&EPA
United States
Environmental Protection
Agency
Air and Radiation
(6604J)
4O2-R-93-O2S
September 1993
EPA's Map of Radon Zones
COLORADO
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EPA'S MAP OF RADON ZONES
COLORADO
RADON DIVISION
OFFICE OF RADIATION AND INDOOR AIR
U.S. ENVIRONMENTAL PROTECTION AGENCY
SEPTEMBER, 1993
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ACKNOWLEDGEMENTS
This document was prepared by the U.S. Environmental Protection Agency's (EPA's)
Office of Radiation and Indoor Air (ORIA) in conjunction with the U.S. Geological Survey
(USGS). Sharon W. White was the EPA project manager. Numerous other people in ORIA
were instrumental in the development of the Map of Radon Zones, including Lisa Ratcliff,
Kirk Maconaughey, R. Thomas Peake, Dave Rowson, and Steve Page.
EPA would especially like to acknowledge the outstanding effort of the USGS
radon team — Linda Gundersen, Randy Schumann, Jim Otton, Doug Owen, Russell
Dubiel, Kendell Dickinson, and Sandra Szarzi — in developing the technical base for the
Map of Radon Zones.
ORIA would also like to recognize the efforts of all the EPA Regional Offices in
coordinating the reviews with the State programs and the Association of American State
Geologists (AASG) for providing a liaison with the State geological surveys. In addition,
appreciation is expressed to all of the State radon programs and geological surveys for their
technical input and review of the Map of Radon Zones.
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II. THE USGS/EPA RADON POTENTIAL
ASSESSMENTS:INTRODUCTION
TABLE OF CONTENTS
I. OVERVIEW
III. REGION 8 GEOLOGIC RADON POTENTIAL
SUMMARY
V. PRELIMINARY GEOLOGIC RADON POTENTIAL
ASSESSMENT OF COLORADO
V. EPA'S MAP OF RADON ZONES -- COLORADO
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OVERVIEW
Sections 307 and 309 of the 1988 Indoor Radon Abatement Act (IRAA) direct EPA to
identify areas of the United States that have the potential to produce elevated levels of radon.
EPA, the U.S. Geological Survey (USGS), and the Association of American State Geologists
(AASG) have worked closely over the past several years to produce a series of maps and
documents which address these directives. The EPA Map of Radon Zones is a compilation of
that work and fulfills the requirements of sections 307 and 309 of IRAA. The Map of Radon
Zones identifies, on a county-by-county basis, areas of the U.S. that have the highest potential
for elevated indoor radon levels (greater than 4 pCi/L).
The Map of Radon Zones is designed to assist national, State and local governments
and organizations to target their radon program activities and resources. It is also intended to
help building code officials determine areas that are the highest priority for adopting radon-
resistant building practices. The Map of Radon Zones should not be used to determine if
individual homes in any given area need to be tested for radon. EPA recommends that all
homes be tested for radon, regardless of geographic location or the zone designation of
the county in which they are located.
This document provides background information concerning the development of the
Map of Radon Zones. It explains the purposes of the map, the approach for developing the
map (including the respective roles of EPA and USGS), the data sources used, the conclusions
and confidence levels developed for the prediction of radon potential, and the review process
that was conducted to finalize this effort.
BACKGROUND
Radon (Rn222) is a colorless, odorless, radioactive gas. It comes from the natural
decay of uranium that is found in nearly all soils. It typically moves through the ground to
the air above and into homes and other buildings through cracks and openings in the
foundation. Any home, school or workplace may have a radon problem, regardless of
whether it is new or old, well-sealed or drafty, or with or without a basement. Nearly one out
of every 15 homes in the U.S. is estimated to have elevated annual average levels of indoor
radon.
Radon first gained national attention in early 1984, when extremely high levels of
indoor radon were found in areas of Pennsylvania, New Jersey, and New York, along the
Reading Prong-physiographic province. EPA established a Radon Program in 1985 to assist
States and homeowners in reducing their risk of lung cancer from indoor radon.
Since 1985, EPA and USGS have been working together to continually increase our
understanding of radon sources and the migration dynamics that cause elevated indoor radon
levels. Early efforts resulted in the 1987 map entitled "Areas with Potentially High Radon
_Leygls_."_ This map was based on Ijmited geologic information only because few indoor radon
measurements were available at the time. The development of EPA's Map of Radon Zones
and its technical foundation, USGS' National Geologic Radon Province Map, has been based
on additional information from six years of the State/EPA Residential Radon Surveys,
independent State residential surveys, and continued expansion of geologic and geophysical
information, particularly the data from the National Uranium Resource Evaluation project.
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Purpose of the Map of Radon Zones
EPA's Map of Radon Zones (Figure 1) assigns each of the 3141 counties in the
United States to one of three zones:
o Zone 1 counties have a predicted average indoor screening level > than
4 pCi/L
o Zone 2 counties have a predicted average screening level > 2 pCi/L and
< 4 pCi/L
o Zone 3 counties have a predicted average screening level < 2 pCi/L
The Zone designations were determined by assessing five factors that are known to be
important indicators of radon potential: indoor radon measurements, geology, aerial
radioactivity, soil parameters, and foundation types.
The predictions of average screening levels in each of the Zones is an expression of
radon potential in the lowest liveable area of a structure. This map is unable to estimate
actual exposures to radon. EPA recommends methods for testing and fixing individual homes
based on an estimate of actual exposure to radon. For more information on testing and fixing
elevated radon levels in homes consult these EPA publications: A Citizen's Guide to Radon,
the Consumer's Guide (o Radon Reduction and the Home Buyer's and Seller's Guide to
Radon,
EPA believes that States, local governments and other organizations can achieve
optimal risk reductions by targeting resources and program activities to high radon potential
areas. Emphasizing targeted approaches (technical assistance, information and outreach
efforts, promotion of real estate mandates and policies and building codes, etc.) in such areas
addresses the greatest potential risks first.
EPA also believes that the use of passive radon control systems in the construction of
new homes in Zone 1 counties, and the activation of those systems if necessitated by follow-
up testing, is a cost effective approach to achieving significant radon risk reduction.
The Map of Radon Zones and its supporting documentation establish no regulatory
requirements. Use of this map by State or local radon programs and building code officials is
voluntary. The information presented on the Map of Radon Zones and in the supporting
documentation is not applicable to radon in water.
Development of the Map of Radon Zones
The technical foundation for the Map of Radon Zones is the USGS Geologic Radon
Province Map. _In prderjp e_xmineJhejadjan,j)jol^^ Sjates^thjLUSGS
began by identifying approximately 360 separate geologic provinces for the U.S. The
provinces are shown on the USGS Geologic Radon Province Map (Figure 2). Each of the
geologic provinces was evaluated by examining the available data for that area: indoor radon
measurements, geology, aerial radioactivity, soil parameters, and foundation types. As stated
previously, these five factors are considered to be of basic importance in assessing radon
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potential and some data are available for each of these factors in every geologic province. The
province boundaries do not coincide with political borders (county and state) but define areas
of general radon potential. The five factors were assigned numerical values based on an
assessment of their respective contribution to radon potential, and a confidence level was
assigned to each contributing variable. The approach used by USGS to estimate the radon
potential for each province is described in Part II of this document.
EPA subsequently developed the Map of Radon Zones by extrapolating from the
province level to the county level so that all counties in the U.S. were assigned to one of
three radon zones. EPA assigned each county to a given zone based on its provincial radon
potential. For example, if a county is located within a geologic province that has a predicted
average screening level greater than 4 pCi/L, it was assigned to Zone 1. Likewise, counties
located in provinces with predicted average screening levels > 2 pCi/L and < 4 pCi/L, and
less than 2 pCi/L, were assigned to Zones 2 and 3, respectively.
If the boundaries of a county fall in more than one geologic province, the county was
assigned to a zone based on the predicted radon potential of the province in which most of
the area lies. For example, if three different provinces cross through a given county, the
county was assigned to the zone representing the radon potential of the province containing
most of the county's land area. (In this case, it is not technically correct to say that the
predicted average screening level applies to the entire county since the county falls in
multiple provinces with differing radon potentials.)
Figures 3 and 4 demonstrate an example of how EPA extrapolated the county zone
designations for Nebraska from the USGS geologic province map for the State. As figure 3
shows, USGS has identified 5 geologic provinces for Nebraska. Most of the counties are
extrapolated "straight" from their corresponding provinces, but there are counties "partitioned"
by several provinces — for example, Lincoln County. Although Lincoln county falls in
multiple provinces, it was assigned to Zone 3 because most of its area falls in the province
with the lowest radon potential.
It is important to note that EPA's extrapolation from the province level to the
county level may mask significant "highs" and "lows" within specific counties. In other
words, within-county variations in radon potential are not shown on the Map of Radon
Zones. EPA recommends that users who may need to address specific within-county
variations in radon potential (e.g., local government officials considering the
implementation of radon-resistant construction codes) consult USGS' Geologic Radon
Province Map and the State chapters provided with this map for more detailed
information, as well as any locally available data.
Map Validation
The Map of Radon Zones is intended to represent a preliminary assessment of radon
potential for the entire United States. The factors that are used in this effort -indoor radon
data, geology, aerial radioactivity, soils, and foundation type — are basic indicators for radon
potential. It is important to note, however, that the map's county zone designations are not
"statistically valid" predictions due to the nature of the data available for these 5 factors at the
county level. In order to validate the map in light of this lack of statistical confidence, EPA
conducted a number of analyses. These analyses have helped EPA to identify the best
situations in which to apply the map, and its limitations.
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Figure 3
Geologic Radon Potential Provinces for Nebraska
Ut> 4 e r 11 e
Loi
Figure 4
NEBRASKA - EPA Map of Radon Zones
Li tea I a J3p u n t y -^
Zoae 1 Zone 2 Zone 3
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One such analysis involved comparing county zone designations to indoor radon
measurements from the State/EPA Residential Radon Surveys (SRRS). Screening averages
for counties with at least 100 measurements were compared to the counties' predicted radon
potential as indicated by the Map of Radon Zones. EPA found that 72% of the county
screening averages were correctly reflected by the appropriate zone designations on the Map.
In all other cases, they only differed by 1 zone.
Another accuracy analysis used the annual average data from the National Residential
Radon Survey (NRRS). The NRRS indicated that approximately 6 million homes in the
United States have annual averages greater than or equal to 4 pCi/L. By cross checking the
county location of the approximately 5,700 homes which participated in the survey, their
radon measurements, and the zone designations for these counties, EPA found that
approximately 3.8 million homes of the 5.4 million homes with radon levels greater than or
equal to 4 pCi/L will be found in counties designated as Zone 1. A random sampling of an
equal number of counties would have only found approximately 1.8 million homes greater
than 4 pCi/L. In other words, this analysis indicated that the map approach is three times
more efficient at identifying high radon areas than random selection of zone designations.
Together, these analyses show that the approach EPA used to develop the Map of
Radon Zones is a reasonable one. In addition, the Agency's confidence is enhanced by results
of the extensive State review process -- the map generally agrees with the States' knowledge
of and experience in their own jurisdictions. However, the accuracy analyses highlight two
important points: the fact that elevated levels will be found in Zones 2 and 3, and that there
will be significant numbers of homes with lower indoor radon levels in all of the Zones. For
these reasons, users of the Map of Radon Zones need to supplement the Map with locally
available data whenever possible. Although all known "hot spots", i.e., localized areas of
consistently elevated levels, are discussed in the State-
specific chapters, accurately defining the boundaries of the "hot spots" on this scale of map is
not possible at this time. Also, unknown "hot spots" do exist.
The Map of Radon Zones is intended to be a starting point for characterizing radon
potential because our knowledge of radon sources and transport is always growing. Although
this effort represents the best data available at this time, EPA will continue to study these
parameters and others such as house construction, ventilation features and meteorology factors
in order to better characterize the presence of radon in U.S homes, especially in high risk
areas. These efforts will eventually assist EPA in refining and revising the conclusions of the
Map of Radon Zones. And although this map is most appropriately used as a targeting tool
by the aforementioned audiences - the Agency encourages all residents to test their homes
for radon, regardless of geographic location or the zone designation of the county in
which they live. Similarly, the Map of Radon Zones should not to be used in lieu of
testing during real estate transactions.
Review Process
The Map of Radon Zones has undergone extensive review within EPA and outside the
Agency. The Association of American State Geologists (AASG) played an integral role in
this review process. The AASG individual State geologists have reviewed their State-specific
information, the USGS Geologic Radon Province Map, and other materials for their geologic
content and consistency.
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In addition to each State geologist providing technical comments, the State radon
offices were asked to comment on their respective States' radon potential evaluations. In
particular, the States were asked to evaluate the data used to assign their counties to specific
zones. EPA and USGS worked with the States to resolve any issues concerning county zone
designations. In a few cases, States have requested changes in county zone designations. The
requests were based on additional data from the State on geology, indoor radon
measurements, population, etc. Upon reviewing the data submitted by the States, EPA did
make some changes in zone designations. These changes, which do not strictly follow the
methodology outlined in this document, are discussed in the respective State chapters.
EPA encourages the States and counties to conduct further research and data collection
efforts to refine the Map of Radon Zones. EPA would like to be kept informed of any
changes the States, counties, or others make to the maps. Updates and revisions will be
handled in a similar fashion to the way the map was developed. States should notify EPA of
any proposed changes by forwarding the changes through the Regional EPA offices that are
listed in Part II. Depending on the amount of new information that is presented, EPA will
consider updating this map periodically. The State radon programs should initiate proper
notification of the appropriate State officials when the Map of Radon Zones is released and
when revisions or updates are made by the State or EPA.
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THE USGS/EPA RADON POTENTIAL ASSESSMENTS: AN INTRODUCTION
by
Linda C.S. Gundersen and R. Randall Schumann
U.S. Geological Survey
and
Sharon W. White
U.S. Environmental Protection Agency
BACKGROUND
The Indoor Radon Abatement Act of 1988 (15 U.S.C. 2661-2671) directed the U.S.
Environmental Protection Agency (EPA) to identify areas of the United States that have the
potential to produce harmful levels of indoor radon. These characterizations were to be based
on both geological data and on indoor radon levels in homes and other structures. The EPA
also was directed to develop model standards and techniques for new building construction
that would provide adequate prevention or mitigation of radon entry. As part of an
Interagency Agreement between the EPA and the U.S. Geological Survey (USGS), the USGS
has prepared radon potential estimates for the United States. This report is one of ten
booklets that document this effort. The purpose and intended use of these reports is to help
identify areas where states can target their radon program resources, to provide guidance in
selecting the most appropriate building code options for areas, and to provide general
information on radon and geology for each state for federal, state, and municipal officials
dealing with radon issues. These reports are nor intended to be used as a substitute for
indoor radon testing, and they cannot and should not be used to estimate or predict the
indoor radon concentrations of individual homes, building sites, or housing tracts. Elevated
levels of indoor radon have been found in every State, and EPA recommends that all homes
be tested for indoor radon.
Booklets detailing the radon potential assessment for the U.S. have been developed for
each State. USGS geologists are the authors of the geologic radon potential booklets. Each
booklet consists of several components, the first being an overview to the mapping project
(Part I), this introduction to the USGS assessment (Part II), including a general discussion of
radon (occurrence, transport, etc.), and details concerning the types of data used. The third
component is a summary chapter outlining the general geology and geologic radon potential
of the EPA Region (Part III). The fourth component is an individual chapter for each state
(Part IV). Each state chapter discusses the state's specific geographic setting, soils, geologic
setting, geologic radon potential, indoor radon data, and a summary outlining the radon
potential rankings of geologic areas in the state. A variety of maps are presented in each
chapter—geologic, geographic, population, soils, aerial radioactivity, and indoor radon data by
~cWnty7 "Fin'allyVlhe'b^ok^^
accompanying description (Part V).
Because of constraints on the scales of maps presented in these reports and because the
smallest units used to present the indoor radon data are counties, some generalizations have
been made in order to estimate the radon potential of each area. Variations in geology, soil
characteristics, climatic factors, homeowner lifestyles, and other factors that influence radon
concentrations can be quite large within any particular geologic area, so these reports cannot
be used to estimate or predict the indoor radon concentrations of individual homes or housing
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tracts. Within any area of a given geologic radon potential ranking, there are likely to be
areas where the radon potential is lower or higher than that assigned to the area as a whole,
especially in larger areas such as the large counties in some western states.
In each state chapter, references to additional reports related to radon are listed for the
state, and the reader is urged to consult these reports for more detailed information. In most
cases the best sources of information on radon for specific areas are state and local
departments of health, state departments responsible for nuclear safety or environmental
protection, and U.S. EPA regional offices. More detailed information on state or local
geology may be obtained from the state geological surveys. Addresses and telephone
numbers of state radon contacts, geological surveys, and EPA regional offices are listed in
Appendix C at the end of this chapter.
RADON GENERATION AND TRANSPORT IN SOILS
Radon (2"Rn) is produced from the radioactive decay of radium (226Ra), which is, in turn,
a product of the decay of uranium (23SU) (fig. 1). The half-life of 222Rn is 3.825 days. Other
isotopes of radon occur naturally, but; with the exception of th&ron (220Rn), which occurs in
concentrations high enough to be of concern in a few localized areas, they are less important
in terms of indoor radon risk because of their extremely short half-lives and less common
occurrence. In general, the concentration and mobility of radon in soil are dependent on
several factors, the most important of which are the soil's radium content and distribution,
porosity, permeability to gas movement, and moisture content. These characteristics are, in
turn, determined by the soil's parent-material composition, climate, and the soil's age or
maturity. If parent-material composition, climate, vegetation, age of the soil, and topography
are known, the physical and chemical properties of a soil in a given area can be predicted.
As soils form, they develop distinct layers, or horizons, that are cumulatively called the
soil profile. The A horizon is a surface or near-surface horizon containing a relative
abundance of organic matter but dominated by mineral matter. Some soils contain an E
horizon, directly below the A horizon., that is generally characterized by loss of clays, iron, or
aluminum, and has a characteristically lighter color than the A horizon. The B horizon
underlies the A or E horizon. Important characteristics of B horizons include accumulation of
clays, iron oxides, calcium carbonate or other soluble salts, and organic matter complexes. In
drier environments, a horizon may exist within or below the B horizon that is dominated by
calcium carbonate, often called caliche or calcrete. This carbonate-cemented horizon is
designated the K horizon in modern soil classification schemes. The C horizon underlies the
B (or K) and is a zone of weathered parent material that does not exhibit characteristics of A
or B horizons; that is, it is generally not a zone of leaching or accumulation. In soils formed
in place from the underlying bedrock, the C horizon is a zone of unconsolidated, weathered
bedrock overlying the unweathered bedrock".
The shape and orientation=of-soil particles-(soil structure)-control-permeability and-affect—
water movement in the soil. Soils with blocky or granular structure have roughly equivalent
permeabilities in the horizontal and vertical directions, and air and water can infiltrate the soil
relatively easily. However, in soils with platy structure, horizontal permeability is much
greater than vertical permeability, and air and moisture infiltration is generally slow. Soils
with prismatic or columnar structure have dominantly vertical permeability. Platy and
prismatic structures form in soils with high clay contents. In soils with shrink-swell clays, air
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and moisture infiltration rates and depth of wetting may be limited when the cracks in the
surface soil layers swell shut. Clay-rich B horizons, particularly those with massive or platy
structure, can form a capping layer that impedes the escape of soil gas to the surface
(Schumann and others, 1992). However, the shrinkage of clays can act to open or widen
cracks upon drying, thus increasing the soil's permeability to gas flow during drier periods.
Radon transport in soils occurs by two processes: (1) diffusion and (2) flow (Tanner,
1964). Diffusion is the process whereby radon atoms move from areas of higher
concentration to areas of lower concentration in response to a concentration gradient. Flow is
the process by which soil air moves through soil pores in response to differences in pressure
within the soil or between the soil and the atmosphere, carrying the radon atoms along with it.
Diffusion is the dominant radon transport process in soils of low permeability, whereas flow
tends to dominate in highly permeable soils (Sextro and others, 1987). In low-permeability
soils, much of the radon may decay before it is able to enter a building because its transport
rate is reduced. Conversely, highly permeable soils, even those that are relatively low in
radium, such as those derived from some types of glacial deposits, have been associated with
high indoor radon levels in Europe and in the northern United States (Akerblom and others,
1984; Kunz and others, 1989; Sextro and others, 1987). In area's of karst topography formed
in carbonate rock (limestone or dolomite) environments, solution cavities and fissures can
increase soil permeability at depth by providing additional pathways for gas flow.
Not all radium contained in soil grains and grain coatings will result in mobile radon
when the radium decays. Depending on where the radium is distributed in the soil, many of
the radon atoms may remain imbedded in the soil grain containing the parent radium atom, or
become imbedded in adjacent soil grains. The portion of radium that releases radon into the
pores and fractures of rocks and soils is called the emanating fraction. When a radium atom
decays to radon, the energy generated is strong enough to send the radon atom a distance of
about 40 nanometers (1 nm = 10'9 meters), or about 2xlO'6 inches—this is known as alpha
recoil (Tanner, 1980). Moisture in the soil lessens the chance of a recoiling radon atom
becoming imbedded in an adjacent grain. Because water is more dense than air, a radon atom
will travel a shorter distance in a water-filled pore than in an air-filled pore, thus increasing
the likelihood that the radon atom will remain in the pore space. Intermediate moisture levels
enhance radon emanation but do not significantly affect permeability. However, high
moisture levels can significantly decrease the gas permeability of the soil and impede radon
movement through the soil.
Concentrations of radon in soils are generally many times higher than those inside of
buildings, ranging from tens of pCi/L to more than 100,000 pCi/L, but typically in the range
of hundreds to low thousands of pCi/L. Soil-gas radon concentrations can vary in response to
variations in climate and weather on hourly, daily, or seasonal time scales. Schumann and
others (1992) and Rose and others (1988) recorded order-of-magnitude variations in soil-gas
radon concentrations between-seasons-in Colorado-and-Pennsylvaniar The most-important
factors appear to be (1) soil moisture conditions, which are controlled in large part by
precipitation; (2) barometric pressure; and (3) temperature. Washington and Rose (1990)
suggest that temperature-controlled partitioning of radon between water and gas in soil pores
also has a significant influence on the amount of mobile radon in soil gas.
Homes in hilly limestone regions of the southern Appalachians were found to have higher
indoor radon concentrations during the summer than in the winter. A suggested cause for this
phenomenon involves temperature/pressure-driven flow of radon-laden air from subsurface
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solution cavities in the carbonate rock into houses. As warm air enters solution cavities that
are higher on the hillslope than the homes, it cools and settles, pushing radon-laden air from
lower in the cave or cavity system into structures on the hillslope (Gammage and others,
1993). In contrast, homes built over caves having openings situated below the level of the
home had higher indoor radon levels in the winter, caused by cooler outside air entering the
cave, driving radon-laden air into cracks and solution cavities in the rock and soil, and
ultimately, into homes (Gammage and others, 1993).
RADON ENTRY INTO BUILDINGS
A driving force '(reduced atmospheric pressure in the house relative to the soil, producing
a pressure gradient) and entry points must exist for radon to enter a building from the soil.
The negative pressure caused by furnace combustion, ventilation devices, and the stack effect
(the rising and escape of warm air from the upper floors of the building, causing a
temperature and pressure gradient within the structure) during cold winter months are
common driving forces. Cracks and other penetrations through building foundations, sump
holes, and slab-to-foundation wall joints are common entry points.
Radon levels in the basement are generally higher than those on the main floor or upper
floors of most structures. Homes with basements generally provide more entry points for
radon, commonly have a more pronounced stack effect, and typically have lower air pressure
relative to the surrounding soil than nonbasement homes. The term "nonbasement" applies to
slab-on-grade or crawl space construction.
METHODS AND SOURCES OF DATA
The assessments of radon potential in the booklets that follow this introduction were
made using five main types of data: (1) geologic (lithologic); (2) aerial radiometric; (3) soil
characteristics, including soil moisture, permeability, and drainage characteristics; (4) indoor
radon data; and (5) building architecture (specifically, whether homes in each area are built
slab-on-grade or have a basement or crawl space). These five factors were evaluated and
integrated to produce estimates of radon potential. Field measurements of soil-gas radon or
soil radioactivity were not used except where such data were available in existing, published
reports of local field studies. Where applicable, such field studies are described in the
individual state chapters.
•GEOLOGIC DATA
The types and distribution, of lithologic units and other geologic features in an
assessment area are of primary importance in determining radon potential. Rock types that
are mosriikely^o^aure indoon-a^
bearing sandstones, certain kinds of fluvial sandstones and fluvial sediments, phosphorites,
chalk, karst-producing carbonate rocks, certain kinds of glacial deposits, bauxite, uranium-rich
granitic rocks, metamorphic rocks of granitic composition, silica-rich volcanic rocks, many
sheared or faulted rocks, some coals, and certain kinds of contact metamorphosed rocks.
Rock types least likely to cause radon problems include marine quartz sands, non-
carbonaceous shales and siltstones, certain kinds of clays, silica-poor metamorphic and
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igneous rocks, and basalts. Exceptions exist within these general lithologic groups because of
the occurrence of localized uranium deposits, commonly of the hydrothermal type in
crystalline rocks or the "roll-front" type in sedimentary rocks. Uranium and radium are
commonly sited in heavy minerals, iron-oxide coatings on rock and soil grains, and organic
materials in soils and sediments. Less common are uranium associated with phusphate and
carbonate complexes in rocks and soils, and uranium minerals.
Although many cases of elevated indoor radon levels can be traced to high radium and
(or) uranium concentrations in parent rocks, some structural features, most notably faults and
shear zones, have been identified as sites of localized uranium concentrations (Deffeyes and
MacGregor, 1980) and have been associated with some of the highest reported indoor radon
levels (Gundersen, 1991). The two highest known indoor radon occurrences are associated
with sheared fault zones in Boyertown, Pennsylvania (Gundersen and others, 1988a; Smith
and others, 1987), and in Clinton, New Jersey (Henry and others, 1991; Muessig and Bell,
1988).
NURE AERIAL RADIOMETRIC DATA
Aerial radiometric data are used to quantify the radioactivity of rocks and soils.
Equivalent uranium (eU) data provide an estimate of the surficial concentrations of radon
parent materials (uranium, radium) in rocks and soils. Equivalent uranium is calculated from
the counts received by a gamma-ray detector from the 1.76 MeV (mega-electron volts)
emission energy corresponding to bismuth-214 (2UBi), with the assumption that uranium and
its decay products are in secular equilibrium. Equivalent uranium is expressed in units of
parts per million (ppm). Gamma radioactivity also may be expressed in terms of a radium
activity; 3 ppm eU corresponds to approximately 1 picocurie per gram (pCi/g) of radium-226.
Although radon is highly mobile in soil and its concentration is affected by meteorological
conditions (Kovach, 1945; Klusman and Jaacks, 1987; Schery and others, 1984; Schumann
and others, 1992), statistical correlations between average soil-gas radon concentrations and
average eU values for a wide variety of soils have been documented (Gundersen and others,
1988a, 1988b; Schumann and Owen, 1988). Aerial radiometric data can provide an estimate
of radon source strength over a region, but the amount of radon that is able to enter a home
from the soil is dependent on several local factors, including soil structure, grain size
distribution, moisture content, and permeability, as well as type of house construction and its
structural condition.
The aerial radiometric data used for these characterizations were collected as part of the
Department of Energy National Uranium Resource Evaluation (NURE) program of the 1970s
and early 1980s. The purpose of the NURE program was to identify and describe areas in the
United States having potential uranium resources (U.S. Department of Energy, 1976). The
NURE aerial radiometric data were -collected-by^aircraft in~whirh^a"
was mounted, flying approximately 122 m (400 ft) above the ground surface. The equivalent
uranium maps presented in the state chapters were generated from reprocessed NURE data in
which smoothing, filtering, recalibrating, and matching of adjacent quadrangle data sets were
performed to compensate for background, altitude, calibration, and other types of errors and
inconsistencies in the original data set (Duval and others, 1989). The data were then gridded
and contoured to produce maps of eU with a pixel size corresponding to approximately 2.5 x
2.5 km (1.6 x 1.6 mi).
II-6 Reprinted from USGS Open-File Report 93-292
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FL1CIIT LINE SPACING Of SURE AERIAL SUKVEYS
2 KM (1 MILE)
5 KM (3 MILES)
2 & 5 KM
10 KM (6 MILES)
5 t 10 KM
NO DATA
Figure 2. Nominal flightline spacings for NURE aerial gamma-ray surveys covering the
contiguous United States (from Duval and others, 1990). Rectangles represent I°x2° quadrangles.
-------
Figure 2 is an index map of NUKE 1° x 2° quadrangles showing the flight-line spacing
for each quadrangle. In general, the more closely spaced the flightlines are, the more area
was covered by the aerial gamma survey, and thus, more detail is available in the data set.
For an altitude of 400 ft above the ground surface and with primary flightline spacing
typically between 3 and 6 miles, less than 10 percent of the ground surface of the United
States was actually measured by the airborne gamma-ray detectors (Duval and others, 1989),
although some areas had better coverage than others due to the differences in flight-line
spacing between areas (fig. 2). This suggests that some localized uranium anomalies may not
have been detected by the aerial surveys, but the good correlations of eU patterns with
geologic outcrop patterns indicate that, at relatively small scales (approximately 1:1,000,000
or smaller) the National eU map (Duval and others, 1989) gives reasonably good estimates of
average surface uranium concentrations and thus can assist in the prediction of radon potential
of rocks and soils, especially when augmented with additional geologic and soil data.
The shallow (20-30 cm) depth of investigation of gamma-ray spectrometers, either
ground-based or airborne (Duval and others, 1971; Durrance, 1986), suggests that gamma-ray
data may sometimes underestimate the radon-source strength in soils in which some of the
radionuclides in the near-surface soil layers have been transported downward through the soil
profile. In such cases the concentration of radioactive minerals in the A horizon would be
lower than in the B horizon, where such minerals are typically concentrated. The
concentration of radionuclides in the C horizon and below may be relatively unaffected by
surface solution processes. Under these conditions the surface gamma-ray signal may indicate
a lower radon source concentration than actually exists in the deeper soil layers, which are
most likely to affect radon levels in structures with basements. The redistribution of
radionuclides in soil profiles is dependent on a combination of climatic, geologic, and
geochemical factors. There is reason to believe that correlations of eU with actual soil
radium and uranium concentrations at a depth relevant to radon entry into structures may be
regionally variable (Duval, 1989; Schumann and Gundersen, 1991). Given sufficient
understanding of the factors cited above, these regional differences may be predictable.
SOIL SURVEY DATA
Soil surveys prepared by the U.S. Soil Conservation Service (SCS) provide data on soil
characteristics, including soil-cover thickness, grain-size distribution, permeability, shrink-
swell potential, vegetative cover, generalized groundwater characteristics, and land use. The
reports are available in county formats and State summaries. The county reports typically
contain both generalized and detailed maps of soils in the area.
Because of time and map-scale constraints, it was impractical to examine county soil
reports for each county in the United States, so more generalized summaries at appropriate
scales were used where-available,. For State-or~regional=scale-radon-characterizations,- soil
maps were compared to geologic maps of the area, and the soil descriptions, shrink-swell
potential, drainage characteristics, depth to seasonal high water table, permeability, and other
relevant characteristics of each soil group noted. Technical soil terms used in soil surveys are
generally complex; however, a good summary of soil engineering terms and the national
distribution of technical soil types is the "Soils" sheet of the National Atlas (U.S. Department
of Agriculture, 1987).
II-8 Reprinted from USGS Open-File Report 93-292
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Soil permeability is commonly expressed in SCS soil surveys in terms of the speed, in
inches per hour (in/hr), at which water soaks into the soil, as measured in a soil percolation
test. Although in/hr are not truly units of permeability, these units are in widespread use and
are referred to as "permeability" in SCS soil surveys. The permeabilities listed in the SCS
surveys are for water, but they generally correlate well with gas permeability. Because data
on gas permeability of soils is extremely limited, data on permeability to water is used as a
substitute except in cases in which excessive soil moisture is known to exist. Water in soil
pores inhibits gas transport, so the amount of radon available to a home is effectively reduced
by a high water table. Areas likely to have high water tables include river valleys, coastal
areas, and some areas overlain by deposits of glacial origin (for example, loess).
Soil permeabilities greater than 6.0 in/hr may be considered high, and permeabilities less
than 0.6 in/hr may be considered low in terms of soil-gas transport. Soils with low
permeability may generally be considered to have a lower radon potential than more
permeable soils with similar radium concentrations. Many well-developed soils contain a
clay-rich B horizon that may impede vertical soil gas transport. Radon generated below this
horizon cannot readily escape to the surface, so it would instead tend to move laterally,
especially under the influence of a negative pressure exerted by'a building.
Shrink-swell potential is an indicator of the abundance of smectitic (swelling) clays in a
soil. Soils with a high shrink-swell potential may cause building foundations to crack,
creating pathways for radon entry into the structure. During dry periods, desiccation cracks in
shrink-swell soils provide additional pathways for soil-gas transport and effectively increase
the gas permeability of the soil. Soil permeability data and soil profile data thus provide
important information for regional radon assessments.
INDOOR RADON DATA
Two major sources of indoor radon data were used. The first and largest source of data is
from the State/EPA Residential Radon Survey (Ronca-Battista and others, 1988; Dziuban and
others, 1990). Forty-two states completed EPA-sponsored indoor radon surveys between 1986
and 1992 (figi 3). The State/EPA Residential Radon Surveys were designed to be
comprehensive and statistically significant at the state level, and were subjected to high levels
of quality assurance and control. The surveys collected screening indoor radon measurements,
defined as 2-7 day measurements using charcoal canister radon detectors placed in the lowest
livable area of the home. The target population for the surveys included owner-occupied
single family, detached housing units (White and others, 1989), although attached structures
such as duplexes, townhouses, or condominiums were included in some of the surveys if they
met the other criteria and had contact with the ground surface. Participants were selected
randomly from telephone-directory listings. In total, approximately 60,000 homes were tested
-in-the-State/EPA surveys.-
The second source of indoor radon data comes from residential surveys that have been
conducted in a specific state or region of the country (e.g. independent state surveys or utility
company surveys). Several states, including Delaware, Florida, Illinois, New Hampshire, New
Jersey, New York, Oregon, and Utah, have conducted their own surveys of indoor radon. The
quality and design of a state or other independent survey are discussed and referenced where
the data are used.
II-9 Reprinted from USGS Open-File Report 93-292
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.
o
ex
I
-------
Data for only those counties with five or more measurements are shown in the indoor
radon maps in the state chapters, although data for all counties with a nonzero number of
measurements are listed in the indoor radon data tables in each state chapter. In total, indoor
radon data from more than 100,000 homes nationwide were used in the compilation of these
assessments. Radon data from State or regional indoor radon surveys, public health
organizations, or other sources are discussed in addition to the primary data sources where
they are available. Nearly all of the data used in these evaluations represent short-term (2-7
day) screening measurements from the lowest livable space of the homes. Specific details
concerning the nature and use of indoor radon data sets other than the State/EPA Residential
Radon Survey are discussed in the individual State chapters.
RADON INDEX AND CONFIDENCE INDEX
Many of the geologic methods used to evaluate an area for radon potential require
subjective opinions based on the professional judgment and experience of the individual
geologist. The evaluations are nevertheless based on established scientific principles that are
universally applicable to any geographic area or geologic setting. This section describes the
methods and conceptual framework used by the U.S. Geological Survey to evaluate areas for
radon potential based on the five factors discussed in the previous sections. The scheme is
divided into two basic parts, a Radon Index (RI), used to rank the general radon potential of
the area, and the Confidence Index (CI), used to express the level of confidence in the
prediction based on the quantity and quality of the data used to make the determination. This
scheme works best if the areas to be evaluated are delineated by geologically-based
boundaries (geologic provinces) rather than political ones (state/county boundaries) in which
the geology may vary across the area.
Radon Index. Table 1 presents the Radon Index (RI) matrix. The five factors—indoor
radon data, geology, aerial radioactivity, soil parameters, and house foundation type—were
quantitatively ranked (using a point value of 1, 2, or 3) for their respective contribution to
radon potential in a given area. At least some data for the 5 factors are consistently available
for every geologic province. Because each of these main factors encompass a wide variety of
complex and variable components, the geologists performing the evaluation relied heavily on
their professional judgment and experience in assigning point values to each category and in
determining the overall radon potential ranking. Background information on these factors is
discussed in more detail in the preceding sections of this introduction.
Indoor radon was evaluated using unweighted arithmetic means of the indoor radon data
for each geologic area to be assessed. Other expressions of indoor radon levels in an area
also could have been used, such as weighted averages or annual averages, but these types of
data were not consistently available for the entire United States at the time of this writing, or
-the-schemes-were-not considered -sufficient^to-provide-a-means-of-consistent-eomparison—
across all areas. For this report, charcoal-canister screening measurement data from the
State/EPA Residential Radon Surveys and other carefully selected sources were used, as
described in the preceding section. To maintain consistency, other indoor radon data sets
(vendor, state, or other data) were not considered in scoring the indoor radon factor of the
Radon Index if they were not randomly sampled or could not be statistically combined with
the primary indoor radon data sets. However, these additional radon data sets can provide a
means to further refine correlations between geologic factors and radon potential, so they are
II-l 1 Reprinted from USGS Open-File Report 93-292
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TABLE 1. RADON INDEX MATRIX, "ppm eU" indicates parts per million of equivalent
uranium, as indicated by NURE aerial radiometric data. See text discussion for details.
INCREASING RADON POTENTIAL
FACTOR
INDOOR RADON (average)
AERIAL RADIO ACnvrTY
GEOLOGY*
SOIL PERMEABILITY
ARCHITECTURE TYPE
POINT VALUE
1
<2pCi/L
< 1.5 ppm eU
negative
low
mostly slab
2
2 - 4 pCi/L
1.5 - 2.5 ppm eU
variable
moderate
mixed
3
>4pCi/L
> 2.5 ppm eU
positive
high
mostly basement
*GEOLOGIC FIELD EVIDENCE (GFE) POINTS: GFE points are assigned in addition to points
for the "Geology" factor for specific, relevant geologic field studies. See text for details.
Geologic evidence supporting: HIGH radon +2 points
MODERATE +1 point
LOW -2 points
No relevant geologic field studies 0 points
SCORING:
Radon potential category
LOW
MODERATE/VARIABLE
HIGH
Probable average screening
Point range indoor radon for area
3-8 points
9-11 points
12-17 points
< 2 pCi/L
2-4pCi/L
>4pCi/L
POSSIBLE RANGE OF POINTS = 3 to 17
TABLE 2. CONFIDENCE INDEX MATRIX
INCREASING CONFIDENCE
FACTOR
INDOOR RADON DATA
AERIAL RADIOACTIVITY
GEOLOGIC DATA
SOIL PERMEABILITY
POINT VALUE
1
sparse/no data
.questionable/no data
questionable
questionable/no data
2
fair coverage/quality
glacial cover
variable
variable
3
good coverage/quality
no glacial cover
proven geol. model
reliable, abundant
SCORING:
LOW CONFIDENCE
MODERATE CONFIDENCE
HIGH CONFIDENCE
4-6 points
7-9 points
10 -12 points
POSSIBLE RANGE OF POINTS = 4 to 12
II-12 Reprinted from USGS Open-File Report 93-292
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included as supplementary information and are discussed in the individual State chapters. If
the average screening indoor radon level for an area was less than 2 pCi/L, the indoor radon
factor was assigned 1 point, if it was between 2 and 4 pCi/L, it was scored 2 points, and if
the average screening indoor radon level for an area was greater than 4 pCi/L, the indoor
radon factor was assigned 3 RI points.
Aerial radioactivity data used in this report are from the equivalent uranium map of the
conterminous United States compiled from NURE aerial gamma-ray surveys (Duval and
others, 1989). These data indicate the gamma radioactivity from approximately the upper 30
cm of rock and soil, expressed in units of ppm equivalent uranium. An approximate average
value of eU was determined visually for each area and point values assigned based on
whether the overall eU for the area falls below 1.5 ppm (1 point), between 1.5 and 2.5 ppm
(2 points), or greater than 2.5 ppm (3 points).
The geology factor is complex and actually incorporates many geologic characteristics. In
the matrix, "positive" and "negative" refer to the presence or absence and distribution of rock
types known to have high uranium contents and to generate elevated radon in soils or indoors.
Examples of "positive" rock types include granites, black shales, phosphatic rocks, and other
rock types described in the preceding "geologic data" section. Examples of "negative" rock
types include marine quartz sands and some clays. The term "variable" indicates that the
geology within the region is variable or that the rock types in the area are known or suspected
to generate elevated radon in some areas but not in others due to compositional differences,
climatic effects, localizeddistribution of uranium, or other factors. Geologic information
indicates not only how much uranium is present in the rocks and soils but also gives clues for
predicting general radon emanation and mobility characteristics through additional factors
such as structure (notably the presence of faults or shears) and geochemical characteristics
(for example, a phosphate-rich sandstone will likely contain more uranium than a sandstone
containing little or no phosphate because the phosphate forms chemical complexes with
uranium). "Negative", "variable", and "positive" geology were assigned 1, 2, and 3 points,
respectively.
In cases where additional reinforcing or contradictory geologic evidence is available,
Geologic Field Evidence (GFE) points were added to or subtracted from an area's score
(Table 1). Relevant geologic field studies are important to enhancing our understanding of
how geologic processes affect radon distribution. In some cases, geologic models and
supporting field data reinforced an already strong (high or low) score; in others, they provided
important contradictory data. GFE points were applied for geologically-sound evidence that
-supports-the-predietion-(but-whith may-contradict-one-or^more-factors)"on"the-basis""of"known™
geologic field studies in the area or in areas with geologic and climatic settings similar
enough that they could be applied with full confidence. For example, areas of the Dakotas,
Minnesota, and Iowa that are covered with Wisconsin-age glacial deposits exhibit a low aerial
radiometric signature and score only one RI point in that category. However, data from
geologic field studies in North Dakota and Minnesota (Schumann and others, 1991) suggest
that eU is a poor predictor of geologic radon potential in this area because radionuclides have
11-13 Reprinted from USGS Open-File Report 93-292
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been leached from the upper soil layers but are present and possibly even concentrated in
deeper soil horizons, generating significant soil-gas radon. This positive supporting field
evidence adds two GFE points to the score, which helps to counteract the invalid conclusion
suggested by the radiometric data. No GFE points are awarded if there are no documented
field studies for the area.
"Soil permeability" refers to several soil characteristics that influence radon concentration
and mobility, including soil type, grain size, structure, soil moisture, drainage, slope, and
permeability. In the matrix, "low" refers to permeabilities less than about 0.6 in/hr; "high"
corresponds to greater than about 6.0 in/hr, in U.S. Soil Conservation Service (SCS) standard
soil percolation tests. The SCS data are for water permeability, which generally correlates
well with the gas permeability of the soil except when the soil moisture content is very high.
Areas with consistently high water tables were thus considered to have low gas permeability.
"Low, "moderate", and "high" permeability were assigned 1, 2, and 3 points, respectively.
Architecture type refers to whether homes in the area have mostly basements (3 points),
mostly slab-on-grade construction (1 point), or a mixture of the'two. Split-level and crawl
space homes fall into the "mixed" category (2 points). Architecture information is necessary
to properly interpret the indoor radon data and produce geologic radon potential categories
that are consistent with screening indoor radon data.
The overall RI for an area is calculated by adding the individual RI scores for the 5
factors, plus or minus GFE points, if any. The total RI for an area falls in one of three
categories—low, moderate or variable, or high. The point ranges for the three categories were
determined by examining the possible combinations of points for the 5 factors and setting
rules such that a majority (3 of 5 factors) would determine the final score for the low and
high categories, with allowances for possible deviation from an ideal score by the other two
factors. The moderate/variable category lies between these two ranges. A total deviation of 3
points from the "ideal" score was considered reasonable to allow for natural variability of
factors—if two of the five factors are allowed to vary from the "ideal" for a category, they
can differ by a minimum of 2 (1 point different each) and a maximum of 4 points (2 points
different each). With "ideal" scores of 5, 10, and 15 points describing low, moderate, and
high geologic radon potential, respectively, an ideal low score of 5 points plus 3 points for
possible variability allows a maximum of 8 points in the low category. Similarly, an ideal
high score of 15 points minus 3 points gives a minimum of 12 points for the high category.
Note, however, that if both other factors differ by two points from the "ideal", indicating
considerable variability in the system, the total point score would lie in the adjacent (i.e.,
moderate7vanablej category. ""
Confidence Index. Except for architecture type, the same factors were used to establish a
Confidence Index (CI) for the radon potential prediction for each area (Table 2). Architecture
type was not included in the confidence index because house construction data are readily and
reliably available through surveys taken by agencies and industry groups including the
National Association of Home Builders, U.S. Department of Housing and Urban
Development, and the Federal Housing Administration; thus it was not considered necessary
11-14 Reprinted from USGS Open-File Report 93-292
-------
to question the quality or validity of these data. The other factors were scored on the basis of
the quality and quantity of the data used to complete the RI matrix.
Indoor radon data were evaluated based on the distribution and number of data points and
on whether the data were collected by random sampling (State/EPA Residential Radon Survey
or other state survey data) or volunteered vendor data (likely to be nonrandom and biased
toward population centers and/or high indoor radon levels). The categories listed in the CI
matrix for indoor radon data ("sparse or no data", "fair coverage or quality", and "good
coverage/quality") indicate the sampling density and statistical robustness of an indoor radon
data set. Data from the State/EPA Residential Radon Survey and statistically valid state
surveys were typically assigned 3 Confidence Index points unless the data were poorly
distributed or absent in the area evaluated.
Aerial radioactivity data are available for all but a few areas of the continental United
States and for part of Alaska. An evaluation of the quality of the radioactivity data was based
on whether there appeared to be a good correlation between the radioactivity and the actual
amount of uranium or radium available to generate mobile radon in the rocks and soils of the
area evaluated. In general, the greatest problems with correlations among eU, geology, and
soil-gas or indoor radon levels were associated with glacial deposits (see the discussion in a
previous section) and typically were assigned a 2-point Confidence Index score. Correlations
among eU, geology, and radon were generally sound in unglaciated areas and were usually
assigned 3 CI points. Again, however, radioactivity data in some unglaciated areas may have
been assigned fewer than 3 points, and in glaciated areas may be assigned only one point, if
the data were considered questionable or if coverage was poor.
To assign Confidence Index scores for the geologic data factor, rock types and geologic
settings for which a physical-chemical, process-based understanding of radon generation and
mobility exists were regarded as having "proven geologic models" (3 points); a high
confidence could be held for predictions in such areas. Rocks for which the processes are
less well known or for which data are contradictory were regarded as "variable" (2 points),
and those about which little is known or for which no apparent correlations have been found
were deemed "questionable" (1 point).
The soil permeability factor was also scored based on quality and amount of data. The
three categories for soil permeability in the Confidence Index are similar in concept, and
scored similarly, to those for the geologic data factor. Soil permeability can be roughly
estimated from grain size and drainage class if data from standard, accepted soil percolation
tests are unavailable; however, the reliability of the data would be lower than if percolation
does not encompass all the factors that affect soil permeability and thus may be inaccurate in
some instances. Most published soil permeability data are for water; although this is
generally closely related to the air permeability of the soil, there are some instances when it
may provide an incorrect estimate. Examples of areas in which water permeability data may
not accurately reflect air permeability include areas with consistently high levels of soil
moisture, or clay-rich soils, which would have a low water permeability but may have a
II- 15 Reprinted from USGS Open-File Report 93-292
-------
significantly higher air permeability when dry due to shrinkage cracks in the soil. These
additional factors were applied to the soil permeability factor when assigning the RI score, but
may have less certainty in some cases and thus would be assigned a lower CI score.
The Radon Index and Confidence Index give a general indication of the relative
contributions of the interrelated geologic factors influencing radon generation and transport in
rocks and soils, and thus, of the potential for elevated indoor radon levels to occur in a
particular area. However, because these reports are somewhat generalized to cover relatively
large areas of States, it is highly recommended that more detailed studies be performed in
local areas of interest, using the methods and general information in these booklets as a guide.
11-16 Reprinted from USGS Open-File Report 93-292
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Durrance, E.M., 1986, Radioactivity in geology: Principles and applications: New York, N.Y.,
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Henry, Mitchell E., Kaeding, Margret E., and Monteverde, Donald, 1991, Radon in soil gas and
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Ronca-Battista, M., Moon, M., Bergsten, J., White, S.B., Holt, N., and Alexander, B., 1988,
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Schery, S.D., Gaeddert, D.H., and Wilkening, M.H., 1984, Factors affecting exhalation of radon
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Schumann, R.R., Peake, R.T,j SchmidtrKM^, and-Owen,-B.E7r 1991,- Correlations of sothgas
and indoor radon with geology in glacially derived soils of the northern Great Plains, in
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report EPA/600/9-9 l/026b, p. 6-23-6-36.
II-18 Reprinted from USGS Open-File Report 93-292
-------
Schumann, R.R., Owen, D.E., and Asher-Bolinder, S., 1992, Effects of weather and soil
characteristics on temporal variations in soil-gas radon concentrations, in Gates, A.E., and
Gundersen, L.C.S., eds., Geologic controls on radon: Geological Society of America
Special Paper 271, p. 65-72.
Sextro, R.G., Moed, B.A., Nazaroff, W.W., Revzan, K.L., and Nero, A.V., 1987,
Investigations of soil as a source of indoor radon, in Hopke, P.K., ed., Radon and its
decay products: American Chemical Society Symposium Series 331, p. 10-29.
Sterling, R., Meixel, G., Shen, L., Labs, K., and Bligh, T., 1985, Assessment of the energy
savings potential of building foundations research: Oak Ridge, Tenn., U.S. Department of
Energy Report ORNL/SUB/84-0024/1.
Smith, R.C., II, Reilly, M.A., Rose, A.W., Barnes, J.H., and Berkheiser, S.W., Jr., 1987,
Radon: a profound case: Pennsylvania Geology, v. 18, p. 1-7.
Tanner, A.B., 1964, Radon migration in the ground: a review, in Adams, J.A.S., and Lowder,
W.M., eds., The natural radiation environment: Chicago, HI., University of Chicago
Press, p. 161-190.
Tanner, A.B., 1980, Radon migration in the ground: a supplementary review, in Gesell, T.F.,
and Lowder, W.M. (eds), Natural radiation environment IE, Symposium proceedings,
Houston, Texas, v. 1, p. 5-56.
U.S. Department of Agriculture, 1987, Principal kinds of soils: Orders, suborders, and great
groups: U.S. Geological Survey, National Atlas of the United States of America, sheet
38077-BE-NA-07M-00, scale 1:7,500,000.
U.S. Department of Energy, 1976, National Uranium Resource Evaluation preliminary report,
prepared by the U.S. Energy Research and Development Administration, Grand Junction,
Colo.: GJO-11(76).
Wanty, Richard B., and Schoen, Robert, 1991, A review of the chemical processes affecting the
mobility of radionuclides in natural waters, with applications, in Gundersen, Linda C.S.,
and Richard B. Wanty, eds., Field studies of radon in rocks, soils, and water: U.S.
Geological Survey Bulletin no. 1971, p. 183-194.
Washington, J.W., and Rose, A.W., 1990, Regional and temporal relations of radon in soil gas to
soil temperature and moisture: Geophysical Research Letters, v. 17, p. 829-832.
White, S.B., Bergsten, J.W., Alexander, B.V., and Ronca-Battista, M., 1989, Multi-State
surveys of indoor 222Rn: Health Physics, v. 57, p. 891-896.
JJ-19 Reprinted from USGS Open-File Report 93-292
-------
APPENDIX A
GEOLOGIC TIME SCALE
Subdivisions {and their symbols)
Eon or
Eonothem
Phanerozoic2
Proterozoic
(Pi
1C)
Archean
/A1
l«J
Era or
Erathem
Cenozoic 2
(Cz)
Mesozoic2
(Mz)
Paleozoic2
(Pi)
M.OOI»
Early
Uli
Micdlt
Afchftin (V)
£«ny
Period, System,
Subperiod, Subsystem
Quaternary
(Q)
Neogene 2
Subperiod or
T.nfery Subsystem IN)
rn Paleogene
Suboeriod or
Subsystem (ft)
Cretaceous
(K)
Jurassic
(J)
Triassic
OS)
Permian
(P)
Pennsylvanian
Carboniferous IP)
(C) Mississippian
(M)
Devonian
(D)
Silurian
IS)
Ordovician
(O)
Cambrian
rC)
Epoch or Series
Holocene
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Late
Early
Late
Middle
Upper
Lower
Age estimates
of boundaries
in mega-annum
(Ma)1
.0 ll.o-i.9J
,.„„ ee ffi'X-fifil
96 195-97}
j UO |Iftn f^cin^n^l
-• — •* Tlfl
-570 3
1 Ranges reflect uncertainties of isotopic and biostrmtigraphic age assignments. Age boundaries not closely bracketed by existing
d»ta shown by ^ Decay constants and isoiopic ratios employed are cited in Steiger and JSger (1977). Designation m.y. used for an
Interval of lime.
'Modifier* (lower, middle, upper or early, middle, late) when used with these hems are informal divisions of the larger unit: th«
first letltr ol th» modifier is lowercase.
3 Rocks older than S70 Ma also ealted Precambrian (pC). a time term without specific rank.
'informal time term without specific rank.
USGS Open-File Report 93-292
-------
APPENDIX B
GLOSSARY OF TERMS
Units of measure
pCi/L (picdcuries per liter)- a unit of measure of radioactivity used to describe radon
concentrations in a volume of air. One picocurie (10~12 curies) is equal to about 2.2 disintegrations
of radon atoms per minute. A liter is about 1.06 quarts. The average concentration of radon in
U.S. homes measured to date is between 1 and 2 pCi/L.
Bq/m3 (Becquerels per cubic meter)- a metric unit of radioactivity used to describe radon
concentrations in a volume of air. One becquerel is equal to one radioactive disintegration per
second. One pCi/L is equal to 37 Bq/m3.
ppm (parts per million)- a unit of measure of concentration by weight of an element in a
substance, in this case, soil or rock. One ppm of uranium contained in a ton of rock corresponds
to about 0.03 ounces of uranium. The average concentration of uranium in soils in the United
States is between 1 and 2 ppm.
in/hr (inches per hour)- a unit of measure used by soil scientists and engineers to describe the
permeability of a soil to water flowing through it. It is measured by digging a hole 1 foot (12
inches) square and one foot deep, filling it with water, and measuring the time it takes for the water
to drain from the hole. The drop in height of the water level in the hole, measured in inches, is
then divided by the time (in hours) to determine the permeability. Soils range in permeability from
less than 0.06 in/hr to greater than 20 in/hr, but most soils in the United States have permeabilities
between these two extremes.
Geologic terms and terms related to the study of radon
aerial radiometric, aeroradiometric survey A survey of radioactivity, usually gamma rays,
taken by an aircraft carrying a gamma-ray spectrometer pointed at the ground surface.
alluvial fan A low, widespread mass of loose rock and soil material, shaped like an open fan
and deposited by a stream at the point where it flows from a narrow mountain valley out onto a
plain or broader valley. May also form at the junction with larger streams or when the gradient of
the stream abruptly decreases.
alluvium, alluvial General terms referring to unconsolidated detrital material deposited by a
stream or other body of running water.
____
sensitive to alpha particles. The film is etched with acid in a laboratory after it is exposed. The
etching reveals scratches, or "tracks", left by the alpha particles resulting from radon decay , which
can then be counted to calculate the radon concentration. Useful for long-term (1-12 months)
radon tests.
amphibolite A mafic metamorphic rock consisting mainly of pyroxenes and(or) amphibole and
plagioclase.
n-21 Reprinted from USGS Open-File Report 93-292
-------
argillite, argillaceous Terms referring to a rock derived from clay or shale, or any sedimentary
rock containing an appreciable amount of clay-size material, i.e., argillaceous sandstone.
arid Term describing a climate characterized by dryness, or an evaporation rate that exceeds the
amount of precipitation.
basalt A general term for a dark-colored mafic igneous rocks that may be of extrusive origin,
such as volcanic basalt flows, or intrusive origin, such as basalt dikes.
batholith A mass of plutonic igneous rock that has more than 40 square miles of surface
exposure and no known bottom.
carbonate A sedimentary rock consisting of the carbonate (COs) compounds of calcium,
magnesium, or iron, e.g. limestone and dolomite.
carbonaceous Said of a rock or sediment that is rich in carbon, is coaly, or contains organic
matter.
charcoal canister A passive radon measurement device consisting of a small container of
granulated activated charcoal that is designed to adsorb radon. Useful for short duration (2-7 days)
measurements only. May be referred to as a "screening" test
chert A hard, extremely dense sedimentary rock consisting dominantly of interlocking crystals of
quartz. Crystals are not visible to the naked eye, giving the rock a milky, dull luster. It may be
white or gray but is commonly colored red, black, yellow, blue, pink, brown, or green.
clastic pertaining to a rock or sediment composed of fragments that are derived from preexisting
rocks or minerals. The most common clastic sedimentary rocks are sandstone and shale.
clay A rock containing clay mineral fragments or material of any composition having a diameter
less than 1/256 mm.
clay mineral One of a complex and loosely defined group of finely crystalline minerals made up
of water, silicate and aluminum (and a wide variety of other elements). They are formed chiefly by
alteration or weathering of primary silicate minerals. Certain clay minerals are noted for their small
size and ability to absorb substantial amounts of water, causing them to swell. The change in size
that occurs as these clays change between dry and wet is referred to as their " shrink-swell"
potential.
concretion A hard, compact mass of mineral matter, normally subspherical but commonly
irregular in shape; formed by precipitation from a water solution about a nucleus or center, such as
a leaf, shell, bone, or fossil, within a sedimentary or fractured rock.
conglomerate A coarse-grained, clastic sedimentary rock composed of rock and mineral
fragments larger than 2 mm, setm~aiiner-~grained~fnalrix of clastic material.
cuesta A hill or ridge with a gentle slope on one side and a steep slope on the other. The
formation of a cuesta is controlled by the different weathering properties and the structural dip of
the rocks forming the hill or ridge.
daughter product A nuclide formed by the disintegration of a radioactive precursor or "parent"
atom.
11-22 Reprinted from USGS Open-File Report 93-292
-------
delta, deltaic Referring to a low, flat, alluvial tract of land having a triangular or fan shape,
located at or near the mouth of a river. It results from the accumulation of sediment deposited by a
river at the point at which the river loses its ability to transport the sediment, commonly where a
river meets a larger body of water such as a lake or ocean.
dike A tabular igneous intrusion of rock, younger than the surrounding rock, that commonly cuts
across the bedding or foliation of the rock it intrudes.
diorite A plutonic igneous rock that is medium in color and contains visible dark minerals that
make up less than 50% of the rock. It also contains abundant sodium plagioclase and minor
quartz.
dolomite A carbonate sedimentary rock of which more than 50% consists of the mineral dolomite
(CaMg(CO3)2), and is commonly white, gray, brown, yellow, or pinkish in color.
drainage The manner in which the waters of an area pass, flow off of, or flow into the soil.
Also refers to the water features of an area, such as lakes and rivers, that drain it.
eolian Pertaining to sediments deposited by the wind.
esker A long, narrow, steep-sided ridge composed of irregular beds of sand and gravel deposited
by streams beneath a glacier and left behind when the ice melted.
evapotranspiration Loss of water from a land area by evaporation from the soil and
transpiration from plants.
extrusive Said of igneous rocks that have been erupted onto the surface of the Earth.
fault A fracture or zone of fractures in rock or sediment along which there has been movement.
fluvial, fluvial deposit Pertaining to sediment that has been deposited by a river or stream.
foliation A linear feature in a rock defined by both mineralogic and structural characteristics. It
may be formed during deformation or metamorphism.
formation A mappable body of rock having similar characteristics.
glacial deposit Any sediment transported and deposited by a glacier or processes associated
with glaciers, such as glaciofluvial sediments deposited by streams flowing from melting glaciers.
gneiss A rock formed by metamorphism in which bands and lenses of minerals of similar
composition alternate with bands and lenses of different composition, giving the rock a striped or
"foliated" appearance.
granite-Broadly-appUed,-any coarsely crystdtin^^^ —
rock. Technically, granites have between 10 and 50% quartz, and alkali feldspar comprises at least
65% of the total feldspar.
gravel An unconsolidated, natural accumulation of rock fragments consisting predominantly of
particles greater than 2 mm in size.
heavy minerals Mineral grains in sediment or sedimentary rock having higher than average
specific gravity. May form layers and lenses because of wind or water sorting by weight and size
IE-23 Reprinted from USGS Open-File Report 93-292
-------
and may be referred to as a "placer deposit." Some heavy minerals are magnetite, garnet, zircon,
monazite, and xenotime.
igneous Said of a rock or mineral that solidified from molten or partly molten rock material. It is
one of the three main classes into which rocks are divided, the others being sedimentary and
metamorphic.
intermontane A term that refers to an area between two mountains or mountain ranges.
intrusion, intrusive The processes of emplacement or injection of molten rock into pre-existing
rock. Also refers to the rock formed by intrusive processes, such as an "intrusive igneous rock".
kame A low mound, knob, hummock, or short irregular ridge formed by a glacial stream at the
margin of a melting glacier; composed of bedded sand and gravel.
karst terrain A type of topography that is formed on limestone, gypsum and other rocks by
dissolution of the rock by water, forming sinkholes and caves.
lignite A brownish-black coal that is intermediate in coalification between peat and
subbituminous coal.
limestone A carbonate sedimentary rock consisting of more than 50% calcium carbonate,
primarily in the form of the mineral calcite (CaCOs).
lithology The description of rocks in hand specimen and in outcrop on the basis of color,
composition, and grain size.
loam A permeable soil composed of a mixture of relatively equal parts clay, silt, and sand, and
usually containing some organic matter.
loess A fine-grained eolian deposit composed of silt-sized particles generally thought to have
been deposited from windblown dust of Pleistocene age.
mafic Term describing an igneous rock containing more than 50% dark-colored minerals.
marine Term describing sediments deposited in the ocean, or precipitated from ocean waters.
metamorphic Any rock derived from pre-existing rocks by mineralogical, chemical, or structural
changes in response to changes in temperature, pressure, stress, and the chemical envkonment.
Phyllite, schist, amphibolite, and gneiss are metamorphic rocks.
moraine A mound, ridge, or other distinct accumulation of unsorted, unbedded glacial material,
predominantly till, deposited by the action of glacial ice.
outcrop That part of a geologic formation-orstructurethalrappears at the surf ace of ^heEarthras
in "rock outcrop".
percolation test A term used in engineering for a test to determine the water permeability of a
soil. A hole is dug and filled with water and the rate of water level decline is measured.
permeability The capacity of a rock, sediment, or soil to transmit liquid or gas.
phosphate, phosphatic, phosphorite Any rock or sediment containing a significant amount
of phosphate minerals, i.e., minerals containing PO4.
11-24 Reprinted from USGS Open-File Report 93-292
-------
physiographic province A region in which all parts are similar in geologic structure and
climate, which has had a uniform geomorphic history, and whose topography or landforms differ
significantly from adjacent regions.
placer deposit See heavy minerals
residual Formed by weathering of a material in place.
residuum Deposit of residual material.
rhyolite An extrusive igneous rock of volcanic origin, compositionally equivalent to granite.
sandstone A clastic sedimentary rock composed of sand-sized rock and mineral material that is
more or less firmly cemented. Sand particles range from 1/16 to 2 mm in size.
schist A strongly foliated crystalline rock, formed by metamorphism, that can be readily split into
thin flakes or slabs. Contains mica; minerals are typically aligned.
screening level Result of an indoor radon test taken with a charcoal canister or similar device,
for a short period of time, usually less than seven days. May indicate the potential for an indoor
radon problem but does not indicate annual exposure to radon.
sediment Deposits of rock and mineral particles or fragments originating from material that is
transported by air, water or ice, or that accumulate by natural chemical precipitation or secretion of
organisms.
semiarid Refers to a climate that has slightly more precipitation than an arid climate.
shale A fine-grained sedimentary rock formed from solidification (lithification) of clay or mud.
shear zone Refers to a roughly linear zone of rock that has been faulted by ductile or non-ductile
processes in which the rock is sheared and both sides are displaced relative to one another.
shrink-swell clay See clay mineral.
siltstone A fine-grained clastic sedimentary rock composed of silt-sized rock and mineral
material and more or less firmly cemented. Silt particles range from 1/16 to 1/256 mm in size.
sinkhole A roughly circular depression in a karst area measuring meters to tens of meters in
diameter. It is funnel shaped and is formed by collapse of the surface material into an underlying
void created by the dissolution of carbonate rock.
slope An inclined part of the earth's surface.
-solution-cavity-A-holerehannel-or-eave-Hke-cavity-formed-by-dissolutiorrof-rockr —
stratigraphy The study of rock strata; also refers to the succession of rocks of a particular area.
surficial materials Unconsolidated glacial, wind-, or waterborne deposits occurring on the
earth's surface.
tablelands General term for a broad, elevated region with a nearly level surface of considerable
extent.
11-25 Reprinted from USGS Open-File Report 93-292
-------
terrace gravel Gravel-sized material that caps ridges and terraces, left behind by a stream as it
cuts down to a lower level.
terrain A tract or region of the Earth's surface considered as a physical feature or an ecological
environment.
till Unsorted, generally unconsolidated and unbedded rock and mineral material deposited directly
adjacent to and underneath a glacier, without reworking by meltwater. Size of grains varies greatly
from clay to boulders.
uraniferous Containing uranium, usually more than 2 ppm.
vendor data Used in this report to refer to indoor radon data collected and measured by
commercial vendors of radon measurement devices and/or services.
volcanic Pertaining to the activities, structures, and extrusive rock types of a volcano.
water table The surface forming the boundary between the zone of saturation and the zone of
aeration; the top surface of a body of unconfined groundwater in rock or soil.
weathering The destructive process by which earth and rock materials, on exposure to
atmospheric elements, are changed in color, texture, composition, firmness, or form with little or
no transport of the material.
n-26 Reprinted from USGS Open-File Report 93-292
-------
APPENDIX C
EPA REGIONAL OFFICES
EPA Regional Offices
State
EPA Region
EPA Region 1
JFK Federal Building
Boston, MA 02203
(617) 565-4502
EPA Region 2
(2AIR:RAD)
26 Federal Plaza
New York, NY 10278
(212) 264-4110
Region 3 (3AH14)
841 Chestnut Street
Philadelphia, PA 19107
(215) 597-8326
EPA Region 4
345 Courtland Street, N.E.
Atlanta, GA 30365
(404) 347-3907
EPA Region 5 (5AR26)
77 West Jackson Blvd.
Chicago, IL 60604-3507
(312) 886-6175
EPA Region 6 (6T-AS)
1445 Ross Avenue
Dallas, TX 75202-2733
(214) 655-7224
EPA Re^wn 7
726 Minnesota Avenue
Kansas City, KS 66101
(913) 551-7604
EPA Region 8
(8HWM-RP)
999 18th Street
One Denver Place, Suite 1300
Denver, CO 80202-2413
(303) 293-1713
EPA Region 9 (A-3)
75 Hawthorne Street
San Francisco, CA 94105
(415) 744-1048
EPA Region 10
1200 Sixth Avenue
Seattle, WA 98101
(202) 442-7660
Alabama 4
Alaska 10
Arizona 9
Arkansas 6
California 9
Colorado 8
. Connecticut 1
Delaware 3
District of Columbia 3
Florida 4
Georgia 4
Hawaii 9
Idaho 10
Illinois 5
Indiana 5
Iowa 7
Kansas .• 7
Kentucky ...4
Louisiana 6
Maine 1
Maryland 3
Massachusetts 1
Michigan 5
Minnesota 5
Mississippi 4
Missouri 7
Montana 8
Nebraska 7
Nevada 9
New Hampshire 1
New Jersey 2
New Mexico 6
New York 2
North Carolina 4
North Dakota 8
Ohio 5
Oklahoma 6
Oregon 10
Pennsylvania 3
Rhode Island 1
South Carolina 4
South Dakota 8
Tennessee 4
Texas 6
Utah 8
Vermont 1
Virginia 3
—-Washington ..-,•... .•.•«..„»„.,.-..,-.,.,. .-,m-..-l-0-
West Virginia 3
Wisconsin 5
Wyoming 8
n-27
Reprinted from USGS Open-File Report 93-292
-------
STATE RADON CONTACTS
May, 1993
Alabama James McNees
Division of Radiation Control
Alabama Department of Public t^^lsh
State Office Building
Montgomery, AL 36130
(205) 242-5315
1-800-582-1866 in state
Alaska Charles Tedford
Department of Health and Social
Services
P.O. Box 110613
Juneau.AK 99811-0613
(907)465-3019
1-800-478-4845 in state
Arizona John Stewart
Arizona Radiation Regulatory Agency
4814 South 40th St.
Phoenix, AZ 85040
(602) 255-4845
Arkansas Lee Gershner
Division of Radiation Control
Department of Health
4815 Markham Street, Slot 30
Little Rock, AR 72205-3867
(501) 661-2301
California J. David Quinton
Department of Health Services
714 P Street, Room 600
Sacramento, CA 94234-7320
(916) 324-2208
1-800-745-7236 in state
Colorado Linda Martin
Department of Health
4210 East llth Avenue
Denver, CO 80220
• -• - -(303)692-3057
1-800-846-3986 in state
Connecticut Alan J. Siniscalchi
Radon Program
Connecu^ut Department of Health
Services
150 Washington Street
Hartford, CT 06106-4474
(203)566-3122
Delaware Marai G. Rejai
Office of Radiation Control
Division of Public Health
P.O. Box 637
Dover, DE 19903
(302) 736-3028
1-800-554-4636 In State
District Robert Davis
of Columbia DC Department of Consumer and
Regulatory Affairs
614 H Street NW
Room 1014
Washington, DC 20001
(202) 727-71068
Florida N. Michael Gilley
Office of Radiation Control
Department of Health and
Rehabilitative Services
1317 Winewood Boulevard
Tallahassee, FL 32399-0700
(904)488-1525
1-800-543-8279 in state
Georgia Richard Schreiber
Georgia Department of Human
Resources
878 Peachtree St., Room 100
Atlanta, GA 30309
(404) 894-6644
1-800-745-0037 in state
Hawaii Russell Takata
Environmental Health Services
Division
591 Ala Moana Boulevard
— Honolulu, ffl 96813-2498
(808) 5864700
E-28 Reprinted from USGS Open-File Report 93-292
-------
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
PatMcGavarn
Office of Environmental Health
450 West State Street
Boise, ID 83720
(208) 334-6584
1-800-445-8647 in state
Richard Allen
Illinois Department of Nuclear Safety
1301 Outer Park Drive
Springfield, IL 62704
(217) 524-5614
1-800-325-1245 in state
Lorand Magyar
Radiological Health Section
Indiana State Department of Health
1330 West Michigan Street
P.O. Box 1964
Indianapolis, IN 46206
(317) 633-8563
1-800-272-9723 In State
Donald A. Plater
Bureau of Radiological Health
Iowa Department of Public Health
Lucas State Office Building
Des Moines, IA 50319-0075
(515)281-3478
1-800-383-5992 In State
Harold Spiker
Radiation Control Program
Kansas Department of Health and
Environment
109 SW 9th Street
6th Floor Mills Building
Topeka, KS 66612
(913) 296-1561
Jeana Phelps
Radiation Control Branch
Department of Health Services
Cabinet for Human Resources
275 East Main Street
-Frankfort,KY-40601
(502) 564-3700
Louisiana Matt Schlenker
Louisiana Department of
Environmental Quality
P.O. Box 82135
Baton Rouge, LA 70£ 34-2135
(504)925-7042
1-800-256-2494 in state
Maine BobStilwell
Division of Health Engineering
Department of Human Services
State House, Station 10
Augusta, ME 04333
(207) 289-5676
1-800-232-0842 in state
Maryland Leon J. Rachuba
Radiological Health Program
Maryland Department of the
Environment
2500 Broening Highway
Baltimore, MD 21224
(410)631-3301
1-800-872-3666 In State
Massachusetts William J. Bell
Radiation Control Program
Department of Public Health
23 Service Center
Northampton, MA 01060
(413) 586-7525
1-800-445-1255 in state
Michigan Sue Hendershott
Division of Radiological Health
Bureau of Environmental and
Occupational Health
3423 North Logan Street
P.O. Box 30195
Lansing, MI 48909
(517) 335-8194
Minnesota Laura Oatmann
Indoor Air Quality Unit
925 Delaware Street, SE
P.O. Box 59040
Minneapolis, MN 55459-0040
(612)~627-5480
1-800-798-9050 in state
H-29
Reprinted from USGS Open-File Report 93-292
-------
Mississippi Silas Anderson
Division of Radiological Health
Department of Health
3150 Lawson Street
P.O. Box 1700
Jackson, MS 39215-1700
(601) 354-6657
1-800-626-7739 in state
Missouri Kenneth V. Miller
Bureau of Radiological Health
Missouri Department of Health
1730 East Elm
P.O. Box 570
Jefferson City, MO 65102
(314)751-6083
1-800-669-7236 In State
Montana Adrian C. Howe
Occupational Health Bureau
Montana Department of Health and
Environmental Sciences
Cogswell Building A113
Helena, MX 59620
(406)444-3671
Nebraska Joseph Milone
Division of Radiological Health
Nebraska Department of Health
301 Centennial Mall, South
P.O. Box 95007
Lincoln, NE 68509
(402)471-2168
1-800-334-9491 In State
Nevada Stan Marshall
Department of Human Resources
505 East King Street
Room 203
Carson City, NV 89710
(702) 687-5394
New Hampshire David Chase
Bureau of Radiological Health
Division of Public Health Services
Health and Welfare Building
Six Hazen Drive
- - Concord, NH 03301 —
(603)271-4674
1-800-852-3345 x4674
New Jersey Tonalee Carlson Key
Division of Environmental Quality
Department of Environmental
Protection
CN415
Trenton, NJ 08625-0145
(609) 987-6369
1-800-648-0394 in state
New Mexico William M. Floyd
Radiation Licensing and Registration
Section
New Mexico Environmental
Improvement Division
1190 St. Francis Drive
Santa Fe,NM 87503
(505) 827-4300
New York William J. Condon
Bureau of Environmental Radiation
Protection
New York State Health Department
Two University Place
Albany, NY 12202
(518) 458-6495
1-800-458-1158 in state
North Carolina Dr. Felix Fong
Radiation Protection Division
Department of Environmental Health
and Natural Resources
70 IBarbour Drive
Raleigh, NC 27603-2008
(919) 571-4141
1-800-662-7301 (recorded info x4196)
North Dakota Arlen Jacobson
North Dakota Department of Health
1200 Missouri Avenue, Room 304
P.O. Box 5520
Bismarck, ND 58502-5520
(701)221-5188
Marcie Matthews
Radiological Health Program
Department of Health
1224 Kinnear Road - Suite 120
Columbus, OH 43212
644-2727 ----------
1-800-523-4439 in state
n-30
Reprinted from USGS Open-File Report 93-292
-------
Oklahoma
Oregon
Pennsylvania
Puerto Rico
Rhode Island
South Carolina
Gene Smith
Radiation Protection Division
Oklahoma State Department of
Health
P.O. Box 53551
Oklahoma City, OK 73152
(405) 271-5221
George Toombs
Department of Human Resources
Health Division
1400 SW 5th Avenue
Portland, OR 97201
(503)731^014
Michael Pyles
Pennsylvania Department of
Environmental Resources
Bureau of Radiation Protection
P.O. Box 2063
Harrisburg, PA 17120
(717) 783-3594
1-800-23-RADON In State
David Saldana
Radiological Health Division
G.P.O. Call Box 70184
Rio Piedras, Puerto Rico 00936
(809) 767-3563
Edmund Arcand
Division of Occupational Health and
Radiation
Department of Health
205 Cannon Building
Davis Street
Providence, RI02908
(401)277-2438
Bureau of Radiological Health
Department of Health and
—Environmental-Control
2600 Bull Street
Columbia, SC 29201
(803)734-4631
1-800-768-0362
South Dakota MikePochop
Division of Environment Regulation
Department of Water and Natural
Resources
Joe Foss Building, Room 217
523 E. Capitol
Pierre, SD 57501-3181
(605) 773-3351
Tennessee Susie Shimek
Division of Air Pollution Control
Bureau of the Environment
Department of Environment and
Conservation
Customs House, 701 Broadway
Nashville, TN 37219-5403
(615) 532-0733
1-800-232-1139 in state
Texas Gary Smith
Bureau of Radiation Control
Texas Department of Health
1100 West 49th Street
Austin, TX 78756-3189
(512) 834-6688
Utah John Hultquist
Bureau of Radiation Control
Utah State Department of Health
288 North, 1460 West
P.O. Box 16690
Salt Lake City, UT 84116-0690
(801) 536-4250
Paul demons
Occupational and Radiological Health
Division
Vermont Department of Health
10 Baldwin Street
Montpelier, VT 05602
(802) 828-2886
1-800-640-0601 in state
Virgin Islands Contact the U.S. Environmental
Protection Agency, Region II
in New York
II-31 Reprinted from USGS Open-File Report 93-292
-------
Virginia Shelly Ottenbrite
Bureau of Radiological Health
Department of Health
109 Governor Street
Richmond, VA 23219
(804) 786-5932
1-800-468-0138 in state
Washington KateColeman
Department of Health
Office of Radiation Protection
Airdustrial Building 5, LE-13
Olympia, WA 98504
(206) 753-4518
1-800-323-9727 In State
West Virginia BeattieL.DeBord
Industrial Hygiene Division
West Virginia Department of Health
151 llth Avenue
South Charleston, WV 25303
(304) 558-3526
1-800-922-1255 In State
Wisconsin Conrad Weiffenbach
Radiation Protection Section
Division of Health
Department of Health and Social
Services
P.O. Box 309
Madison, WI53701-0309
(608) 267-4796
1-800-798-9050 in state
Wyoming Janet Hough
Wyoming Department of Health and
Social Services
Hathway Building, 4th Floor
Cheyenne, WY 82002-0710
(307) 777-6015
1-800-458-5847 in state
11-32 Reprinted from USGS Open-File Report 93-292
-------
STATE GEOLOGICAL SURVEYS
May, 1993
Ernest A. Mancini
Geological Survey of Alabama
P.O. Box 0
420 Hackberry Lane'
Tuscaloosa, AL 35486-9780
(205) 349-2852
Alaska Thomas E. Smith
Alaska Division of Geological &
Geophysical Surveys
794 University Ave., Suite 200
Fairbanks, AK 99709-3645
(907) 479-7147
Arizona Larry D. Fellows
Arizona Geological Survey
845 North Park Ave., Suite 100
Tucson, AZ 85719
(602) 882-4795
Arkansas Norman F. Williams
Arkansas Geological Commission
Vardelle Parham Geology Center
3815 West Roosevelt Rd.
Little Rock, AR 72204
(501) 324-9165
California James F. Davis
California Division of Mines &
Geology
801 K Street, MS 12-30
Sacramento, CA 95814-3531
(916) 445-1923
Colorado Pat Rogers (Acting)
Colorado Geological Survey
1313 Sherman St., Rm 715
Denver, CO 80203
(303) 866-2611
Connecticut Richard C. Hyde
Connecticut Geological & Natural
History Survey
165 Capitol Ave., Rm. 553
HartfordrCT 06106
(203) 566-3540
Delaware Robert R. Jordan
Delaware Geological Survey
University of Delaware
101 Penny Hall
Newark, DE 19716-7501
(302) 831-2833
Walter Schmidt
Florida Geological Survey
903 W. Tennessee St.
Tallahassee, FL 32304-7700
(904)488-4191
Georgia William H. McLemore
Georgia Geologic Survey
Rm. 400
19 Martin Luther King Jr. Dr. SW
Atlanta, GA 30334
(404) 656-3214
Hawaii Manabu Tagomori
Dept. of Land and Natural Resources
Division of Water & Land Mgt
P.O. Box 373
Honolulu, HI 96809
(808) 548-7539
Idaho Earl H. Bennett
Idaho Geological Survey
University of Idaho
Morrill Hall, Rm. 332
Moscow, ID 83843
(208) 885-7991
Illinois Morris W. Leighton
Illinois State Geological Survey
Natural Resources Building
615EastPeabodyDr.
Champaign, EL 61820
(217) 333-4747
Indiana Norman C. Hester
Indiana Geological Survey
611 North Walnut Grove
Bloomington, IN 47405
(812) 855-9350
Iowa Donald L. Koch
Iowa Department of Natural Resources
Geological Survey Bureau
109 Trowbridge Hall
Iowa:CityrIA-52242-1319
(319) 335-1575
Kansas Lee C.Gerhard
Kansas Geological Survey
1930 Constant Ave., West Campus
University of Kansas
Lawrence, KS 66047
(913) 864-3965
11-33 Reprinted from USGS Open-File Report 93-292
-------
Kentucky;
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Donald C. Haney
Kentucky Geological Survey
University of Kentucky
228 Mining & Mineral Resources
Building
Lexington, KY 40506-0107
(606) 257-5500
William E. Marsalis
Louisiana Geological Survey
P.O. Box 2827
University Station
Baton Rouge, LA 70821-2827
(504) 388-5320
Walter A. Anderson
Maine Geological Survey
Department of Conservation
State House, Station 22
Augusta, ME 04333
(207) 289-2801
Emery T. Cleaves
Maryland Geological Survey
2300 St. Paul Street
Baltimore, MD 21218-5210
(410) 554-5500
Joseph A. Sinnott
Massachusetts Office of
Environmental Affairs
100 Cambridge St, Room 2000
Boston, MA 02202
(617) 727-9800
R. Thomas Segall
Michigan Geological Survey Division
Box 30256
Lansing, MI 48909
(517) 334-6923
PriscillaC.Grew
Minnesota Geological Survey
2642 University Ave.
St. Paul, MN 55114-1057
(612) 627-4780
S. Cragin Knox
Mississippi Office of Geology
P.O. Box 20307
Jackson, MS 39289-1307
(601) 961-5500
Missouri James H. Williams
Missouri Division of Geology &
Land Survey
111 Fairgrounds Road
P.O. Box 250
Rolla, MO 65401
(314) 368-2100
Montana Edward T. Ruppel
Montana Bureau of Mines & Geology
Montana College of Mineral Science
and Technology, Main Hall
Butte, MT 59701
(406) 496-4180
Nebraska Perry B. Wigley
Nebraska Conservation & Survey
Division
113 Nebraska Hall
University of Nebraska
Lincoln, NE 68588-0517
(402)472-2410
Nevada Jonathan G. Price
Nevada Bureau of Mines & Geology
Stop 178
University of Nevada-Reno
Reno, NV 89557-0088
(702) 784-6691
New Hampshire Eugene L.Boudette
Dept. of Environmental Services
117 James Hall
University of New Hampshire
Durham, NH 03824-3589
(603) 862-3160
New Jersey Haig F. Kasabach
New Jersey Geological Survey
P.O. Box 427
Trenton, NJ 08625
(609)292-1185
New Mexico Charles E. Chapin
New Mexico Bureau of Mines &
Mineral Resources
Campus Station
Socoiro.NM 87801
-— •---- (505) 835-5420 —
New York Robert H. Fakundiny
New York State Geological Survey
3136 Cultural Education Center
Empire State Plaza
Albany, NY 12230
(518)474-5816
11-34 Reprinted from USGS Open-File Report 93-292
-------
North Carolina Charles H. Gardner
North Carolina Geological Survey
P.O. Box 27687
Raleigh, NC 27611-7687
(919) 733-3833
North Dakota
Ohio
Oklahoma
Oregon
Pennsylvania
Puerto Rico
Rhode Island
John P. Bluemle
North Dakota Geological Survey
600 East Blvd.
Bismarck, ND 58505-0840
(701) 224-4109
Thomas M. Berg
Ohio Dept. of Natural Resources
Division of Geological Survey
4383 Fountain Square Drive
Columbus, OH 43224-1362
(614) 265-6576
Charles J. Mankin
Oklahoma Geological Survey
Room N-131, Energy Center
lOOE.Boyd
Norman, OK 73019-0628
(405)325-3031
Donald A. Hull
Dept. of Geology & Mineral Indust.
Suite 965
800 NE Oregon St. #28
Portland, OR 97232-2162
(503)731-4600
Donald M. HosMns
Dept. of Environmental Resources
Bureau of Topographic & Geologic
Survey
P.O. Box 2357
Harrisburg, PA 17105-2357
(717) 787-2169
Ramdn M. Alonso
Puerto Rico Geological Survey
Division
Box 5887
Puerta de Tierra Station
San Juan, P.R. 00906
_(809)-722-2526
South Carolina Alan-Jon W. Zupan (Acting)
South Carolina Geological Survey
5 Geology Road
Columbia, SC 29210-9998
(803)737-9440
South Dakota C.M. Christensen (Acting)
South Dakota Geological Survey
Science Center
University of South Dakota
Vermillion, SD 57069-2390
(605) 677-5227
Tennessee Edward T. Luther
Tennessee Division of Geology
13th Floor, L & C Tower
401 Church Street
Nashville, TN 37243-0445
(615) 532-1500
Texas William L. Fisher
Texas Bureau of Economic Geology
University of Texas
University Station, Box X
Austin, TX 78713-7508
(512) 471-7721
Utah M. Lee Allison
Utah Geological & Mineral Survey
2363 S. Foothill Dr.
Salt Lake City, UT 84109-1491
(801) 467-7970
Vermont Diane L.Conrad
Vermont Division of Geology and
Mineral Resources
103 South Main St.
Waterbury,VT 05671
(802)244-5164
Stanley S. Johnson
Virginia Division of Mineral
Resources
P.O. Box 3667
Charlottesville, VA 22903
(804)293-5121
J. Allan Cain
Department of Geology
University of Rhode Island
315 Green Hall
Kingston, RI02881
(401) 792-2265
Washington Raymond Lasmanis
Washington Division of Geology &
Earth Resources
Department of Natural Resources
P.O. Box 47007
Olympia, Washington 98504-7007
(206) 902-1450
11-35 Reprinted from USGS Open-File Report 93-292
-------
West Virginia Larry D. Woodfoik
West Virginia Geological and
Economic Survey
Mont Chateau Research Center
P.O. Box 879
Morgantown, WV 16507-0879
(304)594-2331
Wisconsin James Robertson
Wisconsin Geological & Natural
History Survey
3817 Mineral Point Road
Madison, WI 53705-5100
(608)263-7384
Wyoming Gary B. Glass
Geological Survey of Wyoming
University of Wyoming
Box 3008, University Station
Laramie, WY 82071-3008
(307) 766-2286
E-36 Reprinted from USGS Open-File Report 93-292
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EPA REGION 8 GEOLOGIC RADON POTENTIAL SUMMARY
by
R. Randall Schumann, Douglass E. Owen, Russell F. Dubiel, and Sandra L. Szarzi
U.S. Geological Survey
EPA Region 8 includes the states of Colorado, Montana, North Dakota, South Dakota,
Utah, and Wyoming. For each state, geologic radon potential areas were delineated and ranked on
the basis of geologic, soils, housing construction, and other factors. Areas in which the average
screening indoor radon level of all homes within the area is estimated to be greater than 4 pCi/L
were ranked high. Areas in which the average screening indoor radon level of all homes within the
area is estimated to be between 2 and 4 pCi/L were ranked moderate/variable, and areas in which
the average screening indoor radon level of all homes within the area is estimated to be less than
2 pCi/L were ranked low. Information on the data used and on the radon potential ranking scheme
is given in the introduction to this volume. More detailed information on the geology and radon
potential of each state in Region 8 is given in the individual state chapters. The individual chapters
describing the geology and radon potential of the six states in EPA Region 8, though much more
detailed than this summary, still are generalized assessments and there is no substitute for having a
home tested. Within any radon potential area homes with indoor radon levels both above and
below the predicted average likely will be found.
Figure 1 shows a generalized map of the physiographic provinces in EPA Region 8. The
following summary of radon potential in Region 8 is based on these provinces. Figure 2 shows
average screening indoor radon levels by county. The data for South Dakota are from the
EPA/Indian Health Service Residential Radon Survey and from The Radon Project of the
University of Pittsburgh; data for Utah are from an indoor radon survey conducted in 1988 by the
Utah Bureau of Radiation Control; data for Colorado, Montana, North Dakota, and Wyoming are
from the State/EPA Residential Radon Survey. Figure 3 shows the geologic radon potential areas
in Region 8, combined and summarized from the individual state chapters. Rocks and soils in
EPA Region 8 contain ample radon source material (uranium and radium) and have soil
permeabilities sufficient to produce moderate or high radon levels in homes. At the scale of this
evaluation, all areas in EPA Region 8 have either moderate or high geologic radon potential, except
for an area in southern South Dakota corresponding to the northern part of the Nebraska Sand
Hills, which has low radon potential.
The limit of continental glaciation is of great significance in Montana, North Dakota, and
South Dakota (fig. 1). The glaciated portions of the Great Plains and the Central Lowland
generally have a higher radon potential than their counterparts to the south because glacial action
crushes and grinds up rocks as it forms till and other glacial deposits. This crushing and grinding
enhances weathering and increases the surface area from which radon may emanate; further, it
exposes more uranium and radium at grain surfaces where they are more easily leached. Leached
___ uraniujTijnd jadjum jnay_be^ansported dpjyn^^djn the soil below the depth at which it may be
detected by a gamma-ray spectrometer (approximately 30 cm), giving these areas a relatively low
surface or aerial radiometric signature. However, the uranium and radium still are present at
depths shallow enough to allow generated radon to migrate into a home.
The Central Lowland Province is a vast plain that lies between 500 and 2,000 feet above
sea level and forms the agricultural heart of the United States. In Region 8, it covers the eastern
part of North Dakota and South Dakota. The Central Lowland in Region 8 has experienced the
effects of continental glaciation and also contains silt and clay deposits from a number of glacial
ffl-1 Reprinted from USGS Open-File Report 93-292-H
-------
Figure 1. Physiographic provinces in EPA Region 8 (after Hunt, C.W., 1967, Physiography of
the United States: Freeman and Co., p. 8-9.)
-------
100 Miles
Indoor Radon Screening
Measurements: Average (pCi/L)
16 EZj 0.0 to 1.9
76I///V1 2.0 to 4.0
82
Missing Data
Figure 2. Average screening indoor radon levels by county for EPA Region 8. Data for
CO, MT, ND, and WY from the EPA/State Residential Radon Survey; data for UT from
the Utah Bureau of Radiation Control indoor radon survey; data for SD from the EPA/EHS
Indoor Radon Survey and from The Radon Project. Histograms in map legend
indicate the number of counties in each measurement category.
-------
GEOLOGIC
RADON POTENTIAL
Figure 3. Geologic radon potential of EPA Region 8.
-------
lakes. Many of the glacial deposits are derived from or contain components of the uranium-bearing
Pierre Shale. Although many of the soils derived from glacial deposits in the Dakotas contain
significant amounts of clay, the soils can have permeabilities that are higher than indicated by
standard water percolation tests due to shrinkage cracks when dry. In addition, clays tend to have
high radon emanation coefficients because clay particles have a high surface-area-to-volume ratio
compared to larger and(or) more spherical soil grains. These two factors make areas underlain by
glacial deposits derived from the Pierre Shale, and areas underlain by glacial lake deposits, such as
the Red River Valley, highly susceptible to indoor radon problems. Average indoor radon levels in
this province generally are greater than 4 pCi/L (fig. 2). The Central Lowland in Region 8 has
high radon potential.
The Great Plains Province is an extension of the Central Lowlands that rises from 2,000
feet in the east to 5,000 feet above sea level in the west. In Region 8, it covers the western part of
North and South Dakota and the eastern portions of Montana, Wyoming, and Colorado. The
northern part of the Great Plains has been glaciated (fig. 1) and previous comments about
continental glaciation apply. The Great Plains are largely underlain by Cretaceous and Tertiary
sedimentary rocks. In general, the Cretaceous and Tertiary rocks in the southern part of the Great
Plains in Region 8 have a moderate to high radon potential. The Cretaceous Inyan Kara Group,
which surrounds the Black Hills in southwestern South Dakota and northeastern Wyoming, locally
hosts uranium deposits. There are a number of uranium occurrences in Tertiary sedimentary rocks
in the northern part of the Great Plains, such as in the Powder River Basin. The northwestern part
of the Great Plains contains numerous discontinuous uplifts (mountainous areas) that generally
have high radon potential. A few, such as the Black Hills, have uranium districts associated with
them. Average indoor radon levels in this province are greater than 2 pCi/L, with a significant
number of counties having average indoor radon concentrations exceeding 4 pCi/L (fig. 2).
The Northern Rocky Mountains Province (fig. 1) has high radon potential. Generally, the
igneous and metamorphic rocks of this province have elevated uranium contents. The soils
developed on these rocks typically have moderate or high permeability. Coarse-grained glacial
flood deposits composed of sand, gravel, and boulders, which are found in many of the valleys in
the province, also have high permeability. A number of uranium occurrences are found in granite
and chalcedony in the Boulder Batholith; in veins or pegmatite dikes in igneous and metamorphic
rocks near Clancy in Jefferson County, near Saltese in Mineral County, and in the Bitterroot and
Beartooth Mountains, all in Montana. Uranium also occurs in Tertiary volcanic rocks about 20
miles east of Helena, and in the Mississippian-age Madison Limestone in the Pryor Mountains.
County average indoor radon levels generally exceed 4 pCi/L in the province (fig. 2).
The Wyoming Basin Province lies dominantly in Wyoming, but also includes an area of
Tertiary sedimentary rocks in northern Colorado (fig. 1). The Wyoming Basin consists of a
number of elevated semiarid basins separated by small mountain ranges. In general the rocks and
soils have uranium contents greater than 2.5 ppm and host a number of uranium occurrences as
well, Particularly in the Tertiary Fort Union and Wasatch Formations. Average indoor radon levels
~ than^^i/L7figTT)7~The~Wyoming Basin has~a~~
high radon potential.
The Middle Rocky Mountains Province (fig. 1) has both moderate and high radon potential
areas (fig. 3). The southern part of the Middle Rocky Mountains province contains the Wasatch
Range in Utah, which has high radon potential, and the Uinta Mountains and the Overthrust Belt in
Utah and Wyoming, both of which have moderate radon potential. The northern part of the
province contains the Yellowstone Plateau, which is underlain by volcanic rocks containing
ffl-5 Reprinted from USGS Open-File Report 93-292-H
-------
relatively high uranium concentrations. Mountain ranges such as the Grand Tetons and Big Horn
Mountains, which are underlain by granitic and metamorphic rocks that generally contain more
than 2.5 ppm uranium, also occur in this province. County average indoor radon levels are mostly
in the 2-4 pCi/L range (fig. 2). The Yellowstone Plateau, Grand Tetons, and Big Horn Mountains
all have high geologic radon potential.
The Southern Rocky Mountains Province lies dominantry in Colorado (fig. 1). Much of
the province is underlain by igneous and metamorphic rocks with uranium contents generally
exceeding the upper continental crustal average of 2.5 ppm. The Front Range Mineral Belt west of
Denver hosts a number of uranium occurrences and inactive uranium mines. County indoor radon
averages generally are greater than 4 pCi/L, except in the San Juan Mountains in south-central
Colorado, where the county radon averages range from 1 to 4 pCi/L (fig. 3). The Southern Rocky
Mountains generally have high radon potential, with the main exception being the volcanic rocks of
the San Juan volcanic field (located in the southwestern part of the province) which have moderate
radon potential.
The part of the Colorado Plateau Province in Region 8 has a band of high radon potential
and a core of moderate radon potential (figs. 1,3). The band of high radon potential consists
largely of: (1) the Uravan Mineral Belt, a uranium mining district, on the east; (2) the Uinta Basin,
which contains uranium-bearing Tertiary rocks, on the north; and (3) Tertiary volcanic rocks,
which have a high aeroradiometric signature, on the west. The moderate radon potential zone in
the interior part of the province is underlain primarily by sedimentary rocks, including sandstone,
limestone, and shale, which have a low aeroradiometric signature. County average screening
indoor radon levels in the Colorado Plateau are mostly greater than 2 pCi/L (fig. 3).
The part of the Basin and Range Province lying in EPA Region 8 has moderate geologic
radon potential. The part of the province which is in Region 8 is actually a part of the Great Basin
Section of the Basin and Range Province. The entire province is laced with numerous faults, and
large displacements along the faults are common. Many of the faulted mountain ranges have high
aeroradiometric signatures, whereas the intervening valleys or basins often have low
aeroracHometric signatures. Because of the numerous faults and igneous intrusions, the geology is
highly variable and complex. Indoor radon levels are similarly variable, with county averages
ranging from less than 1 pCi/L to more than 4 pCi/L (fig. 3).
m-6 Reprinted from USGS Open-File Report 93-292-H
-------
PRELIMINARY GEOLOGIC RADON POTENTIAL ASSESSMENT OF COLORADO
by
Russell F. Dubiel
U.S. Geological Survey
INTRODUCTION
Colorado is the birthplace of the uranium mining industry in the United States, which
began with the discovery of pitchblende in 1871 in the mine tailings of the Wood mine in the
Central City district of Gilpin County (Chenoweth, 1980). The subsequent development of the
uranium mining industry in Colorado reflects the relative importance and abundance of three
metals: radium, vanadium, and uranium. In 1980 Colorado'ranked fourth in domestic uranium
production behind New Mexico, Wyoming, and Utah (Chenoweth, 1980). Although uranium
mining is not presently economically viable, uranium deposits occur in rocks of many geologic
ages and lithologies in Colorado. Because the uranium- and radium-bearing bedrock and the soils
and alluvium developed from those rocks are widespread in Colorado, and because radon is a
daughter product of uranium decay, many areas in the state have the potential to generate and
transport radon in sufficient concentrations to be of concern in indoor air.
This is a generalized assessment of geologic radon potential of rocks, soils, and surficial
deposits of Colorado. The scale of this assessment is such that it is inappropriate for use in
identifying the radon potential of small areas such as neighborhoods, individual building sites, or
housing tracts. Any localized assessment of radon potential must be supplemented with additional
data and information from the locality. Within any area of a given radon potential ranking, there
are likely to be areas with higher or lower radon levels than characterized for the area as a whole.
Indoor radon levels, both high and low, can be quite localized, and there is no substitute for testing
individual homes. Elevated levels of indoor radon have been found in every state, and EPA
recommends that all homes be tested. For more information on radon, the reader is urged to
consult the local or State radon program or EPA regional office. More detailed information on state
or local geology may be obtained from the State geological survey. Addresses and phone numbers
for these agencies are listed in chapter 1 of this booklet.
PHYSIOGRAPHIC AND GEOGRAPHIC SETTING
The physiography of Colorado (fig. 1) is in part a reflection of the underlying bedrock
geology (fig. 2) (Mallory, 1972). The southern Rocky Mountains form a distinct physiographic
province that extends in a broad north-south belt through Colorado from southeastern Wyoming to
north-central New Mexico. The Rockies rise to more than 14,000 ft, and many of the ranges are
anticlinal, with Precambrian igneous cores flanked by steeply dipping hogbacks of Paleozoic and
Mesozoic sedimentary strata. Large intermontane basins, or parks, separate many of the ranges.
'ThTfrilenh^n'fane'Basinsaregener^lyTiHe^"b~y"TertiaryMia^"Quaternarydeposits. Tnextreme
northern Colorado, the Wyoming Basin province is transitional to the southern Rocky Mountains.
The southern Rocky Mountains separate the Great Plains province in the eastern half of the state
from the part of the Colorado Plateau province that occupies the southwestern corner of the state.
The Great Plains in Colorado are generally underlain by Mesozoic and Cenozoic sedimentary rocks
and rise to about 5,500 ft adjacent to the Rocky Mountains. That part of the Great Plains adjacent
to the Front Range is known as the Colorado Piedmont or the High Plains. The Colorado Plateau
IV-1 Reprinted from USGS Open-File Report 93-292-H
-------
Hgure 1. Major physiographic provinces of the western United States (modified from Mallory,
1972).
-------
100 miles
EXPLANATION
Quaternary sedimentary
and Igneous rocks
Tertiary sedimentary rocks
Tertiary volcanic rocks
Cretaceous
sedimentary rocks
Jurassic, Triassic.
and Paleozoic rocks
p I Precambrlan
sedimentary rocks
Tertiary intrusive rocks
Precambrlan igneous
and metamorphic rocks
Faults
Figure 2. Map showing generalized geology of Colorado (modified from Mallory, 1972).
-------
is a roughly circular area centered about the Four Corners region of Colorado, Utah, Arizona, and
New Mexico, and it extends into southwestern Colorado. The Colorado Plateau consists of highly
dissected plateaus and mesas ranging in elevation from about 5,000 to 11,000 ft, except in the
deepest river canyons. The San Juan Mountains in southwestern Colorado form an isolated range
at the transition between the Colorado Plateau and the southern Rocky Mountains and are
composed primarily of Tertiary volcanic rocks.
Population distribution (fig. 3A, B) and land use in Colorado reflect in part the geology,
topography, and climate of the state (Erickson and Smith, 1985). In 1990, the census indicated
approximately 3.3 million persons residing within Colorado's 103,766 square miles. Thus, the
population density (fig. 3A) is approximately 31 persons per square mile, substantially below the
national average of 65 persons per square mile. Within Colorado, the population is very unevenly
distributed (fig. 3B): some mountainous tracts have virtually no residents, and only a few
ranching and farming families can be found over large areas of both the Great Plains and the
Colorado Plateau provinces. Urban areas are concentrated along the Front Range on the eastern
edge of the Rocky Mountains, extending from Pueblo on the south through Colorado Springs and
Denver to Fort Collins on the north. This distribution reflects Colorado's early history and the rich
mineral deposits of the Rocky Mountains. Mineral wealth provided the major impetus for
settlement in Colorado (Erickson and Smith, 1985), and Denver, Colorado Springs, Golden,
Boulder, and other towns along the Front Range were established at the mountain front as supply,
transportation, and smelting centers for the mining industry in the Rocky Mountains. Other cities
such as Grand Junction, Durango, and many smaUer towns are situated along major rivers that
drain the eastern and western slopes of the Continental Divide. These early transportation
corridors provided access to the mineral districts and continue today as the routes followed by
modern highways. Despite the general decline in the minerals industry in the last decade, many
former mining towns in scenic high-country locations have been rejuvenated and have grown in
population in recent years in response to the outdoor recreation and ski industries.
East of the Rockies on the Great Plains, agricultural activities on irrigated and non-irrigated
cropland, rangeland, or non-irrigated pastureland are the predominate industries; grazing is the
dominant land use in the state (Erickson and Smith, 1985). Along rivers and on high mesas west
of the mountains on the Colorado Plateau, agriculture as a whole is limited, but fruit orchards
sustain a major local industry. Grazing is the dominant land use on the western slope of the
Rockies and on the Colorado Plateau in the southwestern part of the state. The forested Rocky
Mountains and the high mesas of the Colorado Plateau are used extensively both for forest
production and for winter and summer recreation (Erikson and Smith, 1985).
GEOLOGY
Colorado's geology is complex and varies widely from place to place, but in general the
bedrocfc geology i^aracteris^
addition, many of the radiorrietric anomalies noted on the aerial radiometric map (fig. 4; Duval and
others, 1989) can be associated with specific bedrock formations. The following discussion of
geology and soils of Colorado is condensed from Chronic and Chronic (1972), Mallory (1972),
Heil and others (1977), Tweto (1979), several topical papers in Kent and Porter (1980), and Beach
and others (1985).
The Great Plains east of the Rocky Mountains are characterized by relatively undeformed
sedimentary rocks consisting primarily of sandstone, siltstone, and mudstone. The eastern half of
IV-4 Reprinted from USGS Open-File Report 93-292-H
-------
POPULATION (1990)
E3 0 to 10000
E3 10001 to 25000
E3 25001 to 50000
H 50001 to 100000
• 100001 to 467610
Figure 3A. Population of counties in Colorado (1990 U.S. Census data).
-------
Urban Populition in Thousands of Persons One dot represents 1,000 persons
POPULATION
DISTRIBUTION
1980
Savin: U.S. CKIKO a Poouuun. 1MO
Figure 3B. Map showing population distribution of Colorado in 1980 (modified from Erickson
and Smith, 1985).
-------
Figure 4. Aerial radiometric map of Colorado (after Duval and others, 1989). Contourlines at 1.5
and 2.5 ppm equivalent uranium (eU). Pixels shaded from 0 to 6.0 ppm eU at 0.5 ppm ell
increments; darker pixels have lower eU values; white indicates no data.
-------
the Great Plains in Colorado and a large area of the plains adjacent to the mountain front from
Colorado Springs to north of Denver are underlain by Tertiary and Quaternary sedimentary rocks,
whereas the remaining western part of the province is underlain by Cretaceous sandstones, shales,
and limestones. In the southeastern part of Colorado, sedimentary strata consisting of Permian,
Triassic, and Jurassic sandstones, mudstones, and minor limestones are exposed in the drainages
of thePurgatoire and Cimarron Rivers.
The Rocky Mountains, including the southern Rocky Mountains and the Wyoming Basin,
were formed during the Laramide orogeny, a Late Cretaceous to Eocene structural event that
emplaced Precambrian and Cambrian igneous and metamorphic crystalline rocks and minor
Cenozoic volcanic rocks adjacent to Paleozoic and Mesozoic sedimentary strata. The Paleozoic and
Mesozoic sedimentary rocks consist of conglomerate, sandstone, shale, and limestone. The oldest
rocks in Colorado are Precambrian granite intrusive rocks, Precambrian metamorphic gneiss,
schist, and pegmatite, and sedimentary quartzite, slate, and phyllite exposed in the Rocky
Mountains and locally on uplifts in the San Juan Mountains in southwestern Colorado. Paleozoic
and Mesozoic sedimentary strata are steeply dipping where they have been uplifted by the igneous
intrusions and along basement faults reactivated by Laramide structural uplift. Cambrian quartzite,
and Ordovician, Devonian, and Mississippian limestone, dolomite, and minor sandstone are
exposed along the western flank of the Rocky Mountains and in scattered outcrops around the
White River uplift in west-central Colorado. Pennsylvanian, Permian, Triassic, Jurassic, and
Cretaceous conglomerate, sandstone, shale, and minor limestone also are locally uplifted and
exposed along the mountain fronts.
The Colorado Plateau and the Wyoming Basin provinces are underlain by uplifted,
primarily flat-lying, locally folded, deeply eroded sedimentary rocks ranging in age from
Pennsylvanian to Tertiary. Pennsylvanian and Permian rocks are predominantly arkosic
conglomerate, fluvial and eolian sandstone, and minor marine limestone. Triassic strata comprise
marginal-marine sandstone and shale and extensive continental fluvial and lacustrine sandstone,
mudstone, and limestone. Jurassic rocks consist of widely exposed eolian sandstone, marine
limestone and shale, and continental lacustrine and fluvial sandstone and mudstone. Cretaceous
rocks form a thick sedimentary section in Colorado and consist of marine shale, sandstone, and
limestone interfingered with nonmarine fluvial sandstone and shale. Tertiary sedimentary strata are
dominandy lacustrine carbonate and mudstone and minor fluvial sandstone. Tertiary volcanic
rocks of extrusive lava, tuff, breccia, and conglomerate and minor rhyolitic intrusive rocks
compose the San Juan Mountains in southwestern Colorado and are also found in minor exposures
throughout the state.
Uranium deposits (fig. 5A) and production (fig. 5B) in Colorado occur in rocks of many
geologic ages and lithologies. A comprehensive report on the uranium deposits in Colorado, from
which the following discussion is summarized, can be found in Chenoweth (1980). Sedimentary-
hosted uranium deposits are the most common type of uranium occurrence in the State. Uranium-
vanadium deposits in fluvialsandstones of the Salt Wash Member of the Upper Jurassic j^orrisgn
Formation occur in the Uravan mineraTbeft in western Colorado. The Uravan mineral belt is an
arcuate area in Mesa, Montrose, and San Miguel Counties containing an abundance of closely
spaced, high-grade ore deposits. Uranium-vanadium deposits also occur in the Salt Wash east of
Meeker on Coal Creek anticline in Rio Blanco County. Uranium-vanadium deposits in eolian
strata of the Middle Jurassic Entrada Sandstone are known northeast of Rifle in Garfield County,
near Placerville in San Miguel County, and north of Durango at Barlow Creek-Graysill in San Juan
and Dolores Counties. The Oligocene and Miocene Browns Park Formation is a fluvial, arkosic,
IV-8 Reprinted from USGS Open-File Report 93-292-H
-------
RALSTON CREEK-GOLDEN GATE vwSMwSTOK
COCHETOP~ MARSHALL PASS
Figure 5 A. Map showing major uranium mines, significant uranium ore deposits, and uranium
production (modified from Chenoweth, 1980).
-------
EXPLANATION
Precambrian crystalline rocks exposed
URANIUM DEPOSITS OR GROUP OF DEPOSITS
Size (production plus reserves) of deposits that
contains at least 0.1 percent "-"-
More than
1,000,000
•
•
•
Tons
1.000 to
1,000,000
•
•
*
Ho
1,000
D
O
0
Age of
host rock
Tertiary
Cretaceous
and Jurassic
older
Deposits peneconcotdantwilh sedimentary features
of enclosing rocks
Symbol with a vertical stem indicates deposit is in
coaly carbonaceous rock
Veins, breccia zones, and related types of deposits
COaiy caroonatruuiluin
Figure 5B. Map showing major uranium mines and deposits, with known production (modified
from Mallory, 1972).
-------
locally tuffaceous sandstone that hosts uranium deposits near Maybell in Moffat County. The
Tallahassee Creek area on the southeastern flank of the Thirty-nine Mile volcanic field in north-
central Fremont County contains tabular uranium deposits in the Eocene Echo Park Alluvium,
which contains arkosic sandstone and conglomerate, and the Oligocene Tallahassee Creek
Conglomerate, which consists of volcaniclastic conglomerate and tuffaceous sandstone. The
Upper Cretaceous Fox Hills Sandstone and Laramie Formation of the Denver Basin in Weld
County contain roll front-type uranium deposits in fluvial sandstones. The Upper Cretaceous
Dakota Sandstone has produced small amounts of uranium ore near Rabbit Ears Pass in Grand
County, near Badito Cone in Huerfano County, and on the east flank of the Turkey Creek anticline
in the northwest corner of Pueblo County. The Paleocene and Eocene Coalmont Formation has
produced uranium ore near Hot Sulfur Springs in Grand County, and ore has been produced from
the Oligocene Antero Formation near Hartsel in Park County. Minor amounts of uranium ore have
been produced from fracture-controlled, sedimentary-hosted deposits in the Middle Pennsylvanian
to Lower Permian Weber Sandstone and Maroon Formation in Moffat and Park Counties, the
Middle Jurassic Curtis Formation in Moffat County, the Upper Jurassic Morrison Formation in
El Paso County, and the Upper Cretaceous Dakota Sandstone and Laramie Formation in Jefferson
County.
Production from vein-type uranium occurrences in Colorado is subordinate to sedimentary-
hosted deposits, but significant ore bodies occur in the Front Range west of Denver in Precambrian
rocks in Larimer, Boulder, Jefferson, Gilpin, Clear Creek, and Park Counties. Uranium has been
known in the Central City district of Gilpin County since 1871, where pitchblende was first
discovered in the United States on the tailings pile of the Wood mine. Since that time, important
deposits have been found near Jamestown, Ralston Creek, Golden Gate Canyon, and Ideldale.
These deposits, located near the Central City district, are hosted in the Precambrian (Early
Proterozoic) metamorphic complex of the Idaho Springs Formation, which also hosts the
Schwartzwalder uranium mine, the largest uranium mine in Colorado. Other Precambrian rocks
along the Front Range locally have produced ore. Complicated fault-vein relationships produce
uranium in the Marshall Pass area in northern Saguache and southeastern Gunnison Counties.
Uranium also occurs along high-angle normal faults within the Middle and Upper Jurassic Junction
Creek Sandstone and Upper Jurassic Morrison Formation of the Cochetopa area on the northern
margin of the San Juan Mountains in northwestern Saguache County. Minor amounts of uranium
ore have been produced from vein deposits in a variety of host rocks in the Park, Sawatch, and
Sangre de Cristos Ranges and the San Juan and La Plata Mountains.
In addition to the known deposits in Colorado where uranium has been concentrated as ore,
uranium also occurs in several rock formations at concentrations too low to be considered
economic but that may still generate radon at levels considered to be a problem in indoor air. For
example, the Upper Cretaceous Sharon Springs Member of the Pierre Shale, the Upper Cretaceous
Mancos Shale, and the Miocene and Pliocene Ogallala Formation all contain low-level but
consistent concentrations of uranium. Precambrian rocks such as the Middle Proterozic Pikes Peak
"Gramte~nave consistent"ufaHium concentrations and locally Mghe7cbncenSations~along fractures'
faults, and shear zones (Schumann, Gundersen, and others, 1989). Tertiary volcanic rocks and
ash-flow tuffs around calderas in the San Juan Mountains have low-level uranium concentrations.
Many alluvial deposits and soils reworked from uranium-bearing igneous and sedimentary parent
rocks, particularly along the Front Range, have significant potential to generate radon.
IV-11 Reprinted from USGS Open-File Report 93-292-H
-------
SOILS
A generalized soil map of Colorado (fig. 6) compiled from Heil and others (1977) and
Erickson and Smith (1985) indicates that soils in Colorado consist of Mollisols and Aridisols on
the Great Plains; Alfisols, Aridisols, and Inceptisols in the Rocky Mountains; and Entisols, with
minor Alfisols, Mollisols, and Aridisols on the Colorado Plateau. Natrargids (sodium-rich
Aridisols) are the major soil order in the Wyoming Basin in northwestern Colorado. It should be
noted that many of the areas within these generalized soil orders, especially in the Rocky
Mountains and on the Colorado Plateau, consist of bare bedrock with incipient to nonexistent soil
development In general, most soils in Colorado are moderately permeable; however, each soil
order contains individual soil associations that range from slow to rapid permeability. Although
the data in Heil and others (1977) refers most commonly to depth to bedrock in soil associations,
which generally can vary from less than 20 inches to more than 60 inches, a few associations do
indicate depth to seasonal high water table. For the Aridisols and Natrargids, several soil
associations have depth to seasonal high water table from 2 to more than 6 feet. The Entisols and
Mollisols include a few soil associations that have depth to seasonal high water table from 0 to 2
feet. The shrink-swell potential of many of Colorado's soils can affect radon concentrations in
those soils (Schumann and others, 1989). Soils with high shrink-swell potential may cause
building foundations to crack and thus allow radon to enter the structure. Swelling soils, which
often crack as they dry, can have effectively increased soil permeability due to cracks. Several
areas of Colorado have soils with high shrink-swell potential and include areas underlain by
bentonitic Upper Cretaceous marine shales (Benton Formation) and Cretaceous to Tertiary rocks
(Arapahoe and Denver Formations) in the Great Plains, in the Grand Valley on the Colorado
Plateau, and in parts of the Uinta and Piceance basins in the Wyoming Basin province.
INDOOR RADON DATA
Indoor radon data for Colorado (fig. 7; Table 1) from the State/EPA Residential Radon
Survey were compiled from 1986 to 1987 (Colorado Geological Survey, 1991). Data from only
those counties in which five or more measurements were made are presented in figure 7. A map
showing the counties in Colorado (fig. 8) is provided to facilitate discussion of correlations among
the indoor radon data (fig. 7), geology (fig. 2), aerial radiometric data (fig. 4), and soils (fig. 6).
In this discussion, "elevated" refers to screening indoor radon levels greater than 4.0 pCi/L. For
the counties that have sufficient data to be shown on figure 7, the distribution of elevated indoor
radon levels correlates with the bedrock geology and in general with the aerial radiometric data.
Elevated indoor radon levels occur in the Great Plains region underlain by Cretaceous sedimentary
shales and limestones. Elevated indoor radon levels also occur in the High Plains adjacent to the
Front Range on the eastern flank of the Rocky Mountains in areas underlain by Permian, Triassic,
Jurassic, and Cretaceous sedimentary rocks, in areas of alluvium derived from those rocks, and
from the"igneous rocks to" the wesf in tfie~Rocky Mountains. Elevated radon levels also occur "in
the Rocky Mountains, and especially in the Front Range, where the bedrock consists of
Precambrian igneous and metamorphic rocks, some with faults and fracture zones, and numerous
Paleozoic to Cenozoic sedimentary rocks. Elevated indoor radon levels also occur on the Colorado
Plateau in regions underlain by Paleozoic, Mesozoic, and especially Cretaceous sedimentary rocks.
IV-12 Reprinted from USGS Open-File Report 93-292-H
-------
EXPLANATION
[;££j Affisols (moderate permeability)
^^ Inceptisols (moderate permeability)
i?';V!i Entisols (moderate permeability)
$8888 Aridisols (moderate permeability)
I I Mollisols (stow to rapid permeability)
|V£| Natrargids (moderate permeability)
0
J-
150 mi
225km
Figure 6. Map showing generalized soils of Colorado (modified from Erickson and Smith, 1985).
-------
Bsmt. & 1st Floor Rn
% >4pCi/L
21 L
OtolO
11 to 20
21 to 40
41 to 60
61 to 80
Missing Data
or < 5 measurements
Bsmt. & 1st Floor Rn
Average Concentration (pCi/L)
5 L^l
E3
21 L
0.0 to 1.9
2.0 to 4.0
4.1 to 10.0
10.1 to 14.7
Missing Data
or < 5 measurements
100 Miles
Figure 7. Screening indoor radon data from the EPA/State Residential Radon Survey of
Colorado, 1986-87, for counties witlr5or more mrastiremen1srDam~areffbTrr2-7"dayl:harcoal"~
canister tests. Histograms in map legends show the number of counties in each category. The
number of samples in each county (See Table 1) may not be sufficient to statistically characterize
the radon levels of the counties, but they do suggest general trends. Unequal category intervals
were chosen to provide reference to decision and action levels.
-------
TABLE 1. Screening indoor radon data from the EPA/State Residential Radon Survey of
Colorado conducted during 1986-87. Data represent 2-7 day charcoal canister measurements
from the lowest level of each home tested.
COUNTY
ADAMS
ALAMOSA
ARAPAHOE
ARCHULETA
BACA
BENT
BOULDER
CHAFMiE
CHEYENNE
COSTELLA
CROWLEY
CUSTER
DELTA
DENVER
DOUGLAS
EAGLE
ELBERT
EL PASO
FREMONT
GRAND
GUNNISON
HUERFANO
JACKSON
JEFFERSON
KIOWA
KIT CARSON
LA PLATA
LARIMER
LAS ANIMAS
LINCOLN
MESA
MINERAL
MOFFAT
MONTEZUMA
MONTROSE
OTERO
OURAY
PARK
PROWERS
PUEBLO
RIO BLANCO
NO. OF
MEAS.
33
6
64
6
34
16
54
7
18
14
18
' 2
19
40
72
8
21
113
88
23
15
19
6
50
13
8
10
96
27
14
73
3
8
17
22
19
2
9
18
32
16
MEAN
5.7
2.5
6.2
1.6
4.8
2.7
4.2
1.2
6.7
4.8
5.0
1.1
4.2
4.3
7.6
5.7
4.6
4.7
5.0
5.4
3.8
5.0
6.8
5.1
14.7
7.2
5.2
5.5
6.0
4.6
2.7
14.0
2.8
2.6
2.4
3.5
2.8
5.2
2.6
2.5
1.8
GEOM.
MEAN
3.7
2.0
4.4
1.4
2.7
2.6
2.7
0.8
5.3
3.6
4.1
1.1
3.4
3.5
5.6
3.8
2.9
2.7
3.1
2.3
2.1
4.1
4.6
3.6
3.6
5.6
3.1
3.2
3.8
3.1
2.1
5.4
2.4
1.7
1.0
2.5
2.0
2.1
2.1
1.7
1.1
MEDIAN
3.3
1.9
4.8
1.3
3.6
2.7
2.6
1.0
4.9
3.2
4.1
1.1
3.3
4.0
5.2
2.9
4.5
3.0
3.5
4.0
3.7
4.0
4.3
3.5
4.9
7.8
3.8
3.5
4.2
4.1
2.2
3.5
1.9
1.5
1.0
3.0
2.8
2.6
1.9
1.7
1.4
STD.
DEV.
7.3
1.9
6.7
1.0
4.5
0.9
3.9
1.0
5.9
3.5
3.0
0.1
3.0
2.5
6.9
6.0
3.0
6.5
8.8
7.0
3.2
3.9
6.0
4.4
22.3
4.5
4.6
5.2
5.9
4.5
2.0
20.3
1.6
3.2
3.7
3.0
2.8
5.4
2.1
2.3
1.6
MAXIMUM
39.8
6.1
47.9
3.6
17.0
5.0
20.2
3.2
26.9
12.6
11.3
1.2
11.5
11.2
33.5
19.0
10.1
46.4
81.2
34.1
1LO
18.7
15.0
24.1
70.9
13.8
13.5
25.1
27.1
18.4
11.5
37.4
5.7
13.4
16.3
12.7
4.8
14.7
8.7
10.4
7.0
%>4 pCi/L
36
17
55
0
47
6
41
0
67
43
50
0
42
48
63
38
57
36
44
48
40
47
50
48
69
63
50
43
52
50
15
33
25
12
14
21
50
44
22
28
6
%>20pCi/L
3
0
3
0
0
0
2
0
6
0
0
0
0
0
7
0
0
4
1
4
0
0
0
2
23
0
0
3
4
0
0
33
0
0
0
0
0
0
0
0
0
-------
TABLE 1 (continued). Screening indoor radon data for Colorado.
COUNTY
RIO GRANDE
ROUTT
SAGUACHE
SAN MIGUEL
TELLER
WASHINGTON
YUMA
NO. OF
MEAS.
6
14
9
9
3
1
3
MEAN
2.9
5.5
1.4
1.6
99.7
7.8
11.0
GEOM.
MEAN
2.7
3.6
1.2
0.9
57.6
7.8
7.6
MEDIAN
2.7
3.3
1.2
0.8
71.6
7.8
9.5
STD.
DEV.
1.3
5.8
0.8
1.8
104.2
***
9.7
MAXIMUM
5.1
21.4
2.8
5.4
215.0
7.8
21.4
%>4 pCi/L
17
43
0
11
100
100
67
%>20 pCi/L
0
7
0
0
67
0
33
-------
-------
The highest indoor radon levels measured in Colorado as of this writing were greater than
600 pCi/L. These levels are associated with faults and mineralized shear zones in igneous and
metamorphic crystalline rocks in the Front Range of Jefferson County near Conifer (Schumann,
Gundersen, and others, 1989) and in faulted Sharon Springs Member of the Pierre Shale along the
mountain front in Larimer County near Fort Collins, and in the Highlands Ranch subdivision in the
southern Denver metropolitan area. The highest indoor radon value measured in the State/EPA
Residential Radon Survey of Colorado was 215 pCi/L in Teller County (Table 1).
The complex geology in each county of Colorado and the scale of maps used in this report
makes it difficult to characterize individual rock units that may be responsible for the specific
elevated radon levels; the reader is referred to the geologic discussion in this report and should note
that the specific geology at any particular site is critical to discerning the factors responsible for
measured elevated radon levels. Each of the geologic terranes with elevated radon levels
corresponds to areas of anomalously high radiometric signatures on the aerial radiometric map
(fig. 4) that reflect uranium-bearing bedrock or alluvium and soils derived from those rocks.
GEOLOGIC RADON POTENTIAL
A comparison of geology (fig. 2) with aerial radiometric data (fig. 4) and indoor radon data
(fig. 7; Table 1) provides preliminary indications of rock types and geologic features suspected of
producing elevated radon levels. An overriding factor in the geologic evaluation is the abundance
and widespread outcrops of known uranium-bearing and uranium-producing formations in the
state (fig. 5; Chenoweth, 1980). Because of the widespread occurrence of uranium-bearing rock
formations and alluvium, and soils derived from them, virtually all areas of Colorado have the
potential for some indoor elevated radon levels; however, even in areas underlain by rocks known
to contain uranium, other mitigating factors locally may interact to produce an environment that
does not have elevated radon levels. Colorado has many uranium-bearing rocks throughout the
state, as discussed in the geology section of this report (fig. 5), but all of those rocks are not highly
uraniferous at every locality. The following list is an overview of the rocks that are most likely to
produce elevated indoor radon levels. In the Great Plains, sedimentary rocks such as the Upper
Cretaceous and Paleocene Dawson Arkose, and various Cretaceous sedimentary rocks including
the Dakota Formation, Fox Hills Sandstone, and Laramie Formation, the Pierre Shale (especially
the Sharon Springs Member), all have the potential to produce locally elevated indoor radon levels.
In addition, the Upper Cretaceous and Paleocene Denver Formation and the Upper Cretaceous
Arapahoe Formation, along with Tertiary and Quaternary alluvium and soils derived from these
rocks and from uplifted Paleozoic and Mesozoic sedimentary rocks and Precambrian igneous rocks
in the Rocky Mountains also have potential for producing locally elevated radon levels.
In the Rocky Mountains, outcrops of Precambrian igneous and metamorphic crystalline
rocks such as the Pikes Peak Granite, the Silver Plume Granite, and the Idaho Springs Formation,
particularly wherethey arejracture^fojufed^^^
- -- —•- ------ - - - - -_->•-—•--«-- • - f.._. _J j__^.l__.rt.1rt T T»i1i-P*-«yI TDnl^a
concentrations of uranium minerals and to produce elevated radon levels. Uplifted Paleozoic and
Mesozoic sedimentary rocks, and smaller outcrops of Tertiary volcanic and sedimentary rocks, are
also locally uraniferous and may produce elevated radon levels.
On the Colorado Plateau and in the Wyoming Basin, many rock formations are known to
produce uranium ore and to locally contain low-level concentrations of uranium where ore is not
present. Outcrops of the Salt Wash Member of the Morrison Formation are probably the most
likely to contain significant uranium orebodies, but many other formations have produced uranium
IV-18 Reprinted from USGS Open-File Report 93-292-H
-------
occurrences in Colorado. Locally, the Middle Jurassic Entrada Sandstone, the Oligocene and
Miocene Browns Park Formation, the Eocene Echo Park Alluvium, the Oligocene Tallahassee
Creek Conglomerate, and the Paleocene and Eocene Coalmont Formation all have potential to
produce elevated radon levels. In the San Juan Mountains, various extrusive volcanic rocks locally
contain above-average uranium concentrations that may produce elevated radon levels.
Ground water in contact with uranium-bearing bedrock has the potential to accumulate
radon and to contribute to indoor radon levels (Nazaroff and Nero, 1988). Municipal water
treatment generally dissipates radon accumulations in water supplies, but individual wells used as a
source of domestic water that are located in bedrock with high uranium concentrations can
contribute significant levels of radon to indoor air (Hess and others, 1990; Lawrence and others,
1989, in press). In Colorado, domestic water wells that tap ground water in uranium-bearing
bedrock, especially the fractured, faulted, or sheared Precambrian rocks of the .Rocky Mountains,
have the potential to significantly contribute to elevated indoor radon levels (Lawrence and others,
1989), and waterborne radon levels as high as 3,000,000 pCi/L have been found in the Lyons,
Colorado, area. The Ogallala aquifer, a principal source of ground water on the Great Plains, and
the Dakota Group aquifer (Vinckier, 1982), located in the Canon City embayment of Fremont and
Pueblo Counties, also contain low-level uranium concentrations. In such areas, ground water may
contribute significantly to indoor radon but on a highly variable basis depending on water usage.
SUMMARY
For purposes of assessing the radon potential of the state, Colorado can be divided into
nine general areas (termed Area 1 through Area 9), delineated on the basis of similar geology and
other factors listed in Table 2 (see figure 9 and Table 2) and scored with a Radon Index (RI), a
semi-quantitative measure of radon potential, and an associated Confidence Index (CI), a measure
of the relative confidence of the assessment based on the quality and quantity of data used to make
the evaluations. For further details on the ranking schemes and the factors used in the evaluations,
refer to the Introduction chapter to this booklet.
Areas 1, 2,3, 4, and 6 each have high radon potential (RI=15, 14, 13, 13, and 12,
respectively) associated with a high or moderate confidence index (CI=11, 10, 9, 9, and 8,
respectively), and area 5 has a high radon potential (RI=12) with a moderate confidence index
(CI=8) on the basis of high indoor radon measurements, high surface radioactivity as evidenced by
the aerial radiometric data, and the presence of rock formations such as Precambrian granite,
Jurassic sandstone and limestone, or Cretaceous to Tertiary sandstone, shale, and volcanic rock
that are known to contain or produce uranium. Area 1 encompasses the Rocky Mountains and
contains primarily Precambrian granite that has low but consistent uranium concentrations and
abundant shear zones that are known to produce radon in several areas (Schumann, Gundersen,
and others, 1989); it also contains outcrops of sedimentary rocks shed from the granitic highlands.
Area 2 just east of the Rocky Mountains in central Colorado contains primarily outcrops of the
DawsbfrAfkoseTKaf was sKM^~aUuvTal"'fan and riverIdeposits sourcecTIn trie^ffite'mcoiiitainslo"
the west. Area 3 is primarily underlain by marine shales of the Mancos Shale and by Tertiary
sandstones. Area 4 is underlain by variable geology and includes uranium-bearing Jurassic
sedimentary rocks of the Uravan mineral belt. Area 5 is underlain primarily by Tertiary sandstone,
primarily the Ogallala Formation, and in part, is covered to varying degrees by windblown eolian
sand and silt (loess) deposits. Both the bedrock and the loess have the capacity to contribute to
high radon values, whereas the thicker eolian sand generally is associated with relatively lower
IV-19 Reprinted from USGS Open-File Report 93-292-H
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radon values. Area 6 in eastern Colorado contains variable geology including Quaternary
deposits, Cretaceous marine shales that locally have a high radiometric signature, and small areas
of older sedimentary rocks.
Areas 7, 8, and 9 have moderate radon potential (RI=11 for each area) associated with
moderate confidence indices (CI=9, 9, and 8, respectively). Area 7 in southwestern Colorado
contains primarily volcanic rocks of the San Juan volcanic field. Area 8 comprises three parts of
western Colorado: sedimentary outcrops of the easternmost Uinta Mountains, west of the Rocky
Mountains, and the northern part of the San Juan Basin in southwestern Colorado. Area 9
contains primarily Teriary sedimentary rocks.
This is a generalized assessment of the State's geologic radon potential and there is no
substitute for having a home tested. The conclusions about radon potential presented in this report
cannot be applied to individual homes or building sites. Indoor radon levels, both high and low,
can be quite localized, and within any radon potential area there will likely be areas with higher or
lower radon potential that assigned to the area as a whole. Any local decisions about radon should
not be made without consulting all available local data. For additional information on radon and
how to test, contact your State radon program or EPA regional office. More detailed information
on state or local geology may be obtained from the State geological survey. Addresses and phone
numbers for these agencies are listed in chapter 1 of this booklet.
IV-21 Reprinted from USGS Open-File Report 93-292-H
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TABLE 2. Radon Index (RI) and Confidence Index (CI) scores for geologic radon potential areas
of Colorado.
FACTOR
ENDOOR RADON
RADIOACTIVITY
GEOLOGY
SOIL PERM.
ARCHITECTURE
GFE POINTS
TOTAL
RANKING
FACTOR
INDOOR RADON
RADIOACTIVITY
GEOLOGY
SOIL PERM.
ARCHITECTURE
GFE POINTS
TOTAL
RANKING
FACTOR
INDOOR RADON
RADIOACTIVITY
GEOLOGY
SOIL PERM.
ARCHITECTURE
GFE POINTS
TOTAL
Areal
RI CI
3
3
3
2
3
1
15
3
3
3
2
11
HIGH HIGH
Area 4
RI CI
2
3
3
2
3
0
13
2
3
2
2
9
HIGH MOD
Area?
RI CI
2
1
3
2
3
0
11
2
3
2
2
9
Area 2
RI CI
3
3
3
2
3
0
14
3
3
2
2
10
HIGH HIGH
Area 5
RI CI
3
2
3
2
2
0
12
1
3
2
2
8
HIGH MOD
Area 8
RI CI
2
2
2
2
3
0
11
2
3
2
2
9
Area 3
RI CI
2
3
3
2
3
0
13
.z
3
2
2
9
HIGH MOD
Area 6
RI CI
3
2
3
2
2
0
12
Z
3
2
2
9
HIGH MOD
Area 9
RI CI
2
3
2
2
2
0
11
1
3
2
2
8
RANKING MOD MOD
RADON INDEX SCORING:
Radon potential category
MOD MOD
Point range
LOW * 3-8 points
MODERATE/VARIABLE 9-11 points
HIGH > 11 points
Probable screening indoor
radon average for area
< 2 pCi/L
2-4pCi/L
> 4 pCi/L
Possible range of points = 3 to 17
CONFIDENCE INDEX SCORING:
LOW CONFIDENCE
MODERATE CONFIDENCE
HIGH CONFIDENCE
4-6 points
7-9 points
10 - 12 points
Possible range of points = 4 to 12
IV-22 Reprinted from USGS Open-File Report 93-292-H
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REFERENCES CITED IN THIS REPORT
AND GENERAL REFERENCES PERTAINING TO RADON IN COLORADO
Alter, H.W., 1980, Track etch radon ratios to soil uranium and a new uranium abundance
estimate, in Gesell,T.F., and Lowder, W.M. eds., Natural radiation environment IE; Vol.
1, Proceedings of International Symposium on the Natural Radiation Environment,
Houston, TX, April 23-28, 1978: DOE Symposium Series 1, p. 84-89.
Asher-Bolinder, Sigrid, Owen, D.E., and Schumann, R.R., 1990, Pedologic and climatic controls
on Rn-222 concentrations in soil gas, Denver, Colorado: Geophysical Research Letters v
17, p. 825-828.
Asher-Bolinder, S., Owen, D.E., and Schumann, R.R., in press, A preliminary evaluation of
environmental factors influencing day-to-day and seasonal soil-gas radon concentrations, in
Gundersen, L.C.S., and Wanty, R.B., eds. Field studies of radon in natural rocks, soils,
and water: U.S. Geological Survey Bulletin, 26 p.
Beach, R.A., Gray, A.W., Peterson, E.K., and Roberts, C.A., 1985, Availability of federal land
for mineral exploration and development in western states-Colorado, 1984: U.S. Bureau
of Mines Special Publication, 40 p.
Bell, M.W., Allen, J.W., Pacer, J.C., and Roberts, E.H., 1981, Drilling-mud emanometry; a new
technique for uranium exploration: U.S. Department of Energy Report GJBX-273(81),
50 p.
Bell, M.W., Pacer, J.C., and Roberts, E.H., 1981 , Drilling-mud emanometry, a new technique
for uranium exploration: American Association of Petroleum Geologists Bulletin v 65 D
898. '*'
Chenoweth, W.L., 1980, Uranium in Colorado, in Kent, H.C., and Porter, K.W., eds.,
Colorado geology: Rocky Mountain Association of Geologists, p. 217-224.
Chronic, J. and Chronic, H., 1972, Prairie, peak, and plateau: Denver, Colorado, Colorado
Geological Survey, 126 p.
Colorado Energy Research Institute, 1983, Radon gas levels in metropolitan Denver homes:
Prepared for the Executive Committee, Legislative Council of the Colorado General
Assembly, 42 p.
Colorado Geological Survey, 1991, Results of the 1987-88 EPA supported radon study in
-c-ol°rado,_withadiscussion-onGeology-:.jColoradoGeological-Survey-Open-Fil©Jleport
91-4, 51 p. F
Colorado Land Use Commission, 1973, A land use program for Colorado: Denver, Colorado,
Colorado Land Use Commission, 247 p.
Cothern, C.R., and Rebers, P.A., eds., 1990, Radon, radium, and uranium in drinking water,
286 p.
IV-23 Reprinted from USGS Open-File Report 93-292-H
-------
Duval, J.S., Jones, W.J., Riggle, F.R., and Pitkin, J.A., 1989, Equivalent uranium map of
conterminous United States: U.S. Geological Survey, Open-File Report 89-478, 10 p.
Erickson, K.A., and Smith, A.W., 1985, Atlas of Colorado: Colorado Associated University
Press, 73 p.
Evans, H.B., 1957, Natural radioactivity of the atmosphere (Colorado Plateau and Texas): U.S.
' Geological Survey TEI-700, 268-272 p.
Felmlee, J.K,. and Cadigan, R.A., 1979, Radium and uranium concentrations and associated
hydrogeochemistry in ground water in southwestern Pueblo County, Colorado: Geological
Society of America, Abstracts with Programs, v. 11, p. 272.
Fisher, J.C., 1976, Application of track etch radon prospecting to uranium deposits, Front Range,
* Colorado, w Weiss, A., ed., World mining and metals technology: Proceedings of third
joint meeting of the Mining and Metallurgical Institute of Japan and the American Institute
of Mining, Metallurgical and Petroleum Engineers,World Mining and Metals Technology
Denver, CO, Sept. 1-3, 1976, p. 95-112.
Franklin, J.C., and Marquardt, R.F., 1976, Continuous radon gas survey of the Twilight Mine:
U.S. Bureau of Mines, Technical Program Report 93, 16 p.
Gerlach, A.C., ed., 1970, The National Atlas of the United States of America: Washington, D.C.,
U.S. Geological Survey, 417 p.
Gingrich, J.E., and Fisher, J.C., 1976, Uranium exploration using the track-etch method:
Proceedings of exploration for uranium ore deposits, Vienna, Austria, March 29: April 2,
1976, IAEA, Proceedings Series 434, p. 213-227.
Hazle, A.J., 1987, Colorado; the legacy of uranium mining: Environment, v. 29, p. 13, 15, 17,
37-39.
Heil, R.D., Romine, D.S., Moreland, D.C., Dansdill, R.K., Montgomery, R.H., and Cipra,
J.E., 1977, Soils of Colorado: Fort Collins, Colorado, Colorado State University
Experiment Station in Cooperation with the Soil Conservation Service-USD A, 40 p.
Hess, C.T., Vietti, M.A., Lachapelle, E.B., and Guillemette, J.F., 1990, Radon transferred from
drinking water into house air, in C.R. Cotherm and P.A. Rebers, eds., Radon, radium
and uranium in drinking water: Chelsea, Michigan, Lewis Publishers, p. 51-67.
Holub, R.F., DrouUard, R.R,. Bprak, XB^Lnkret J&^
Radon-222 and 222Rn progeny concentrations measured in an energy-efficient house
equipped with a heat exchanger: Health Physics, v. 49, p. 267-277.
Jaacks, J.A., and Klusman, R.W., 1981, Seasonal and short-term variations in gas emission at an
aseismic site: Eos, Transactions, American Geophysical Union, v. 62, p. 962-963 .
IV-24 Reprinted from USGS Open-File Report 93-292-H
-------
Jaacks, J.A., and Klusman, R.W., 1984, A comparison of instantaneous versus integrative
techniques in soil-gas sampling, in Anonymous, ed., Exploration for ore deposits of the
North American Cordillera; Abstracts with Program: Proceedings of Symposium of the
Association of Exploration Geochemists, Reno, NV, Mar. 25-28, 1984, p. 45.
Jacoby, G.C., Jr., Simpson, H.J., Mathieu, G., and Torgersen, T., 1979, Analysis of
groundwater and surface water supply interrelationships in the Upper Colorado River basin
using natural radon-222 as a tracer: John Muir Institute, 46 p.
Kent, H.C. and Porter, K.W., eds., 1980, Colorado Geology: Denver, Colorado, Rocky
Mountain Association of Geologists, , 258 p.
Klusman, R.W., 1981, Seasonal and short-term variations in gas emission from the Earth: Eos,
Transactions, American Geophysical Union, v. 62, p. 1033-1034 .
Klusman, R.W. and Jaacks, J.A., 1987, Environmental influences upon mercury, radon and
helium concentrations in soil gases at a site near Denver, Colorado: Journal of
Geochemical Exploration, v. 27, p. 259-280.
Klusman, R.W. and Webster, J.D., 1981, Preliminary analysis of meteorological and seasonal
influences on crustal gas emission relevant to earthquake prediction: Bulletin of the
Seismological Society of America, v. 71, p. 211-222.
Landa, E.R., 1983, Radon concentrations in the indoor air of earth sheltered buildings in
Colorado, in Boyer, L.L., ed., Proceedings of First International Earth Sheltered Buildings
Conference, Sydney, Australia: Architectural Extension and University Center for Energy
Research, Oklahoma State University, p. 275-279.
Landa, E.R., 1984, Radon in earth-sheltered structures: Underground Space, v. 8, p. 264-269.
Lawrence, E.P., Wanty, R.B. and Briggs, P.H., 1989, Hydrologic and geochemical processes
governing distribution of U-238 series radionuclides in groundwater near Conifer, CO:
Geological Society of America, Abstracts with Programs, v. 21, p. A144.
Lawrence, E.P., Wanty, R.B., and Nyberg, P., in press, Contribution of 222Rn in domestic water
supplies to 222Rn in indoor air in homes in Colorado: Health Physics, 20 p.
Mallory, W.W., 1972, Geologic Atlas of the Rocky Mountain Region: Denver, Rocky Mountain
Association of Geologists, p. 331.
,JSurfac^^
Geophysics, v. 48, p. 806.
Nazaroff, W.M., and Nero, A.V., eds., 1988, Radon and its daughter products in indoor air,
518 p.
Nelson-Moore, J.L., Collins, D.B., and Hornbaker, A.L., 1978, Radioactive mineral occurrences
of Colorado and bibliography: Denver, Colorado, Colorado Geological Survey, 1054 p.
IV-25 Reprinted from USGS Open-File Report 93-292-H
-------
Nielson, K.K,. and Rogers, V.C., 1986, Surface water hydrology considerations in predicting
radon releases from water-covered areas of uranium tailings ponds, in Abt, S.R., Nelson,
J.D., Shepherd, T.A., Wardwell, R.E., and van Zyl, D., eds., Proceedings of the 8th
annual symposium on geotechnical and geohydrological aspects of waste management:
Fort Collins, Colorado, Feb. 5-7, 1986, p. 215-222.
Norton, S.A., Hess, C.T., Blake, G.M., Morrison, M.M., and Baron, J., 1985, Excess
unsupported 210Pb in lake sediment from Rocky Mountain lakes; a groundwater effect:
Canadian Journal of Fisheries and Aquatic Sciences, v. 42, p. 1249-1254.
Otton, J.K., 1989, Using geology to map and understand radon hazards in the United States:
United States Geological Survey Yearbook, p. 52-54.
Otton, J.K., Schumann, R.R., Owen, D.E. and Chleborad, A.F., 1988, Geologic assessments of
radon hazards; a Colorado case history, in Marikos, M.A., and Hansman, R.H., eds.,
Geologic causes of natural radionuclide anomalies: Proceedings of GEORAD Conference,
Geologic Causes of Natural Radionuclide Anomalies, St. Louis, MO, Apr. 21-22,1987,
p. 167.
Owen, D.E. and Asher-Bolinder, S., 1988, Assessment of natural phenomena producing
fluctuations and variations in soil-gas Radon-222 concentrations: Geological Society of
America, Abstracts with Programs, v. 20, p. A354.
Reimer, G.M., 1985, Gaseous emanations associated with sandstone-type uranium deposits, in
Finch, W.I., and Davis, J.F., eds., Geological environments of sandstone-type uranium
deposits: Report of the working group on uranium geology organized by the International
Atomic Energy Agency, U.S. Geological Survey, Denver, CO, p. 335-346.
Reimer, G.M., and Rice, R.S., 1977, Linear-traverse surveys of helium and radon in soil gas as a
guide for uranium exploration, central Weld County, Colorado: U.S. Geological Survey,
Open-File Report 77-589,10 p.
Schumann, R.R., Asher-Bolinder, S., and Owen, D.E., 1989, Factors influencing seasonal
variations in soil-gas radon concentrations in a fine-grained soil: Geological Society of
America, Abstracts with Programs, v. 21, p. 65.
Schumann, R.R., Gundersen, L.C.S., Asher-Bolinder, S., and Owen, D.E., 1989, Anomalous
radon levels in crystalline rocks near Conifer, Colorado: Geological Society of America,
Abstracts with Programs, v. 21, p. A144-A145.
Schumann, R.R., Owen,JD.E> and Asher-Bolinder7S.,-t989rWeatherfaetors-affecting-soil-gas—
radon concentrations at a single site in the semiarid western U.S., in Osborne, M.C., and
Harrison, J., eds., Proceedings of the 1988 EPA Symposium on Radon and Radon
Reduction Technology, v. 2, EPA Report EPA/600/9-89/006B, p. 3-1 to 3-13.
IV-26 Reprinted from USGS Open-File Report 93-292-H
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Spitz, H.B., Cohen, N., and Wrenn, M.E., 1975, Non-occupational radiation exposures from
radon-222 and daughters to residents of Grand Junction, Colorado, annual report, July 1,
1974 to June 30, 1975; Volume 2, p. (unknown).
Stevens, D.N., Rouse, G.E., and De Voto, R.H., 1970, Radon in soil gas; three uranium
exploration case histories in the western United States: Proceedings of the Canadian
Institute of Mining and Metallurgy, Geologic Division-Society of Economic Geology:
Toronto, Third International Geochemical Exploration Symposium, Program and
Abstracts, p. 57.
Stevens, D.N., Rouse, G.E. and De Voto, R.H., 1971, Radon-222 in soil gas; three uranium
exploration case histories in the western United States: Canadian Institute of Mining and
Metallurgy, Special Volume, v. 11, p. 258-264.
Stieff, L.R., Stieff, C.B., and Nelson, R.A., 1987, Field measurements of in situ 222Rn
concentrations in soil based on the prompt decay of the 214Bi counting rate: Nuclear
Physics, v. 1, p. 183-195.
Streufert, R.K., and Ohl, J.P., 1989, Colorado metal mining activity map with directory:
Colorado Geological Survey, Special Map 25, scale 1:500,000.
Swindle, R.W., 1977, Radon daughter control in the Uravan mineral belts, m Kim, Y.S. ed.,
Uranium mining technology: Proceedings of First conference on uranium mining
Technology, University of Nevada, Conferences and Institutes, Reno, Nev., April 24-29,
1977, (unpaginated).
Tewto, O., 1979, Geologic map of Colorado: U.S. Geological Survey, scale 1:500,000.
Tripp, R.M., 1944, Radon survey of the Fort Collins anticline (Colorado) (abst): Dallas Digest,
p. 67.
Varani, F.T., Jelinek, R.T., and Correll, R.J., 1987, Occurrence and treatment of uranium in
point of use systems in Colorado, in Graves, B. ed., Radon, radium, and other
radioactivity in ground water: National Water Well Association Conference: Radon,
radium and other radioactivity in ground water, Somerset, NJ, Apr. 7-9,1987, p. 535-
546.
Vinckier, T.A., 1982, Hydrogeology of the Dakota Group aquifer with emphasis on the radium-
226 content of its contained ground water, Canon City Embayment, Fremont and Pueblo
Counties, Colorado: Colorado Geological Survey, Open File 82-3, 80 p.
IV-27 Reprinted from USGS Open-File Report 93-292-H
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EPA's Map of Radon Zones
The USGS1 Geologic Radon Province Map is the technical foundation for EPA's Map
of Radon Zones. The Geologic Radon Province Map defines the radon potential for
approximately 360 geologic provinces. EPA has adapted this information to fit a county
boundary map in order to produce the Map of Radon Zones.
The Map of Radon Zones is based on the same range of predicted screening levels of
indoor radon as USGS' Geologic Radon Province Map. EPA defines the three zones as
follows: Zone One areas have an average predicted indoor radon screening potential greater
than 4 pCi/L. Zone Two areas are predicted to have an average indoor radon screening
potential between 2 pCi/L and 4 pCi/L. Zone Three areas are predicted to have an average
indoor radon screening potential less than 2 pCi/L.
Since the geologic province boundaries cross state and county boundaries, a strict
translation of counties from the Geologic Radon Province Map to the Map of Radon Zones
was not possible. For counties that have variable radon potential (i.e., are located in two or
more provinces of different rankings), the counties were assigned to a zone based on the
predicted radon potential of the province in which most of its area lies. (See Part I for more
details.)
COLORADO MAP OF RADON ZONES
The Colorado Map of Radon Zones and its supporting documentation (Part IV of this
report) have received extensive review by Colorado geologists and radon program experts.
The map for Colorado generally reflects current State knowledge about radon for its counties.
Some States have been able to conduct radon investigations in areas smaller than geologic
provinces and counties, so it is important to consult locally available data.
Although the information provided in Part IV of this report -- the State chapter entitled
"Preliminary Geologic Radon Potential Assessment of Colorado" -- may appear to be quite
specific, it cannot be applied to determine the radon levels of a neighborhood, housing tract,
individual house, etc. THE ONLY WAY TO DETERMINE IF A HOUSE HAS
ELEVATED INDOOR RADON IS TO TEST. Contact the Region 8 EPA office or the
Colorado radon program for information on testing and fixing homes. Telephone numbers
and addresses can be found in Part II of this report.
V-l
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