radon removal from aeration-system off-gas remains unresolved and would probably be of greatest concern in urban areas.

Airborne release at facilities that treat groundwater can expose operators to high concentrations of radon (Fisher and others 1996). Ironically, the problem has been identified at plants that treat groundwater for contaminants other than radon (such as iron). In a survey of 31 water-treatment plants in Iowa, Fisher and others (1996) found that processes such as filtration, backwashing, and regeneration cause radon release directly into the plant. Their results suggest that the air in all facilities that treat groundwater should be monitored for radon and that ventilation should be investigated as a means of reducing worker exposure.

Microbial and Disinfection-Byproducts Risks

Treated water that leaves aeration systems might contain increased bacterial counts (Kinner and others 1990; 1989). On the basis of the pending Groundwater Disinfection Rule (GWDR) (EPA 1992b), disinfection would be required in cases where the heterotrophic plate count exceeded 500 colony-forming units per milliliter. In addition, aeration systems can have periodic problems with high coliform counts in the treated water as a result of the transfer of bacteria from air to water during treatment. That might also necessitate disinfection to comply with the coliform rule in distribution systems (Drago 1998).

EPA did not mention the potential need for disinfection of the effluent from aeration systems in its 1991 proposed rule, nor did it consider disinfection in its cost estimates for radon treatment. The SAB (1993) criticized the agency for neglecting to do so, and EPA has since added these costs (EPA 1994b). The technology to disinfect groundwater is well developed, and disinfection systems already exist in some communities or are being added to meet the requirements of the pending GWDR. The commonly used disinfection methods include chlorination (such as with sodium hypochlorite or gaseous chlorine) and ultraviolet irradiation.

Although disinfection reduces the health risk resulting from microbial contamination of drinking water, it has its own associated risks, especially if chlorination is used. Groundwater might contain organic carbon at 0.5 to 2 mg L-1 (Cornwell and others 1999; Miller and others 1990; Kinner and others 1990; 1989). Addition of chlorine to water that contains such natural organic matter could result in the formation of disinfection byproducts (DBPs) (that is, trihalomethanes, THMs—such as chloroform, dibromochloromethane, dichlorobromomethane, and bromoform—and other compounds at lower concentrations) in the range of 10–50 µg L-1. THMs are regulated in water under the Disinfection By-Products Rule because they are known to cause cancer in rats and mice. As a result, EPA has established potency values for these compounds. Disinfecting the effluent of aeration systems that remove radon from water increases the risk of exposure to disinfection byproducts. The SAB (1993) criticized EPA for not



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