From: Public Services and Procurement Canada
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.
The implementation of soil mixing involves the following steps:
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.
Soil mixing produces little residue and no discharge. However, the selected complementary technology may produce residues and/or discharges.
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.).
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.
Note:
Stabilization / solidification can be used as a complementary technology for the treatment of polychlorinated biphenyls.
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.
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).
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:
The production of secondary products, depending on the secondary technology selected, may require the establishment of a secondary treatment unit.
The following sites provide application examples:
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).
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
Air/steam—by-products
Emissions monitoring at the source, evaluation of potential risks associated to by-products, development of intervention plan.
Runoff
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
Groundwater—chemical/ geochemical mobilization
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
Accident/Failure—damage to public services
Record checks and prior permits for soil mixing, development of drilling and emergency procedures.
Accident/Failure—leak or spill
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions.
Accident/Failure—fire or explosion
Other
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
