Radon-222 Content of Natural Gas Samples from 

Upper and Middle Devonian Sandstone and Shale 

Reservoirs in Pennsylvania: Preliminary Data

By E.L. Rowan and T.F. Kraemer

Open-File Report Series 2012–1159

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Rowan, E.L., and Kraemer, T.F., 2012, Radon-222 Content of natural gas samples from 

Upper and Middle Devonian sandstone and shale reservoirs in Pennsylvania: Preliminary 

data: U.S. Geological Survey Open-File Report 2012–1159. Available only at 

http://pubs.usgs.gov/of/2012/1159.

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iii

Contents

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

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

Sample Collection .............................................................................................................................2

Analytical Method..............................................................................................................................2

Sources of Analytical Error................................................................................................................3

Results and Discussion.....................................................................................................................3

References Cited ..............................................................................................................................4

Table

1 Radon activity of natural gas samples at STP (standard temperature and pressure) from 

11 wells in Pennsylvania, reservoir name and age, lithology, and approximate reservoir 

depth....................................................................................................................................6
1

Radon-222 Content of Natural Gas Samples from 

Upper and Middle Devonian Sandstone and Shale 

Reservoirs in Pennsylvania: Preliminary Data

By E.L. Rowan and T.F. Kraemer

Abstract

Samples of natural gas were collected as part of a study of formation water 

chemistry in oil and gas reservoirs in the Appalachian Basin. Nineteen samples (plus two 

duplicates) were collected from 11 wells producing gas from Upper Devonian sandstones

and the Middle Devonian Marcellus Shale in Pennsylvania. The samples were collected 

from valves located between the wellhead and the gas-water separator. Analyses of the 

radon content of the gas indicated 222Rn (radon-222) activities ranging from 1 to 79

picocuries per liter (pCi/L) with an overall median of 37 pCi/L. The radon activities of 

the Upper Devonian sandstone samples overlap to a large degree with the activities of the 

Marcellus Shale samples.

Introduction

The U.S. Geological Survey (USGS) is conducting ongoing research to 

characterize the geochemistry of produced waters from the major oil- and natural-gas 

producing reservoirs in the Appalachian Basin. In the course of this work, 19 gas 

samples, plus two duplicates, were collected from wells producing from sandstone 

reservoirs in the Upper Devonian Bradford and Elk Groups and from the Middle 

Devonian Marcellus Shale. 

Radon is a radioactive noble gas that occurs in the uranium and thorium decay 

series. It is chemically inert and is preferentially partitioned into a gas phase relative to 

water (Kraemer, 1986). The only isotope of radon with a significantly long half-life is 

222Rn (t1/2 = 3.8 days1

), making it the most likely radon source for exposure of the general

public. 222Rn occurs in the 238U decay chain and forms directly from the alpha decay of 

226Ra, a radionuclide that is commonly present in the formation waters of sedimentary 

basins including the Appalachian Basin (Kraemer and Reid, 1984; Fisher, 1998; Rowan 

and others, 2011). 

A recent report by Resnikoff (2012) has led to increased interest in possible 

human exposure to radon as a component of natural gas in household settings. The report,

however, relied on theoretical calculations utilizing limited data from geologic analogs. 

A decision was made to release our small and preliminary dataset because, to the authors’ 

 1 National Nuclear Data Center (http://www.nndc.bnl.gov/chart).
2

knowledge, measurements of radon in natural gas at the wellhead have not previously 

been published for the Appalachian Basin. 

Sample Collection

Samples were taken from producing gas wells, from valves located between the 

wellhead and the gas-water separator, at gas line pressure, which was typically between 

50 and 500 pounds per square inch (psi). Gas samples were taken only from points past

which gas had flowed continuously for at least 24 hours, to avoid possible stagnant 

accumulations of gas and thus the possibility of underestimating 222Rn content due to its 

rapid decay. The samples were collected in stainless steel cylinders of 200- or 500-

milliliter volume. Care was taken to fill and then release gas from the cylinders several 

times before collecting the sample to remove any air or traces of the previous sample. 

Gas was allowed into or out of the cylinder only at low flow rates to avoid (1) 

entrainment of water into the cylinder and (2) precipitation of hydrocarbon liquids or 

water as a result of adiabatic cooling. The line pressure was noted, and collection time 

was recorded to allow calculation of the radon decay in the elapsed interval between 

sample collection and analysis. 

Analytical Method

The samples were analyzed for 222Rn in the laboratory using a Pylon AB-5 radon 

field-portable scintillation measurement system consisting of Lucas cells (Lucas, 1957), a 

photomultiplier tube, and a radon extraction and transfer unit (not used in this procedure).

Use of the Lucas cell for radon measurement is an established approach in field and 

laboratory settings (Mathieu and others, 1988; Hamlat and others, 2003; Papastefanou, 

2007; Smith and others, 2008). 

The Lucas cell was evacuated and then attached to a sample cylinder with in-line 

valves, expansion tubing, and a pressure gage. The Lucas cell was filled with sample by 

opening the cylinder to the evacuated expansion tubing and then closing the cylinder 

valve and opening the valve to the Lucas cell. After the pressure stabilized, the process 

was repeated if necessary until the cell was filled with natural gas to approximately 

atmospheric pressure. The final pressure was recorded and the Lucas cell placed on the 

AB-5 photomultiplier tube to count total alpha activity. The counting interval was 90 

minutes and was repeated as time permitted. Radon activities were calculated from the 

counting data after correcting for the efficiency of the Lucas cell, the cell volume, the 

recorded pressures, and the time interval between sample collection and counting. 

To test whether the radon counting efficiencies were the same for the natural gas 

samples as in air, we recorded counts from a radon source, first in air, then in methane to

approximate the composition of the natural gas. The radon source consisted of a 226Rabearing solution obtained from the National Institute of Standards and Technology 

(NIST) in a gas-permeable plastic tube that was placed in a flask and sealed. An 

evacuated Lucas cell was attached to the flask, filled either with air or with methane, and 

radon counts were recorded. Two such tests were conducted in air and two in methane. 

The ratio of radon counts recorded in methane versus counts recorded in air was 0.68. 

Accordingly, the resulting radon activities of the samples were divided by 0.68, 
3

increasing the values to correct for the fact that the measurements were made in natural 

gas (predominantly methane) with cell efficiencies calibrated for air.

Sources of Analytical Error

The cumulative error from sources inherent in the measurements is estimated to 

be ±25 percent for samples with radon activities >10 pCi/L, and possibly greater in

samples with lower radon activities. The main sources of potential analytical error are 

summarized below. 

1. Imperfect evacuation of the Lucas cell before filling with sample gas (about 10

percent systematic error). The pump used was capable of reaching about -27 inches of 

mercury representing approximately 90 percent evacuation.

2. Use of an uncalibrated electronic pressure gage to measure final sample pressure. 

The pressure gage was adjusted to a zero reading on the digital display if necessary 

before each filling procedure (systematic error). 

3. Variation in Lucas cell volume and efficiency relative to values reported by the 

manufacturer resulting in random errors of approximately ±2 percent for volume and 

±3 percent for efficiency. 

4. Use of an empirically derived ratio between count rates in air versus methane, as 

described above (systematic error).

5. Measurement of low radon activities relative to cell backgrounds. The measurement 

error increases as the difference between sample and background count rates 

decreases. Many of the natural gas samples had intrinsically low radon activities, 

which decreased further due to radioactive decay in the interval between sample 

collection and analysis (random error).

6. Cylinder blanks were not measured and therefore could not be used to correct the 

final radon activities. Possible radium contamination in the cylinder could represent 

an additional and ongoing source of radon generated by 226Ra decay. However, most 

of the samples analyzed approached near-zero activity after 30 days, consistent with 

unsupported 222Rn whose half-life is 3.8 days. The error associated with the possible 

presence of radium is therefore assumed to be negligible. 

Results and Discussion

Table 1 lists the 222Rn activities of the gas samples at standard temperature and 

pressure (25 oC and 1 atmosphere) at the time of sample collection; that is, they have 

been corrected to account for radon decay in the time elapsed between sample collection 

and laboratory measurement. The reservoirs sampled include sandstones within the 

Upper Devonian Bradford and Elk Groups and the Middle Devonian Marcellus Shale. 

Reservoir lithology and approximate depth are also listed for each well. The 222Rn 

activities of the samples range from 1 to 79 picocuries per liter (pCi/L); an overall

median of 37 pCi/L was calculated using the average values for two sets of duplicate 

samples. 

The 222Rn activities of samples from the Upper Devonian sandstones overlap 

those of the Marcellus Shale to a large degree. The Upper Devonian sample activities 
4

range from 7 to 65 pCi/L with a median of 42 pCi/L, while the Marcellus sample 

activities range from 1 to 79 pCi/L with a median of 32 pCi/L. Although no meaningful 

difference in the 222Rn activities for the two groups of samples is apparent from the data 

presented here, the total number of samples is too small to reliably characterize the 

reservoirs or to yield statistically valid results. 

The activity of radon in natural gas in the reservoir is ultimately determined by 

the abundance of its source, 226Ra, whose decay to 222Rn is independent of pressure. As 

radon forms, it preferentially enters a gas phase (if present) and establishes a 

concentration dependent upon its rate of supply from 226Ra in the reservoir and the rate of 

222Rn decay within the gas (Kraemer, 1986). Other factors being equal, including the 

226Ra content of the aqueous and solid phases in the reservoir, lower radon activities 

would be expected in gas originating from a high pressure reservoir than a low pressure 

reservoir due to the greater relative expansion of the gas from the higher pressured 

reservoir to surface conditions.

When interpreting the significance of the radon activities, the 3.8-day half-life 

should be taken into account. Based on the definition of a half-life, radon activity decays 

to 1/(2n

) over n half-lives. For example, in 2 half-lives (7.6 days) from the time the gas 

leaves the reservoir, the radon activity would decay to 1/(22), or 25 percent of its original 

activity. The highest radon activity reported here, approximately 79 pCi/L, would decay 

to 19.8 pCi/L over this time period. Additionally, in a household setting, radon that 

originated from natural gas would be diluted with air, although those calculations are 

outside the scope of this report. For comparison, the U.S. Environmental Protection 

Agency (EPA) threshold for remediation of radon in indoor air is 4 pCi/L2.

The key to better understanding radon, its sources, and behavior in hydrocarbon 

reservoirs lies with the collection and interpretation of additional data. Examples of the 

types of data that might be collected include (1) time-series measurements to document 

temporal variability in radon activity with changing water and gas production rates and 

changing reservoir pressures, (2) measurements on samples from reservoirs in a single 

formation (and lithology) but from a range of depths and pressures to confirm the role of 

pressure and volume expansion in reducing the radon activity at surface conditions, and 

(3) analyses of radon in gas samples more broadly across the Appalachian Basin to define 

regional variations with source rock thermal maturity, gas composition (for example, 

wetness), and other factors.

References Cited

Fisher, R.S., 1998, Geologic and geochemical controls on naturally occurring radioactive 

materials (NORM) in produced water from oil, gas, and geothermal operations: 

Environmental Geosciences, v. 5, p. 139-150.

Hamlat, M.S., Kadi, H., Djeffal, S., and Brahimi, H., 2003, Radon concentrations in 

Algerian oil and gas industry: Applied Radiation and Isotopes, v. 58, p. 125-130.

Kraemer, T.F., 1986, Radon in unconventional natural gas from Gulf Coast geopressuredgeothermal reservoirs: Environmental Science and Technology, v. 20, p. 939-942.

Kraemer, T.F., and Reid, D. F., 1984, The occurrence and behavior of radium in saline 

formation water of the U.S. Gulf Coast region: Isotope Geoscience, v. 2, p. 153-174.

 2 http://www.epa.gov/radon/aboutus.html.
5

Lucas, H. F., 1957, Improved low-level alpha-scintillation counter for radon: Review of 

Scientific Instruments, v. 28, no. 9, p. 680-683.

Mathieu, G., Biscayne, P., Lupton, R., and Hammond, D. E., 1988, System for 

measurement of 222Rn at low level in natural waters: Health Physics, v. 55, p. 989-992.

Papastefanou, C., 2007, Measuring radon in soil gas and groundwaters: a review: Annals 

of Geophysics, v. 50, p. 569-578.

Resnikoff, M., 2012, Radon in natural gas from Marcellus Shale: NEOGAP, Network for 

Oil & Gas Accountability and Protection, 14 p., available at 

http://www.neogap.org/neogap/2012/01/18/radon-in-natural-gas-report-resnikoff_1-10-

12/

Rowan, E.L., Engle, M.A., Kirby, C.S., and Kraemer, T.F., 2011, Radium content of oiland gas-field produced waters in the northern Appalachian Basin (USA): Summary and 

discussion of data: U.S. Geological Survey Scientific Investigations Report 2011-5135, 

31p.

Smith, C. G., Cable, J. E., Martin, J., and Moutusi, R., 2008, Evaluating the source and 

seasonality of submarine groundwater discharge using a radon-222 pore water transport 

model: Earth and Planetary Science Letters, v. 273, p. 312-322.
6

Table 1. Radon activity of natural gas samples at STP (standard temperature and pressure) from 

11 wells in Pennsylvania, reservoir name and age, lithology, and approximate reservoir depth. [In 

the Sample ID column, letters indicate individual wells, and numbers indicate that multiple samples 

were collected from a given well. Two duplicate samples were collected at well A. pCi/L, picocuries 

per liter; ft, feet]

Sample ID

222Rn Activity 

(pCi/L) Reservoir Name and Age Lithology

Approximate 

Reservoir 

Depth (ft)

A-1 35 Marcellus Shale, Middle Devonian Black shale 8,000

A-2 40 Marcellus Shale, Middle Devonian Black shale 8,000

A-2, duplicate 45 Marcellus Shale, Middle Devonian Black shale 8,000

A-3 79 Marcellus Shale, Middle Devonian Black shale 8,000

A-3, duplicate 79 Marcellus Shale, Middle Devonian Black shale 8,000

A-4 76 Marcellus Shale, Middle Devonian Black shale 8,000

A-5 1 Marcellus Shale, Middle Devonian Black shale 8,000

A-6 3 Marcellus Shale, Middle Devonian Black shale 8,000

B-1 38 Marcellus Shale, Middle Devonian Black shale 8,000

B-2 1 Marcellus Shale, Middle Devonian Black shale 8,000

B-3 4 Marcellus Shale, Middle Devonian Black shale 8,000

C 30 Marcellus Shale, Middle Devonian Black shale 8,000

D 65 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

E 46 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

F 42 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

G-1 21 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

G-2 7 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

H 37 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

I 50 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

J 32 Bradford and Elk Groups, Upper Devonian Sandstone 4,000

K 49 Bradford and Elk Groups, Upper Devonian Sandstone 6,000