Radon: What You—and Your Customers—Should Know
Reprinted from Opflow, Vol. 22, No. 2 (February 1996), by permission. Copyright © 1996, American Water Works Association. For additional information relevant to the drinking water industry, visit AWWA's web site, or call 800-926-7337.
by Robert M. Kick, Sandra L. Potter, Rená D. Bass, and Matthew D. Klaus
Over the last 10 years the term radon has become part of our everyday lives. Many people have limited knowledge about radon and might be concerned if it has been detected in their public or private drinking water supply. The purpose of this article is to help you understand the basics of radon, which you, in turn, can use to help educate your community.
What Is Radon?
Radon is a colorless, tasteless, odorless, naturally occurring gas. It is also radioactive. Radon is an inert gas like helium, which means it does not chemically react with other substances. Therefore, radon may be present in, and move readily through, rocks, soil, groundwater, and air. Radon normally escapes from the ground in small concentrations into the atmosphere where, it dissipates harmlessly. Elevated concentrations of radon, however, can exist if this gas is trapped, for example, in our homes or businesses. In this manner, radon may become a health concern.
Where Does Radon Come From?
Radon comes from the natural breakdown (radioactive decay) of three radioactive isotopes: uranium-238, uranium-235, and thorium-232. These isotopes are called parent radionuclides because they are the source of other elements as they decay. These isotopes are typically found in igneous and metamorphic rocks, such as granite and gneiss (pronounced "nice"), and in sedimentary rocks such as organic-rich black shale, phosphatic rock, and coal. Radon levels may also be found in areas where uranium and phosphate mining wastes were used as fill material for construction.
Areas of the United States in which significantly elevated levels of radon have been found include: portions of the Appalachian Mountains, particularly granitic gneisses in western Pennsylvania, New Jersey, and New York; phosphate mining areas of central Florida; and uranium-rich areas of the Colorado Plateau. Other areas of the country, not as intensely studied as the above areas, may also contain significant radon levels.
Rock type does not solely indicate whether or not radon may be a concern in any particular area. Given a source for radon, this gas must have a migration pathway by which it can move from the source to sensitive receptors, such as humans. The presence of fractures or faults in the rock formations, the permeability and porosity of soil overlying the bedrock, and the presence of groundwater in the rock and soils affect the movement and therefore the amount of radon gas in an area. These factors tend to obscure our ability to predict radon occurrence.
Why Is Radon a Concern?
Uranium-238 is the most common parent of radon gas because this isotope comprises more than 99 percent of uranium and thorium isotopes found on Earth. Uranium-238 is radioactive, which means other elements are created as it decays ultimately to non-radioactive lead. As each decay occurs, creating a new element, an alpha or beta particle is also emitted. These particles are part of the "radiation" emitted from radioactive elements. Radon-222 is one of the radioactive daughter elements of uranium-238 decay.
The time required for half of a radioactive element to decay is called a half-life. Some radioactive elements such as uranium-238 have a long half-life (4.5 billion years) and so are not very radioactive; therefore, the probability of a decay occurring at a particular moment is low. Radon-222, in comparison, has a half-life of only 3.8 days. This means that it is relatively unstable and more likely to decay and emit radiation at any particular moment. The cumulative half-life of the next six radon decay products is only about 48 minutes. Thus, as radon decays, a cascade of emissions results, not just from the radon but also from other short-lived radioactive daughter products. This "burst" of radiation from radon and its daughter products is one aspect that makes exposure to radon so potentially dangerous.
Where Is Radon Found?
Radon can exist in rock, soil, water and air. Radon forms as the uranium and thorium in the rock and soil decay. Thus rock and soil host radon and act as a source for its introduction into water and air.
Radon occurs in groundwater, but not surface water. Its absence in surface water is related to its volatility — radon simply escapes from surface water into the atmosphere. Radon in groundwater is dissolved; the way carbon dioxide in a carbonated beverage is dissolved. It will stay in a solution until the pressure on the groundwater is decreased, at which time it will "bubble out." Due to the short half-life of radon and typically slow rate of natural groundwater flow, radon in groundwater typically cannot migrate far from its source. Therefore, if high concentrations of radon occur in groundwater, the presence of high concentrations of the parent radionuclides in the aquifer also exist.
Radon can also be found in the air. In the open atmosphere radon dissipates and ultimately decays. Significant concentrations of radon in air only occur when radon is trapped in an area with poor ventilation such as a mine, cave, or building.
Why Is Radon a Health Concern?
Radon gas has several characteristics, summarized below, that make it a special health concern:
First, radon is an invisible gas with no odor or other property we can sense, so it is not readily detectable without special instruments or tests. Because it is a gas, it can be inhaled into our lungs, which contain some of the most sensitive tissue in the body.
Second, radon emits radiation and in so doing produces radioactive daughter products. Most radon atoms that we breathe are harmless because they are exhaled before they can decay. However, some radon decay will occur in the lung, resulting in the emission of an alpha particle and the first daughter product, polonium-218.
Polonium-218 has an electrostatic charge and readily becomes attached to dust and smoke. The dust or smoke particles with radon decay products attached may then become lodged in the lining of the lungs. Once lodged, the residence time in the lungs for these particles is longer than the cumulative half-life of the radon decay products. The decay of polonium and subsequent daughter products then results in the irradiation (exposure to radiation) of sensitive lung tissue.
The US Environmental Protection Agency (USEPA) has estimated that radon causes 5,000 to 20,000 lung cancer deaths each year. Approximately 800 deaths annually are attributed to the effects of radon in groundwater. The Surgeon General has stated that radon is the second-leading cause of lung cancer in the United States.
Third, radon becomes trapped in confined spaces. Radon concentrations in the atmosphere never reach dangerous levels because air movement dissipates the radon gas. However, radon can become concentrated in mines, caves, and buildings with poor ventilation. Radon buildup can be a greater problem in newer buildings that are more airtight and energy efficient. Holding other factors constant, older, leaky, less energy-efficient buildings tend to have less radon buildup than more modern buildings.
Radon gas can find its way into buildings through small basement cracks or other foundation penetrations such as utility pipes. Radon gas can also be released from water used in the building, for example, from showers. Without adequate ventilation, the radon gas then accumulates and occupants become exposed.
How Is Radon in Groundwater Measured?
As previously stated, when a radioactive isotope decays, two things happen: (1) energy is released as radiation, and (2) a new isotope is formed. The amount of radiation emitted from decay of a radioactive isotope is measured in picocuries per litre (pCi/L) of air or water.
A curie is a commonly used measurement of radioactivity and a picocurie is a very small amount of radiation equal to one trillionth of a curie. One picocurie is equal to the amount of radiation released from the decay of approximately 2 radon atoms in one minute.
To determine the amount of radon in groundwater, a sample of the groundwater must be collected. The sample should reach the laboratory as soon as possible, preferably within 24 hours after collection. Using procedures approved by the USEPA, the laboratory will count the amount of alpha radiation released as the radon in the water sample decays. The measured emissions can be related to the radon concentration, reported in pCi/L. The typical cost for this test ranges from $50 to $100.
Who Regulates Radon Levels in Drinking Water and Air?
USEPA, under Amendments to the Safe Drinking Water Act of 1986, is required to identify contaminants of concern and propose maximum contaminant levels (MCLs) for public water systems. Radon was one of the contaminants for which an MCL was to be established. On July 18, 1991, USEPA proposed an MCL for radon of 300 pCi/L; however, this MCL has not been promulgated into law. In USEPA's 1993 Appropriation Bill, Congress required USEPA to study the risks of human exposure to radon, the costs for controlling or mitigating that exposure, and the risks posed by treating water to remove radon.
Congress placed these requirements on USEPA because of public concern over the costs to be incurred by public water systems in the control of radon in drinking water. The concern was a result of the greater threat of radon from indoor air, which was not being addressed except through voluntary measures. The results of this evaluation and a review of the report by the Science Advisory Board are presented in "Report to the United States Congress on Radon in Drinking Water — Multimedia Risk and Cost Assessment of Radon" (March 1994).1 State and local laws for the acceptable levels of radon exposure may be more stringent than federal proposed levels.
Since the major concern of radon in water is the release of the radon into air, USEPA recommends that actions be taken to reduce the airborne radon level if more than 4 pCi/L of radon is detected (USEPA, 1993).2 The quantity of 4 pCi/L is a level of concern and is not an enforceable limit.
Should I Be Concerned About Radon in My Water Supply?
In recent years, well surveys by private water utilities and USEPA have detected radon in groundwater throughout the United States. Radon has been detected in groundwater from some private wells at levels as high as 2.6 million pCi/L (Lamarre, 1989).3 According to the USEPA, however, the average radon concentration in public water systems is within the range of 200 to 600 pCi/L (Hurlburt, 1989).4 Radon is most likely to be present in water from private wells or from community water systems serving fewer than 500 people. Larger systems usually provide some kind of w2ater treatment that removes the radon.
The health risk from ingesting radon-contaminated groundwater is generally considered minimal because most of the radon escapes at the faucet or water outlet, leaving only minimal amounts in the water itself. In addition, the majority of the radon remaining in the water will pass out of the body before it can decay and damage tissue.
The main concern over radon in water supplies is its effect on indoor air quality. Radon gas escapes when water is no longer under pressure. It is most easily released to the atmosphere from heated and agitated water such as from showerheads, dishwashers, and washing machines.
To estimate the effect of radon release from groundwater on indoor air quality, it is generally thought that approximately 10,000 pCi/L of radon in water will normally produce a concentration of about 1 pCi/L of radon in air (Hurlburt, 1989).5 Given USEPA's estimate of radon concentration in public water systems of 200 to 600 pCi/L, the contribution to the air should be approximately 0.02 to 0.06 pCi/L (Hurlburt, 1989).6 The addition of this amount of radon is not considered a significant contribution, whether or not airborne radon is already considered a problem.
However, it should be noted that radon levels in water, if sufficiently elevated, can significantly affect airborne radon concentrations. Therefore, each water supply source should be tested to determine if it could represent a significant concern.
Can Radon Be Removed From Water?
The good news about radon is that it is relatively easy to remove from water. There are several options available for the treatment of radon in water, including storage, adsorption on granular activated carbon, and aeration. Choosing the correct option depends on radon levels present, the nature of your water supply system, and available financial resources.
Storage. Simply storing water in a tower or large distribution system for several days to a week provides time for radon to volatilize or harmlessly decay. This is why people who obtain their drinking water from rivers, lakes, or reservoirs have little to worry about. For some water distribution systems, however, it may not be possible or practical to hold the water as long as necessary to correct a significant radon problem.
Granular activated carbon. Granular activated carbon (carbon) has been shown to be very effective in removing radon from water. Water is run through a carbon filter or held over a carbon bed that adsorbs the radon out of the water. It is important that the water is not back-flushed over the carbon filter, as radon or radon decay products could be released into the water. In general, the amount of radon removed from the water is directly proportional to the length of time the water is in contact with the carbon.
Carbon filters have been very effective in dealing with radon problems for small systems, such as a private well, where the filter can be effective for several years. Larger water systems, however, may require large or multiple units, which are expensive.
Another concern associated with carbon use is the collection of radioactive decay products over time. In particular two decay products, lead-210 and polonium-210, have long half-lives. Although it is unlikely that the radioactive levels in the carbon would become dangerous, the spent carbon may need to be handled as a low-level radioactive waste. Consultation with the appropriate federal, state, or local regulatory agency is necessary to determine how to dispose of spent filters. The potential extra cost of handling the waste as radioactive should be taken into account when choosing a treatment option.
Aeration. Another effective treatment option for radon is aeration. Because radon is a gas, it will separate from water within a reasonable amount of time when exposed to air. One method of aeration is to use a packed tower aeration column. Water is trickled down a packed tower and air is pumped up to meet the water. This method has been shown to remove 95 percent or more of radon from the water (Dixon and Lee, 1989; Lamarre, 1989).7, 8
Another effective method for aeration is to use cascading tray aerators. Though the radon removal efficiency is less than for a packed tower aeration column, cascading tray aerators are a simple and inexpensive way to treat large volumes of water. This aeration system has been shown to remove 75 percent or more of radon from water (Dixon and Lee, 1989).9
When using aeration to treat a radon problem, it is important that the air from the system is vented away from areas where it could be inhaled by people. Radon is then released into the air where it dissipates, leaving no radioactive waste to manage. With proper venting, aeration can be a safe, effective, and cost-effective treatment for radon.
Other methods. Other treatment options include cation exchange, reverse osmosis, and water softening treatment. Because of the high costs associated with these methods, they are normally used at the point of use (for example, the kitchen sink) or by large producers when required. Surface water can also be mixed with groundwater to reduce radon levels.
Robert M. Kick, primary scientist; Sandra L. Potter, associate geoscientist; Rená D. Bass, associate scientist; and Matthew D. Klaus, assistant engineer, work for The Forrester Group, Inc., an environmental management consulting firm in Springfield, MO. The firm specializes in environmental business management, litigation support, regulatory compliance, Superfund, and RCRA management. For more information about radon in water supplies, contact the authors at (417) 864-6444 (phone) or by fax (417) 864-6445.
