When Contaminate Hydrogeology Intersects with Geotechnical Engineering - Lessons Learned During In-Situ Remediation

ABSTRACT: building foundations and how they may influence in-situ remediation approaches. A site located within the Iroquois Sand Plain near Lake Ontario that comprised a three story commercial building constructed in the 1950s (a third floor was added in the 1980s) had tetrachloroethylene (PCE) concentrations in groundwater in excess of the Ontario Ministry of Environment, Conservation, and Parks site condition standards. The property owner wished to renovate the building to allow for a commercial use on the ground floor and residential use on the upper two floors. Remediation of PCE impacted groundwater was required to allow for this redevelopment. An in-situ remediation program was designed that consisted of temporary well point injections of Zero Valent Iron (ZVI), as a slurry, and permanent wells for injection of EHC-L, in the basement of the building and in the parking lot at the rear of the building. During the initial phase of ZVI injections, ZVI was observed to be daylighting through existing floor cracks and in adjacent monitoring wells. In order to address and prevent ZVI daylighting, a denser slurry was injected during a subsequent injection event. EHC-L was injected into the permanent injection wells during this second ZVI injection event. During this second event, cracking in the drywall was observed on the walls on the ground floor above the injection area. Injection work was ceased and in order to address the situation, a geotechnical investigation to assess the cause of the cracking was completed. Based on the results of the investigation, structural supports were installed to support a structural column that was identified to be a concern. Modifications to the in-situ remediation program were made (low pressure injections) and remediation work continued.