From: Public Services and Procurement Canada
Impermeable walls are installed by excavating a deep trench and replacing the soil with impermeable or low hydraulic conductivity materials. Impermeable walls are not a treatment technology, but are generally used to limit the migration of contaminants. They can also be used to isolate a source of contamination to prevent the dispersion of contaminants to groundwater. Thus, impermeable walls may surround the contamination or be constructed hydraulically upstream or downstream of the contamination. These walls are used to contain the contaminated groundwater, to redirect it, or to act as a barrier during the implementation of an in situ treatment system.
The materials used to form a wall may be a mixture including bentonite, bentonite cement, bentonite pozzolan, attapulgite, organically modified bentonite, or a slurry geotextile that has a low permeability and a chemical resistant barrier.
To increase the effectiveness of a wall of this type, it is preferable that the base of the wall penetrates a layer of low-permeability geological material, such as a layer of clay that extends to the contact with sound rock.
The technology of an impermeable wall can include:
On-site storage may include impermeable materials, fuels, lubricants and other site materials required for the operation of machinery or equipment for the process. When walls are built, little material is kept on site unless the system requires an active component such as a treatment system.
This technology does not produce any residues other than the soils that were excavated for the placement of the walls. Soils excavated to create the trench may be contaminated and therefore will need to be treated or disposed of.
Notes:
Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.
Preliminary or treatability testing and pilot testing may be required to evaluate the permeability and the compatibility of the wall materials with contaminants, water and vapours.
The installation of impermeable walls is applicable to the majority of dissolved contaminants that migrate into groundwater, as long as the contaminants do not restrict the occupancy of the environment, the quality of the environment and the life of living organisms in the long term. Furthermore, the technology is effective in the presence of porous, fractured and homogeneous soils.
The period for which the impermeable wall is kept in place can be very long and even indefinite. As long as the source of contamination is present, the presence of the impermeable wall may be required.
Impermeable walls can deteriorate over time and allow contaminated groundwater to migrate. A monitoring network for surrounding groundwater and off-gas migration is often required.
In addition, activities carried out on the site, or its uses, must not compromise the integrity of the wall.
The impermeable wall does not produce secondary products.
Impermeable walls do not treat contamination. They are a contamination containment measure. Treatment of source area contaminants by in situ or ex situ technologies may be required.
The following links provide application examples:
Impermeable walls have been used for several decades as a mitigation strategy and as a means to control long-term contamination migration. These walls are 95% effective in containing uncontaminated groundwater. However, in the presence of contaminated groundwater, certain types of contaminants can degrade the wall’s components and thus reduce its long-term effectiveness.
Unavailable for this fact sheet
Composed by : Josée Thibodeau, M.Sc, National Research Council
Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada
Updated Date : April 30, 2014
Latest update provided by : Nathalie Arel ing., M.Sc., Frédéric Gagnon CPI., Sylvain Hains ing., M.Sc., Golder Associates Ltd.
Updated Date : March 24, 2022
