Zero-Valent Iron Media
Needham, MA: Permeable Reactive Barrier Installation By The Bio-Polymer Slurry Method, Mdl Site
This project included the construction of approximately 19,000 square feet of permeable reactive wall to depths of 56 feet, designed to intercept a plume contaminated with chlorinated hydrocarbons, primarily TCE. The permeable reactive wall backfill was a mixture of zero-valent iron filings and sand emplaced across the path of the contaminated plume. The wall was designed to have a permeability greater than the surrounding soils, allowing the natural groundwater to passively flow through the wall while the iron filings reduce the contaminants to harmless end products. Geo-Con installed the wall using the Bio-Polymer (BP) slurry trench method. This method includes excavating a deep narrow trench supported by biodegradable slurry during excavation and prior to backfilling. The backfill mix was prepared in an off-site pugmill and delivered to the site in road trucks prior to placement into the trench. Following backfill installation, the degradable slurry was broken and reverted back to water leaving a permeable wall.
The wall was installed down to bedrock through soil, boulders and cobbles to a maximum depth of 60 feet. The installation took place on a public street within 50 feet of several residences and within 10 feet of active utility services. Other work that took place in the congested construction area following closing of the heavily traveled street (17,000 vehicles per day) for 3 months, included; traffic control; police detail; relocation of the stormdrain; temporary relocation of water and sewer services; temporary relocation of phone, electric and cable services; installation of inclinometers and monitoring wells; handling, stabilization and disposal of all excavated spoils; and pavement restoration. The trench was installed with one of Geo-Con's Linkbelt 7400 hydraulic excavator with an extended boom and stick, capable of excavating over 70 deep.
To effectively manage public relations, the DEP, the Needham Health Department and Geo-Con set up a town meeting prior to project commencement to explain all aspects of the project including scheduling and safety and to answer any questions from residents. Regular weekly meetings were held with local residents for the duration of the project to address any individual concerns and resolve any issues. Construction work was restricted to the hours of 7:00 a.m. until 5:30 p.m. to control noise in the community. Driveway access for local residents was maintained at all times as well as dust control.
In addition, Geo-Con and the DEP met with the Needham Conservancy District on several occasions to discuss, apply and receive an Order of Conditions to perform the project scope of work and to adequately address potential environmental concerns due to adjacent wetlands and wildlife preserve.