<p>Lead contamination in drinking water remains a serious public health risk in the United States, yet public water systems with aging lead service lines still serve many households. Replacement efforts have been slow, limited by financial and logistical barriers. In 2018, the state of Wisconsin implemented a public infrastructure policy providing financial assistance for the voluntary replacement of lead service lines, aiming to improve compliance with the Safe Drinking Water Act (SDWA) and reduce associated health risks. This study estimates the causal effect of the policy using a synthetic difference-in-differences (SDID) approach, comparing SDWA violations in Wisconsin’s public water systems to those in a synthetic control. The results indicate that the policy reduced the violations by about 28.7% relative to the control group’s average post-treatment violations—suggesting improved regulatory compliance and safer drinking water. An event study analysis confirms the robustness of these findings. The results provide new empirical evidence on the effectiveness of state-level interventions in enhancing public health compliance and demonstrate the value of the SDID method as a robust tool for environmental policy evaluation.</p>