From: Public Services and Procurement Canada
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.
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:
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.
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.
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.
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.
This technology also applies to radioactive contaminants as well as light and dense liquids in the non-aqueous phase.
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.
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.
The following sites provide application examples:
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.
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
Atmospheric/Steam Emissions—Non-point Sources
Doesn’t apply
N/A
Air/steam—by-products
Runoff
Groundwater—displacement
Modelling the effects of injection and pumping required, and monitoring groundwater migration
Groundwater—chemical/ geochemical mobilization
Monitoring of groundwater migration
Groundwater—by-product
Monitoring water quality
Accident/Failure—damage to public services
File checks and licensing prior to excavation or drilling, development of excavation procedures and emergency response
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 : 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
