Fact sheet: Soil mixing—chemical process—in situ

From: Public Services and Procurement Canada

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Description

Soil mixing is an in situ soil remediation technique that consists of the introduction of a mixing equipment into contaminated soils, for mixing and the injection of a treatment solution. The treatment solution may contain a variety of amendments, depending on the type of contamination and the objectives of the treatment. It may be portland cement or pozzolan, organic binders (bitumen, polyethylene, paraffin and other polyolefins), biological treatment agents (compounds with progressive release of oxygen, nutrients and microorganisms), chemical oxidants or zero-valent iron. These amendments are generally injected into the soil in the form of a liquid sludge but can also be mixed with the soil in solid form using a mixing equipment such as an auger which also serves to homogenize the soil mixture/amendments. Steam can also be injected at the time of mixing.

In situ soil mixing with chemical processes is generally combined with other treatment techniques, such as steam extraction with air injection (ambient or hot), solidification and stabilization, chemical reduction (zero-valent iron) and/or chemical oxidation.

Sources:

Implementation of the technology

The implementation of soil mixing involves the following steps:

  • Mobilization and access to the site.
  • Installation of equipment for the preparation of amendments.
  • The creation of treatment zones with a diameter ranging from 0.9 to 10 m allowing soil mixing up to depths of 18 m.
  • The installation of the facilities necessary for the complementary treatment technique.
  • The demobilization of the site at the end of the remediation work.

Materials and Storage

Soil mixing is carried out using specialized machinery that may require specific installation conditions.

On-site storage is generally limited to quantities of fuel and lubricant or site supplies during the time of the soil mixing work.

Amendments, which depend on the contaminant to be treated, can also be stored on the site.

If complementary technology is used, materials and storage related to this technology may be left on-site.

Residues and Discharges

Soil mixing produces little residue and no discharge. However, the selected complementary technology may produce residues and/or discharges.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Alkalinity
  • Organic matter content
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

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

Recommended trials for detailed characterization

Chemical trials

  • Evaluation of the matrix oxidant demand

Notes:

On-site treatment trials will establish the efficiency of the technology and the parameters that influence the treatment time and cost (e.g. residence time, pump flow rate, requirements for pre-treatment, etc.).

Physical trials

  • Vapour survey
  • Evaluation of optimal mixing rates

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:

Treatability testing is recommended to evaluate the effectiveness of potential amendments and to determine the optimum dosage. In addition, it may be necessary to validate the stability of the mixing blend for several chemical and physical parameters such as hydraulic conductivity, leaching potential, unconfined compression, etc.

Applications

  • Applicable for all soil types including soils with low permeability or low hydraulic conductivity, such as clays and silt as well as marine or lacustrine sediments.

Applications to sites in northern regions

  • Remote sites require greater mobilization and higher on-site monitoring costs.
  • Even mixing of the soil can be difficult if the frost has penetrated deeply into the soil to be treated.
  • Conditions in northern regions could also limit the implementation of complementary technologies to soil mixing.

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
Applies

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
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
With restrictions

Notes:

Note:

Stabilization / solidification can be used as a complementary technology for the treatment of polychlorinated biphenyls.

Treatment time

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

Long-term considerations (following remediation work)

Depending on the type of contamination, amendment and complementary technology selected, follow-up or re-sampling of soils may be required to evaluate treatment effectiveness and need for further amendments or additional complementary treatment.

Secondary by-products and/or metabolites

Soil mixing technology enhances the volatilization of volatile and semi-volatile organic contaminants. Depending on the complementary technology selected, secondary products can be generated (chemical oxidation can generate secondary products, depending on the chosen oxidizing agent and the contaminants present).

Limitations and Undesirable Effects of the Technology

  • Requires surface access at all locations where the soil is contaminated. Soil contamination under buildings or other infrastructure cannot be addressed by in situ soil mixing.
  • The efficiency of the technology is reduced when the contamination is located in permeable soils located under the water table.
  • Soil mixing efficiency is reduced as the depth of contamination increases.
  • Dense soil with large rocks or boulders will reduce the soil mixing capacity.
  • High concentrations of some contaminants may limit the binding process or be toxic to microorganisms.

Complementary technologies that improve treatment effectiveness

Based on the characteristics of the site, soil mixing technology could be combined with various in situ remediation technologies. The most common technologies combined with soil mixing are:

  • in situ chemical oxidation;
  • injection of air or hot air with a vapour extraction system;
  • injection of hot water or steam with a vapour extraction system;
  • solidification or stabilization of soils.

Required secondary treatments

The production of secondary products, depending on the secondary technology selected, may require the establishment of a secondary treatment unit.

Application examples

The following sites provide application examples:

Performance

Soil mixing combined with a base-catalyzed sodium persulphate treatment was used on xylene and pesticide impacted soils on a site in Robbinsville, New Jersey. A total of 2,100 m3 of soils was treated, down to a 4.6 m below ground surface (Deep Foundations Institute, 2014)

On another project in East Rutherford, New Jersey, soil mixing combined with potassium permanganate oxidation was used to treat 5,700 m3 of TCE impacted soils down to a depth of 5.8 m (Deep Foundations Institute, 2014).

During a demonstration at the Portsmouth Gaseous Diffusion Plant, U.S.A., soil mixing technology combined with in situ hydrogen peroxide treatment was found to treat approximately 72% of the volatile organic compounds’ mass in 75 min. of operation. The treatment was applied at depths down to 5 m (U.S. Department of Energy, 1996).

In another study at a Kansas City Plant, U.S.A., soil mixing combined with in situ potassium permanganate oxidation was able to achieve 83% removal of trichloroethylene in unsaturated soil, and 69% removal of trichloroethylene in saturated soil over a 2-day period (U.S. EPA, 1998).

Measures to improve sustainability or promote ecological remediation

  • Use of energy-efficient equipment.
  • Depending on the complementary technology selected, other means may apply.
  • Select locally produced amendments

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Doesn’t Apply

Monitoring and Mitigation

Dust

Applies

Emissions monitoring at the source (choice of parameters, types of samples and type of intervention [source, risk or local requirements]).

Atmospheric/Steam Emissions—Point Sources or Chimneys

Doesn’t apply

N/A

Atmospheric/Steam Emissions—Non-point Sources

Applies

Emissions monitoring at the source (choice of parameters, types of samples and type of intervention [source, risk or local requirements]).

Air/steam—by-products

Applies

Emissions monitoring at the source, evaluation of potential risks associated to by-products, development of intervention plan.

Runoff

Applies

When adding amendments, minimize water production (choice of parameters, types of samples and type of intervention [depending on source, risk or local requirements]).

Groundwater—displacement

Doesn’t apply

N/A

Groundwater—chemical/ geochemical mobilization

Applies

The addition of amendments may affect groundwater. Downstream monitoring of the site may be required (choice of parameters, types of samples and type of intervention [depending on source, risk or local requirements]).

Groundwater—by-product

Doesn’t apply

N/A

Accident/Failure—damage to public services

Applies

Record checks and prior permits for soil mixing, development of drilling and emergency procedures.

Accident/Failure—leak or spill

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.

Accident/Failure—fire or explosion

Doesn’t apply

N/A

Other

Doesn’t apply

N/A

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 : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1