Fact sheet: Solidification/stabilization—in situ

From: Public Services and Procurement Canada

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Description

In situ solidification/stabilization systems are used to limit the spread of contaminants in soil and groundwater by stabilizing and/or solidifying the contaminated soil. However, this technology is not intended for the treatment or removal of the contaminated material. As such, these systems are applicable to many types of organic and inorganic contaminants, including non-aqueous phase liquids (NAPL).

Solidification/stabilization systems consist of mixing deep or shallow contaminated soil with stabilizers and/or binding agents such as Portland cement, pozzoland, ash, lime, bentonite clay. Considerations must be made with respect to the compatibility of the contaminants and the materials used. The soil and contaminants will be stabilized chemically (stabilization) and/or bound physically (solidification) which reduces or eliminates leaching of contaminants. These treatments prevent off-site contaminant migration and expansion of the contaminated zone.

Stabilization involves transforming the chemical properties of the contaminants within the soil matrix. The contaminants are transformed into compounds that have lower water solubility, mobility and toxicity. In certain cases, a binding agent may be added to the contaminated material to maximize the stabilization process.

Solidification involves transforming the physical properties of the contaminated soil by the addition of binding agents which compact the matrix, change the pore volume and reduce the hydraulic conductivity. The contaminants are then trapped in the soil and binding agent mixture. Solidification does not actively promote chemical changes in the contaminants.

There are typically two types of in situ solidification/stabilization techniques used:

The first involves mixing a binding/stabilizing agent with contaminated soils using an auger;

The second technique involves the high pressure injection of a solubilized binding/stabilizing agent to force the binder into the contaminated soil matrix, which involves forcing the binder into the soil pore space using high-pressure grout injection pipes.

If volatile elements are present or if there is a potential for the production of gas emissions during the stabilization/solidification process, a gas emissions collection and treatment system may be required.

Internet Link:

Centre for Public Environmental Oversight (CPEO)—Techtree: Stabilization/Solidification—Chemical.

Implementation of the technology

This technology relies on locking the contaminants in low permeability, high strength stabilized and/or solidified monolithic blocks. In situ solidification/stabilization activities may include:

  • mobilization, site access and temporary facilities
  • characterization and delineation of the contaminant source (contaminated soil, waste material, or groundwater plume)
  • mixing of additives or soil amendments to physically, chemically or biologically stabilize soil contamination
  • hydrogeological characterization (depth and thickness of underlying aquitard and groundwater flow pattern)
  • preliminary trials, bench scale or pilot scale tests to get the right mix and formulation
  • treatability studies are generally required
  • monitoring of contamination migration from the solidification/stabilization system

Soil mixing for in situ solidification/stabilization can be accomplished with excavator buckets, rotary drum mixers or augers. Auger mixing provides the highest level of quality control and is the only method capable of stabilizing materials deeper than 4.5 m below the work platform. Dewatering might be required to allow proper soil mixing below water table level.

Soil amendments for in situ stabilization include additives such as Portland cement, pozzoland, bentonite, polymers and may also consist of organics (for example, biosolids, manure and compost), pH control amendments (for example, lime, wood ash, or coal combustion products), or mineral soil amendments (for example, foundry sand, steel slag, cement kiln dust, fly ash and gypsum).

In situ solidification is most commonly performed through the addition of Portland cement alone, or in combination with other additives: blast furnace slag, cement kiln dust, fly ash, bentonite clay and activated carbon to name a few examples. Generally, permeability reduction and strength increase are the most important factors; with their objectives of achieving permeability less than 1x10-6 cm/s and a strength greater than 345 kPa. The solidification process may or may not involve a chemical bonding between the toxic contaminant and the solidification additive. 

This technology may require capping or covering, engineering controls, and/or institutional controls, especially if the solidified material contains radioactive contaminants, where a soil cover sufficiently thick to absorb gamma radiation is required.

Effectiveness of in situ solidification/stabilization relies on the successful implementation during the construction phase. The formulations developed based on treatability tests must be achieved in the field. It is important to ensure that the correct proportions are attained and that sufficient mixing is imparted to the material.

Materials and Storage

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

Storage of materials used may include stockpiles of solidification materials (blast furnace slag, cement kiln dust, fly ash, bentonite clay, activated carbon, etc.) and/or stabilization materials (Portland cement, pozzoland, polymers, biosolids, manure, compost, lime, wood ash, coal combustion products, etc.)

Waste and Discharges

The stabilized contaminated soil is not generally considered as a residual, though it remains in situ. There is potential for liquid and gaseous residuals leaving the in situ solidification/stabilization system, however, the monitoring and management of these residues are required as part of the remediation system design

There is limited potential for windblown dust that can occur from the construction activities, trench excavation or stockpiles, which for example, may also deposit directly on downwind surfaces (stormwater can also be impacted by dust)

Diverted and collected/treated stormwater is typically passed into the local stormwater system

Regular inspection and monitoring is required due to the potential leaks from the solidification/stabilization system

Recommended analyses for detailed characterization

Chemical analysis

  • Organic carbon content
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Soil granulometry
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

None.

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
  • Volume of contaminated material to treat
  • 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:

There are no predefined procedures for this treatment approach. A treatability study is therefore recommended before treatment. Treatability studies provide information on the optimal type and quantity of stabilizers/binders to add to maximize the solidification/stabilization treatment according to the type of contaminated matrix and volume of material to be treated. Treatability studies will also enable to assess if emissions of contaminants during construction work will occur and require mitigation as well as to assess the volume increase that may occur as a result of adding the stabilizing/binding agents.

Applications

The solidification/stabilization technology is applicable to contamination within the saturated and vadose zones. As mentioned earlier, dewatering might be required to allow proper soil mixing below water table level.

The solidification/stabilization technology is generally easier to implement in sandy, silty or gravely soils, than in soils with high clay content as it is easier to achieve a uniformity of mixing in the former, while the latter may tend to leave residual clay balls of unmixed and untreated material.

The target contaminants are generally metals and free radicals. The application of this technology has been successfully performed to treat metals and radioactive materials as well as organic contaminants of concern including non-volatile and semi-volatile compounds such as chlorinated ethenes, petroleum hydrocarbon constituents, polychlorinated biphenyls, pesticides, and dioxins and furans. However, it has not been demonstrated to treat volatile organic compounds. This technology is in constant evolution and may be applicable to a wider range of contaminants in the future.

Applications to sites in northern regions

Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short seasonal work windows

The use of certain additives (surfactants for example) in conjunction with Portland cement as a stabilizing/solidifying agent has been demonstrated to help resist the negative impacts of the freeze/thaw cycles

Frequent rain and freeze/thaw cycles may reduce the lifetime of the stabilized/solidified material and may increase contaminant mobilization

Enforcement of institutional controls, if required, and long-term monitoring may be challenging and costly. Telemetry can be used for remote monitoring of site conditions

Institutional controls are frequently very applicable to remote northern sites provided that the underlying risk assessment accounts for northern lifestyles, cultures and unique ecological systems

Treatment type

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

State of technology

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

Target contaminants

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

Notes:

For Phenolic compounds, applies to pentachlorophenol (PCP) only.

Treatment time

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

Notes:

The estimated treatment time is largely dependent on the size of the contaminated area. It can take from a few days to several months depending on numerous site-specific factors, including:

  • depth and volume of contamination requiring treatment
  • contaminant distribution
  • mixing method
  • subsurface soil characteristics including particle size distribution and density
  • quantity of mixing equipment available on-site;

    diameter of auger (in case of auger mixing)

    presence of subsurface utilities or debris

    weather conditions

Long-term considerations (following remediation work)

Long term monitoring of in situ solidification-stabilization system may be required to assess its effectiveness

Long term groundwater and vapor monitoring should be performed to confirm the integrity of the solidification/stabilization system over time

Site use may be modified or restrictions may apply (for example, planting deep-rooted trees or other subsurface activities would require approval). It may be warranted to place signs to indicate the solidified/stabilized area

Secondary by-products and/or metabolites

There is no by-product production during in situ solidification/stabilization. Contaminants are not degraded by the treatment and are still present within the contaminated site after treatment. The contaminants are immobilized within the matrix and are not inclined to migrate as long as the integrity of the solidified/stabilized matrix is maintained.

Limitations and Undesirable Effects of the Technology

  • Solidification/stabilization technology is a remediation technique which only controls contaminant migration
  • Performance of solidification/stabilization technologies depends upon the site characteristics and cannot be estimated
  • Depth of contaminants may limit some types of application processes. Auger mixing is limited to depths less than 30 metres. Costs can be become prohibitive at depths greater than 18 metres
  • Mixing concrete with used oil and tar might cancel the cement hydration process, which will require the addition of an agent to counter the effect. There will be a potential for heat to be produced which will increase volatilization of certain contaminants (gas emissions)
  • The stabilized/solidified material may be susceptible to leaching over time
  • This technique applies to contaminated soils from the saturated and vadose zones. Thus, dewatering may be required, which can add to overall project cost
  • Mineral salts, strong acids or strong bases within the contaminated matrix may interfere with the solidification/stabilization process
  • The presence of large rocks or debris within the matrix to treat may interfere with the solidification/stabilization process
  • It can be difficult to formulate an effective binder for heterogeneous waste mixtures
  • Soil heterogeneity can limit the depth of application of the solidification/stabilization technique
  • Soil amendments used for in situ stabilization can also affect the pH and soil organic content
  • Binder injection and mixing must be controlled to minimize the spread of contaminants to clean areas
  • Groundwater and vapor monitoring are part of the solidification/stabilization system design
  • Site use may be modified or restrictions may apply (for example, planting deep-rooted trees or other subsurface activities would require approval). It may be warranted to place signs to indicate the solidified/stabilized area
  • Weathering of the stabilized media may occur, resulting in reduced capacity to maintain the materials immobilized
  • In situ solidification/stabilization reduces water infiltration from surface run-off and precipitation through the soil
  • Some processes result in a significant volume increase (up to double the original)
  • The depth of contaminants may limit some types of application: reagent delivery and effective mixing are difficult and confirmatory sampling can be more difficult
  • Volatile organic compounds generally are not immobilized
  • These processes may not be effective on some organics (for example, semi-volatile organic compounds and pesticides) that can inhibit the chemical bonding of stabilizers or the mechanical bonding of solidifying agents
  • Certain wastes are incompatible with variations of this process. Treatability studies are generally required
  • The solidified material may hinder future site use

Complementary technologies that improve treatment effectiveness

  • Physical separation to remove the uncontaminated soil particle size fraction(s) will reduce the volume of material to be treated
  • Addition of specific agents can maximize the stabilization and solidification of the treated matrix
  • A vacuum system to collect gas emissions may be required when the treatment enhances volatilization of contaminants

Required secondary treatments

Groundwater must be monitored over the long term to prevent any leaching of contaminants from the treated areas

Collected gas emissions may require treatment before being exhausted into the atmosphere

Application examples

Application examples are available at these links:

Unites States Environmental Protection Agency. 2000. Innovative Remediation Technologies: Field-Scale Demonstration Projects in North America, 2nd Edition, Year 2000 Report. EPA 542-B-00-004.

Cement Association of Canada. Remediation Technology:  Solidification/Stabilization.

Bates, E. and Hills C. 2015. Stabilization and Solidification of Contaminated Soil and Waste:  A Manual of Practice. Hygge Media.

Performance

The performance of several stabilizers and binding agents is well documented, having been proven to prevent contaminant migration while being easy to apply at low cost.

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 (for example, cement kiln dust from cement manufacturing)

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

Composed by : Martin Désilets, B.Sc., National Research Council

Updated by : Martin Désilets, B.Sc., National Research Council

Updated Date : April 1, 2008

Latest update provided by : Daniel Charette, P.Eng., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017

Version:
1.2.1