On May 22, 2017, the U.S. Environmental Protection Agency (EPA) added subsurface intrusion (SsI) to the list of pathways evaluated under the Hazard Ranking System (HRS) rule (EPA, 2017a). The HRS is a numerical scoring framework that the EPA uses under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, also known as Superfund) to assess the relative potential of sites to pose a threat to public health or the environment. The HRS score is determined by evaluation of one or more of the migration and/or exposure pathways. The pathways are scored using three categories, (1) likelihood of exposure/likelihood of release, (2) waste characteristics, and (3) targets. One or more pathways can be scored for a site, and sites with an overall score of 28.5 or higher can be proposed for inclusion on the National Priority List (NPL).
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