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Radon Source
Radon Formation
Radon Movement
Radon Entry Into Buildings
Radon In Water
Radon Potential
As mentioned above, uranium is the first element in a
long series of decay to produce radium and radon. All rocks contain some
uranium, although most contain just small amount – between 1 and 3 parts per
million (ppm) of uranium. In general, the uranium content of a soil will be
about the same as the uranium content of the rock from which the soil is
derived.
Some types of rocks have higher than average uranium
contents. These include light-colored volcanic rocks, granites, dark shales,
sedimentary rocks and their soils may contain as much as 100 ppm uranium.
The higher the uranium level is in an area, the greater the chances are that
houses in the area have high levels of indoor radon. But some houses in
areas with lots of uranium in the soil have low levels of indoor radon, and
other houses on uranium-poor soils have high levels of indoor radon.
Clearly, the amount of radon in a house is affected by factors in addition
to the presence of uranium in the underlying soil.
Since uranium is the source, wherever rocks and soils it
is present, so are radon and radium. Each atom of radium decays by ejecting
from its nucleus an alpha particle composed of two neutrons and two protons.
As the alpha particle is ejected, the newly formed radon atom recoils in the
opposite direction, just as a high-powered rifle recoils when a bullet is
fired. Alpha recoil is the most important factor affecting the release of
radon from mineral grains.
The location of the uranium atom in the mineral grain
(how close it is to the surface of the grain) and the direction of the
recoil of the radon atom (whether it is toward the surface or the interior
of the grain) determine whether or not the newly formed radon atom enters
the pore space between mineral grains. If a radium atom is deep within a big
grain, then regardless of the direction of recoil, it will not free the
radon from the grain, and the radon atom will remain embedded in the
mineral. Even when a radium atom is near the surface of a grain, the recoil
will send the radon atom deeper into the mineral if the direction of recoil
is toward the grain’s core. However, the recoil of some radon atoms near the
surface of a grain is directed toward the grain’s surface. When this
happens, the newly formed radon leaves the mineral and enters the pore space
between the grains or the fractures in the rocks.
The recoil of the radon atom is quite strong. Often newly
formed radon atoms enter the pore space, cross all the way through the pore
space, and become embedded in nearby mineral grains. If water is present in
the pore space, however, the moving radon atom slows very quickly and is
more likely to stay in the pore space.
Because radon is a gas, it has much greater mobility than
uranium and radium, which are fixed in the solid matter in rocks and soils.
Radon can more easily leave the rocks and soils by escaping into fractures
and opening in rocks and into the pore spaces between grains of soil.
The ease and efficiency with which radon moves in the
pore space or fracture effects how much radon enters a house. If radon is
able to move easily in the pore space, then it can travel a great distance
before it decays, and it is more likely to collect in high concentrations
inside a building.
The method and speed of radon’s movement through soils is
controlled by the amount of water present in the pore space (the soil
moisture content), the percentage of pore space in the soil (the porosity),
and the “interconnectedness” of the pore spaces that determines the soil’s
ability to transmit water and air (called soil permeability).
Radon can move through cracks in rocks and through pore
spaces in soils. Radon moves more rapidly through permeable soils, such as
coarse sand and gravel, than through impermeable soils, such as clays.
Fractures in any soil or rock allow radon to move more quickly.
Some radon atoms remain trapped in the soil and decay to
form lead: other atoms escape quickly into the air.
Radon in water moves slower than radon in air. The
distance that radon moves before most of it decays is less than 1 inch in
water-saturated rocks or soils, but it can be more than 6 feet, and
sometimes tens of feet, through dry rocks or soils. Because water also tends
to flow much more slowly through soil pores and rock fractures than does
air, radon travels shorter distances in wet soils than in dry soils before
it decays.
For these reasons, homes in areas with drier, highly
permeable soils and bedrock, such as hills slopes, mouths and bottoms of
canyons, coarse glacial deposits, and fractured or cavernous bedrock, may
have high levels of indoor radon. If the radon content of the air in the
soil or fracture is in the “normal” range, the permeability of these areas
permits radon-bearing air to move greater distance before it decays, and
thus contributes to high indoor radon.
Radon moving through soil pore spaces and rock fractures
near the surface of the earth usually escapes into the atmosphere. Where a
house is present, however, soil air often flows toward its foundation for
three reasons: (1) difference in air pressure between the soil and the
house; (2) the presence of openings in the house’s foundation; and (3)
increases in permeability around the basement (if one is present).
In constructing a house with a basement, a hole is dug,
footings are set, and coarse gravel is usually laid down as a base for the
basement slab. Then, once the basement walls have been built, the gap
between the basement walls and the ground outside is filled with material
that often is more permeable than the original ground. This filled gap is
called disturbed zone.
Radon moves into the disturbed zone and the gravel bed
underneath from the surrounding soil. The backfill material in the disturbed
zone is commonly rocks and soil from the foundation site, which also
generate and release radon. The amount of radon in the disturbed zone and
gravel bed depends on the amount of uranium present in the rock at the site,
the type and permeability of soil surrounding the disturbed zone and
underneath the gravel bed, and the soil’s moisture content.
The air pressure in the ground around most houses is
often greater than the air pressure inside the house. Thus, air tends to
move from the disturbed zone and the gravel bed into the house through
openings in the house’s foundation. All house foundations have openings such
as cracks, utility entries, seams between foundation materials, and
uncovered soil in crawl spaces and basements.
Most houses draw less than one percent of their indoor
air from the soil; the remainder comes from outdoor air, which is generally
quite low in radon. Houses with low indoor air pressures, poorly sealed
foundations, and several entry points for soil air, however, may draw as
much as 20 percent of their indoor air from the soil. Even if the soil air
has only moderate levels of radon, levels inside the house may be very high.
Radon can also enter a home through the water system.
Water in rivers and reservoirs usually contains very little radon, because
it escapes into the air; so homes that rely on surface water usually do not
have a radon problem from their water. In big cities, water processing in
large municipal systems aerates the water, which allows radon to escape, and
also delays the use of water until most of the remaining radon has decayed.
In many areas of the country, however, ground water is
used as the main water supply for homes and communities. These small public
water works and private domestic wells often have closed systems and short
transit times that do not remove radon from the water or permit it to decay.
This radon escapes from the water to the indoor air as people take showers,
wash clothes or dishes, or otherwise use water.
The areas most likely to have problems with radon in
ground water are areas that have high levels of uranium in the underlying
rocks. For example, granites in various parts of the United States are
sources of high levels of radon in ground water that is supplied to private
water supplies.
In areas where the main water supply is from private
wells and small public water works, radon in ground water can add radon to
the indoor air.
Radon dissolves easily in water. When taken from deep
natural wells in affected areas, water may contain considerably more radon
than mains tap. When water containing radon is agitated, it releases the
radon molecules into the room air - a significant source if this water is
used for showering, washing and cleaning. Mains water contains far less
radon because the cleaning process it undergoes allows most of the dissolved
radon to escape. As yet, there is no evidence showing any risk associated
with drinking radon dissolved in water.
You can get an idea as to how concerned you should be
about radon in your house by learning about the geology of the site and its
radon potential. If your house is in an area with a high potential for
radon, then chances are that your house may have an indoor radon problem.
However, as we have discussed, the way a house is built can increase the
risk - so even in areas of low radon potential, some houses can have
unhealthy radon levels.
Scientists evaluate the radon potential of an area and
create a radon potential map by using a variety of data. These data include
the uranium or radium content of the soils and underlying rocks and the
permeability and moisture content of the soils. Usually, maps of these
factors are not available. Other indirect sources of information about these
factors, such as geologic maps, maps of surface radioactivity, and soil
maps, are used.
Another type of information that scientists use in
determining the radon potential of an area is radon measurements of local
soil air. Existing indoor radon data for homes also are useful. These data
are the most direct information available about indoor radon potential. Even
though the houses that have been sampled may not be typical for the area and
exact location information for measured houses is seldom available.
::TOP::
Knowing the types of rock and soil at a site helps a geologist to
determine its radon potential.
Adapted from: U.S. Environmental Protection Agency
- 1992, A Citizen’s guide to Radon: The Guide to
Protecting Yourself and Your Family from Radon (2nd Edition)
- 1993, Home Buyer’s and Seller’s Guide to Radon
- 1992, Consumer’s Guide to Radon Reduction: How to
Reduce Radon Levels in Your Home
- 1992, National Residential Radon Survey: Summary
Report
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