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| Figure 1. Radon decay chain and half lives (modified from Clark and Briar, 1993). |
Radon is a concern of many residents in Wyoming because
breathing air containing high levels of radon poses a health
risk. Radon is second only to cigarette smoking in causing
lung cancer in the United States and is estimated to cause
between 5,000 and 20,000 lung-cancer fatalities annually (Cothern,
1987). A 1992 study by the U.S. Environmental Protection
Agency (USEPA) reported that many homes in Wyoming exceeded
the recommended level of 4 pCi/L (picocuries per liter) for
indoor air. In Natrona County, 10 percent of the indoor-air
levels of radon exceeded 4 pCi/L, whereas in Lincoln County,
51 percent of the indoor-air levels of radon exceeded 4 pCi/L
(U.S. Environmental Protection Agency, 1993). One source of
radon in indoor air is ground water. Consequently, the U.S.
Geological Survey (USGS), in cooperation with Converse
County, Laramie Rivers, Platte County,
Saratoga-Encampment-Rawlins, and Star Valley Conservation
Districts; the Sublette County Public Health Nursing
Service; and the town of Torrington, Wyoming, collected
water samples from wells in Albany, Carbon, Converse,
Goshen, Lincoln, Platte, and Sublette Counties to measure
radon concentrations in some of Wyoming's ground water.
Radon is a colorless, odorless, tasteless gas that
naturally occurs in rocks, soil, water, and air. Radon is
produced by the radioactive decay of uranium-238 to
radium-226 (fig. 1). Radium-226 further decays to produce
isotopes of radon, including radon-222. Radioactive decay is
a natural, spontaneous process in which an atom of one
radioactive element breaks down to form another element or
isotope. The rate of an isotope's decay is defined by its
half-life, or the time it takes for one half of a given
amount of the element to decay. Radon-222 (referred to as
radon in this report) has a half-life of 3.8 days.
The ultimate source of radon is uranium and most rocks contain some uranium. Rocks typically contain between 1 and 3 ppm (parts per million) of uranium (Otton, 1992). In Wyoming, some rocks and sediments derived from various rock types can contain higher amounts of uranium. Rock types typically having high amounts of uranium (as much as 100 ppm) include: granite, light-colored volcanic rocks, dark shales, sedimentary rocks that contain phosphate or uranium deposits, and metamorphic rocks derived from these rocks (Otton, 1992). In addition, sediment and soil derived from these rocks can have high amounts of uranium. Because radon is a gas, it is very mobile in the environment and can easily move through fractures and openings in rocks and into the pore spaces in an aquifer or soil. The amount of radon in ground water is related to several factors, including the amount of uranium available in the source rocks, the location of the radium atom in the mineral grain (how close it is to the grain's surface), and the physical properties of the aquifer materials, such as porosity. High radon concentrations are most prevalent in water in the eastern United States (Lindsey and Ator, 1996; Zapecza and Szabo, 1988), but high concentrations also have been detected in the Rocky Mountain region (Clark and Briar, 1993).
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| Figure 2. Radon entering a home through a water system (modified from Otton, 1992). |
Radon can enter homes from the rocks and soil through
cracks, vents, or other openings in basement floors or
walls. Radon can also enter homes through a water system
(fig. 2). When radon is confined by tightly sealed areas, it
can become concentrated. Homes with private wells are more
likely to have increased exposure to radon from water
compared to homes that rely on surface water or to homes
served by a public ground-water supply. Homes that rely on
surface water for their water supply usually do not have
radon problems because radon is released from water on
contact with the atmosphere. Homes served by large, public
ground-water supplies generally have a lower risk of radon
exposure because some of the radon is released into the air
as a result of pumping, the water is treated, or because
some of the radon naturally decays while in the distribution
system. People can be exposed to radon from ground water in
two ways: 1) inhalation of radon that has been released from
the water during household activities when the water is
heated or agitated, such as showering, washing clothes, or
washing dishes; and 2) ingestion by drinking the water. The
risk from radon exposure by ingestion is considered much
lower than by inhalation.
During 1996-97, the USGS collected 59 water samples for analysis of radon in ground water in Wyoming (fig. 3). Domestic wells were selected for sampling in seven counties: Albany (11 wells), Carbon (7 wells), Converse (6 wells), Goshen (12 wells), Lincoln (10 wells), Platte (6 wells), and Sublette (7 wells). Some points in figure 3 are obscured by the close proximity of wells sampled in Albany, Goshen, and Lincoln Counties. Of the wells sampled, 25 were completed in unconsolidated deposits, 33 were completed in non-granitic bedrock, and 1 was completed in granitic bedrock. Unconsolidated deposits include: 1) alluvial deposits left from streams, consisting of cobbles, gravels, sands, and clays that are loosely arranged; and 2) terrace deposits, also left from streams, which occur as a step-like ledge along a stream margin and mark a former, higher water level. The depth of wells sampled ranged from 20 to 510 feet. Samples were collected using methods described by Koterba and others (1995). Samples were analyzed using liquid scintillation at the USGS National Water-Quality Laboratory in Arvada, Colorado.
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| Table 1. Radon concentrations in ground water in Wyoming, 1996-97 [radon concentrations in picocuries per liter]. |
Results of the radon sampling, by county, are shown in
table 1. Concentrations of radon in relation to the seven
counties are shown in figure 3 and to the three geologic
units in figure 4. Radon concentrations in ground water
samples were variable across the State and ranged from 150
to 8,200 pCi/L. The median concentration is used to compare
radon concentrations in different counties. When
concentrations are ranked from lowest to highest, the median
concentration is in the middle. If there were an odd number
of samples, the median is in the middle. If there were an
even number of samples, the median is the average of the two
middle samples. The lowest median radon concentration in
ground water was from wells in Platte County (520 pCi/L),
whereas the highest median concentration was from wells in
Converse County (1,300 pCi/L). Radon concentrations in
ground water from alluvial or terrace deposits ranged from
270 to 3,600 pCi/L (fig. 4). Radon concentrations in ground
water from non-granitic bedrock wells ranged from 150 to
7,000 pCi/L. The lowest radon concentration (150 pCi/L) in
ground water was from limestone bedrock in Lincoln County.
The highest radon concentration (8,200 pCi/L) in ground
water was from granitic bedrock in Albany County. The USEPA
has not established a Maximum Contaminant Level (MCL) for
radon in drinking water. The old, proposed MCL of 300 pCi/L
for public-water supplies is currently (1998) under review.
For every 10,000 pCi/L of radon in water, it is estimated
about 1 pCi/L is released to the air (Prichard, 1987).
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| Figure 3. Radon concentrations in ground water in Wyoming, 1996-97. |
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| Figure 4. Radon concentrations in ground water from different rock types in Wyoming, 1996-97. |
The USEPA and Surgeon General recommend homes below the
third floor be tested for indoor radon that may be entering
the home through the basement (U.S. Environmental Protection
Agency and Centers for Disease Control, 1992). In homes
where high levels of radon are measured in the air (greater
than 4 pCi/L) and where water is supplied by a private well,
the USEPA recommends that water from the well be tested as a
potential source for radon. If high radon concentrations are
detected in the water, the most common remediation
techniques are aeration and granular activated-carbon
systems. Additional information about radon is available
from the Wyoming Department of Health in Cheyenne, Wyoming.
The reader is referred to the following references and internet addresses for additional information concerning radon in ground water and air in the home.
Otton, J.K., 1992, The geology of radon: U.S. Geological Survey. U.S. Environmental Protection Agency, 1992, Consumer's guide to radon reduction - How to reduce radon levels in your home. U.S. Environmental Protection Agency, 1993, EPA's Map of radon zones - Wyoming. U.S. Environmental Protection Agency, 1993, Home buyer's and seller's guide to radon. U.S. Environmental Protection Agency and Centers for Disease Control, 1992, A citizen's guide to radon - The guide to protecting yourself and your family from radon (2d ed.).
Clark, D.W., and Briar, D.W., 1993, Radon in ground water of western Montana: U.S. Geological Survey, Open-File Report 93-64, 2 p. Cothern, C.R., 1987, Estimating the health risks of radon in drinking water: Journal of American Water Works Association, v. 79, no 4., p. 153-158. Lindsey, B.D., and Ator, S.W., 1996, Radon in ground water of the lower Susquehanna and Potomac River Basins: U.S. Geological Survey, Water-Resources Investigations Report 96-4156, 6 p. Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, Ground-water data- collection protocols and procedures for the National Water-Quality Assessment Program Selection, installation, and documentation of wells, and collection of related data: U.S. Geological Survey Open-File Report 95-398, 69 p. Otton, J.K., 1992, The geology of radon: U.S. Geological Survey, General Interest Publications of the U.S. Geological Survey, 28 p. Prichard, H.M., 1987, The transfer of radon from domestic water to indoor air: Journal of American Water Works Association, v. 79, no 4., p. 159-161. U.S. Environmental Protection Agency, 1992, Consumer's guide to radon reduction - How to reduce radon levels in your home: EPA-402-K92-003, 17 p. U.S. Environmental Protection Agency and Centers for Disease Control, 1992, A citizen's guide to radon - The guide to protecting yourself and your family from radon (2d ed.): EPA-402-K92-001, 15 p. ___1993, EPA's Map of radon zones - Wyoming: EPA 204-R-93-003, 32 p. Zapecza, O.S., and Szabo, Zoltan, 1988, Natural radioactivity in ground water - A review in National Water Summary 1986--Hydrologic events and ground-water quality: U.S. Geological Survey Water-Supply Paper 2325, p. 50-57.
The following Internet/World Wide Web information is also
available:
USGS Radon Page
http://energy.cr.usgs.gov
USEPA Radon Page
http://www.epa.gov/iaq/radon/
National Safety Council Environmental
Health Center, Air Quality Program
http://www.nsc.org/ehc/airqual.htm
For more information, contact:
District Chief
U.S. Geological Survey, WRD
2617 E. Lincolnway, Suite B
Cheyenne, Wyoming 82001
Email:
state_rep_wy@usgs.gov
Internet:
http://wy.water.usgs.gov/welcome.html
By: Melanie L. Clark and Cheryl Eddy Miller
Email:
mlclark@usgs.gov or cemiller@usgs.gov
Layout by: Suzanne C. Roberts
This fact sheet is also available in pdf format: 
fs07998.pdf (191K)
