Impacts of the U.S. Safe Drinking Water Act Arsenic Rule on Arsenic Occurrence ^†

This study provides the first retrospective analysis of the causal impacts of the 2001 US Safe Drinking Water Act (SDWA) Arsenic Rule on arsenic (As) occurrence in public drinking water systems. Using publicly available SDWA compliance data, we employ a difference-in-differences strategy that compares As concentrations before and after the promulgation of the rule for treatment relative to control water systems, defining treatment based on whether a water system had ever sampled As concentrations above the rule's 10 μg per liter (μg/L) threshold before the Rule was published. We find that annual mean As concentrations in treatment systems fell by almost 5 μg/L (p < 0.01) after 2001 relative to control systems, representing a decrease of about 38% compared to pretreatment levels. In light of the Rule's projected net costs at the time it was promulgated, assessment of the Rule's realized impacts using actual measured As concentrations from a nationwide sample is an important first step in retrospective benefit-cost analysis.