discussing the cancer risks associated with exposure to disinfection byproducts; to date, the agency has not done so.

According to Wallace (1997), finished water produced from surface water tends to have higher THM concentrations than finished water from groundwater supplies. One of the concerns of this committee with regard to methods of reducing radon exposure is the potential for increased exposure to THMs if radon mitigation results in the use of chlorine to disinfect the water to satisfy the pending GWDR. To examine this issue, the committee made a screening level estimate of the relative change in cancer risk associated with surface water and groundwater.

Chloroform concentrations are monitored extensively at water-treatment plants, but only sporadically in residential tapwater. Wallace (1997) has reviewed a number of surveys of water-supply concentrations of THMs. Among them, the Community Water Supply Survey provides representative and comprehensive results (Brass and others 1981). The (average and median) values of chloroform, dibromochloromethane, dichlorobromomethane, and bromoform in supplies derived from surface water were 90 (60), 12 (6.8), 5 (1.5), and 2.1 (<1) µg L^-1; respectively, and the average values of chloroform, dibromochloromethane, dichlorobromomethane, and bromoform in supplies derived from groundwater were 8.9, 5.8, 6.6, and 11 µg L^-1; respectively, average concentrations all below the detection limit.

The committee used those reported concentrations with unit dose factors for inhalation, ingestion, and dermal uptake and with EPA cancer potencies of the compounds to make approximate risk estimates. The average lifetime cancer risk associated with a surface-water system is around 1 × 10^-4 and the corresponding risk associated with a groundwater system is around 5 × 10^-5, smaller than the 1 × 10^-4 risk of lung cancer posed by inhalation of radon released from water (see chapter 5). The calculations are summarized in appendix D.

Aeration during radon treatment increases the pH of water (Kinner and others 1990; 1989). The increase has been attributed to the removal of CO₂ from the water. In a study of aeration units used for VOC treatment, the American Water Works Association (AWWA 1991) reported that the effect of CO₂ removal, with the greater stability of CaCO ₃ at the higher pH, negated the effect of the increased oxygen concentration in water. There was no increase in the corrosivity of the water. At one very small water-supply system in Colorado, aeration of the water to remove radon actually eliminated the need for addition of lime to prevent corrosion (Tamburini and Habenicht 1992). At a small system in New Hampshire, aeration resulted in a decrease in corrosivity and a reduction in the lead and copper measured in the drinking water (personal communication, D. Chase, Department of Health and Human Services, Bureau of Radiological Health, August