HRS Subsurface Intrusion

Subsurface Intrusion Graphic

This website provides some key information for members of the public and U.S. Environmental Protection Agency (EPA or Agency) professionals who have responsibilities for implementing the program. This information is specific to the addition of Subsurface Intrusion (SsI), which includes vapor intrusion, to the Hazard Ranking System (HRS).

Current Updates: Federal Register Notice of Rulemaking - Addition of Subsurface Intrusion (SsI) to the HRS

The EPA Administrator, Gina McCarthy, signed the rulemaking to add SsI to the HRS on December 7, 2016. This rule was published in the Federal Register (FR) on January 9, 2017. 

The EPA has published other documents as supporting information to this final rule on the subsurface intrusion. These documents include the following:

The EPA Administrator, Gina McCarthy, signed the proposed rulemaking to add SsI to the HRS on February 3, 2016, and the EPA published the following rule in the FR on February 29, 2016, proposing to add SsI to the HRS.

Addition of SsI to the HRS

Related Links | Listening Sessions

Subsurface intrusion is the migration of hazardous substances, pollutants and contaminants from the unsaturated zone and/or the surficial ground water into overlying structures. While subsurface intrusion can take multiple forms, the most common form of subsurface intrusion is vapor intrusion. The EPA is considering the addition of a new screening mechanism to the Hazard Ranking System (HRS) to enable sites with subsurface intrusion contamination to be evaluated for placement on the National Priorities List (NPL).

In a May 2010 report (EPA's Estimated Costs to Remediate Existing Sites Exceed Current Funding Levels, and More Sites are Expected to Be Added to the National Priorities List, GAO Report to Congressional Requesters, GAO-10-380, May 2010), the Government Accountability Office (GAO) concluded that if vapor intrusion sites are not assessed and, if needed, listed on the NPL, there is the potential that contaminated sites with unacceptable human exposure will not be acted upon. GAO recommended that the EPA Administrator determine the extent to which the EPA will consider vapor intrusion in listing NPL sites and how this will affect the number of NPL sites listed in the future. 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 CERCLA. The HRS is the principal mechanism the EPA uses to place sites on the CERCLA NPL.

The addition would enable the HRS to directly consider the human exposure to contaminants that enter building structures through the subsurface environment. When hazardous substances, pollutants or contaminants are spilled on the ground or otherwise migrate to the subsurface, they can move in the subsurface environment and eventually enter buildings as a gas or vapor, or even as a liquid in some cases. Dry cleaning solvents and industrial de-greasers are products that contain hazardous substances that when released to the environment, can migrate into the soil and subsurface environment, enter buildings by seeping through cracks in basements, foundations, sewer lines and other openings and ultimately result in human exposures. Vapor intrusion is a particular concern because concentrations of vapors can rise to a point where the health of residents or workers in those buildings could be at risk. Intrusion of contaminants in a non-vapor state may also be a pathway of concern because of the potential for human exposure to the liquids, the resulting precipitates, or associated vapors.

Many sites on the NPL that have subsurface intrusion problems were placed on the NPL by evaluation of pathways other than a subsurface intrusion mechanism. There are other contaminated sites that did not qualify for placement on the NPL under the current HRS. However, these sites may qualify for placement on the NPL if the threat from subsurface intrusion was included in the HRS. A new HRS mechanism would enable the EPA to identify situations in which individuals are exposed or potentially exposed to vapor or other contaminant intrusion in dwellings, work places or other structures or enclosures.

The Agency conducted public outreach activities, including facilitating public listening sessions, providing public information documents and establishing a website with more information regarding this rulemaking. The Agency considered the information gathered from the January 31, 2011, Federal Register Notice, listening sessions and other sources before making the decision to issue a proposed rulemaking to add subsurface intrusion to the HRS. The Agency requested comments only regarding this addition to the HRS, and it is not considering changes to the remainder of the HRS. Please note that the comment period for the January 31, 2011, Federal Register Notice ended on April 16, 2011, 11:59 PM ET.

HRS SsI Related Links

HRS SsI Listening Sessions

The EPA held four listening sessions following publication of the January 31, 2011, Federal Register Notice to allow interested parties to present feedback on the potential HRS addition. This input was considered by the Agency as it determined whether to move forward with this addition to the HRS.

The first listening session was held in Arlington, VA on February 24, 2011. The second listening session was held in San Francisco, CA on March 16, 2011. The third listening session was held in Albuquerque, NM on March 30, 2011, and the fourth listening session was held in Edison, NJ on April 7, 2011.

For a transcript and copy of the slide presentation from the Arlington, VA listening session, click here:

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