Addressing uncertainty and conflicting cost estimates in revising the arsenic MCL

The current effort to revise the arsenic drinking water standard is one of the first times that the promulgation of a Maximum Contaminant Level (MCL) for drinking water has been influenced explicitly by benefit-cost considerations. Different stakeholders have developed different estimates of the costs, benefits, and appropriate decision-making criteria for a lower standard. In this study, alternative analyses prepared by the U.S. EPA and by independent researchers are compared. The large discrepancies in the aggregate national cost estimates are shown to result largely from differences in the engineering cost estimates for arsenic treatment processes. Further research is needed to resolve these discrepancies. Alternative regulatory approaches, such as providing point-of-use treatment or exempting water systems with high household compliance costs, yield only modest improvement in the overall cost-effectiveness of lower standards but are effective at addressing serious affordability problems for the small percentage of (primarily small) water systems where these problems are predicted to occur. The U.S. EPA may wish to provide more explicit guidance to state regulators and to water utilities as to the conditions under which these options will be acceptable.