Vermont Agency of Natural Resources
Department of Environmental Conservation
103 South Main Street
Waterbury, Vermont 05671
Fact Sheet - Radioactivity Map
1. Why make a radioactivity map of Vermont?
In 2000, groundwater from a number of private wells
in the towns of Milton and Colchester was found to have elevated radioactivity.
Subsequent to this discovery, the Vermont Geological Survey investigated
the geologic literature and found that some previous geophysical and
geological surveys had identified elevated radioactivity (anomalies)
in this area. The earlier studies, most of which were related to uranium
exploration, consisted of ground-based and airborne geophysical (Geiger
Counter) surveys between 1951-1988. The anomaly areas detected via the
airborne survey corresponded well with most of the areas with elevated
groundwater radioactivity. Further geologic mapping conducted during
the summer of 2001 by the Vermont Geological Survey confirmed the presence
of elevated radioactivity on the ground within the airborne anomalies
as well as in the vicinity of previous ground-based anomalies. The coincidence
of radioactive anomalies from earlier surveys with areas where elevated
radioactivity was detected in private bedrock wells in Milton and Colchester,
suggested that it would be useful to assemble the existing radioactivity
data for the entire state of Vermont.
2. Is there a connection between elevated radioactivity
measured via airborne and ground-based surveys and wells with elevated
In the towns of Milton and Colchester, completed work
suggests that there is a correlation between the elevated radioactivity
areas discovered via airborne and ground-based studies and elevated
radioactivity in groundwater wells.
3. Where does the existing
information on radioactivity come from?
During the 1970s and 1980s, the U.S. Department of
Energy sponsored a program called the National Uranium Resource Evaluation
that sought to explore for uranium throughout the United States.
This exploration focussed on finding areas that would be economically
favorable for uranium deposits. The NURE survey that covered most of
Vermont also covered the states of Connecticut, Rhode Island, Massachusetts
and parts of New York, New Hampshire, and Maine. The NURE investigations
consisted of three separate surveys which were: 1) airborne geophysical
surveys, 2) ground-based geophysical surveys coupled with geochemical
analysis of uranium bearing rocks, and 3) analysis of the uranium and
thorium content of sediments in streams. Airborne surveys were completed
over all of Vermont except the northeasternmost corner whereas ground-based
geophysical and rock geochemistry surveys and stream sediment surveys
focussed on the southern half of Vermont. Airborne geophysical surveys
of the southern half of Vermont were also flown in 1964 by the U.S.
Other sources of data on radioactivity include: 1) Mineral Resource
Data System (MRDS) for Vermont which is a cooperative U.S. Geological
Survey/Vermont Geological Survey database of all known mineral occurrences
in Vermont from which uranium occurrences were extracted and 2) Dept.
of Environmental Conservation, Water Supply Division database of Public
Water Supply Wells with elevated radioactivity.
4. What are the categories on the radioactivity
There are four categories on the radioactivity map
which are designed to separate the different methods of acquiring the
geophysical and geologic data. There are also two sets of data points.
Category1 means that elevated
radioactivity was measured in all or parts of these areas via ground-based
geophysical or geological studies. In other words, elevated radioactivity
was measured on the ground surface by geologists conducting surveys
with Geiger Counters in all or parts of these areas. Ground-based surveys
where direct measurements of elevated radioactivity are taken are considered
to be the most reliable data. Rock samples were taken from some of these
areas and were geochemically analyzed and found to have elevated amounts
of uranium and thorium. Groundwater wells in some of these areas were
found to have elevated radioactivity.
Category 2 means that elevated radioactivity
was measured indirectly in these areas via airborne or stream sediment
surveys and was not measured directly via ground-based surveys. Airborne
radioactivity surveys are conducted by airplanes flying at a relatively
constant altitude and speed above the earth's surface towing a Geiger
Counter. Because of the altitude and speed of the airplane, the areas
of elevated radioactivity (anomalies) determined from the air may be
larger than the source of the radioactive anomaly on the ground.
Stream sediment survey teams were responsible for sampling sediments
in streams over large areas. The sediments were geochemically analyzed
to determine the amounts of uranium and thorium present. Areas where
the uranium and thorium content of the stream sediments was significantly
elevated relative to the surrounding areas were outlined as anomalies.
Because these sediments may be transported significant distances along
the stream valley away from their source, the current location of the
anomalies may not reflect the radioactivity of the areas where they
Since the areas of elevated radioactivity shown as Category 2 have not,
to our knowledge, been checked via ground-based geological or geophysical
surveys, we have distinguished these areas from Category 1 areas and
consider them to be less reliable than Category 1.
Category 3 means that elevated radioactivity
was not measured in these areas other than in elevated public water
supply wells and uranium occurrences. Category 3 also includes the unsurveyed
areas between flight lines.
Category 4- No Data means that no known geological/geophysical
radioactivity survey data exists for this area.
Public water supply wells with unadjusted gross alpha greater than
or equal to 15 picocuries/liter* are shown as brown crosses. *Prior
to January 2002, 15 picocuries/liter
was the EPA level that triggered further testing.
Uranium occurrences are sites where uranium was found during
ground-based surveys for the NURE program or sites where uranium was
listed as the commodity of interest in the Mineral Resource Data System
(MRDS) of the U.S. Geological Survey; these occurrences are shown as
blue crossed circles.
5. If I live within Category 1 or 2 areas, does
it mean that I will have elevated radioactivity in my drinking water?
Because many wells with elevated radioactivity in the Milton and Colchester
area correlate with airborne and ground-based radioactivity anomalies,
there is reason to believe that this correlation may be a valid for
other areas of Vermont as well. The only way to know for sure if one
has a radioactivity problem is to test. Even in areas where there are
numerous wells with elevated radioactivity, the problem wells are scattered
over a large area. In addition, there are cases where a particular well
has a radioactivity problem and the neighboring well does not.
6. If I live in a Category 3 area, should I
be worried about radioactivity in my well water?
Category 3 includes areas where elevated radioactivity was not been
reported as well as unsurveyed areas between flight lines. There are
public water supply wells with elevated radioactivity in category 3
areas as well as uranium occurrences. People in Category 3 areas should
also consider testing their water.
7. What does the map show?
The map is a compilation of areas where ground-based and airborne geophysical
and geological surveys have indicated the presence of elevated radioactivity
relative to surrounding areas. Public water supplies with elevated radioactivity
and uranium occurrences are also shown. Since elevated radioactivity
areas determined via these surveys in the Milton and Colchester area
coincide with many of the locations of groundwater wells with radioactivity
problems, we decided to show the statewide distribution of elevated
Although the Vermont Department of Health recommends that all private
well owners test their wells routinely for radioactivity, the map can
provide some guidance to people who are uncertain whether or not to