfavorable, the home was from time to time 

uninhabitable. 

Figure 1. A carbon dioxide cloud that has settled to floor level. (In this photograph, condensation allows the dry-ice-generated 

carbon dioxide cloud to be visible; however, in most settings, carbon dioxide generated from natural soil processes will not be 

visible.)
Introduction 3

Pike County is in southwestern Indiana. The geologic 

materials underlying the county are part of the Illinois Basin, 

a broad depression of Cambrian through Permian sedimentary 

rocks. In many southwestern Indiana and southern Illinois 

locations, surface-mining techniques are used to extract coal 

deposits associated with the Illinois Basin.

Mean monthly temperatures at Spurgeon, Ind., about 8 mi 

southwest of the study site, range from about 30°F in January 

to about 78°F in July. At Spurgeon, normal annual precipitation is 46.6 in.; normal monthly precipitation ranges from 

about 2.6 in. in February to about 5.2 in. in May (Midwest 

Regional Climate Center, 2009). 

The relevant history of the study site begins with the 

establishment of a coal mine in 1986. At this mine, the 

Houchin Creek Coal Member of Petersburg Formation and the 

Survant Coal Member of Linton Formation were extracted by 

means of standard surface-mining practices. Mining activity at 

the study site ended in 1992, and the land was then reclaimed 

following modern reclamation practices (Indiana Geological 

Survey, Coal Mine Information System, 2009). Reclamation involved returning the stripped overburden—spoil—to 

the mined area, capping the spoil with approximately 36 in. 

of soil, and establishing vegetation on the soil surface. After 

reclamation, the land was sold for residential development. 

Currently, land use in the broader study area includes widely 

spaced single-family units (less than one per 5 acres overall) 

constructed within an area of agricultural land, rangeland, and 

forest land (Homer and others, 2004). 

The study-site home was constructed with a walk-out 

basement and is positioned where a level landscape surface 

meets a hillside that gently slopes down to a 7-acre lake. The 

lake is approximately 300 ft from the home, and the elevation of the lake’s surface is approximately 20 ft lower than 

the home’s basement floor. The occurrence of extensive and 

long-lasting puddles after even moderate rain showers (less 

than 1⁄4 in.), indicates that the level surfaces of the study-site 

landscape are poorly drained.

Study Design

One aspect of the mission of the U.S. Geological Survey (USGS) is to provide reliable scientific information that 

describes natural systems and helps to minimize the loss of 

life and property that may be impacted by natural hazards. In 

most cases, this involves documenting and analyzing widely 

recognized natural hazards such as floods, earthquakes, and 

volcanic activity. However, on occasion previously unrecognized phenomenon, such as CO2

 intusion, emerge as a newly 

identified natural hazard. Following the guidelines within our 

strategic science direction, the USGS works with its cooperative partners to document these emerging hazards and ensure 

that scientific methods are effectively applied to better understand the phenomenon and minimize loss.

To aid in understanding the processes that control the 

potential for CO2 to invade and accumulate in homes built on 

or near reclaimed coal mines, the USGS and the IDNR Division of Reclamation entered into a cooperative investigation of 

this phenomenon and of steps that might be taken to provide 

effective mitigation. This investigation included three primary 

study components:

Figure 2. The study-site home in east-central Pike County, Ind., where the intrusion of carbon dioxide as a soil gas has posed a health risk. 
4 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

1. Documenting the environmental factors that resulted 

in elevated concentrations of CO2 within the study-site 

home. This was accomplished by documenting CO2 flux 

from soils across the study-site landscape; determining 

the chemistry of spoil samples and masonry materials 

collected at the study site; establishing an onsite, continuously recording meteorological observation platform; 

attempting to measure ground-water-level fluctuations 

within onsite nested monitoring wells; and evaluating CO2

data collected within the study-site home.

2. Documenting the effectiveness of mitigation approaches 

that could be applied to reduce CO2 levels in homes where 

CO2 accumulation is a concern. This was accomplished 

by documenting CO2 levels within the study-site home 

prior to and in between each successively more complex 

mitigation approach.

3. Documenting site characteristics for all Indiana homes 

with known CO2-accumulation issues and using those 

data to map other locations where this phenomenon could 

potentially occur. (The methods and results of component 

3 are not presented in this report.)

Because carbon dioxide accumulation represents an acute 

health risk, homeowners try to quickly identify and apply 

effective mitigation when they first recognize this phenomenon in their home. Therefore, in most cases, there is no opportunity to collect the scientific data required to understand this 

phenomenon. As mentioned above, for the Pike county studysite home, it was thought that commonly applied mitigation 

methods would not effectively reduce the within-home CO2

concentrations. Therefore, this home represented the one site 

where data could be collected to document the CO2-intrusion 

phenomenon. These data are expected to reveal details regarding CO2

 intrusion at the study-site and have a broader application to other structures where this phenomenon may occur.

Purpose and Scope

An investigation of the phenomenon of CO2 accumulation in a structure constructed on a reclaimed surface coal 

mine was completed. The purpose of this report is to present 

data and results for two components of that investigation: 

the description of the environmental conditions that lead to 

elevated CO2 concentrations in an east-central Pike County 

home, and a determination of the effectiveness that three mitigation methods had on reducing indoor CO2

 concentrations to 

acceptable levels.

Methods of Study

To date, there have been few investigations of the fundamental characteristics and controls for the phenomenon of CO2

accumulation in homes built on top of or near reclaimed coal 

mines. Therefore, when this investigation was begun, it was 

recognized that numerous lines of inquiry would be required 

to fully pursue this subject and that not all lines of inquiry 

would lead to equal benefit. In this section of the report, 

methods used to collect geological, meteorological, and gasconcentration data are described. In later sections of the report, 

the results and significance of these lines of inquiry and the 

data they produced are discussed.

Geological Data Collection

One goal of this investigation was to characterize the 

environmental setting of the study site. Means of addressing 

this included measurements to document CO2 flux from surface soils; a monitoring-well network to facilitate water-sample collection, documentation of groundwater-flow directions, 

calculations of vertical and horizontal groundwater gradients, 

and fluctuations of groundwater levels; and spoil sampling to 

allow for chemical characterization of the geologic materials 

of the study site.

Soil-Gas Flux

To complete a survey of CO2 flux from the study site’s 

soil surface, an evenly spaced grid of sampling cells (fig. 3) 

was established by using a handheld compass, a mason’s line, 

measuring tapes and wheels, and surveyor’s wire flags. The 

study-site house defined the approximate center of the sampling grid. A mason’s line and compass were used to establish 

straight lines that ran parallel to the outside walls of the studysite house. Surveyor’s wire flags and distance-measuring 

devices where then used to mark the center of each 40-ft by 

40-ft sampling cell. The entire grid of sampling cells consisted 

of nine base lines, each identified by a letter (A through I). 

Each base line included nine sampling cells, identified by a 

number (1 through 9). Although the grid design included 81 

sampling cells, physical obstructions on the study-site landscape prevented data collection at eight cells (A1, A2, A3, 

B1, B2, B3, C1, and F5). This resulted in data being collected 

at 73 sampling cells. A few cells were visited and measured 

multiple times to ensure that the results were not biased by 

instrument drift.
Methods of Study 5

Study-site

house

A4 A5 A6 A7 A8 A9

B4 B5 B6

C2 C3 C4 C5 C6

D1 D2 D3 D4 D5 D6

E1 E2 E3 E4 E5

B7 B8 B9

C7 C8 C9

D7 D8 D9

F1 F2 F3 F4

G1 G2 G3 G4 G5 G6 G7

F6 F7

E6 E7

H1 H2 H3 H4 H5 H6

I1 I2 I3 I4 I5 I6

E8 E9

F8 F9

G8 G9

H7 H8 H9

I7 I8 I9

MW1d

MW1s

MW2d

MW2s

MW3d

MW3s

MOP

EXPLANATION

H2 Center of yard-survey 

data-collection cell

MW1d Monitoring-well number and 

relative depth (deep or shallow)

MOP Meteorological observation

platform

0 40 80 120 FEET

0 20 40 METERS

N

Gravel driveway

200 feet to

lake's edge

KNOX

PIKE

NAPDIN22

study site

INDIANA

Study area

Spurgeon, Ind.

Figure 3. Location of outdoor data-collection sites relative to the study-site house, Pike County, Ind.
6 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

To measure CO2 flux near the center of each sampling 

cell, a LI-COR LI-8100 portable soil-gas-flux meter (LICOR Environmental, Lincoln, Nebr.) was used. To complete 

measurements with this device, a sampling collar was seated 

in the top 2 in. of soil at the locations where soil-gas-flux measurements were to be completed. The LI-8100 gas-collection 

chamber was then, in turn, placed on each sampling collar, 

and the device’s pneumatic bellows were used to create a seal 

between those two components of the sampling train. During 

each data-collection session, the operator could evaluate initial 

datasets to determine an appropriate duration for instrumentpurging and data-collection times. LI-COR-developed 

software, designed to support the LI-8100, included a variety 

of tools to evaluate measurement results and presented CO2-

flux data in units of micromoles per square meter per second 

(μmol/m2/s).

In addition to measuring CO2 flux for each sampling 

cell, field personnel also measured and recorded air temperature, barometric pressure, and soil pH. Air temperature was 

measured with a mercury thermometer, and barometric pressure was measured with a laboratory-grade barometer. Using 

the method recommended by the manufacturer, soil pH was 

measured with a handheld pH meter. This process involved 

combining equal weights of soil and de-ionized water (approximately 20 g) in a clean beaker, stirring the mixture to form a 

uniform paste, and then gently stirring with the pH probe until 

a stable reading of pH was obtained.

Well Construction and Groundwater-Level 

Measurements

The services of an environmental drilling contractor 

were used to establish boreholes and install monitoring wells. 

The temporary network of monitoring wells was designed 

to allow collection of water samples and measurements of 

ground-water-level fluctuations at the study site. The network 

included a total of six 2-in.-diameter wells installed as nested 

pairs at each of three drilling locations (fig. 3). Each well pair 

consisted of a deep well, screened from approximately 33 to 

38 ft below the land surface, and a shallow well, screened 

from 13 to 18 ft below the land surface. (The water-well log 

for the homeowners’ water-supply well indicated that bedrock 

was 38 ft below land surface. This information was used to 

identify the maximum drilling depth when the drilling contract 

was established.) Locational data for the monitoring wells is 

presented in appendix 1 at the back of this report.

The boreholes were drilled with 4.25-in. inside-diameter 

hollow-stem augers and a drill rig mounted on an all-terrain 

vehicle (fig. 4). Each well (table 1) was constructed with a 

5-ft-long polyvinylchloride (PVC) well screen and enough 

PVC well riser to extend the top of the well approximately 2 ft 

above the land surface. A well-screen sand filter was established 

in each well and extended approximately 2 ft above the top of 

the screen. A pump and tremie pipe were used to fill the well 

annulus with bentonite grout from the top of the sand filter to a 

level approximately 2 ft below the land surface. At each well, 

the remaining 2 ft of well annulus was filled with drill cuttings.

Figure 4. Drilling 

activities for installation 

of a temporary network 

of monitoring wells at the 

study site in east-central 

Pike County, Ind.
Methods of Study 7

The latitude and longitude of each well was established 

by using a handheld global positioning device (GPS), which 

referenced the WGS 84 map datum. The relative elevations for 

each water-level measurement point were established by using 

conventional surveying techniques, an auto-level, and a stadia 

rod.

During monthly site visits, an electric measuring tape 

(commonly referred to as an eTape) was used to document 

water levels in the monitoring wells. (At the beginning of 

this investigation, it was thought that these data would allow 

for determinations of groundwater-flow directions, hydraulic 

gradients, and groundwater-level fluctuations; however, five of 

the six monitoring wells were found to be consistently dry, so 

these calculations could not be completed.)

Spoil Sampling and Chemical Analysis

During the borehole-drilling process described above, 

grab samples of the reclaimed coal-mine spoil were collected 

from various depths below the land surface (appendix 1). The 

collected samples were briefly described for color and textural 

characteristics, placed in properly labeled sampling jars, and 

shipped to the USGS Central Mineral and Environmental 

Resources Science Center, where they were analyzed for 

major ions by use of x-ray-diffraction techniques.

Meteorological Data Collection

Conversations with homeowners who have experienced 

acute CO2-intrusion events and a review of the relevant 

literature revealed that certain meteorological conditions may 

trigger these events. Therefore, meteorological data were 

collected at the study site to document local meteorological 

conditions and the pH of rainwater at the study site.

Meteorological Observation Platform

At the study site, a meteorological observation platform 

(figs. 3 and 5) was constructed and relevant data were collected from November 2008 to March 2010. These data 

included wind direction, wind speed, air temperature, barometric pressure, relative humidity, and precipitation. The first 

five of the listed parameters were measured by using a Vaisala 

Weather Transmitter WXT520 (Vaisala Inc., Woburn, Mass.). 

Precipitation was measured by using a tipping-bucket rain 

gage. Each data parameter was collected hourly and transmitted via satellite to USGS computers for processing and longterm storage.

Precipitation pH

A total of five rain samples were collected at the study 

site, and field measurements were completed to document the 

pH of those samples. The samples were collected by placing a clean 12-in.-diameter high-density polyethylene funnel 

and a collection beaker in the study-site yard during selected 

rainstorms. After a sufficient volume of water was collected 

(approximately 150 mL), the pH of the sample was measured 

with a LaMotte portable pH meter. This pH meter was calibrated with standard buffer solutions of pH 7 and pH 4 before 

each use.

Within-Residence Data Collection

Because of potential dangers associated with working 

in a CO2-rich environment, a number of precautions were 

required during site visits. To ensure the safety of all study 

personnel, CO2 and O2 levels were monitored continuously 

when investigation activities required entry to the study-site 

home. The CO2 monitor has a display window that allows the 

user to continually monitor CO2 levels and avoid locations 

where concentrations are above the 5,000-ppm threshold. The 

O2 monitor projects audible, visual, and tactile alarms when O2

levels fall below 19.5 percent. 

Table 1. Selected characteristics of monitoring wells at the study site, Pike County, Indiana.

Well name Latitude Longitude

Screened interval 

(feet below land 

surface)

Relative 

land-surface 

elevation1 

(feet)

Height of measurement point above land 

surface (feet)

MW1d 38°19′42′′ 87°08′22′′ 33–38 200.00 1.92

MW1s 38°19′42′′ 87°08′22′′ 13–18 199.62 1.52

MW2d 38°19′40′′ 87°08′27′′ 34–39 188.43 0.90

MW2s 38°19′40′′ 87°08′27′′ 13–18 189.03 1.62

MW3d 38°19′42′′ 87°08′26′′ 33–38 198.36 1.27

MW3s 38°19′42′′ 87°08′26′′ 13–18 199.15 1.94

1

Elevations are referenced to an arbitrary local datum; the measurement point of well MW1d was defined as the arbitrary local datum of 

200.00 feet.
8 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

Gas Concentrations

Personnel from IDNR Division of Reclamation placed 

and maintained a Draeger X-am 7000 gas monitor (Draeger 

Safety Inc., Pittsburgh, Pa.) to continuously record CO2 and O2

concentrations at a fixed location in the basement of the studysite home. This device was placed approximately 18 in. above 

floor level in the room that consistently has shown some of the 

highest CO2 concentrations when a soil-gas-intrusion event 

occurs. The instrument was configured to record gas concentrations every 10 minutes. In keeping with the manufacturer’s 

recommendation, the sensors of this device were calibrated 

every 90 to 120 days or more frequently if the acquired data 

indicated a need for recalibration (Randy Hoffman, Indiana 

Department of Natural Resources, Division of Reclamation, 

oral commun., 2010).

 On a biweekly schedule, within-residence gas-concentration data were downloaded from the Draeger unit, and a 

copy of all downloaded data was provided to the USGS for 

processing and long-term storage. During processing of these 

data, all CO2-concentration values collected on a given day 

(midnight to midnight) were examined to identify the single 

highest value for that day. Likewise, all O2-concentration 

values collected on a given day were examined to identify the 

single lowest value for that day. It is these daily maximum and 

minimum values of CO2 and O2, respectively, that are archived 

in the USGS National Water Information System database and 

referenced in this report.

Several attempts were made to use a Bacharach model 

2810 portable CO2 analyzer (Bacharack, Inc., New Kensington, Pa.) to further document CO2

 levels in the basement and 

garage of the study-site home. This analyzer uses infrared 

absorption to measure CO2 concentrations, was calibrated in 

fresh air prior to each use, and can be used where CO2 levels 

range from 0 to 10,000 ppm. However, during these attempts 

it became apparent that even slight disturbances during data 

collection were profoundly affecting the measured values and 

prevented this instrument from delivering reproducible results. 

These disturbances included changes in relative humidity, 

operator exhalation, movement of people and pets in the 

home, operation of ceiling fans, status of doors and windows as either open or closed, and furnace or air-conditioner 

activity.

Masonry Sampling and Chemical Analysis

Samples were collected from the masonry components of 

the study-site home and analyzed to determine their chemical 

composition. In this effort, a total of 10 cores were collected. 

These cores ranged from 1 to 4 in. in diameter and were 

obtained by masonry-cutting professionals using handheld 

electric drills and diamond-tipped coring bits. The cores were 

collected from the garage’s concrete slab and the basement’s 

block walls and concrete slab. These sampling locations were 

selected because they represented points where carbonatebased components of the study-site home were in contact 

Figure 5. Meteorological observation platform designed for hourly transmission of local meteorological data 

measured at the Pike County, Ind. study site.
Occurrence of Within-Residence Carbon Dioxide 9

with soil. The collected cores were sent to the USGS Central 

Mineral and Environmental Resources Science Center where 

they were analyzed for major ions by using x-ray-diffraction 

techniques. After the cores were obtained, the resultant holes 

were filled with a concrete-strength, nonshrink epoxy grout to 

ensure that these holes were well sealed and would not leak 

gases or moisture to the home’s interior.

Occurrence of Within-Residence 

Carbon Dioxide

Because the study of the CO2-intrusion phenomenon is 

in its infancy and many of the underlying controls are not yet 

well documented, current researchers sometimes explore lines 

of inquiry that lead to dead ends or need to take a trial and 

error approach as they employ generally untested methods of 

data collection. This certainly was the case for the investigation described herein. As part of this investigation, several 

lines of inquiry were explored that did little to further the 

understanding of the CO2-intrusion phenomenon. In the interest of brevity, a full description of these lines of inquiry and 

their resultant data are not presented in this report; rather, the 

bulleted items below list these study components and briefly 

describe why they produced results of minimal value. 

• At the start of this investigation, the study team 

expected that groundwater-level fluctuations may be 

forcing soil gases toward the study-site home during 

periods of substantial recharge. Therefore, a network of 

six monitoring wells was established at the study site 

to create an opportunity to measure groundwater levels. However, in the course of this investigation, five of 

the six wells were consistently dry, and data collected 

during this investigation were not able to support or 

refute the concept that groundwater fluctuations may 

be physically influencing CO2 intrusion.

• A survey of CO2 flux from the surface soils of the study 

site was completed to determine whether a few discrete 

locations—hot spots—were responsible for generating a significant proportion of the CO2

 emitted from 

the study-site landscape. To the contrary, the results 

of this survey revealed a fairly uniform distribution of 

CO2 flux from the surface soils of the study site. The 

measured flux values, which ranged from 1.36 to 6.08 

μmol/m2/s, showed much less variability than values 

measured by researchers working at other CO2-accumulation sites (Kwame Awuah-Offei, Missouri University of Science and Technology, oral commun., 2010). 

Therefore, rather than identifying hot spots where 

corrective measures could be focused, the data showed 

that CO2 flux was, to a large extent, evenly distributed 

across the study-site landscape.

• While conducting the CO2-flux survey, the study team 

also measured soil pH at each location where CO2-flux 

values were determined to see whether, as might be 

expected, there was a strong correlation between soil 

pH and CO2 flux rates. However, the resultant data did 

not reveal a correlation (R2 = 0.0013) and, similar to 

the uniform distribution of CO2-flux rates, all soil pH 

values at the study site fell within a fairly narrow range 

(4.32 to 6.83) and all were acidic (pH values below 7).

• Spoil samples were collected from a variety of depths 

below the land surface (appendix 1), and their chemical 

compositions were measured to determine whether the 

site’s geological materials—mostly spoil derived from 

a shale source—had a unique chemical composition 

that could cause them to produce inordinate amounts 

of CO2 as a soil gas. However, the spoil-chemistry 

data did not reveal any outstanding characteristics; the 

median loss-on-ignition value—which can be used to 

approximate carbonate content—for all spoil samples 

was 8.5 percent. This value is well within the range of 

geochemical determinations reported for other shales 

or shale-derived materials and indicates that unusual 

spoil chemistry at the study site was not controlling the 

CO2-intrusion process.

• Likewise, the chemical composition of samples collected from the masonry components of the study-site 

home were determined to learn whether these materials 

had a unique chemical quality that would cause them 

to produce inordinate amounts of CO2 (appendix 1). As 

with the spoil samples mentioned above, this was not 

found to be the case.

Although the above-mentioned lines of inquiry did not 

substantially advance our understanding of the CO2-intrusion 

phenomenon, they did allow some components of the environmental setting to be excluded from the list of potential determinant factors. A review of all data revealed that coupling of 

meteorological data and within-residence CO2 data (appendix 

1, table 1–5) allowed for identification of the natural factors 

that most strongly control the occurrence and timing of CO2-

intrusion events. (For completeness, the second page of table 

1–5 contains the daily minimum O2 values that were measured 

in the study-site home. Because CO2 tends to displace O2, 

either of these datasets (CO2 or O2) can be used to document 

the CO2-intrusion phenomenon; however, for consistency 

within this report, only the CO2 data are discussed.)
10 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

Meteorological Controls

In most locations rainwater is acidic—has a pH less than 7. 

This acidity is commonly the result of chemical processes in the 

lower atmosphere that cause rainwater to combine with available CO2

, forming carbonic acid. Data collected by the National 

Atmospheric Deposition Program (NADP; 2009) show that, for 

most locations in the United States, rainwater pH is less than 6, 

and in the eastern United States, rainwater pH is typically less 

than 5. For data collected in 2007 by the NADP at their monitoring location IN22 in Knox County, Ind. (approximately 34 mi 

from the study site), the annual mean rainwater pH value was 

4.6 (National Atmospheric Deposition Program, 2008).

The NADP results are in close agreement with the measurements of rainwater-pH collected at the study site (table 

2). Between December 2008 and March 2010, a total of five 

samples were collected at the study site for rainwater-pH 

determinations, and all pH measurements were 4.6. One component that can promote the generation of CO2

 as a soil gas 

is a source of acidic water in the presence of a carbonate rich 

substrate. Clearly, with a pH of 4.6, this input of acidic water 

is available at the study site.

For the study-site home, CO2-intrusion events were found 

to be most closely tied to specific meteorological conditions. 

These include, but may not be limited to, rapid drops in barometric pressure, rainstorms, and cold weather. These conditions 

often occur simultaneously and all can, individually or collectively, contribute to the problem of CO2

 intrusion to a structure. 

Meteorological and within-residence CO2-concentration 

data for March 25 to April 25, 2009, are plotted in figure 6. At 

the top of the figure are vertical bars showing daily maximum 

CO2 concentrations in the basement of the study-site home. 

(The “action level” line corresponds to a CO2 concentration 

of 5,000 ppm, the broadly accepted health standard for CO2.) 

Barometric-pressure data are presented as a continuous trace 

through the middle of the figure. Daily precipitation totals 

are presented as vertical bars at the bottom of the figure. 

Examination of this plot reveals that, within this 1-month 

period, substantial and rapid drops in barometric pressure were 

accompanied by rainfall and spikes in the CO2 concentration 

on numerous occasions. This general pattern was consistently 

seen in the study-site home.

Table 2. Determinations of pH for rainwater 

samples collected at the study site, Pike County, Ind.

Sampling date Rainwater pH 

(standard units)

December 9, 2008 4.6

April 28, 2009 4.6

May 14, 2009 4.6

March 23, 2010 4.6

March 29, 2010 4.6

0

1

2

960

980

1,000

1,020

0

1

2

DAILY MAXIMUM 

CARBON DIOXIDE, 

IN PERCENT

BAROMETRIC PRESSURE,

IN MILLIBARS

PRECIPITATION,

IN INCHES

3/25/2009 3/30/2009 4/5/2009 4/10/2009 4/15/2009 4/20/2009 4/25/2009

DATE

Action level

Figure 6. CO2-concentration, barometric-pressure, and precipitation data collected from March 25 to April 25, 2009, 

at the study site in east-central Pike County, Ind. (Carbon dioxide values that were near background levels and trace 

amounts of precipitation are not plotted so that meaningful values can be shown at an appropriate scale.)
Occurrence of Within-Residence Carbon Dioxide 11

Conceptual Model

Data collected at the study site and a review of the 

relevant literature led to the development of a conceptual 

model which, as depicted in figure 7, shows those conditions 

that drive the CO2-intrusion process. The significance of the 

controlling components (rapid drops in barometric pressure, 

rainstorms, windy conditions, and cold weather) shown in this 

model are briefly described below.

Soil gases naturally move from areas of relative high 

pressure to areas of relative low pressure. When a weather front 

causes a rapid drop in barometric pressure to an area, all atmospheric and near-surface gases respond to its effect. The gases 

inside a structure tend to rise under reduced barometric pressure 

and, as they rise, soil gases surrounding the structure’s foundation are drawn in by the resultant pressure gradient. Soil gases 

will enter the home via any pathway that allows their transmission: cracks in the house’s floor or walls, expansion joints 

between footers and slabs, locations where plumbing components pass through the outer shell of the structure, and places 

where porous construction materials allow gas passage.

A syringe in a beaker of water (fig. 8) can be used as 

a physical model to illustrate this process. When a force 

is applied to withdraw the plunger from the syringe, a low 

pressure is generated within the syringe, and water from the 

beaker is drawn in to fill the potential void. In this physical 

model, the syringe represents the structure that is influenced 

by an approaching low-pressure front, and water in the beaker 

represents the surrounding soil gases that are drawn into the 

structure as a result of the established pressure gradient.

CO2

CO2 CO2

CO2

CO2

CO2 CO2

CO2 CO2

CO2

CO2

CO2 CO2 CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2 CO2

CO2

CO2 CO2

Bedrock surface

Land

surface Carbon dioxide

accumulates

in basement

CO2 CO2

CO2

RAPID DROP IN

BAROMETRIC 

Reclaimed

coal-mine

spoil

Compression of soil gases

Infiltration and percolation

of rainwater

Wetting front

Compressed soil gases

are displaced to areas

of low pressure

(Lisse Effect)

WIND and COLD

cause a low-pressure

"stack effect"

within structures

Wind

RAIN

PRESSURE

Figure 7. Conceptual model depicting four of the environmental conditions that may lead to 

carbon dioxide intrusion to structures: rapid drops in barometric pressure, rain, wind, and cold.

Figure 8. Syringe in a beaker of water, used to illustrate the 

effect of how a drop in barometric pressure draws soil gases 

into a structure.
12 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

During rainstorms, a phenomenon known as the Lisse 

effect (Guo and others, 2008) may act to drive soil gases, 

including CO2, toward a structure or dwelling. With intense 

rainfall, infiltrating rainwater traps and compresses soil gases 

which migrate toward areas of low pressure. Because precipitation falling on a house is diverted by manmade drainage 

systems (the roof, gutters, downspouts, and perimeter drains), 

the materials directly beneath the footprint of a house do not 

receive as much infiltrating precipitation as surrounding soils. 

Therefore, the drier soils beneath a house represent an area 

of relative low pressure where soil gases tend to migrate and 

collect (fig. 7). 

Soil-gas intrusion is also expected during windy and cold 

weather (Walker, 2010). During windy conditions a phenomenon, described by the Bernoulli Principle, causes low pressure 

to form inside a structure as wind blows across the structure’s 

large and small openings. During cold-weather conditions, 

heating the air inside a structure causes the air to rise and a 

low-pressure condition to be generated inside the structure 

(U.S. Environmental Protection Agency, 2010). Therefore, 

similar to the effect created by an approaching low-pressure 

front, windy and cold-weather conditions can cause soil gases 

to be drawn into a structure in response to an established pressure gradient. 

In homes where CO2 accumulation is a recognized problem, observations reported by the occupants invariably point 

to these specific meteorological conditions. The symptoms 

associated with CO2 intrusion (and oxygen deficiency) are 

typically most pronounced when atmospheric conditions combine to produce periods of low atmospheric pressure, intense 

rain, and windy conditions during cold-weather periods. When 

these conditions are coincident and at their worst, CO2 can be 

present at concentrations sufficient to cause respiratory 

problems throughout a structure or home.

Attempted Mitigation

Often, the least costly approach to addressing a 

natural hazard is avoidance. In locations where the 

CO2-intrusion hazard can be anticipated, avoidance 

may be accomplished by favoring construction on a 

crawl space and avoiding basements when residential 

construction is planned. (To date, all CO2-intrusion 

case studies known to the author consistently link this 

phenomenon to homes with basements.)

Where construction of a basement is planned, several 

steps can be taken to lessen the problem or facilitate mitigation if soil-gas intrusion is found to occur. These steps are 

described in a variety of building-guideline publications 

(Nebraska Department of Health and Human Services, 2007; 

US Environmental Protection Agency, 2009a, b) and include 

the following (fig. 9):

• Pouring the basement slab on a layer of aggregate. This 

aggregate can serve as a soil-gas collection layer if an 

active mitigation system is to be installed.

• Placing a vapor barrier between the basement slab and 

the sub-slab aggregate.

• Placing plumbing lines and an electrical junction box at 

strategic locations to accommodate a future mitigation 

system if one becomes necessary.

• Sealing all joints, cracks, and surfaces where soil gases 

may enter the basement.

• Installing gas-proof floor drains and sump-pit covers to 

inhibit gas intrusion.

Where CO2 intrusion becomes an identified problem, the 

services of soil-gas-intrusion professionals will be required to 

design and install a mitigation system. In most locations, these 

professionals will be identified as offering radon-mitigation 

services. In the 1970s, an entire industry designed to provide 

radon mitigation grew around the recognition that radon was 

a soil gas that could collect in homes and pose a health risk to 

the inhabitants. Now, as CO2 becomes recognized as another 

soil gas that poses a potential health risk, radon-mitigation 

specialists are commonly called for CO2 mitigation.

3- to 4-inch PVC

to vent sub-slab

soil gases

Seal joints, cracks, 

and slab surface

Electrical junction 

box in attic, ready

for potential fan

4-inch-thick layer of aggregate

to allow capture of sub-slab gases

Vapor barrier

Airtight covers

for sumps and

floor drains

Figure 9. New-construction steps 

that can be taken to reduce soil-gas 

intrusion or facilitate mitigation. 
Attempted Mitigation 13

When CO2 intrusion was first recognized as a concern at 

the study-site home and a radon-mitigation specialist was consulted, it appeared that, for the study-site home, the standard 

approach for radon mitigation might not resolve the CO2-intrusion concern. The standard approach to radon mitigation is to 

depressurize the air space beneath a basement’s slab (this is 

the air found between the clasts of a sub-slab aggregate layer) 

and exhaust the soil gases that collect in the sub-slab space to 

a safe location above the roofline of the home. At the studysite home, the basement slab was poured without an underlying layer of aggregate and, therefore, without a sub-slab 

airspace to depressurize.

As part of the investigation described herein, a series of 

steps were taken to determine whether other soil-gas mitigation approaches could reduce in-home CO2

 concentrations 

to acceptable levels. In this report, three of those mitigation 

approaches (block-wall depressurization, block-wall and subslab depressurization, and block-wall and sub-slab pressurization) are described and data are presented to evaluate their 

effectiveness.

Block-Wall Depressurization

On April 28, 2009, a mitigation system was established 

to depressurize the block walls of the study-site basement. 

A small pit was excavated adjacent to the study-site home 

and an electric drill was used to remove a 4-in.-diameter core 

from one of the wall blocks. A length of 4-in. PVC pipe was 

inserted in the cavity of that block and a RadonAway RP 265 

fan (RadonAway Inc., Ward Hill, Mass.) was connected to 

depressurize the basement’s block walls (fig. 10) and exhaust 

the evacuated air to a point just below the home’s roofline. 

The RP 265 fan is a high-volume fan rated to move air at up 

to 327 cubic feet per minute (ft3/min). To demonstrate that this 

device was effective at depressurizing the entire perimeter of 

the basement, a small hole was drilled in the basement wall at 

the opposite corner of the house and non-thermal smoke was 

puffed near that hole. The smoke was directly observed being 

drawn into the drilled hole, thereby confirming that the mitigation system was depressurizing the basement’s block walls. 

Figure 10. Mitigation system established to depressurize the block walls of the study-site home, Pike County, Ind.
14 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

To determine whether this depressurization effort was 

sufficient to protect the home from CO2 intrusion, in-home 

CO2-concentration data were collected for a period of 2 

weeks. During this period, the RP265 fan was run continuously. Data collected on May 8, 2009, revealed that a rapid 

drop in barometric pressure and accompanying rain were still 

capable of producing a threatening intrusion of CO2 (fig. 11). 

On this date, CO2 levels in the study-site basement reached a 

maximum of 0.8 percent, well above the targeted maximum of 

0.5 percent. It was therefore concluded that, for the study-site 

home, block-wall depressurization alone did not provide sufficient protection.

Block-Wall and Sub-Slab Depressurization

On May 13, 2009, the block-wall depressurization system 

described above was augmented by a system designed to draw 

soil gases from beneath the garage and basement slabs of 

the study-site home. Similar to the block-wall system, subslab depressurization was established by coring through the 

two slabs and installing a single fan to draw soil gases from 

beneath both slabs. In this case the fan was a high-pressure 

RadonAway GP501, capable of moving 80 ft3/min of air at a 

pressure equivalent to 2 in. of water column (WC). This combined system of block-wall and sub-slab depressurization was 

run continuously for a period of 5 months. 

Conditions experienced during the warm-weather period 

of July to mid-September 2009 did not produce any threatening CO2

-intrusion events; however, it was recognized that, for 

two reasons, this was not the appropriate time of year to evaluate the ultimate effectiveness of any mitigation system. First, 

during warm months, the furnace in the study-site home does 

not run, and therefore its potential to create a within-home 

low-pressure condition—“stack effect”—and contribute to 

CO2 intrusion cannot be evaluated. Secondly, because there is 

little temperature contrast in the summer between southern and 

northern latitudes of North America, there is little opportunity 

for strong barometric pressure gradients to form (in meteorological parlance, a barotropic condition). In the spring and fall 

of most years, when southern and northern temperature contrasts are more pronounced (a condition of baroclinicity) (John 

Kwiatkowski, National Weather Service, oral commun., 2009), 

large and rapid pressure drops can occur when midlatitude 

cyclones develop and move across North America. Therefore, 

0

1

2

960

980

1,000

1,020

4/15/2009 4/19/2009 4/23/2009 4/27/2009 5/1/2009 5/5/2009 5/9/2009

0

1

2

DAILY MAXIMUM 

CARBON DIOXIDE,

IN PERCENT

BAROMETRIC PRESSURE, 

IN MILLIBARS

PRECIPITATION,

IN INCHES

Block-wall

depressurization

DATE

Action level

Figure 11. CO2-concentration, barometric-pressure, and precipitation data collected from April 15 to May 

10, 2009, at the study site in Pike County, Ind. Block-wall depressurization was established on April 28, 2009. 

Data collected on May 8, 2009, revealed that this depressurization did not eliminate the CO2-intrusion threat. 

(Carbon dioxide values that were near background levels and trace amounts of precipitation are not plotted 

so that meaningful values can be shown at an appropriate scale.)
Attempted Mitigation 15

because CO2 intrusion is in part triggered by drops in barometric pressure, this phenomenon is more likely to occur in the 

spring and fall, and mitigation systems are best evaluated then.

To determine whether the combination of block-wall and 

sub-slab depressurization was producing effective mitigation, 

in-home CO2-concentration data were plotted with barometric 

pressure and precipitation data for September 1 to October 13, 

2009 (fig. 12). Data collected from September 26 to October 

12, 2009, revealed that rapid and substantial drops in barometric pressure were still capable of producing threatening 

CO2-intrusion events. In this 17-day period, there were 9 days 

where CO2 concentrations were above the target threshold of 

0.5 percent. It was therefore determined that, for the study-site 

home, the combination of block-wall and sub-slab depressurization did not provide sufficient protection.

0

2

4

6

960

980

1,000

1,020

9/1/2009 9/11/2009 9/21/2009 10/1/2009 10/11/2009

0

1

2

3

DATE

PRECIPITATION,

IN INCHES

BAROMETRIC PRESSURE,

IN MILLIBARS

DAILY MAXIMUM 

CARBON DIOXIDE,

IN PERCENT

Block-wall and sub-slab depressurization

were active through this entire period.

Action level

Figure 12. CO2-concentration, barometric-pressure, and precipitation data collected from September 1 to 

October 13, 2009, at the study site in Pike County, Ind. A combination of block-wall and sub-slab depressurization 

was established on May 13, 2009. Data collected from September 26 to October 12, 2009, revealed that this 

expanded depressurization effort did not eliminate the CO2-intrusion threat. (Carbon dioxide values that were 

near background levels and trace amounts of precipitation are not plotted so that meaningful values can be 

shown at an appropriate scale.)

Block-Wall and Sub-Slab Pressurization

On October 21, 2009 the two fans that had been used to 

draw gases from within the block walls and beneath the slabs 

of the study-site home were reversed. This reversal of air-flow 

direction was an attempt to pressurize the block walls and the 

void spaces immediately below the slabs, thereby preventing 

soil gases from entering the study-site home.

To determine whether block-wall and sub-slab pressurization was producing the desired mitigation effect, in-home 

CO2-concentration data were plotted with barometric pressure 

and precipitation data for October 5 to November 5, 2009 (fig. 

13). Data collected between October 22 and 30, 2009, revealed 

that this pressurization system was not able to prevent CO2

intrusion to the study-site home when atmospheric conditions 

were conducive to intrusion.
16 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

Limitations Regarding Extrapolation of 

Mitigation Results

It should be noted that the results obtained at the Pike 

County, Ind., study site might not be reproduced if the same 

mitigation methods were applied at another structure where 

CO2 intrusion is known to occur. Many site-specific factors, 

including foundation characteristics, building materials, differential settling of spoil, physical and geochemical properties 

of the spoil, proximity of the water table, and reclamation 

methods, can profoundly influence CO2 generation and intrusion. Therefore, though the three mitigation methods described 

above did not prevent CO2 intrusion at the study site, they 

might be more beneficial at another site or sites.

Summary and Conclusions

In recent years, CO2 intrusion has become recognized as 

a potentially serious health threat where homes are constructed 

on or near reclaimed coal mines. When CO2 invades the living space of a home, it can collect near the floor, displace the 

oxygen there, and produce an oxygen-deficient environment. 

For the individuals living in a home that is experiencing this 

phenomenon, the severity of the health threat is closely tied to 

the level of oxygen deficiency. In a slightly oxygen-deficient 

environment, symptoms may include headaches, dizziness, 

or confused thinking. In severe cases of oxygen deficiency, 

individuals can lose consciousness or die.

Several lines of inquiry were pursued to determine the 

environmental factors that most influenced the CO2-intrusion 

phenomenon at a study site in Pike County, Ind. From the data 

collected at the study-site home, CO2-intrusion events were 

found to be most closely tied to rapid drops in barometric 

pressure and rainstorms. Results in the relevant literature indicate that wind and cold weather also contribute to the phenomenon of soil-gas intrusion. With this recognition, a conceptual 

model was developed to illustrate how these four meteorological conditions can individually or collectively contribute to the 

CO2-intrusion phenomenon.

To evaluate their effectiveness, three mitigation methods 

were applied to the study-site home: block-wall depressurization, block-wall and sub-slab depressurization, and block-wall 

and sub-slab pressurization. Block-wall depressurization was 

accomplished by drilling a 4-in.-diameter hole in one of the 

Figure 13. CO2-concentration, barometric-pressure, and precipitation data collected from October 

5 to November 5, 2009, at the study site in Pike County, Ind. A combination of block-wall and sub-slab 

pressurization was established on October 21, 2009. Data collected between October 22 and 30, 2009, revealed 

that this pressurization effort did not eliminate the CO2-intrusion threat. (Carbon dioxide values that were near 

background levels and trace amounts of precipitation are not plotted so that meaningful values can be shown 

at an appropriate scale.)

0

2

4

6

960

980

1,000

1,020

10/5/2009 10/15/2009 10/25/2009 11/4/2009

0

1

2

3

DATE

PRECIPITATION,

IN INCHES

BAROMETRIC PRESSURE,

IN MILLIBARS

Action level

Block-wall and sub-slab

pressurization

DAILY MAXIMUM 

CARBON DIOXIDE,

IN PERCENT
References Cited 17

basement-wall blocks and attaching an exhaust fan designed 

for radon mitigation. This fan was run continuously for a period 

of 2 weeks. To establish block-wall and sub-slab depressurization, the system described above was augmented by a fan which 

drew soil gases from beneath the garage and basement slabs of 

the study-site home. This combined system of block-wall and 

sub-slab depressurization was run continuously for a period of 5 

months. To establish block-wall and sub-slab pressurization, the 

air-flow direction of the two fans was reversed, thereby attempting to pressurize the block walls and the void spaces immediately below the slabs and prevent soil gases from entering the 

study-site home. This pressurization system was run continuously for a period of 1 month.

In each case, it was found that these methods did not 

ensure a safe environment within the study-site home. Even 

with these mitigation methods active, CO2 intrusion resulted 

when meteorological conditions caused a rapid drop in 

barometric pressure and accompanying rain. It should be 

noted that, although these mitigation methods did not provide 

sufficient protection for the study-site home, they might be 

effective for other structures. 

References Cited

Ehler, W.C., 2002, Dangerous atmosphere created by strip 

mine spoil: Office of Surface Mining, Federal Reclamation 

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18 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil

Table 1–1. Locational data for spoil samples collected at the carbon 

dioxide-accumulation study site in Pike County, Ind.

Sample 

identification Latitude Longitude

Sampling 

depth (feet 

below land 

surface)

MW1d-1 38°19′42′′ 87°08′22′′ 2

MW1d-2 38°19′42′′ 87°08′22′′ 7

MW1d-3 38°19′42′′ 87°08′22′′ 13

MW1d-4 38°19′42′′ 87°08′22′′ 23

MW1d-5 38°19′42′′ 87°08′22′′ 38

MW1d-6 38°19′42′′ 87°08′22′′ 38.5

MW2d-1 38°19′40′′ 87°08′27′′ 3

MW2d-2 38°19′40′′ 87°08′27′′ 8

MW2d-3 38°19′40′′ 87°08′27′′ 18

MW2d-4 38°19′40′′ 87°08′27′′ 29

MW2d-5 38°19′40′′ 87°08′27′′ 39

MW3d-1 38°19′42′′ 87°08′26′′ 3

MW3d-2 38°19′42′′ 87°08′26′′ 8

MW3d-3 38°19′42′′ 87°08′26′′ 18

MW3d-4 38°19′42′′ 87°08′26′′ 23

MW3d-5 38°19′42′′ 87°08′26′′ 28

MW3d-6 38°19′42′′ 87°08′26′′ 38

Table 1–2. Results of major-ion analysis of spoil samples collected at the study site in Pike County, Ind.

[All data are expressed as percent of sample, by weight]

Sample 

identificationAl2O3 CaO Cr2O3 Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 TiO2

Loss on 

ignition

MW1d-1 13.6 0.46 0.01 6.61 2.16 1.05 0.11 0.67 0.20 66.5 0.94 7.48

MW1d-2 16.1 0.40 <0.01 6.09 2.78 1.83 0.08 1.03 0.13 62.8 1.03 6.98

MW1d-3 16.8 0.68 <0.01 6.19 2.92 1.76 0.10 0.96 0.15 61.8 0.94 7.83

MW1d-4 15.8 0.83 0.01 6.36 2.73 1.66 0.09 0.96 0.19 61.2 0.93 8.77

MW1d-5 16.2 0.58 <0.01 6.58 2.72 1.75 0.10 0.99 0.18 60.3 1.00 8.67

MW1d-6 15.3 0.61 0.02 6.19 2.53 1.52 0.09 0.87 0.19 62.6 0.95 8.12

MW2d-1 16.4 0.66 0.01 6.41 2.85 1.64 0.10 0.90 0.15 60.7 0.93 8.72

MW2d-2 14.6 1.31 0.01 5.11 2.45 1.56 0.09 1.11 0.15 66.3 0.94 6.66

MW2d-3 17.2 0.53 0.01 6.64 2.99 1.66 0.11 0.86 0.16 60.0 0.94 8.46

MW2d-4 14.0 1.14 0.01 5.18 2.16 1.44 0.07 0.83 0.22 64.9 0.91 8.30

MW2d-5 14.0 0.81 0.01 6.37 2.28 1.52 0.09 0.94 0.14 65.4 0.94 7.31

MW3d-1 12.9 0.24 0.01 5.78 2.20 0.84 0.06 0.76 0.12 69.1 0.82 7.40

MW3d-2 19.5 0.66 0.02 5.13 2.10 1.22 0.06 0.59 0.20 54.9 0.94 14.80

MW3d-3 17.5 0.52 <0.01 7.27 3.04 1.93 0.12 0.95 0.17 57.6 0.95 10.40

MW3d-4 16.1 0.76 0.02 6.80 2.80 1.87 0.11 0.99 0.31 57.8 0.95 11.40

MW3d-5 16.7 0.55 <0.01 7.10 2.84 1.87 0.12 1.03 0.18 59.8 1.02 8.93

MW3d-6 16.2 1.22 0.01 7.23 2.83 1.67 0.11 0.79 0.27 57.2 0.86 10.70

Appendix 1. Detailed Descriptive and Analytical Data From the Study Site.
Appendix 1. Detailed Descriptive and Analytical Data From the Study Site. 19

Table 1–3. Description of materials and locations where masonry samples were 

collected from the study-site home in Pike County, Ind.

Sample identification Material sampled Location of sample collection

HRM1 Concrete Garage floor

HRM2 Concrete Garage floor

HRM3 Block Basement wall (near ceiling)

HRM4 Block Basement wall (near floor)

HRM5 Concrete Basement floor

HRM6 Concrete Basement floor 

HRM6D Concrete Basement floor (split duplicate)

HRM7 Concrete Basement floor

HRM7D Concrete Basement floor (split duplicate)

HRM8 Concrete Basement floor

HRM9 Block Garage foundation (inside)

HRM10 Block Basement wall (outside)

Table 1–4. Results of major-ion analysis of masonry samples collected from the study-site home in Pike County, Ind.

[All data are expressed as percent of sample, by weight]

Sample 

identificationAl2O3 CaO Cr2O3 Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 TiO2

Loss on 

ignition

HRM1 4.55 18.1 <0.01 2.10 0.92 4.63 0.05 0.77 0.07 50.1 0.20 18.3

HRM2 5.57 13.3 <0.01 2.02 1.10 2.23 0.02 1.22 0.07 61.6 0.22 12.5

HRM3 2.28 28.1 <0.01 1.10 0.50 2.31 0.03 0.30 0.04 38.8 0.14 25.3

HRM4 2.26 28.6 0.02 1.12 0.45 1.97 0.03 0.27 0.05 39.4 0.13 24.6

HRM5 5.38 16.2 0.01 1.97 1.06 3.42 0.04 1.01 0.08 54.3 0.20 15.9

HRM6 4.16 19.5 <0.01 1.63 0.74 5.04 0.07 0.76 0.06 45.8 0.16 20.6

HRM6D 5.11 19.0 <0.01 2.18 0.90 3.81 0.05 0.97 0.07 49.1 0.22 18.0

HRM7 4.39 18.2 <0.01 1.96 0.84 4.26 0.05 0.83 0.08 49.6 0.19 18.7

HRM7D 4.46 17.7 <0.01 1.74 0.83 3.49 0.04 0.77 0.08 52.3 0.18 17.4

HRM8 5.03 14.3 <0.01 2.01 1.04 2.74 0.03 0.94 0.08 58.7 0.21 14.2

HRM9 2.31 27.1 <0.01 1.12 0.47 2.18 0.04 0.31 0.05 41.5 0.17 24.0

HRM10 2.38 28.5 <0.01 1.18 0.46 1.83 0.03 0.32 0.05 40.1 0.13 24.4
20 Occurrence and Attempted Mitigation of Carbon Dioxide in a Home Constructed on Reclaimed Coal-Mine Spoil Table 1–5. Daily maximum carbon dioxide percentages and daily minimum oxygen percentages within the study-site home, February 18–December 30, 2009. —Continued CARBON DIOXIDE, MAXIMUM PERCENT DAY JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

1 --- --- 0.00 0.00 0.20 0.00 0.00 0.00 0.00 1.20 0.00 0.002 --- --- 0.30 1.90 0.30 0.00 0.00 0.00 0.00 1.20 0.00 2.403 --- --- 0.40 0.70 0.00 0.30 0.00 0.00 0.00 0.20 0.00 0.404 --- --- 0.50 0.70 0.00 0.90 0.40 --- 0.00 0.00 0.00 0.005 --- --- 0.30 1.00 0.00 0.40 0.30 0.00 0.00 0.00 0.00 0.006 --- --- 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.20 0.00 0.407 --- --- 0.30 0.00 0.00 0.00 0.00 0.20 0.20 0.40 0.00 0.608 --- --- 0.00 0.20 0.80 0.20 0.00 0.40 --- 5.00 0.00 4.809 --- --- 0.00 0.30 0.00 0.00 0.00 0.40 --- 2.60 0.00 1.90

10 --- --- 0.30 0.60 0.00 0.00 0.00 0.20 --- 0.20 0.00 0.00

11 --- --- 0.00 0.00 0.00 5.00 0.00 0.20 --- 0.30 0.00 ---

12 --- --- 0.00 0.40 0.00 0.40 0.00 0.00 --- 0.80 0.30 ---13 --- --- 0.30 0.60 0.00 0.00 0.00 0.00 --- 0.20 0.20 ---14 --- --- 0.30 0.00 0.00 0.00 0.00 0.00 --- 2.10 0.00 0.6015 --- --- 0.00 0.00 0.00 0.00 0.30 0.20 0.00 2.40 0.00 0.0016 --- --- 0.00 0.00 0.00 --- 0.00 0.20 0.00 --- 0.00 0.0017 --- --- 0.00 0.00 0.00 --- 0.20 0.00 0.00 --- 0.00 0.7018 --- 0.30 0.20 0.00 0.00 --- 0.00 0.00 0.00 --- 0.00 1.6019 --- 0.00 0.00 1.00 0.00 --- 0.00 0.20 0.00 --- 0.00 0.3020 --- 0.30 0.00 0.80 0.00 --- 0.00 0.20 0.00 --- 0.00 0.0021 --- 0.50 0.00 0.00 0.00 --- 0.00 0.00 0.00 --- 0.00 0.0022 --- 0.00 0.20 0.00 0.00 --- 0.20 0.00 0.00 1.70 0.00 0.0023 --- 0.30 0.00 0.00 0.00 --- 0.00 0.00 0.00 1.70 0.00 0.0024 --- 0.50 0.00 0.00 0.00 --- 0.00 0.00 0.00 0.00 0.00 1.4025 --- 0.00 0.00 0.00 0.00 --- 0.00 0.00 0.00 --- 0.00 1.5026 --- 0.50 0.00 0.00 0.50 --- 0.00 0.00 0.60 --- 0.00 0.0027 --- 0.30 0.00 0.00 0.40 --- 0.00 0.00 1.20 --- 0.00 0.0028 --- 0.20 0.40 0.00 0.30 --- 0.00 0.00 1.10 0.00 0.00 ---29 --- --- 0.00 0.00 0.00 --- 0.00 0.00 0.00 1.20 0.00 0.3030 --- --- 0.00 0.40 0.80 0.00 0.00 0.00 0.00 0.90 0.00 1.2031 --- --- 1.20 --- 0.50 --- 0.40 0.00 --- 0.00 --- ---
Appendix 1. Detailed Descriptive and Analytical Data From the Study Site. 21

Table 1–5. Daily maximum carbon dioxide percentages and daily minimum oxygen percentages within the study-site home, February 18–December 30, 2009.—Continued

OXYGEN, MINIMUM PERCENT

DAY JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

1 --- --- 20.40 20.90 19.80 20.90 20.90 20.90 20.60 20.10 20.60 20.502 --- --- 20.00 16.50 19.00 20.90 20.90 20.90 20.60 20.20 20.50 18.403 --- --- 19.10 18.80 19.90 20.90 20.90 20.90 20.60 20.60 20.60 20.604 --- --- 18.40 19.80 20.90 20.20 20.90 --- 20.60 20.60 20.50 20.605 --- --- 19.40 18.30 20.90 20.90 20.90 20.50 20.60 20.60 20.60 20.506 --- --- 20.40 20.90 19.90 20.90 20.90 20.60 20.60 20.10 20.40 20.207 --- --- 19.50 20.90 20.90 20.90 20.90 20.60 20.60 20.60 20.50 20.108 --- --- 20.20 20.10 15.80 20.60 20.90 20.50 --- 18.00 20.60 17.209 --- --- 20.00 20.00 20.90 20.90 20.90 20.50 --- 19.90 20.60 19.90

10 --- --- 19.50 18.70 20.90 20.90 20.90 20.50 --- 20.60 20.60 20.60

11 --- --- 20.90 20.90 20.90 17.60 20.90 20.60 --- 20.50 20.60 ---

12 --- --- 20.20 19.40 20.30 20.90 20.90 20.60 --- 20.10 20.30 ---13 --- --- 18.80 18.90 19.60 20.90 20.90 20.60 --- 20.50 20.30 ---14 --- --- 18.50 20.90 20.90 20.90 20.90 20.60 --- 19.30 20.60 19.8015 --- --- 20.60 20.90 20.00 20.90 20.60 20.60 20.60 19.20 20.60 20.9016 --- --- 20.90 20.60 19.50 --- 20.90 20.60 20.90 --- 20.30 20.6017 --- --- 20.10 20.40 20.90 --- 20.90 20.60 20.90 --- 20.50 19.8018 --- 19.90 19.50 20.50 20.90 --- 20.90 20.60 20.90 --- 20.60 19.2019 --- 20.90 20.90 16.60 20.90 --- 20.90 20.60 20.90 --- 20.60 20.2020 --- 19.90 20.50 17.80 20.90 --- 20.90 20.60 20.90 --- 20.50 20.9021 --- 19.10 20.30 20.60 20.90 --- 20.90 20.60 20.60 --- 20.50 20.9022 --- 20.90 19.40 20.90 20.90 --- 20.90 20.60 20.60 20.00 20.30 20.3023 --- 19.80 19.40 20.50 20.90 --- 20.90 20.60 20.60 20.00 20.60 20.3024 --- 18.30 20.40 20.90 20.90 --- 20.90 20.60 20.60 20.60 20.30 18.7025 --- 20.90 20.30 20.90 20.00 --- 20.90 20.60 20.60 --- 20.60 19.4026 --- 18.20 19.60 20.90 20.50 --- 20.90 20.60 20.40 --- 20.60 20.6027 --- 19.40 19.40 20.90 20.60 --- 20.90 20.60 20.20 --- 20.60 20.6028 --- 20.20 15.20 20.90 20.90 --- 20.90 20.60 20.30 20.60 20.50 ---29 --- --- 20.90 20.60 20.90 --- 20.90 20.60 20.60 20.00 20.60 20.9030 --- --- 20.00 19.10 20.20 20.90 20.90 20.60 20.60 20.30 20.60 20.1031 --- --- 19.10 --- 20.90 --- 20.60 20.60 --- 20.60 --- ---
Robinson—Occurrence and Attempted Mitigation of CO2 in a Home Constructed on Reclaimed Coal-Mine Spoil—Scientific Investigations Report 2010–5157

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