RADON FROM GROUND WATER TO INDOOR AIR: THE REAL AND REGULATORY PATHWAYS

Scientific and regulatory uncertainties have delayed promulgation of a regulatory standard for radon in drinking water for over 15 years.  The U.S. Environmental Protection Agency proposed regulations for radon in 1991 and in 1999, and final action on a proposed rule is not anticipated until 2009.  Uncertainty in quantifying the transfer of dissolved radon into airborne concentrations has contributed to the regulatory uncertainty, although a 1999 National Academies report recommends EPA continue to use the 1.0 x 10^-4 as the best central estimate of the transfer coefficient.  The NAS report acknowledges, however, a need for better, more nationally representative data on the water-to-air transfer coefficient. The pathway to a regulatory decision is hindered, perhaps more so, by the precedent of setting a standard for a contaminant in one medium, drinking water, that is aimed largely at reducing the health impact in another medium, indoor air, and by the regulatory framework required in the 1996 amendments to the Safe Drinking Water Act (SDWA).   Those amendments introduced the concept of a multimedia mitigation program that gives states and water utilities some flexibility in mitigating health risks from radon in cases where the waterborne radon concentration exceeds the maximum contaminant level (MCL) but is less than the alternative maximum contaminant level (AMCL).  A multimedia approach recognizes another significant difficulty inherent to regulating radon in water.  Soil gas, not ground water, is the dominant pathway for radon entering homes and radon in water only adds a small increment to the overall indoor air concentration.  At present the amended SDWA compels EPA to set a regulation for radon in drinking water, although practically speaking it is probably not the agency's highest priority.  The question arises as to whether regulating radon in ground water is worth the effort.