CASE STUDY: TCE, DCE and Vinyl Chloride Remediation in Weathered Shale

Using Injection Wells and Direct Push Technology to create a Permeable Reactive Barrier with Zero-Valent Iron and Emulsified Vegetable Oil to remediate chlorinated ethene groundwater impacts from an on-site source.

Background

At a manufacturing plant in Toronto’s Islington district, chlorinated ethene groundwater impacts, in the form of TCE, DCE and vinyl chloride, were discovered through environmental testing performed in preparation for selling the property. The contamination, originating from manufacturing processes, was potentially flowing down-gradient onto the adjacent property at a groundwater flow velocity of approximately 8 metres per year. The site owner required an expedient remediation solution in order to minimize environmental damage and complete the property sale. The building itself, as well as an electrical transformer compound, restricted site access, making traditional installation methods impractical and remediation challenging.

Location: Toronto, Ontario, Canada

Client: Industrial

Duration: 2 Weeks

Project Value: $115,000 CDN

Geology: Silty Sand and Weathered Shale

Plume Width: Approx. 100 m

Approach

The industrial landowner hired an environmental consulting firm to characterize the site. At the conclusion of the investigation, the consultant recommended a Permeable Reactive Barrier. They tendered the project for competitive bid to three companies. IRSL earned the project based on their extensive experience with Permeable Reactive Barriers, which coupled practical experience with an innovative design.

Permeable Reactive Barrier using Injection Wells and Direct Push Technology

To treat the chlorinated ethene (TCE, DCE and vinyl chloride) groundwater impacts flowing from the source on site, IRSL designed and installed a 100 m long Permeable Reactive Barrier (PRB) using Injection Wells and Direct Push Technology (DPT) to administer a Zero-Valent Iron and Emulsified Oil mixture.

Analysis

To develop an optimal Permeable Reactive Barrier design, IRSL used their proprietary analytical model, which incorporated observed contaminant concentrations, groundwater flow rates, reaction rates, temperature and inorganic parameters, within the shale, soil and groundwater. Through this process, they discovered variations in gradient and flow direction, which had been previously overlooked, and significantly impacted the design.

Construction

Based on their analysis, IRSL created and implemented the design. They successfully constructed a Permeable Reactive Barrier, using both injection wells and direct push technology, in two weeks without impacting the operating facility.

Injection Wells: To remediate the contamination in the weathered shale, IRSL installed a series of injection wells in a 100 m barrier perpendicular to the groundwater flow.

Direct Push Technology: To remediate the contamination in the overlying silty sand, they used Direct Push Technology at various depths and intervals to create a barrier.

Treatment

Into the shale and overburden, IRSL injected a mixture of Zero-Valent Iron (ZVI) and Emulsified Vegetable Oil (EVO). Through the process of Chemical Reduction, the ZVI donated electrons to the chlorinated ethenes, transforming them into harmless compounds. The EVO stimulated Anaerobic Biodegradation. Together, the two simultaneous processes degraded the TCE, DCE and vinyl chloride to below the applicable regulatory standards.

Monitoring

An independent consultant monitored the process and conducted a rigorous Quality Assurance-Quality Control program throughout the project.

Details

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.

Chemical Reduction with Zero-Valent Iron (ZVI)

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.

Enhanced Anaerobic Bioremediation with Emulsified Vegetable Oil (EVO)

Enhanced anaerobic bioremediation creates conditions in which naturally occurring or injected micro-organisms, in the absence of oxygen, will feed on contaminants, breaking them down into less-toxic compounds and by-products such as chloride, elemental sulfur, hydrogen gas, methane, and sulfide. EVO: A cost-effective slow-release electron donor with greater hydrogen release efficiency than other electron donors, Emulsified Vegetable Oil donates electrons to enhance the reductive dechlorination of the chlorinated ethenes and their daughter products by native bacteria into such harmless by-products as ethene, chloride, and water. Emulsification ensures small, uniform droplet sizes that distribute evenly throughout the plume.

Challenges

  • The restrictive location of the PRB complicated the installation. IRSL leveraged its extensive installation experience to realize significant cost savings for the client.
  • The weathered shale made delivering the ZVI-EVO mixture into the chlorinated ethene plume a challenge, which required the development of an innovative delivery method.
  • Unknowns associated with the geology and hydrogeology required a robust remedial design.

Results

  • The chlorinated ethene plume was treated to below the applicable regulatory standards.
  • The Permeable Reactive Barrier incurred no Operations & Maintenance costs.

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