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by James K. Otton, Linda C.S. Gundersen, and R. Randall SchumannTABLE OF CONTENTS
High levels of indoor radon are found in every state. We know from medical and environmental studies that radon can be a health risk, primarily as a cause of lung cancer. Radon comes from the soil, rock, and water around us. Because levels of radon vary from place to place, and because houses differ in their vulnerability to radon, it is important that all homes be measured for radon. These pages present important geological information about radon; how it forms, the kinds of rocks and soils it comes from, and how it moves through the ground or is carried by water into buildings, and explains the way geologists estimate the radon potential of an area.  (116
kb) Radon gas, a radioactive product of uranium, can reach high
levels in some houses, depending on the local geology and house
construction.Radon is a gas produced by the radioactive decay of the element radium. Radioactive decay is a natural, spontaneous process in which an atom of one element decays or breaks down to form another element by losing atomic particles (protons, neutrons, or electrons). When solid radium decays to form radon gas, it loses two protons and two neutrons. These two protons and two neutrons are called an alpha particle, which is a type of radiation. The elements that produce radiation are called radioactive. Radon itself is radioactive because it also decays, losing an alpha particle and forming the element polonium. Elements that are naturally radioactive include uranium, thorium, carbon, and potassium, as well as radon and radium. Uranium is the first element in a long series of decay that produces radium and radon. Uranium is referred to as the parent element, and radium and radon are called daughters. Radium and radon also form daughter elements as they decay. The decay of each radioactive element occurs at a very specific rate. How fast an element decays is measured in terms of the element "half-life", or the amount of time for one half of a given amount of the element to decay. Uranium has a half-life of 4.4 billion years, so a 4.4-billion-year-old rock has only half of the uranium with which it started. The half-life of radon is only 3.8 days. If a jar was filled with radon, in 3.8 days only half of the radon would be left. But the newly made daughter products of radon would also be in the jar, including polonium, bismuth, and lead. Polunium is also radioactive - it is this element, which is produced by radon in the air and in people's lungs, that can hurt lung tissue and cause lung cancer.
Radioactivity is commonly measured in picocuries (pCi). This unit of measure is named for the French physicist Marie Curie, who was a pioneer in the research on radioactive elements and their decay. One pCi is equal to the decay of about two radioactive atoms per minute. Because the level of radioactivity is directly related to the number and type of radioactive atoms present, radon and all other radioactive atoms are measured in picocuries. For instance, a house having 4 picocuries of radon per liter of air (4 pCi/L) has about 8 or 9 atoms of radon decaying every minute in every liter of air inside the house. A 1,000-square-foot house with 4 pCi/L of radon has nearly 2 million radon atoms decaying in it every minute. Radon levels in outdoor air, indoor air, soil air, and ground water can be very different. Outdoor air ranges from less than 0.1 pCi/L to about 30 pCi/L, but it probably averages about 0.2 pCi/L. Radon in indoor air ranges from less that 1 pCi/l to about 3,000 pCi/L, but it probably averages between 1 and 2 pCi/L. Radon in soil air (the air that occupies the pores in soil) ranges from 20 or 30 pCi/L to more than 100,000 pCi/L; most soils in the United States contain between 200 and 2,000 pCi of radon per liter of soil air. The amount of radon dissolved in ground water ranges from about 100 to nearly 3 million pCi/L. Why do radon levels vary so much between indoor air, outdoor air, soil air, and ground water? 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 ground water. 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. 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 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 was derived.
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 soil. Radon formation 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.
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. Radon movement 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 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 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.
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 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. 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 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 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 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.
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, and 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.
Source of information on radon potential Geologic maps These groups prepare and publish geologic maps; local geologic maps are often available in the earth-science libraries of these groups and in the earth-science map collections of large regional libraries. U.S. Geological Survey State geological agencies Colleges and universities Radioactivity maps These groups prepare and publish radioactivity maps. These maps are often available in the earth-science libraries of these groups and in the earth-science map collections of large regional libraries. U.S. Geological Survey Sate geological agencies Soil surveys These groups prepare and publish soil surveys. Other soil data from surficial geologic and engineering maps are prepared and published by geoscience agencies. Many published soil surveys are in local libraries. U.S. Soil Conservation Service (now called Natural Resources Conservation Service) County extension offices Indoor radon data These agencies often have indoor radon data, which they make available to the public in summary form. State Health Departments or Departments of Environmental Protection County or city health departments Geologic maps A geologic map shows the type of rocks and geologic structures in a specific area. Because different types of rocks have different amounts of uranium, a geologic map can indicate to a geologist the general level of uranium or radium to be expected in the rocks and soils of the area. Such maps are especially important in showing where rocks with high levels of uranium occur. Because radon that enters buildings usually comes from the upper several feet of the earth's surface, knowing the radon levels of the near-surface (surficial) materials is important. Surficial geologic and engineering maps show and describe these surface materials for many parts of the United States. These maps are useful for understanding the physical properties of the materials at the surface, like permeability, but are generally not as useful for determining what the uranium concentrations in the surface materials might be.
Radioactivity maps Radioactivity maps give an indication of the uranium levels of surface materials. The most common type of radioactivity map is an aeroradioactivity map, which is based on radioactivity measurements made from an aircraft flying at low altitude with instruments that measure the radioactive energy radiating from the ground. There is a good match between areas identified on aeroradioactivity maps as having high levels of surface uranium and areas for which high levels of indoor radon have been reported. In some parts of the country, however, swamps and marshes are abundant. In many of these areas, the soils at the surface are full of water, which blocks the radiation of energy. The average amount of radiated energy detected for these areas is lower than it would be if the soils were dry, and the uranium content of the soils and the radon potential are likely to be underestimated in these areas. A large amount of aeroradioactivity data was collected as part of a U.S. Department of Energy program to evaluate the uranium resources of the United States. Most of the energy detected during these flights was from rocks and soils within 800 feet of flight lines that were spaced 1 to 6 miles apart. Many major metropolitan areas were not covered by the survey because of flight restrictions. Therefore, only a small part of the surface of the United States was measured. The data from this survey, however, give a good indication of the background uranium concentration of soils and rocks underlying most of the United States. The digital data from the survey were processed by the U.S. Geological Survey to produce a map showing the uranium content of surface materials in the conterminous United States (the lower 48 States). The smallest data point on the map covers about 1.6 by 1.6 miles, limiting the amount of detail that can be seen. (You can tell how parts of a region, a State, or possibly a county vary in surface uranium concentration, but you can't tell how uranium varies from neighborhood to neighborhood or from house to house. Soil-air radon data Scientists also measure radon in soil air. These data give direct evidence about soil radon, but extensive sets of these data are not commonly available. The two basic methods for measuring the radon concentration of soil air are the same as those used to measure radon in buildings. Both methods measure the alpha particles produced by the decay of the radon in the air. One method involves burying a passive device, such as a charcoal cannister or an alpha-track detector, in the soil and leaving it open to the soil air. This method allows long-term measurements, but the devices can be affected strongly by soil moisture. In the other method, a sample of soil air is collected from a probe driven into the ground, and the radon in the sample is measured by using electronic equipment. This method provides data quickly, but these short-term measurements may vary greatly due to daily, weekly, and seasonal changes in soil and atmospheric conditions that are averaged out during long-term measurements.
Soil-air methods require specialized equipment because soil-air data are sensitive to many conditions and factors, such as the depth of measurement. Radon levels vary widely in the top 2 to 3 feet of soil because of variations in soil moisture and the amount of radon that escapes to the atmosphere. Making measurements at 3 feet or deeper avoids many of the problems related to near-surface conditions, but it may be difficult in some soils. Soil surveys The best data on the physical properties of soils are in soil surveys published by the Soil Conservation Service of the U.S. Department of Agriculture in cooperation with state and county officials. Soil surveys provide descriptions and maps of the soils that underlie the areas described. Modern soil surveys include permeability data for the mapped soils at varying depths. In older soil reports, no permeability data are given, and soil names and statements regarding internal drainage must be used to estimate permeability. Indoor radon data Indoor radon has been measured in many houses, schools, and commercial buildings across the United States. For the most part, these measurements have been made by private homeowners using passive detection devices purchased at a nearby store, ordered by phone, or ordered through the mail. Radon concentrations in some homes and businesses are being measured by private companies as part of real estate transactions. Many local, State, and Federal agencies are measuring radon in buildings for which they are responsible. Most indoor radon measurements are confidential transactions between homeowners and measurement vendors. The data from these private measurements are not generally available to the public. When they are available, the data are usually given as summaries by state, county, or zip code. Nonetheless, these summaries are useful in determining which regions of the counties, states, or United States seem likely to have elevated indoor radon levels. By careful examination and correlation, scientists can evaluate the effects of varying geology and soils on actual readings of indoor radon. The indoor radon information can be used as an additional aid to create a radon potential map or it can be used as a way of expressing the radon potential of areas mapped by the geologist. However, differences in house construction also can cause variations in the indoor radon levels.
Radon potential maps Scientists create radon potential maps by combining a variety of data, such as the locations of rocks containing high levels of uranium, locations of fractures, aeroradioactivity data, soil data on permeability and radon content, and indoor radon data. Not all of these types of data are available for every area, and radon potential maps for different areas may vary if they are based on different types of data. For instance, radon potential maps and data sets prepared by the U.S. Geological Survey (USGS) of Montgomery and Prince Georges Counties, Maryland, and Fairfax County, Virginia, are based on different data. The radon potential of Montgomery County was estimated by USGS geologists using measurements of soil radioactivity, measurements of soil-air radon, general geologic and soil maps, and indoor radon measurements reported by homeowners. Generalized Description of Rock Types -Coastal Plain (115
million years to recent time) - Gravel, sand, and clay, deposited by rivers
and the ocean, generally gray, green, or black. In Prince Georges County, indoor radon data were available, but the geologic maps were much less detailed than those of Montgomery County and good aeroradioactivity data were not available. Therefore, USGS geologists measured soil-air radon and surface radioactivity to create a radon data base for the county. The radon potential of Fairfax County was estimated on the basis of an aeroradioactivity map and detailed soil maps that were available for the county and an indoor radon survey that the county conducted. Three levels of radon potential were identified in the counties. Low radon potential means that the majority of homes contain less than 4 pCi/L of indoor radon. Moderate radon potential indicates that one-third to one-half of the homes have more than 4 pCi/L. High Radon potential means that the majority of homes contain more than 4 pCi/L.
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Preface
What
is Radon?
The
Geology of Radon
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