Studies of the geology of radon include
research into how uranium and radon sources are distributed
in rocks and soils, how radon forms in rocks and soils,
and how radon moves. Studying how radon enters buildings
from the soil and through the water system is also an
important part of the geology of radon.
To understand the geology of radon - where it forms,
how it forms, how it moves - we have to start with its
ultimate source, uranium. All rocks contain some uranium,
although most contain just a 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
The bright-yellow mineral tyuyamunite
is one of the most common uranium ore minerals. This
specimen, which is less than 3 inches across, came from
the Ridenour mine, Arizona, near the Grand Canyon
Some types of rocks have higher than
average uranium contents. These include light-colored
volcanic rocks, granites, dark shales, sedimentary rocks
that contain phosphate, and metamorphic rocks derived
from these rocks. These rocks and their soils may contain
as much as 100 ppm uranium. Layers of these rocks underlie
various parts of the United States.
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
Just as uranium is present in all rocks and soils, so
are radon and radium because they are daughter products
formed by the radioactive decay of uranium.
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.
A radium atom decays to radon by
releasing an alpha particle, containing two neutrons
and two protons, from its nucleus.
The location of the radium 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.
For most soils, only 10 to 50 percent
of the radon produced actually escapes from the mineral
grains and enters the pores. Most soils in the United
States contain between 0.33 and 1 pCi of radium per
gram of mineral matter and between 200 and 2,000 pCi
of radon per liter of soil air.
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 openings
in rocks and into the pore spaces between grains of
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 moves more rapidily 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
For these reasons, homes in areas with drier, highly
permeable soils and bedrock, such as hill slopes, mouths
and bottoms of canyons, coarse glacial deposits, and
fractured or cavernous bedrock, may have high levels
of indoor radon. Even if the radon content of the air
in the soil or fracture is in the "normal"
range (200-2,000 pCi/L), the permeability of these areas
permits radon-bearing air to move greater distances
before it decays, and thus contributes to high indoor
Radon entry into buildings
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) differences 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 a disturbed
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
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 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
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 in water
Radon can also enter home through their water systems.
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. A very
rough rule of thumb for estimating the contribution
of radon in domestic water to indoor air radon is that
house water with 10,000 pCi/L of radon contributes about
1 pCi/L to the level of radon in the indoor air.
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.
Source: Environmental Protection