At a site where a sub-slab depressurization (SSD) system had been in operation, yet indoor concentrations still exhibited periodic TCE risk exceedances using current assessment methods, a continuous monitoring platform was deployed. Within one day of VaporSafe™ operation, the project personnel determined that concentrations varied widely throughout the day. A few weeks into the monitoring campaign the SSD was turned off to study potential impacts, which was expected to include a rise on indoor TCE concentrations. To the surprise of project personnel, SSD operation increased Vapor Intrusion risks!
Lessons: SSD can actually increase risk if not properly designed, if preferential pathways are not accounted for, and if appropriate chemistry testing is not performed during implementation.