The U.S. Environmental Protection Agency (EPA) has prepared this Health Effects Support Document for Terbacil to support a determination regarding whether to regulate terbacil with a National Primary Drinking Water Regulation (NPDWR). The available data on occurrence, exposure, and other risk considerations suggest that, because terbacil does not occur in public water systems at frequencies and levels of public health concern, regulating terbacil will not present a meaningful opportunity to reduce health risk. EPA will present a determination and further analysis in the Federal Register Notice covering the CCL proposals.