Fact sheet: Soil washing, leaching, or chemical extraction—in situ

From: Public Services and Procurement Canada

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Description

In situ soil washing, also known as soil leaching or chemical extraction, includes in situ soil remediation technologies that use a washing solution (e.g. solvents, acids, chelating agents, polymers) to mobilize organic or inorganic contaminants towards a groundwater recovery system.

In situ soil washing involves injecting the washing solution into the saturated zone upgradient of the contamination and pumping it downgradient, in order to facilitate its migration through the contaminated area and allow the recovery of the washing solution with the contaminants. When the contamination is localized in the vadose zone, the washing solution can be injected directly from the soil surface above the contaminated area. Soil washing is often used to complete the treatment of groundwater or to improve the performance of conventional pumping and groundwater treatment techniques.

Washing solutions may include water, surfactants, cosolvents, acids, bases, chemical oxidants, chelating agents or organic solvents. Surfactants and cosolvents, or a mixture thereof, are the most commonly used. The selection of the washing solution and the efficiency of this solution depend on the physical properties of the contaminants present, the groundwater geochemistry and the nature of the soils in place.

Sources:

Implementation of the technology

In situ soil washing requires the installation of several elements allowing the injection and recovery of the extraction solution. Thus, wells or trenches must be arranged upstream of the contamination zone to allow the injection of the washing solution by gravity or pressure. The equipment necessary for this injection (tanks, pumps, piping, etc.) must also be set up at proximity. Wells or trenches must also be built downstream of the contaminated area in order to pump underground water and recover the solution that has migrated through the contamination. Pumping equipment and treatment of pumped water will be set up at proximity. In some cases, the extraction solution is separated from the extracted and reused water while in others, the water is simply treated and discarded.

The location, depth and quantity of injection and extraction wells depend on geological factors and engineering considerations. Some equipment, such as the treatment system, will have to be transported and built on the site.

The characteristics of the contaminants as well as the properties of the soil to be treated must be known in order to determine the type and concentration of the optimal extraction solution.

The implementation of an in situ soil washing treatment system may include:

  • Site preparation for the installation of equipment.
  • The installation of wells, trenches or injection and extraction drains.
  • Implementation of systems for mixing, maintaining and distributing the washing solution.
  • Construction of the containment (physical or hydraulic) or extraction system.
  • Implementation of the surface water treatment system (separation of the washing solution and treatment of groundwater).
  • The discharge of treated water.

Materials and Storage

  • Injection and extraction wells are constructed using traditional/standard methods and equipment readily available for well installation, drainage, or utility work.
  • The treatment system can be built on-site or pre-assembled and delivered to the site inside a container, trailer or truck.
  • The operation of the treatment system requires energy and products such as washing solutions, water, etc.
  • Development activities for this type of system generally have little impact but may require on-site storage, including the washing solution, the residues produced (treated soil, treated groundwater, etc.), the treatment unit and the chemicals associated with the treatment, if applicable.

Residues and Discharges

Pumped and treated groundwater must meet the applicable criteria for its discharge.

Contaminated fumes or vapours resulting from the process wastewater treatment are collected and treated.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic carbon content
  • Cation exchange capacity (CEC)
  • Tessier's sequential extraction for metals
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Contaminant adsorption coefficient
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
    • etc.
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Chemical trials

  • Laboratory adsorption testing
  • Soil washing/flushing trials

Physical trials

  • Evaluation of operating pressure/vacuum

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
  • Volume of contaminated material to treat
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient
  • Hydrogeological modelling

Notes:

Laboratory tests are required to determine the parameters that influence the washing reaction (washing solution concentration and composition), the treatment time, and the treatment cost.

Hydraulic studies are also necessary to obtain a reliable recovery system to ensure that all of the injected washing solution and contaminants are recovered.

Applications

  • In situ soil washing applies to the treatment of dissolved and residual contamination as well as non-aqueous phase liquids (light and dense).
  • Allows treatment of residual contamination within the vadose and saturated zone.
  • Suitable for a wide range of organic and inorganic contaminants such as volatile and semi-volatile organic compounds, polychlorinated biphenyls, pesticides, and heavy metals.
  • Can treat organic and inorganic compounds simultaneously.
  • The washing solution remaining in the soil after the remediation treatment may improve the solubility and bioavailability of contaminants for eventual bioremediation treatment.
  • Soil washing is efficient in permeable sandy or gravelly and homogeneous soils.

Applications to sites in northern regions

The use of the technology in northern regions may be limited, as the injection and extraction of water and washing solutions in the wells could be affected by temperature. A low temperature limits the degradation of contaminants, which could influence the efficiency of the treatment system. There could also be equipment breakages. Storage of chemicals, including surfactants, may be affected by low temperatures.

In addition, remote sites require greater mobilization, resulting in higher on-site monitoring costs. Equipment availability is limited and work windows are relatively short.

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
Does not exist
Dissolved contamination
Applies
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
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

Notes:

Notes:

This technology also applies to radioactive contaminants as well as light and dense liquids in the non-aqueous phase.

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)

At the end of the treatment, the residual contamination in the washing solution must be evaluated, treated and disposed appropriately, according to the regulations in place.

Secondary by-products and/or metabolites

Generally, soil washing doesn’t produce by-products. However, these techniques may create anaerobic conditions within a soil matrix. Under these conditions, some compounds are transformed into toxic substances. For example, under acidic and anaerobic conditions, arsenic can form a very toxic gas, arsine.

Limitations and Undesirable Effects of the Technology

  • Only applicable for permeable and homogeneous soils; fine soil particles such as clay or silt should not exceed 20% of the total mass of the soil.
  • High organic matter content reduces the performance of soil washing techniques.
  • The depth of the contamination can significantly increase the costs of using this technology.
  • The hydraulic conductivity must be higher than 10-3 cm/sec. In particular cases, this technology may be applicable in soils with a hydraulic conductivity ranging from 10-3 to 10-5 cm/sec.
  • Geological heterogeneity and underground infrastructure can encourage movement of the extraction solution through preferential pathways.
  • May produce large volumes of contaminated solution and water.
  • Lack or misinterpretation of hydrogeological data and migration preferential pathways may result in unwanted contaminant(s) migration during and after the soil washing treatment.
  • At the end of the treatment, the presence of residual components in the extraction solution may be unacceptable to site remediation regulations.
  • Contaminated soils located in the vadose zone can be difficult and potentially impossible to treat.
  • Consumption of large volumes of water.
  • The pumped washing solution may be difficult and complex to separate to recover the surfactant.

Complementary technologies that improve treatment effectiveness

  • Soil washing can be used in conjunction with many other in situ soil remediation technologies, such as biostimulation.
  • A closed-loop recirculation system (pumping/injection) can reduce the volume of extraction solution required.

Required secondary treatments

  • Groundwater must be pumped and treated above ground to eliminate all of the washing solution and dissolved or emulsified contaminants.
  • When possible, the active compounds of the extraction solution may be recycled (example: surfactant, alcohol). In other cases, they may be neutralized or separated before disposing of the treated groundwater.

Application examples

The following sites provide application examples:

Performance

In situ soil washing techniques can be successfully applied to a wide range of contaminants. Several private companies offer in situ soil washing technologies. Each of these companies has their own variation of the technology and their own type of solvents. Some offer systems that can be fully automated.

The effectiveness of contaminant removal will depend on the contaminant and the type of soil as well as the geochemistry of the aquifer. Halogenated, semi-volatile, non-halogenated volatiles and non-volatile metals are among the groups of chemical compounds successfully treated by this technology.

Vadose treatment efficiency is limited because of the difficulty of obtaining a direct contact between the soils and the washing solution.

Measures to improve sustainability or promote ecological remediation

  • Use of renewable energy and low-energy equipment.
  • Optimizing equipment selection based on-site conditions to reduce equipment size and energy consumption.
  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Using telemetry for remote monitoring of site conditions and limiting the number of visits.
  • Process optimization to reduce waste and consumables and recycling of the washing solution.
  • Optimization of the water consumption of the treatment system.
  • Using a closed-loop pump-injection system to limit the volume of washing solution required.

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 (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Atmospheric/Steam Emissions—Point Sources or Chimneys

Applies

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

Atmospheric/Steam Emissions—Non-point Sources

Doesn’t apply

N/A

Air/steam—by-products

Applies

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

Runoff

Doesn’t apply

N/A

Groundwater—displacement

Applies

Modelling the effects of injection and pumping required, and monitoring groundwater migration

Groundwater—chemical/ geochemical mobilization

Applies

Monitoring of groundwater migration

Groundwater—by-product

Applies

Monitoring water quality

Accident/Failure—damage to public services

Applies

File checks and licensing prior to excavation or drilling, development of excavation procedures and emergency response

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

Applies

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

Other

Doesn’t apply

N/A

References

Author and update

Composed by : Josée Thibodeau, M.Sc, National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : March 27, 2013

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