Fact sheet: Impermeable/slurry walls

From: Public Services and Procurement Canada

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Description

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.

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Implementation of the technology

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:

  • Mobilization, site access and temporary facilities;
  • Characterization and delineation of the contaminant source (contaminated soil, waste material or groundwater plume);
  • Hydrogeological characterization (such as depth and thickness of underlying aquitard and groundwater flow pattern);
  • Bench-scale or laboratory trials for testing of the compatibility of slurry mixture or other barrier wall materials with the contaminants (in other words, chemical resistance of barriers to contaminants), permeability to water and/or soil vapour and other applicable physical and chemical properties;
  • Construction of low-permeability barrier wall to contain or redirect groundwater flow.
  • Installation of groundwater pumping or soil vapour venting wells or sumps as part of the overall containment strategy;
  • Construction of capping layers and potentially vapour ventilation and/or leachate or water collection systems;
  • Monitoring.

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).

Materials and Storage

  • Method relies on traditional/commonly available civil/earthworks construction equipment and methods for the construction of impermeable walls. Chemicals used in slurry walls include additives such as cement, bentonite and polymers. For capping, materials used include asphalt, concrete, geomembranes, mineral fill and rock, clay and/or vegetated topsoil;
  • Storage of materials used may include stockpiles of capping materials (mineral fill, clay, geomembrane, etc.), slurry and solidification materials (cement, bentonite, etc.); and stabilization materials (such as flyash, cement kiln dust, etc.). On-site storage is typically limited to small amounts of fuel and lubricant (daily fuelling is typically from a mobile tank) as well as miscellaneous construction site supplies.

Waste and Discharges

  • The contained NAPL, soil and/or groundwater are not generally considered as residuals, though they remain in situ;
  • There is potential for liquid and gaseous residuals leaving the containment/capping system. The monitoring and management of these residuals is required as part of the remediation system design;
  • There is limited potential for windblown dust that can occur from trench excavation or stockpiles, which for example, may also deposit directly on downwind surfaces and stormwater;   
  • Diverted and collected/treated stormwater is typically passed into the local stormwater system;
  • Depending on the contaminant type, there is potential for the generation of volatile organic compounds (VOCs) in soil vapour from volatilization of contamination and off-gases (such as, carbon dioxide [CO2], methane [CH4], hydrogen sulphide [H2S] and potentially others reduced sulphur compounds from degradation of organic) in the containment area. The collection and treatment of off-gases or vapours must be part of the system design.  

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Conductivity
  • Dissolved salt concentration in water
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Soil granulometry

Recommended trials for detailed characterization

Hydrogeological trials

  • Permeability test
  • Pumping trials
  • Tracer tests

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.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Notes:

Notes:

  • Laboratory trials are recommended to evaluate compatibility of the contaminants in contact with slurry wall materials or to test new materials.
  • Hydrogeological modelling is recommended to model the groundwater flow before and after the installation of the impermeable wall considering variations in climate.

Applications

  • Applicable to inorganic and organic contaminants;
  • Applicable for dissolved and NAPL contamination;
  • For moderate or long-term application.

Applications to sites in northern regions

  • Impermeable/slurry walls have significant advantages over conventional groundwater treatment systems in remote and northern sites without access to utilities or local operations and maintenance labour;
  • Initial construction may be problematic due to high mobilization costs, limited local construction capacity and short work windows;
  • Northern systems require climate-appropriate design, including consideration of deep freezing, permafrost, spring melt and frost heave;
  • This technology can be used in isolated regions without services or electricity.

Treatment type

Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Does not exist
Control
Applies
Dissolved contamination
Applies
Free Phase
Applies
Physical
Applies
Residual contamination
Does not exist
Resorption
Does not exist
Thermal
Does not exist

State of technology

State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Applies
Metals
Applies
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Applies
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

Treatment time

Treatment timeApplies or Does not apply
Less than 1 year
Does not apply
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Applies

Notes:

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.

Long-term considerations (following remediation work)

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.

Secondary by-products and/or metabolites

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.   

Limitations and Undesirable Effects of the Technology

  • Implementing impermeable walls requires the use of heavy equipment and machinery;
  • The technology only contains contaminants within a specific area; it does not treat or destroy them;
  • The costs of implementing impermeable walls increase with the depth of contamination;
  • The soil-bentonite slurry is susceptible to strong acids, bases, salt solutions and some organic chemicals;
  • There is the potential for the slurry walls to degrade or deteriorate over time;
  • Long-term monitoring of groundwater in the area to identify potential leaks in the slurry wall is required and also serves to assess its effectiveness;
  • Anaerobic conditions may arise from reduced oxygen environments depending on the type of contamination and capping materials used. This would affect the pH and redox conditions within the containment/capping system;  
  • The low permeability slurry walls will affect the local groundwater flow. This leads to refractive flow, mounding, diving and a range of other behaviours. Designers seek to predict and exploit these changes in groundwater flow, with varying levels of success; 
  • Capping may be required to reduce water infiltration from surface run-off and precipitation through the soil;
  • There is potential for the migration of contaminated water or vapours from the contained area if there is damage to the capping material (such as erosion, cracks, or holes, or problems with the vegetation);
  • Site use restrictions (such as planting deep-rooted trees or other subsurface activities would require approval). It may be warranted to place signs to indicate limits of capped or slurry wall areas.

Complementary technologies that improve treatment effectiveness

  • Surface capping is often used in conjunction with slurry walls to prevent surface water infiltration;
  • Other technologies such as in situ permeable wall technologies can be used to treat contamination;
  • Upstream pumping wells can be added to improve the confinement.

Required secondary treatments

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.

Application examples

These studies review the performance of slurry walls installed at waste sites:

Performance

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.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and energy-efficient machinery for construction of the system;
  • Schedule optimization for resource sharing and fewer days of mobilization;
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions;
  • Consideration of locally available and/or recycled materials in the design;
  • Use of waste or by-products of industrial processes, if appropriate, as additives or amendments (e.g. cement kiln dust from cement manufacturing);
  • If capping system is used, consider capping systems that use soil amendments and vegetation to reduce infiltration and contaminant migration; 
  • If capping system is used and management of vapours is required, consider design of passive soil vapour venting systems that utilize high permeability venting materials and appropriately sized wind turbines or fans.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

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.

Updated Date : April 30, 2014

Date Modified: