By Jane Wilson
Radon is a naturally occurring, colorless, odorless and tasteless gas formed from the normal radioactive decay of uranium in the Earth’s rocks and soil. Inhalation exposure to radon has been demonstrated to increase the risk of lung cancer, especially in smokers, and ingestion of radon is suspected to increase the risk of development of stomach cancer. Radon typically enters buildings via soil migration, but it can also be present at significant levels in groundwater sources.
The 1996 amendments to the Safe Drinking Water Act required the U.S. Environmental Protection Agency (USEPA) to establish a health-based primary drinking water regulation for radon. The USEPA published the proposed rule for radon on Nov. 2, 1999. This rule is unique in that multiple approaches are used to mitigate a drinking water contaminant. It established two regulatory values, a “traditional” Maximum Contaminant Level (MCL) set at 300 picocuries per liter (pCi/L), and an Alternative MCL (AMCL) set at 4,000 pCi/L. The approach used in the radon rule is intended to encourage reduction of radon in indoor air, which is the larger source of human exposure, while also reducing the risks posed by radon in drinking water. States and public water systems are given two compliance options under this rule:
- States may elect to develop Multimedia Mitigation (MMM) programs to address the indoor air health risks of radon; under this approach, public water systems are required to reduce radon levels in drinking water to 4,000 pCi/L or less.
- If a state doesn’t elect to develop an MMM, individual public water systems may develop their own local MMM programs and reduce radon levels in drinking water to 4,000 pCi/L; otherwise, they’re required to reduce radon to 300 pCi/L.
More information regarding the proposed radon rule can be found at the USEPA website: http://www.epa.gov/safewater/radon.html.
The majority of the cancer risk attributed to radon-contaminated drinking water is associated with inhalation of radon gas. Still, the risk attributed to ingestion of radon containing water at its mean occurrence level is greater than the risks attributed to ingestion of other carcinogenic drinking water contaminants, such as chloroform and carbon tetrachloride, at their mean occurrence concentrations. For this reason, a protocol has been developed for inclusion in ANSI/NSF Standard 53 Drinking Water Treatment Units-Health Effects for radon reduction testing of point-of-use (POU), activated carbon units.
A carbon unit works to entrap the radon from water passing through it, and the radon subsequently decays into other species such as Lead210, Polonium210, and Bismuth210. The radon decay process releases gamma radiation to the area around the carbon filter. While a carbon filter has a limited capacity for the removal of radon, the decay of radon into other progeny essentially regenerates the sites that can again bind radon. The longer the unit sits between challenges—i.e., use, the greater the carbon recovers its ability to remove radon.
Numbers as guides
The influent challenge level for radon has been set at 4,000 pCi/L, a radon activity that’s equal to the AMCL proposed by the USEPA. For consumers on public water supplies, influent water radon activity shouldn’t exceed this level based on the centralized water treatment standards dictated by the USEPA. The acceptance criterion for the effluent water has been set at 300 pCi/L, equal to the USEPA’s MCL. The Standard 53 protocol is limited to evaluation of POU treatment devices, since above a radon activity of 4,000 pCi/L the inhalation risk from radon in drinking water starts to exceed that of the radon activity in ambient air and point-of-entry treatment is indicated.
Since most basic water quality parameters such as pH, temperature and total organic carbon don’t seem to influence the ability of a carbon filter to remove radon, the influent challenge water for the Standard 53’s testing protocol doesn’t require specific control of these parameters. It does require use of a natural source water containing radon. Several areas of the United States have been shown to contain elevated levels of radon, including the Appalachian, Rocky Mountain and New England states. Use of a naturally occurring source will also alleviate testing laboratories’ need to obtain special licensing for handling radium to generate the radon in the influent water.
The protocol dictates that test units are conditioned with the influent challenge water for a period of 21 days or until a steady state of radon adsorption/decay is achieved. The test systems are cycled four times over a 12-hour test period per day. For this test, a cycle is defined as 25 percent of the manufacturer’s daily production rate at the maximum specified flow rate. Influent and effluent samples for radon analysis are taken during the first and last test cycles daily. Also, a sample for analysis of Polonium210 as an indicator of radon progeny retention is required daily from the last cycle. Analysis is specified as liquid scintillation counting—a method of analyzing radiation—according to the methods cited in the USEPA’s proposed rule.
Points of protocol
To protect consumers from excessive radioactivity in their residences due to accumulation of radioactive species on the carbon filter, the protocol requires measurement of gamma radiation from the unit at a distance of six inches at the end of testing. The measured gamma radiation is required to be below 0.034 millirems per hour, based on an 8-hour exposure day for 365 days per year, which is the current Nuclear Regulatory Commission standard. The protocol also requires an assessment of projected “end of life” radioactivity on the filter through use of the USEPA Carbdose modeling program. This computer-modeling program can estimate the radiation from the treatment unit at a specified distance and as a function of time of filter operation. The Standard 53 test requires the filter be evaluated for a one-year use period. Instruction and labeling requirements will also stipulate a minimum filter replacement frequency of once per year. This should also ensure used filters don’t have a level of radioactivity that would require hazardous waste disposal arrangements.
Testing of POU activated carbon treatment units to the Standard 53 protocol will provide consumers who wish to minimize their ingestion of radon with a means of identifying appropriate treatment devices. The protocol is currently under ballot by the Joint Committee on Drinking Water Treatment Units (DWTUs) and is expected to be finalized by October 2001.
About the author
Jane Wilson is the senior project manager for Water and Environmental Standards in the NSF International Standards Department. She has been with NSF since 1990. Her bachelor’s degree in medical technology and a master’s degree in public health (MPH) are both from the University of Michigan. NSF is located in Ann Arbor, Mich. Wilson can be reached at (734) 827-6825, (734) 827-6831 (fax) or email: [email protected].