The geology of radon helps explain why radon levels can vary so greatly between indoor air, outdoor air, soil air, groundwater, and even in different homes in the same area.
Why do some houses have high levels of indoor radon while nearby houses do not?
The reasons lie primarily in the geology of radon — the factors that govern the occurrence of uranium, the formation of radon, and the movement of radon, soil gas, and groundwater. 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 understanding the geology of radon.
Uranium: The Source
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 to 3 parts per million (ppm). In general, the uranium content of a sample of soil will be about the same as the uranium content of the rock from which the soil was derived.
photo by Karen Wenrich
The bright yellow mineral tyuyamunite is one of the most common uranium-ore minerals. The photo above shows a specimen less than 3 inches wide which came from the Ridenour Mine in Arizona, near the Grand Canyon.
Some types of rocks have a higher-than-average uranium content. They include light-colored volcanic rocks, granites, dark shale, sedimentary rocks that contain phosphate, and metamorphic rocks derived from these rocks. These rocks and their soils may contain as much as 100 ppm of uranium. Layers of these rocks underlie various regions in the United States.
There are three major classifications of rocks: (top) igneous; (center) sedimentary; and (bottom) metamorphic.
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 mere presence of uranium in the underlying soil.
Just as uranium is present in all rocks and soils, so, too 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 into 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 toward 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% of the radon produced actually escapes from the mineral grains and enters the pores. Most soils in the United States contain between 0.33 to 1 pCi of radium per gram of mineral matter, and between 200 to 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 of 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 soil.
The ease and efficiency with which radon moves in the pore space or fracture affect just 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 are controlled by the amount of water present in the pore space (the soil’s moisture content), the percentage of pore space in the soil (the porosity of the soil), 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 clay. 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 more slowly than radon in air. The distance that radon moves before most of it decays is less than 1 inch in water-saturated rocks and soils, but it can move more than 6 feet, and sometimes tens of feet, through dry rocks and soils. Because water also tends to flow more slowly through soil pores and rock fractures than does air, radon travels shorter distances through wet soils than through dry soils before it decays.
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 within the “normal” range (200 to 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.
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 zone.”
Radon moves from the surrounding soil into the disturbed zone and the gravel bed underneath. The backfill material in the disturbed zone is commonly made up of rocks and soil from the foundation site. These 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 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 crawlspaces and basements.
Most houses draw less than 1% 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% 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 the home through its water system. Water from 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 the 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, groundwater 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, do the dishes, and use water in general. A rule of thumb for estimating the contribution of radon from domestic water to the indoor level of airborne radon is that 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 groundwater 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 groundwater that is given off to private water supplies.