When a shopping plaza was being put on the market, a preliminary investigation by others found very high levels of a chemical tetrachloroethene (PCE) beneath the floor of an active dry cleaner tenant. The shopping plaza owner hired St.Germain to help determine if the contamination problem was under control before the commercial property could be sold.
St.Germain completed an initial sampling of sub-slab vapor at the dry cleaner and sub-slab vapor and indoor air at two adjacent units in the shopping plaza, as well as soil vapor sampling outside of it. PCE levels beneath the dry cleaner were in the millions of ug/m3. There was no evidence of contamination outside of the building footprint, so it was concluded that the contaminant source was restricted to the sub-slab beneath the dry cleaner. Low levels of PCE were found in the adjacent unit’s sub-slab vapor, but the PCE level in indoor air in that unit was much higher than expected from sub-slab migration. Further investigation determined that there was a gap above the false ceiling between the dry cleaner and the adjacent unit, allowing vapors from the dry cleaning machinery to migrate directly into that other unit. Once that gap was filled, the indoor air levels of PCE declined significantly.
To improve things further, St.Germain installed a soil vapor extraction (SVE) system beneath the dry cleaner. After several months of the SVE being in operation, the sub-slab vapor PCE levels have declined by orders of magnitude. It is anticipated that the SVE system can be permanently shut down within the next year or two.