From: Public Services and Procurement Canada
Soil mixing (biological process) is a remediation technology used for the treatment of contaminated surface soils (ideally less than 4 metres below the ground surface) impacted with organic compounds. This technology is mainly applicable to soil in the unsaturated zone.
This technology consists of periodic mixing of the contaminated soils to provide the required amendments to support and stimulate the biodegradation processes of organic contaminants. Volatilization can occur during the mixing process, although this is not the objective of the technology.
To promote the biodegradation of contaminants, the addition of amendments, fertilizers or structuring agents to stimulate microbial activity (nutrients), improve aeration, adjust pH and/or control soil water content may be necessary. These amendments are generally injected into the soil in the form of a slurry but can also be mixed into the soil in solid form using augers, which also serve to homogenize the soil/amendment mixture.
Modifications of this technology can be applied, such as the installation of air injection wells to provide an additional, constant supply of air to aerobic microorganisms.
Soil mixing can be combined with other remediation technologies, such as steam extraction with air injection (ambient or hot), solidification and stabilization, chemical reduction (zerovalent iron), and/or chemical oxidation.
Internet links:
Implementation of this technology may include:
On-site storage may include fuels, lubricants and other site materials required for operating the machinery and equipment for the implementation of the technology.
Soil mixing is carried out using specialized machinery that may require special installation conditions.
Depending on the type of contaminant to be treated, amendments may also be stored on-site. If a complementary technology is used, materials and storage related to this technology can be stored on-site.
When vapour and/or water treatment components and treatment systems are incorporated into the technology or if water is to be contained in watertight containers, they may also be stored on-site.
Soil mixing (a biological process) produces little waste and no discharge in general. However, waste could be generated if soil mixing is combined with a complementary technology.
Excess soil may be generated depending on the mass and volume of amendments added and the final slope and compaction requirements.
Vapour discharge may be released from equipment exhaust or the volatilization of contaminants present in the soils. Workers who find themselves in the potential presence of odours or volatile compounds must take precautions to prevent gas emanations by monitoring concentrations, using adequate personal protective equipment and/or by conducting soil mixing work in colder temperatures. On some sites, soil dewatering may be necessary depending on mixing depth, hydraulic conductivity and the type and extent of contaminants and reagents/amendments. Dewatering water may need to be treated before discharge into the environment or collected in containers and disposed of off-site.
Notes:
Depending on the selected amendments and complementary technologies, it may be necessary to evaluate the impact of products on several chemical and physical parameters, such as hydraulic conductivity, leaching potential, load-bearing capacity, etc.
The technology is particularly well suited when the site has a small surface area and the targeted soils are superficial and easy to reach. Soil mixing is generally used for organic contaminants in low-permeability soils where in situ bioremediation cannot be applied. For the technology to be effective, soil conditions must meet the following criteria:
Monocyclic Aromatic Compounds: the technology applies with restrictions as they are mostly volatilized before being biodegraded.
Soil mixing operations have a potential impact on wildlife and aquatic habitats, as well as on the organisms present, especially on species at risk. Documentation of recovery may require long-term monitoring of the site.
Depending on the contaminants, amendments and complementary technologies in place, a monitoring program, including soil sampling, may be required to assess the treatment’s performance, evaluate the need to add amendments, and ensure the performance of complementary treatments, if applicable.
Long-term performance surveillance can include groundwater quality monitoring by installing monitoring wells downgradient from the treatment area to ensure the absence of contamination lixiviation and the ongoing performance success of the technology. Long-term monitoring can also include an assessment of the physical integrity of the disturbed area and the preservation of its geotechnical properties over time. The integrity of the disturbed soil zone can also be validated by long-term monitoring of gaseous emissions. In addition, long-term monitoring may require periodic respirometry tests to measure the evolution of in situ biodegradation rates over time.
Generally, biodegradation of organic contaminants does not generate any deleterious secondary by-products or metabolites. For example, the biodegradation of petroleum hydrocarbons generates harmless products for the environment and human health, such as carbon dioxide and water.
However, toxic intermediate degradation by-products may be generated depending on the site conditions and the contaminant characteristics. Furthermore, the biodegradation processes may lead to changes in the geochemical conditions and increase the mobility of some chemical parameters.
It should be noted that mixing contaminated soils promotes the volatilization of volatile and semi-volatile organic compounds.
Additionally, if a complementary technology is selected, by-products can be generated.
Soil mixing (biological process) can be combined with different in situ soil treatment technologies depending on the site’s specific characteristics and contamination. The following technologies are the most commonly applied simultaneously with soil mixing:
Application examples are available at these links:
The treatment time required for site remediation with soil mixing (biological process) varies according to the type of contaminants and their concentrations (and the required reduction rate), the density and type of native microbial populations, and the physical and chemical properties of the soil.
Monitoring during remediation should be carried out after each soil mixing operation. This will allow assessment of the effectiveness of the soil mixing in meeting the remediation project’s requirements and objectives.
The minor and major potential human health exposure pathways are presented in the following table.
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site and Off-Site)
Monitoring
Mitigation Measures according to Residency or Transport Media
Site preparation
Stockpiling, loading, and unloading of amendments
Soil mixing
Dust
Inhalation of Dust
Dust Monitoring
Vapours
Contaminated soils
Surface water
Groundwater
Animals and Plants
Inhalation of Vapours
Air Quality Monitoring
Animals and Plants (including fish, shellfish, and wildlife)
Country Foods Consumption
Surface Water Monitoring
Animal and Plant Tissue Monitoring
Groundwater (migration/leaching of contaminants and amendments)
Ingestion of Drinking Water
Groundwater Monitoring
Surface water (amendment run-off)
Dermal Contact
Incidental Ingestion
Contaminated Soils
Composed by : Magalie Turgeon and Karine Drouin, M.Sc. National Research Council Canada.
Latest update provided by : Frédérick de Oliveira, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.
Latest update date :March 31, 2024
