Because a relatively low maximum contaminant level (approximately 300 pCi/L) is expected to be set for radon in the near future, research was conducted with liquid scintillation counting to determine whether this method of analysis could be used for low levels of radon. Counting with an optimized window and an optimized water-to-fluor ratio resulted in achievement of a lowest quantifiable level of 150 pCi radon/L. Lower levels can be quantified using a longer counting period.

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Impending new maximum contaminant levels (MCLs) for radionuclides, plus increased concern for radon in the air inside homes, have sparked new interest in these substances. An assessment of research needs,∗ which also provided background information on completed and ongoing research projects, showed that Rn‐222 represents the most serious threat to health of all the radionuclides in drinking water, leading to the anticipation that the new MCLs for these substances could be set at a relatively low level. Small public and private supplies were identified as being more vulnerable to radionuclide contamination than public systems serving more than a few hundred people, but more investigation is needed to assess the extent of the exposure and its associated risk. The previous belief that high levels of Rn‐222 are confined to a few geographic areas was found to be false, largely because of a lack of a nationwide data base.

Two processes were investigated in the laboratory to determine their efficiency for removing radon from household water supplies. Granular activated carbon (GAC) adsorption was found to be extremely effective as a result of an adsorption‐decay steady state that is established quickly and continues for years. Because bed replacement is not a factor for practical purposes, GAC is the most cost‐effective process for radon removal. The GAC bed, however, adsorbs radon progeny as the radon decays, and it becomes a source of gamma radiation. This problem is believed to be manageable for the vast majority of potential applications. Diffused bubble aeration was found to be as effective as GAC, with removals of greater than 99 percent being practical. Although more costly than GAC, aeration does not have the problem of gamma activity buildup.

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