On December 8, 2016, the Environmental Protection Agency (EPA) formally added subsurface intrusion (SsI) to its Hazard Ranking System (HRS) as required under the Comprehensive, Environmental Response, Compensation, and Recovery Act of 1980 (CERCLA) (40 CFR Part 300, Appendix A).

SsI is a term that describes the migration of hazardous substances through subsurface soil or water into overlying structures that affects indoor air quality. The term encompasses both:

Vapor intrusion is more common and more dangerous than groundwater intrusion. When contaminants evaporate into the air within a structure, the vapors or gasses can be inhaled by building occupants, which can raise lifetime risk of cancer or chronic disease. These harmful vapors typically enter buildings by seeping through cracks in basements, foundations, sewer lines, pipes, and other openings.

Subsurface intrusion is typically addressed through the use of vapor mitigation systems, such as:

Vapor mitigation systems are temporary solutions until a more permanent remediation is performed.

The HRS is the primary program used to evaluate sites for placement on the National Priorities List (NPL). The NPL is a list of sites that have known or threatened releases of hazardous substances that are a priority for further investigation to determine if remedial action is required. Only sites included on the NPL are eligible for federal funds under the Superfund program for long-term cleanup. However, not every site listed on the NPL must be remediated.

The HRS was last revised on December 14, 1990 and only included four exposure pathways:

At the time of the 1990 HRS revision, it was not scientifically possible to accurately measure the environmental or public health threat of SsI. EPA only addressed SsI concerns if the site was experiencing other contamination problems.

Under the HSR, sites are assessed and scored based on the environmental threat level of the property. A score of 28.5 will qualify, but not necessarily obligate, a site for listing on the NPL.

Before the current revisions, many sites with SsI contamination were not included on the NPL and consequently did not receive federal remediation funds. Without federal assistance, states may not have the financial ability to appropriately remediate sites with SsI contamination, potentially exposing the public to harmful environmental conditions.

Sites that are currently on, or that are proposed to be added to, the NPL are not affected by the addition of SsI to the HRS. The EPA does not plan to reevaluate sites that previously missed the 28.5 cutoff score, unless new information becomes available or changing site circumstances require new sampling to be performed.

Commercial real estate owners with potential vapor or groundwater intrusion issues, including properties with current or former dry cleaning or petroleum businesses, should consult environmental counsel as well as engineering or other professionals to address potential vapor intrusion issues.

The addition of SsI to the HRS may increase the number of properties included on the NPL, although the EPA does not expect it to materially affect the number of properties added to the NPL each year.

It is also expected that the costs to government agencies conducting assessments will increase, which may cause the EPA to conduct fewer site evaluations each year.

For the full text of the EPA's announcement, see Addition of a Subsurface Intrusion Component to the Hazard Ranking System.

For a list of frequently asked questions published by the EPA, see Subsurface Intrusion Addition to the Hazard Ranking System: Questions and Answers.

For more information on environmental law, see Practice Note, Environmental Law: Overview.