CASE STUDY: TCE, DCE and Vinyl Chloride Remediation with a Permeable Reactive BarrierUsing a Permeable Reactive Barrier with Zero-Valent Iron to remediate chlorinated ethene groundwater impacts from an off-site source.
At an active industrial brownfield site in Toronto’s Yorkdale district, chlorinated ethene groundwater impacts, in the form of TCE, DCE and vinyl chloride, were discovered through standard testing performed in preparation for redevelopment of the property. The unidentified source of the contamination originated off-site and flowed down-gradient onto the property at a groundwater flow velocity of approximately 5-10 metres per year. The developer required an expedient remediation solution in order to complete the property’s redevelopment.
Location: Toronto, Ontario, Canada
Client: National Developer
Duration: 1 Week
Project Value: $175,000 CDN per year
Geology: Silty Sand
Plume Size: Approx. 40 m
The national developer hired an environmental consulting firm to characterize the site. The consultant recommended a Permeable Reactive Barrier. They contracted IRSL directly based on their extensive experience with designing and installing Permeable Reactive Barriers.
Permeable Reactive Barrier with Zero-Valent Iron
To capture and treat the chlorinated ethene plume flowing from an up-gradient source, IRSL designed and supervised the installation of a 40 m long Permeable Reactive Barrier (PRB), which used Zero-Valent Iron, in a silty sand unconfined aquifer.
To develop an optimal Permeable Reactive Barrier design, IRSL used their proprietary analytical model, which incorporates contaminant concentrations, groundwater flow rates, reaction rates, temperature and inorganic parameters, within the soil and groundwater. Through this process, they discovered variations in gradient and flow direction, which had been previously overlooked by other in-situ contractors, and significantly impacted the design.
Based on their analysis, IRSL created the design, which was implemented by an independent general contractor. They successfully installed the Permeable Reactive Barrier in 3 days without impacting the operating facility. Constructed using a cut-and-fill method to a depth of 6 m, the 40 m long PRB was connected into an underlying aquitard.
The barrier used a mixture of zero-valent iron and gravel to create a highly permeable zone compared to the surrounding geologic media. The zero-valent iron degraded the chlorinated ethenes of concern (TCE, DCE and vinyl chloride) to below the applicable regulatory standards. IRSL monitored the process and conducted a rigorous Quality Assurance-Quality Control program throughout the project.
Permeable Reactive Barriers (PRB)
Also known as a Permeable Reactive Treatment Zone (PRTZ), a Permeable Reactive Barrier (PRB) is a physical barrier, usually installed by excavating a trench perpendicular to groundwater flow, which is infilled with a reactive mixture designed to treat the compounds of concern within the groundwater. The barrier may also be constructed using a funnel and gate design or closely spaced boreholes or injection wells. A relatively new development for PRBs is to create them via closely spaced direct push injection points. IRSL has designed and installed PRBs for chlorinated ethenes, ethanes, petroleum hydrocarbons, 1,4 dioxane, phenols, chromium, arsenic, boron, various heavy metals, nitrate and phosphate.
The other half of a redox reaction, chemical reduction results in the gain of electrons. The injected/placed reactants in the reaction are oxidized, losing electrons; whereas the other contaminant is reduced, gaining electrons. In In-Situ Chemical Reduction (ISCR), reducing compounds—compounds that donate electrons—are injected into the subsurface to transform contaminants into harmless compounds.
- The restrictive location complicated the PRB installation. IRSL leveraged its extensive installation experience to advise the general contractor, resulting in a significant cost savings for the client.
- Variations in gradient and flow direction significantly impacted the design, requiring careful consideration.
- The chlorinated ethene plume was treated to below the applicable regulatory standards.
- The Permeable Reactive Barrier incurred no Operations & Maintenance costs.