HRS Subsurface Intrusion Component

HRS Revised to Include a Subsurface Intrusion Component

Former EPA Administrator Gina McCarthy signed the rulemaking to add a subsurface intrusion component to the Hazard Ranking System (HRS) on December 7, 2016. This rule was published in the Federal Register (FR) on January 9, 2017. This action took effect on May 22, 2017.

The HRS is the principal mechanism the EPA uses to place sites on the National Priorities List (NPL). The HRS is Appendix A to the National Oil and Hazardous Substances Pollution Contingency Plan (NCP), which the EPA promulgated on July 16, 1982 (47 FR 31180) pursuant to section 105(a)(8)(A) of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).

The addition of the subsurface intrusion (SsI) component to the HRS meets the congressional mandate in CERCLA to identify releases of hazardous substances at sites that warrant further investigation. The investigation will determine if Superfund remedial authority is needed to address unacceptable risks. This addition now enables the EPA to consider human exposure to hazardous substances or pollutants and contaminants that enter regularly occupied structures through subsurface intrusion when evaluating a site for placement on the NPL.

Subsurface Intrusion

Subsurface intrusion is the migration of hazardous substances or pollutants and contaminants from the unsaturated zone in the subsurface and/or the surficial ground water into overlying structures. While subsurface intrusion can occur through multiple mechanisms, the most common form of subsurface intrusion occurs as vapor intrusion.

When hazardous substances, or pollutants and contaminants are spilled on the ground or otherwise enter into the subsurface, they can migrate in the subsurface environment and eventually enter buildings as a gas or vapor, or, in some cases, even as a liquid. Dry cleaning solvents and industrial de-greasers are examples of products that contain hazardous substances that when released to the environment, can migrate into the subsurface environment, and enter into buildings by seeping through cracks in basements, foundations, sewer and utility lines and other openings and ultimately result in human exposures.

Vapor intrusion is a significant concern because vapors migrate readily through the subsurface into overlying structures even if the source of the vapors is located well below the base of the structure. As a result, contaminant concentrations in those structures can rise to a point where the health of the occupants, residents or workers could be at risk.

HRS SsI Supporting Documents

In support of the Final Rule, the EPA published the following documents:

• Technical Support Document (TSD) (75 pp, 2.8 MB, About PDF)
• TSD Appendix A: Conceptual Site Modeling/Sensitivity Analysis (22 pp, 4.0 MB, About PDF)
• TSD Appendix B: Site Summaries and Scoring Information (71 pp, 1.4 MB, About PDF)
• Regulatory Impact Analysis (RIA)  (63 pp, 1.5 MB, About PDF)
• Response to Comments Support Document (PDF)(119 pp, 1.7 MB, About PDF)
• Frequently Asked Questions (FAQs) (PDF)​(4 pp, 54 K, About PDF)