Formatted Title
Significant Return on Investment Achieved by Successfully Remediating a Challenging Chlorinated Solvent Site
Background/Objectives
Tetrachloroethylene (PCE) and related degradation products were identified in soil, groundwater and soil vapor at a shopping center in British Columbia at concentrations above the applicable regulatory standards both on and off site beneath sensitive receptors including residential properties and a school.
Prior to remediation, PCE concentrations in groundwater were greater than 10% of solubility and indicative of potential dense, non-aqueous phase liquid (DNAPL) resulting in the site being classified as “High Risk”. This made it unavailable for redevelopment and essentially unmarketable for the site owners. A highly effective remedial approach was needed to realize the value of the real estate asset.
Previous remedial activities completed at the site included undertaking a remedial excavation of 7,800 yd3 of PCE-impacted upper till soil in source area on and off site to north and the installation of a perimeter groundwater pump and treat system prevent further migration of groundwater plume off site in the lower sand unit.
Approach/Activities
A pilot-scale injection program of an activated carbon-based remedial amendment was completed in the “worst-case” area at the site in June 2019. Injections took place from approximately 30 to 60 feet below grade in “heaving sand” conditions using an innovative two-step drilling and injection method. Verification soil borings showed good distribution of the amendment in the subsurface across the targeted vertical injection interval. Post-injection groundwater quality monitoring completed over the subsequent three months showed that PCE concentrations in groundwater had dropped to the point that the “High Risk” designation of the site could be removed (i.e., an initial reduction on PCE concentrations from over 25,000 µg/L to less than 12,000 µg/L).
Full-scale remediation activities were then completed over an approximate three-month period. A total of 52,000 lbs of remedial amendment in 100,000 gallons of suspension using 120 injection points with injection intervals ranging in depth from 30 to 70 ft deep. Throughout the full-scale injection program many obstacles needed to be overcome including drilling refusal, injection tip clogging, oversaturation of the subsurface, excessive back pressures, and amendment daylighting to ground surface.
Results/Lessons Learned
In September 2021, after completion of the innovative full-scale on-site and off-site in situ remedial injection program, the perimeter groundwater pump and treat system was able to be shut down, eliminating the need for expensive on-going operation, maintenance and monitoring activities.
Starting in October 2021, groundwater monitoring wells were sampled quarterly for a year. Results indicated that the groundwater plume had significantly decreased in size and was considered stable. Contaminant concentration in the “worst case” area revealed a greater than 99.9% reduction. The consultant is currently in the process of completing a risk assessment and applying for a risk-based Certificate of Compliance for the site. Contaminant conditions have improved such that the site can now be used for mixed commercial and residential redevelopment.
The site owners have a marketable asset whose increased value is worth considerably more than the cost of the entire remediation and verification monitoring program and are now looking forward to what the site can become without the previous significant restrictions on land use.
This presentation will illustrate how the client, consultant and contractor worked collaboratively as a team to tackle this extremely complex and challenging site. It will provide an overview of the chemical and geological characteristics of the site, the methodology of the novel remedial injection technique used, details on how project technical obstacles were overcome, and a comparison of pre- and post-remediation groundwater quality data that highlight this remedial success story.