Public Services and Procurement Canada
Impermeable barriers (or slurry walls) are used to contain contaminated groundwater, divert contaminated groundwater from a drinking water intake, divert uncontaminated groundwater flow, or provide a barrier for a groundwater treatment system. This technology is not intended for the treatment or removal of the contaminants; it contains the dissolved or free-phase contamination within groundwater or avoids spreading of the contamination. Slurry walls are applicable to many types of organic and inorganic contaminants including non-aqueous phase liquids (NAPL) and are often used where the waste mass is too large for treatment or where soluble and mobile contaminants pose an imminent threat to a source of drinking water or surface water.
Slurry walls consist of vertically excavated trenches that are filled with slurry. The slurry forms a barrier that reduces groundwater flow. Most slurry walls are constructed with soil, bentonite and water mixtures. Walls of this composition provide a barrier with chemical resistance and low permeability. Other wall compositions, such as cement/bentonite, pozzolan/bentonite, attapulgite, organically modified bentonite, or slurry/geomembrane composites, may be used if greater structural strength is required or if chemical incompatibility between bentonite and site contaminants exist. Considerations must be made with respect to the compatibility of the contamination type and the materials used in the construction of the barrier walls.
The most effective application of the slurry wall for site remediation or pollution control is to construct the base of the slurry wall into a low permeability layer such as clay or bedrock. This operation provides for an effective foundation with minimum leakage potential.
Containment through barrier walls are usually accompanied by capping of the overlying soil to reduce upward migration of vapours (if present) from the soil; and reduce infiltration of precipitation and leaching of contaminants into groundwater. If volatiles are present or there is potential for production of gases then a vapour collection and treatment system may be necessary. Depending on-site conditions, a leachate collection and treatment system may also be required. A groundwater collection system consisting of wells, sumps or trenches may also be required.
Impermeable walls are used to contain contamination by means of surrounding (or partially surrounding) the contaminated area with low permeability vertical barriers to prevent migration of contaminants present as NAPL, or groundwater or soil vapour plumes. Vertical containment may also be used to divert the flow of uncontaminated groundwater around the contaminated area.
Systems may include:
Slurry walls are a common technology and are vertically excavated trenches that are 0.6 m to 1.2 m wide with depths of up to 30 m and can be constructed using augers, draglines, clamshells or other types of excavating equipment. Vertical containment walls can be constructed using several technologies including: jet grouting; slurry wall construction using conventional excavation and replacement, or clamshell excavation; cutter soil mixing; trench cutter; and sheet piling.
The bentonite slurry is first used to stabilize the trench during construction, followed by backfilling the trench with the soil-bentonite mixture to create the low permeability wall. The most common practice is to base (or “key-in”) the slurry wall into the underlying aquitard or bedrock. Although “hanging” walls or horizontal bottom barriers can be constructed, their use is less frequent. The type of wall and whether it is hanging or keyed into an aquitard will depend on the type and distribution of contamination (such as dense NAPL versus light NAPL). The slurry walls can be made to surround the contaminant source (most common), or to divert the uncontaminated groundwater flow, or divert the flow of contaminated groundwater from a drinking water intake, or divert the flow of contaminated groundwater or soil vapour migration from preferential pathways (i.e. utilities).
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.
The time to install a slurry wall is largely dependent on the size of the contaminated area. It can take from a few days to several months depending on the extent and depth of the slurry wall.
Treatment time might be extremely long even indefinite. As long as a contaminant source remains, the barrier may be required.
The slurry walls can degrade with age and result in leakage of the contaminants. Thus, monitoring of the surrounding groundwater and possibly the vapour emissions, depending on the types of contaminants, are often required.
The slurry walls themselves do not produce secondary by-products.
However, if surface capping is used in conjunction with a slurry wall, there is potential for gas generation within the containment system, for example, from anaerobic conditions that result in CH4 production from degrading organic.
The slurry wall technology does not treat the contamination. Therefore, in situ or ex situ remediation treatments are necessary and must be selected as a function of site and contamination characteristics.
These studies review the performance of slurry walls installed at waste sites:
Slurry walls are a full-scale technology that has been used for decades as mitigation strategy and as a long-term measure for controlling contamination migration. The technology has demonstrated its effectiveness in containing greater than 95% of uncontaminated groundwater (United States Federal Remediation Technologies Roundtable. 2002. Remediation Technologies Screening Matrix and Reference Guide, Version 4.0. Physical Barriers.). However, in contaminated groundwater applications, specific contaminant types may degrade the slurry wall components and reduce the long-term effectiveness.
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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 : Daniel Charette, P.Eng., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.