A study of the occurrence and distribution of dissolved radon in the ground
water of Chester County was undertaken by the U.S. Geological Survey (USGS)
in cooperation with the Chester County Water Resources Authority and the
Chester County Health Department. The results of this study are published
in a technical report by Senior (1998). This fact sheet summarizes the
key findings presented in the technical report. Much of the background
information on radon was taken from Lindsey and Ator (1996).
What is Radon?
Radon is a naturally occurring, colorless, odorless gas that is soluble
in water. It is produced through the radioactive decay of uranium and
radium, which is naturally present in soil and in minerals in bedrock.
Radon is radioactive, which means that it breaks down or decays
to form other elements. The rate of radons radioactive decay is
defined by its half-life, which is the time required for one half of the
amount of radon present to break down to form other elements. The half-life
of radon is 3.8 days. A common unit of radioactivity measurement is picocuries
per liter. One picocurie per liter is about equal to the decay of two
atoms of radon per minute in each liter of air or water. This fact sheet
refers to the amount of radon measured in air or water in picocuries per
liter as the radon concentration.
Radon concentrations generally differ among different rock types and can
vary considerably within the same geologic formation. Radon moves from
its source in rocks and soils through voids and fractures. It can enter
buildings as a gas through foundation cracks or it can dissolve in ground
water and be carried to buildings through the use of water-supply wells.
Why Study Radon?
The Surgeon General of the United States has recognized exposure to radon
gas as being second only to cigarette smoking as a cause of lung cancer
(U.S. Environmental Protection Agency, 1992). Radon gas can cause lung
cancer if inhaled because its decay products can accumulate in the lungs
and damage lung tissue. As radon decays, it produces several short-lived
elements that also are radioactive. Radon, together with these decay products,
emits alpha particles that can damage lung tissue. Although most radon
is exhaled before it can do much damage, the decay products can remain
trapped in the respiratory system attached to dust, smoke, and other fine
particles from the air. Eventually, the accumulation of these radioactive
elements in constant, close contact with lung tissue can cause cancer.
Ingestion of radon also may cause health problems (Crawford-Brown, 1990;
While radon gas commonly enters the air in homes through basements, ground
water can carry additional radon into homes and other buildings, increasing
the health risk associated with radon in the air. Dissolved radon is easily
released into the air when the water is used for showering, cleaning,
and other everyday purposes (fig. 1). Milry and Cothern (1990) estimate
the degassing of radon adds 1 pCi/L (picocuries per liter) to air for
each 10,000 pCi/L in water. In homes built with better insulation and
better seals on windows and doors, radon has less chance to be ventilated
to the outside and can become concentrated to dangerous levels in indoor
air. Most radon escapes from the water at the faucet or other point of
use, leaving little in the water itself. The radon that escapes from the
water adds to the radon that enters the home through the basement, and,
in some cases, the water may contribute a large part of the radon that
is present in a home. A study by Mose and others (1990) found that cancer
occurrences increase as the amount of radon in household water increases.
The U.S. Environmental Protection Agency (USEPA) has not yet established
a Maximum Contaminant Level (MCL) for radon in drinking water.
|Figure 1. Radon entering
a home through the water system (Modified from Otton, 1992, by Lindsey
and Ator, 1996).
Water-borne radon commonly is a concern only for those who use well water
for their water supply. In 1990, about 35 percent of the population of
Chester County approximately 130,000 people) relied on ground water from
individual home wells as a source of water supply (Chester County Planning
Commission, 1996). Because of its short half-life, radon in some public
water supplies utilizing ground water may decay to low concentrations
before the water is delivered to users, especially if the water has been
treated. Also, public suppliers commonly use surface-water supplies, which
generally have very low radon concentrations (Zapecza and Szabo, 1988).
Where Did We Test for Radon?
Since 1986, analysis for radon has been a standard component of ground-water-quality
studies conducted by the USGS in cooperation with the Chester County Water
Resources Authority and the Chester County Health Department. Water samples
collected from November 1986 through September 1996 from 534 wells in
Chester County were analyzed for radon for studies done in cooperation
with Chester County and Commonwealth of Pennsylvania agencies. Most wells
sampled were household wells equipped with submersible pumps. Samples
were collected from the household plumbing system, and, if present, filters
or treatment systems were bypassed.
Water samples from all major geologic units in Chester County were analyzed
for radon. The geologic units that underlie the county have been grouped
into six geohydrologic groups (Sloto, 1994) on the basis of similar geologic,
water-bearing, and ground-water-flow system characteristics (except for
group six). These groups are (1) Triassic-age sedimentary rocks (Stockton
Formation, Hammer Creek Formation, and other sandstone, shale, and conglomerate
units), (2) carbonate rocks (Conestoga Limestone and other limestone,
dolomite, and marble units), (3) quartzite rocks (Chickies Quartzite and
Setters Formation), (4) schist and phyllite (including Octoraro Phyllite,
Peters Creek Schist, and Wissahickon Schist), (5) gneiss, and (6) all
other rocks (including ultramafite/serpentinite) (fig. 2). The last geohydrologic
group (all other rocks) includes geologic units of small areal extent
that are not important water-bearing units, are intrusive rocks, or do
not fit into other categories.
|Figure 2. Generalized geologic map of Chester
County showing geohydrologic groups.
The sample distribution in the 759-square-mile county averaged one per
1.4 square miles, although the distribution was not areally uniform because
some geologic units were sampled more extensively for previous studies.
Sample density also varied among geologic units, and a few units were
not sampled because of their small areal extent. In addition, 28 wells
were sampled more than once, including 1 well sampled monthly for 3 years,
4 wells sampled quarterly for 1 year, and 5 wells sampled bi-monthly for
more than 1 year.
What Did We Find?
Ground-water samples collected from 1989 to 1996 were analyzed at the
USGS National Water-Quality Laboratory in Arvada, Colo., and samples collected
from 1986 to 1988 were analyzed at the University of Maine. Sample results
were analyzed to determine patterns in the radon concentration among the
geologic units of Chester County. Because the radon concentration in ground
water from each geologic unit is highly variable, concentrations are reported
as the median concentration. The median is the concentration at which
half of the samples have a lower concentration and half have a higher
concentration. Median radon concentrations are useful to draw general
conclusions about the presence of radon in ground water in a particular
geologic unit but should not be used to predict concentrations at specific
A map of geologic units grouped by ranges in median radon concentrations
in ground water is shown in figure 3. Although this map is useful in assessing
the general radon potential of ground water in each geologic unit, differences
within a geologic unit are apparent when studied in more detail. For example,
in southern Chester County, ground water in the northern part of the Wissahickon
Schist contains higher median radon concentrations than ground water in
the southern part of the Wissahickon Schist (Senior, 1996).
The lowest radon concentration measured was 50 pCi/L, which is about
equal to the laboratorys minimum reporting level for the analytical
method. The median concentration was 1,500 pCi/L. The maximum concentration
measured was 53,000 pCi/L. Water from about 60 percent of the wells sampled
contained radon concentrations at or above 1,000 pCi/L. This is higher
than the study by Lindsey and Ator (1996), which reported that radon concentrations
exceeded 1,000 pCi/L in water from 31 percent of wells sampled in the
lower Susquehanna and Potomac River Basins. Water from about 12 percent
of wells sampled in Chester County contained radon concentrations greater
than 5,000 pCi/L and water from about 2 percent of the wells sampled contained
radon concentrations greater than 10,000 pCi/L.
Radon concentrations differ by rock type. The geohydrologic groups with
the highest median radon concentrations in ground water include schist
and phyllite (2,400 pCi/L) and quartzite (2,150 pCi/L). The geohydrologic
groups with lowest median radon concentrations in ground water include
carbonate rocks (540 pCi/L) and other rocks (group 6) (360 pCi/L). Water
from wells in gneiss had a median radon concentration of 1,000 pCi/L,
and water from wells in Triassic-age sedimentary rocks had a median radon
concentration of 1,300 pCi/L. Of all the individual geologic units sampled,
water in the Peters Creek Schist has the highest median radon concentration
(4,400 pCi/L), and water in the ultramafic/serpentinite unit (group 6)
has the lowest median radon concentration (150 pCi/L).
Some aquifers with high median concentrations of radon in water represent
a large percentage of the land area in Chester County. The Peters Creek
Schist, the geologic unit that underlies the second largest land area
of all the geologic units in the county (about 70 square miles, or 9 percent
of the county), has the highest median ground-water radon concentration
(4,400 pCi/L) of all the geologic units in the county. Ground water from
the Octoraro Phyllite, which underlies about 48 square miles, or 6 percent
of the county, has a median ground-water radon concentration of 3,000
pCi/L. The Wissahickon Schist, the geologic unit that underlies the largest
land area of all the geologic units in the county (184 square miles, or
about 24 percent of county area), has a median ground-water radon concentration
of 1,400 pCi/L.
Statistical relations were tested between radon concentrations and well
characteristics, such as depth or yield, and between radon concentrations
and concentrations of other chemical constituents analyzed in water samples.
Radon concentrations generally did not correlate with well characteristics,
the pH of water, or concentrations of dissolved major ions and other chemical
constituents in the water samples.
|Figure 3. Concentration of radon in ground
water in Chester County (Geologic map form Sloto, 1994)
Why is Radon Higher in Some Areas Than in Others?
An important factor affecting the amount of radon in ground water in
Chester County is the underlying geologic unit. Many factors that affect
the formation and movement of radon in ground water, such as uranium content,
grain size, permeability, and the nature and extent of fracturing in the
host rock, are functions of rock type (Otton, 1992). Rocks such as schist
typically contain higher concentrations of uranium than rocks such as
limestone; therefore, they can be expected to have higher ground-water
Ground-water radon concentrations are highly variable, even within individual
geologic units. The range of radon concentrations probably reflects the
variable distribution of uranium or radium in the aquifer and the variable
distribution of aquifer properties. Radon concentrations can range up
to three orders of magnitude in water from a single geologic unit and
can differ significantly from well to well locally. Some apparent differences
in the observed range of radon concentrations may be because of variable
sample size. The largest range of radon concentrations was in geologic
units (Chickies Quartzite and Wissahickon Schist) with the largest number
of samples and, therefore, the most likely to include extreme values.
For geologic units with 10 or more samples, the range in radon concentrations
was least in water in the Stockton Formation, Hammer Creek Formation,
and the Conestoga Limestone, which may indicate that uranium or radium
concentrations in water-bearing zones of these aquifers are less variable
than in other geologic units.
Do Radon Concentrations Vary With Time?
Radon concentrations in ground water vary with time because of factors
such as dilution by recharge or changes in contributing areas of the aquifer
because of pumping (Senior, 1998). Data on temporal variability of radon
in ground water is available for 28 wells that were sampled more than
once. The maximum difference in radon concentrations in two samples collected
from the same well was 5,100 pCi/L, and the median difference in radon
concentrations in two samples collected from the same well was 350 pCi/L.
Radon concentrations differed by more than 50 percent in water from 36
percent of the wells sampled more than once and differed by more than
100 percent (a factor of two) in water from 18 percent of the wells sampled
more than once.
Thirteen wells were sampled periodically while pumping to assess radon
variability during the period of sample collection. Some variability may
be caused by short-term fluctuations in concentration while pumping, although
this variability usually was relatively low or the result of sampling
error. The difference between measured radon concentrations for samples
collected during pumping (typically about 1 hour in duration) was less
than 15 percent in most (85 percent) of the 18 wells sampled multiple
times while pumping.
Wells sampled at regular intervals provide data on seasonal variability.
Of the 10 wells that were sampled at regular intervals for at least 1
year, seasonal fluctuations in radon concentrations were observed in water
samples collected from only 1 well. For most wells, radon concentrations
did not exhibit a strong seasonal pattern of fluctuation (fig. 4) and
also were not consistently related to changes in depth to water.
|Figure 4. Variability with time in the concentration
of radon dissolved in water from a well in northern Chester County
What Does This Information on Radon Mean to
Me As A Homeowner?
Although radon concentrations in ground water in Chester County are
more likely to be higher in certain areas relative to others, the only
way to be certain of the radon concentration in the water supplied by
any well is to have the water tested. Although differences in median radon
concentration between geologic units are apparent, local variability is
high within each unit. Therefore, prediction of radon concentration at
an individual well is difficult. The USEPA and the Surgeon General of
the United States recommend that the basement, first floor, and second
floor of all homes be tested for airborne 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 indoor radon levels are
measured and where water is supplied by a privately owned well, the USEPA
further recommends that the well water be tested as a potential contributing
source of the airborne radon. If a large percentage of the radon in your
house is from your water, the USEPA recommends installing a water treatment
system to remove radon. Both homes and water supplies can be treated to
reduce radon levels (U.S. Environmental Protection Agency, 1992).
Where Can I Get More Information?
Chester County Planning Commission, 1996, Water resources use
and service in Chester County: Chester County Water Resources Management
Plan-Phase 2, [variously paginated].
Cothern, C.R., 1987, Estimating the health risks of radon in drinking
water: Journal of the American Water Works Association, v. 79, no.
4, p. 153-158.
Crawford-Brown, D.J., 1990, Analysis of health risk from ingested
radon, in Cothern, C.R. and Rebers, P.A., eds., Radon, radium, and
uranium in drinking water: Chelsea, Mich., Lewis Publishers, p.
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.
Mills, W.A., 1990, Risk assessment and control management of radon
in drinking water, in Cothern, C.R. and Rebers, P.A., eds., Radon,
radium, and uranium in drinking water: Chelsea, Mich., Lewis Publishers,
Milry, Paul, and Cothern, C.R., 1990, Scientific background for
development of regulations for radionuclides in drinking water,
in Cothern, C.R., and Rebers, P.A., eds., Radon, radium, and uranium
in drinking water: Chelsea, Mich., Lewis Publishers, p. 1-16.
Mose, D.G., Mushrush, G.W, and Chrosinak, C., 1990, Radioactive
hazard of potable water in Virginia and Maryland: Bulletin of Environmental
Contamination and Toxicology, v. 44, no. 4, p. 508-513.
Otton, J.K., 1992, The geology of radon: U.S. Geological Survey,
General Interest Publications of the U.S. Geological Survey, 28
Schumann, R.R., 1993, Geologic radon potential of EPA Region 3 -
Delaware, Maryland, Pennsylvania, Virginia, and West Virginia: U.S.
Geological Survey Open-File Report 93-292-C, 185 p.
Senior, L.A., 1996, Ground-water quality and its relation to hydrogeology,
land use, and surface-water quality in the Red Clay Creek Basin,
Piedmont Physiographic Province, Pennsylvania and Delaware: U.S.
Geological Survey Water-Resources Investigations Report 96-4288,
_____1998, Radon-222 in the ground water of Chester County, Pennsylvania:
U.S. Geological Survey Water-Resources Investigations Report 98-4169,
Senior, L.A., and Vogel, K.L., 1995, Radium and radon in ground
water in the Chickies quartzite, southeastern Pennsylvania: U.S.
Geological Survey Water-Resources Investigations Report 92-4088,
Sloto, R.A., 1994, Geology, hydrology, and ground-water quality
of Chester County, Pennsylvania: West Chester, Pa., Chester County
Water Resources Authority Water-Resource Report 2, 127 p.
Swistock, B.R., Sharpe, W.E., and Robillard, P.D., 1993, A survey
of lead, nitrate, and radon contamination of private individual
water systems in Pennsylvania: Journal of Environmental Health,
March 1993, v. 55, no. 5., p. 6-12.
U.S. Environmental Protection Agency and Centers for Disease Control,
1992, A citizens guide to radonThe guide to protecting
yourself and your family from radon (2d ed.): EPA 402-K92-001, 15
U.S. Environmental Protection Agency, 1992, Consumers guide to
radon reductionHow to reduce radon levels in your home: EPA
402-K92-003, 17 p.
_____1993, Home buyers and sellers guide to radon:
EPA 402-R-93-003, 32 p.
_____1994, National primary drinking water standards: EPA 810-F-94-001A,
_____1997, Withdrawal of the proposed national primary drinking
water regulation for radon-222: Office of Ground Water and Drinking
Water, Federal Register Document 97-20666.
Zapecza, O.S., and Szabo, Zoltan, 1988, Natural radioactivity
in ground water A review: U.S. Geological Survey Water-Supply
Paper 2325, p. 50-57.
Ronald A. Sloto and Lisa A. Senior1998
More detailed information on radon in the ground water of Chester
County is included in the technical report by Senior (1998). Government
contacts and additional reference materials for more information
about radon in water or radon in general are listed below. For current
information on USEPA MCLs for radon in water, contact the
USEPA Safe Drinking Water Hotline (1-800-426-4791). Contacts for
individual state and county agencies are included for information
on current regulations. The data on which this fact sheet is based
can be obtained by contacting the USGS office in Malvern, Pa. (610-647-9008)
For additional information, contact:
Director, USGS Pennsylvania Water Science Center
U.S. Geological Survey, WRD
840 Market Street
Lemoyne, PA 17043-1586
Chester County Water Resources Authority
Government Services Center, Suite 270
P.O. Box 2747
601 Westtown Road
West Chester, PA 19380-0990
Environmental Health Director
Chester County Health Department
Government Services Center, Suite 295
P.O. Box 2747
601 Westtown Road
West Chester, PA 19380-0990
Safe Drinking Water Hotline - USEPA, Office of Water and Drinking
Water, 4601 Resource Center, 401 M Street S.W., Washington, D.C.,
20460, (800) 426-4791
United States Environmental Protection Agency,
Region 3, 1650 Arch Street, Philadelphia, PA,
19103-2029, (215) 814-9800
National Radon Hotline, Box 33435, Washington, D.C., 20035-0435,
Pennsylvania Department of Environmental Protection, Bureau of
Radiation Protection, Harrisburg, PA,
(800) 237-2366 or (717)783-3594
Internet/World Wide Web Information:
USGS Radon Home Page:
USEPA Radon Information:
USEPA Radon Publications:
National Safety Council, Environmental Health Center,
Radon and Indoor Air Quality:
USGS Pennsylvania Water Science Center Home Page: