In a final rule, the EPA has added a subsurface intrusion component to the hazard ranking system (HRS) the Agency uses when considering whether to add sites to the Superfund’s National Priorities List (NPL). Until this action, the HRS, which was promulgated in 1990, did not consider the threat posed by subsurface intrusion when evaluating the risk to human health and the environment from a contaminated site. Under the HRS, each of multiple risk pathways is given a numeric score; a total score will be from 0 to 100. Sites that score 28.5 and above are eligible for listing on the NPL. According to the Agency, when the HRS was issued, available science and sampling methods were not considered sufficient to evaluate subsurface intrusion threats for scoring purposes.
As interpreted by the EPA, subsurface intrusion refers to the migration of hazardous substances from the subsurface environment into overlying structures. The phrase covers both vapor intrusion, the most common form of intrusion, and intrusion of contaminants contained in groundwater that enters a structure. The Agency notes that groundwater intrusion is an emerging issue that is only now being comprehensively investigated.