The Southeast Hennepin Area Groundwater and Vapor Site (Minneapolis, Minn.) is bounded to the north by Broadway Street NE, to the east by Industrial Boulevard NE, to the south by Hennepin Avenue and to the west by Taft Street NE. The primary contaminant of concern is trichloroethylene (TCE). Perchloroethylene (PCE) has also been detected, but at lower levels of concern.
TCE and PCE are solvents used for metal cleaning, plating, vehicle repair and other industrial processes. PCE is also a commonly used solvent in the dry cleaning industry. TCE can have short-term health risks, and both TCE and PCE have the potential to cause long-term human health risks.
What’s the problem?
In 2001, TCE was detected in the groundwater at the property located at 359 Hoover Street NE. TCE was reported at a concentration of 3,500 micrograms per liter (µg/l). It was determined that the contamination was coming from off site. There were no groundwater receptors at the time of the investigation and soil vapor intrusion had not yet been identified as a potential problem.
In 2009, TCE was detected in the groundwater at an auto repair shop located at 2314 East Hennepin Ave. at a concentration of 1,620 µg /l. The shop is located southwest (down-gradient) from property at 359 Hoover St. Soil vapor samples were not collected during the investigation.
In 2013 and 2014, TCE was found in soil vapor samples collected below residential basement floors — also referred to as sub-slab samples — at properties east (up-gradient) of the General Mills Superfund site testing area. As a result, the MPCA has collected additional sub-slab samples from residential and commercial properties around both the SE Hennepin Superfund site and the General Mills Superfund site to identify properties with elevated sub-slab soil vapor concentrations. Properties with elevated sub-slab soil vapors have the potential for chemical vapor intrusion into the buildings and may require building mitigation to address vapor intrusion risk.
Human health risks
When chemical vapors enter buildings through the foundation, the amount of chemical vapor in indoor air is usually not high enough to affect most people’s health. The possible health effects from breathing TCE depends on the amount in indoor air, how long people breathe it, and whether and when a pregnant woman is exposed.
The MDH is most concerned about women in the first trimester of pregnancy because TCE exposures may increase the risk of heart defects to the baby.
Exposure to TCE for a long period may also affect the immune system. Long-term exposures to TCE may also increase the risk of certain types of cancer based on studies in workers or animals breathing very high levels of these contaminants (thousands of times greater than what may be found at vapor intrusion sites).
For more information regarding health and vapor intrusion please visit the MDH website.
Groundwater sampling results
During the fall and winter of 2014, a groundwater investigation was completed by General Mills, at the request of the MPCA. Monitoring wells were installed both northeast (up-gradient) and southwest (down-gradient) of the General Mills site.
Results for the groundwater samples collected from monitoring wells located northeast of the General Mills site indicated that TCE was detected at a concentration of 3,290 µg/l in a monitoring well located northeast of the former General Mills property. The Health Risk Limit (HRL), established by the MDH, is 0.4 µg/l for TCE. No drinking water wells have been affected.
The MPCA completed several groundwater investigations from 2015-2019 to evaluate the extent of the TCE contamination in the groundwater and to identify potential sources of the contamination. TCE was detected in the groundwater at concentrations as high as 5,300 µg/l. Additional groundwater investigation is necessary to determine the full extent and magnitude of the groundwater contamination.
Soil vapor sampling results
Between 2014 and 2020, sub-slab vapor samples were collected at 36 residential buildings and 31 commercial buildings in the site area. The highest TCE concentration detected in a sub-slab vapor sample from a residential property was 109 µg/m3, and the highest TCE concentration detected in a sub-slab sample from a commercial property was 8,240,000 µg/m3.
The current Intrusion Screening Value (ISV) for TCE at residential properties is 2.1 µg/m3 and the current ISV for TCE at commercial properties is 7.0 µg/m3. The sub-slab vapor concentrations requiring a mitigation system at residential properties is 33x the residential ISV or 70 µg/m3 for TCE. The sub-slab vapor concentration where a mitigations system would be required or additional investigation would need to be completed at a commercial property is 33x the commercial ISV, or 230 µg/m3 for TCE.
As part of MPCA’s updated sampling protocol, two sets of sub-slab samples are collected at the residential and commercial properties in the area; one set during the heating season, and one set during the non-heating season. For more information, refer to MPCA’s Best Management Practices for Vapor Investigation and Building Mitigation Decisions webpage.
TCE concentrations were greater than the action levels at three of the sampled residences and 21of the sampled commercial properties. Three residential properties have vapor mitigation systems installed. Sub-slab mitigation systems have been installed at 12 commercial properties; 9 commercial properties need mitigations systems to be installed, and 17 commercial properties either need initial or additional vapor sampling.
Planned investigation activities
Most of the work to date, focusing on assessing the potential vapor intrusion risk at residential and commercial properties in the area has been completed. Based on past and current groundwater data, there appears to be one or more sources of PCE and TCE north of Hennepin Ave., up-gradient of the General Mills Superfund site. These sources may be contributing to the contaminated groundwater plume and require further investigation. The MPCA also continues to evaluate the potential for vapor intrusion at the residential and commercial properties on either side of Hennepin Ave.
Additional groundwater monitoring investigation is scheduled to take place during 2021 to identify the source and extent of the TCE contamination in the groundwater.
Additional sub-slab vapor sampling is proceeding at the residential and commercial properties. Samples are being collected at properties that have provided access to the MPCA. The MPCA is continuing attempts to obtain access at the properties that have not responded to the agency's requests.
Al Timm: 651-757-2786 or email@example.com
Andri Dahlmeier: 651-757-2718 or firstname.lastname@example.org
Minnesota Department of Health
651-201-4897 or email@example.com