Successful Mitigation of Soil Vapor Intrusion
In a mixed use area in Minnesota, there stood a 40,000 square foot chemical distribution warehouse. Chlorinated volatile organic compounds (CVOCs) were discovered in an off-site groundwater plume extending from the warehouse. In an adjacent property, there was a daycare facility, which is considered a sensitive receptor. Antea Group was in charge of conducting additional investigation into the matter.
Antea Group installed 27 soil vapor monitoring points, 21 of which were permanent, in the area between the warehouse and the daycare.
Sub-slab soil gas and indoor air sampling was conducted at both buildings along with ambient outdoor air sampling. Inside the warehouse, sub-slab sampling revealed TCE and PCE concentrations greater than 33 times the industrial Intrusion Screening Value (ISV) and some indoor air samples exceeded the TCE industrial ISV. In the daycare, PCE was detected in sub-slab samples at concentrations greater than 33 times the residential ISV. PCE was not detected in the daycare indoor air samples.
ISVs are numerical values representing the safe amount of a chemical in indoor air for people who use a building. They are provided by the Minnesota Pollution Control Agency or MPCA.
Antea Group then installed sub-slab depressurization systems (SSDs) at the warehouse and the daycare buildings. SSD reduces the pressure in the sub-slab environment by exhausting sub-slab gases before they move through floor cracks or openings. An SSD system consists of one or more pipes attached to a fan or blower thereby creating a suction. Toxic vapors are prevented from intruding into an affected building.
Evaluation of the effectiveness of the SSDs indicated that both buildings were successfully mitigated. Antea Group developed Property Summary Reports including Operation and Maintenance Manuals for each building to document the assessments and solutions provided. In February 2018, Antea Group submitted a report to the MPCA summarizing the assessment and remediation of the soil vapor and groundwater impacts and detailing the vapor intrusion assessment and mitigation. The report positions the site for closure concerning the vapor intrusion pathway.