of radon from water to air) in the case of aeration and the retention of radionuclides (gamma-ray exposure and waste disposal) in the case granular activated carbon (GAC) adsorption. If water must be treated to meet either the AMCL or the MCL, disinfection might be required to meet the pending groundwater rule. In this case, the risk associated with the disinfection byproducts, as estimated by the committee, will be smaller than the risk reduction gained from radon removal. The committee has estimated the equivalent gamma dose from a GAC system designed to remove radon from a public water supply. The dose depends heavily on the details and geometry of the system and should be predicted with an extended-source model that can be modified to simulate the actual dimensions of the treatment units.
The 1996 Safe Drinking Water Act Amendments permit EPA to establish an alternative maximum contamination level (AMCL) if the MCL is low enough so that the contribution of waterborne radon to the indoor radon concentration is less than the national average concentration in ambient air. The AMCL is defined such that the waterborne contribution of radon to the indoor air concentration is equal to the radon concentration in outdoor air, which is taken to be the national average ambient radon concentration. In the situations where radon concentrations in water are greater than the MCL but less than the AMCL, states or water utilities can develop a multimedia approach to health risk reduction. The EPA is required to publish guidelines including criteria for multimedia approaches to mitigate radon in indoor air that result in a reduction in risk to the population living in the area served by a public water supply that contains radon in concentrations greater than the MCL. The committee has examined some of the implementation issues involved in a multimedia mitigation approach through a sequence of scenarios that explore the possible options.
The MCL will be determined by EPA based on a variety of considerations including their risk assessment, measurement technology, and best available treatment options and thus, a specific value has not yet been determined. The ratio of the average ambient radon air concentration to the transfer coefficient defines the AMCL. On the basis of the committee's recommended values for the average ambient radon concentration and the average transfer coefficient, the AMCL would be 150,000 Bq m-3 (about 4,000 pCi L-1). Water in excess of the AMCL must be mitigated at least to the AMCL, and alternative means can then be used to provide a health-risk reduction equivalent to what would be obtained by mitigation of the water to the MCL. However, because of the relatively small cost difference between mitigating the water to the AMCL and to the MCL, the committee believes that in most cases multimedia mitigation programs will probably not be considered for public water supplies with water concentrations in excess of the AMCL. For high radon concen-