EPA has proposed to add the Rockwell International Wheel & Trim Site in Grenada, Mississippi to the National Priorities List, the first site added based solely on the risk to human health from vapor intrusion from subsurface contamination, which impacts indoor air quality.

The National Priorities List is designed to identify the contaminated sites that pose the highest risk to human health or the environment so that EPA can focus its efforts on these sites. The primary mechanism for listing a site on the National Priorities List is a score above 28.5 on the Hazard Ranking System (HRS). The HRS is a scoring system used to quantify the risk posed by a site in a way that can be compared to other sites. The types of risks that previously have been quantified in the HRS are exposures to contaminated soil, surface water, groundwater, and ambient air. Effective May 2017, this list was expanded by adding the risk of exposure to indoor air impacted by subsurface intrusion. In the final rule adding subsurface intrusion to the HRS, EPA stated: “Without an evaluation of threats posed by subsurface intrusion contamination, the HRS is not a complete assessment because it omits a known pathway of human exposure to contamination.”

The Rockwell site was the location of a chrome plating facility from 1966 to the early 2000s, and is currently the location of a metal stamping operation. Based on the site narrative, wastes containing trichloroethylene (TCE), toluene, and metals such as chrome were released to a wetland adjacent to a creek, to surface water via an outfall ditch, and to a sludge lagoon. TCE is a volatile organic compound (VOC), which means that it will move into air under normal conditions; as a result, it can move from groundwater or soil to soil vapor and then to the indoor air of a building. TCE is present at the Rockwell site in indoor air in the still-active manufacturing facility.

The HRS score for the Rockwell site is 50 based solely on subsurface intrusion, which is well above the threshold for listing of 28.5. The site’s HRS score is based largely on the results of indoor air sampling that show TCE and other VOCs in indoor air in the manufacturing facility on-site, which is occupied by employees 24 hours a day, 7 days a week. While there are contaminants in other media, including the presence of VOCs in surface water at levels that caused the state agency to issue a no contact order, and VOCs and chromium contamination in groundwater, these were not included in the HRS score.

It is not clear to what extent the addition of vapor intrusion to the HRS score will increase the number of sites proposed for or added to the National Priorities list. However, potentially responsible parties should anticipate that vapor intrusion issues is will continue to play a more prominent role in all phases of the Superfund program from listing decisions to investigation and cleanup to site closure.