This paper compares EPA’s ex ante cost analysis of the 2001 maximum contaminant limit (MCL) for Arsenic in Drinking Water to an ex post assessment of the costs. Because comprehensive cost information for installed treatment technologies or other mitigation strategies pursued by water systems to meet the new standard is not available, this case study relies upon ex post cost data from EPA Demonstration Projects, capturing a total of 50 systems across the US. Information shared by several states and independent associations on the types (but not costs) of treatment technologies used by systems is also summarized. Comparisons of predicted costs to realized costs using our limited data yield mixed results. Plotting the capital cost data from the Demonstration Projects against the cost curves for the compliance technologies recommended for smaller systems, we find that the EPA methodology overestimated capital costs in most cases, especially as the size of the system increases (as measured by the design flow rate).
