On January 9, 2017, EPA issued a final rule adding subsurface intrusion (SsI) as a component to the Hazard Ranking System (HRS), which is the mechanism that is used for determining the eligibility of sites for CERCLA’s National Priorities List (NPL). The rule allows the threat of subterranean vapor migration into regularly occupied structures to be considered when deciding if the risk posed by a particular site is great enough to warrant listing.
Previously, the only exposure pathways that EPA and state and tribal organizations that assess sites for inclusion on the NPL were permitted to take into account were ground water migration, surface water migration, air migration and soil exposure. SsI is being folded into the existing soil exposure pathway, which is being restyled as the Soil Exposure and Subsurface Intrusion pathway.
The addition of SsI to the HRS at this time was driven by the development in technologies that allow SsI risks to be quantifiably and comprehensively measured in an efficient matter. EPA has long been aware of the health risks of SsI but could not justify considering it as part of the HRS, which requires the availability of data that can be used to objectively compare thousands of disparate sites.
The rule could result in an increase in the number of sites being listed on the NPL. Sites with an HRS score of 28.50 are eligible and that number will not change. Under the new rule, however, potential SsI that would in the past have been ignored can add points to a site’s HRS score and there are no corresponding reductions elsewhere. EPA expects that in practice, however, it will result in a shift in the makeup of sites on the NPL without causing more listings during any particular interval.
The rule will not affect sites that have already been evaluated for addition to the NPL. Sites that are already on, or proposed to be put on, the NPL have already been found to pose enough risk through other exposure pathways to warrant listing or proposal for listing. Sites that were dropped from the listing process at some earlier stage of evaluation will not be reevaluated.