U.S. Geological Survey - The Geology of Radon


Radon Potential

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.
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.
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.
Indoor radon data
These agencies often have indoor radon data, which they make available to the public in summary form.

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.

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.



[Previous Page] [Table of Contents] [Radon Home] [Next Page]


URL: http://energy.cr.usgs.gov/radon/georadon/4.html
Server information: webman@energy.cr.usgs.gov
13 October 1995