Fact sheet: Frozen walls

From: Public Services and Procurement Canada

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Description

The principle of the frozen barrier is to artificially freeze the water contained in the pores of the soil. Its freeze causes a decrease in permeability and can form an impermeable barrier. This barrier prevents the migration of contaminants.

To do this, a piping system consisting of refrigerating tubes which circulates a refrigerant, such as liquid carbon dioxide, is installed in the area where the migration of contaminants must be limited. The tubes are installed in boreholes, downstream from the contaminated area and perpendicular to the direction of groundwater flow, to prevent the migration of contaminants. These freezing boreholes are called thermoprobes, freezing probes or freezing wells.

This system permits chilling of the soil matrix to temperatures ranging from -20 °C to -40 °C. By thermal conduction, the groundwater in soil pores surrounding the refrigerant tubes freezes to a thickness of up to 20 m. The aquifer is thereby frozen, and acts as an impermeable wall containing the dissolved contamination. This type of wall can be installed without excavation and down to depths of 300 m. Soil freezing can also be achieved by using a thermosiphon (a passive heat transfer device used for permafrost stabilization) if the ambient temperatures are sufficiently cold.

Sources:

Implementation of the technology

A frozen barrier involves the installation of wells and piping that allow refrigerant circulation within freezing probes installed through the soil, in order to freeze the water in the soil and produce a barrier.

The implementation of such a system may include:

  • Mobilization, access to the site and setting up temporary facilities.
  • Drilling and installation of freezing probes and piping for the refrigeration system.
  • The installation of temperature probes in soils.
  • The installation of an insulated and impermeable surface or membrane on the surface of the targeted area.
  • The layout of the refrigeration unit and its connection to the piping.
  • The performance of pressure tests in the pipes and the injection of the refrigerant.

Materials and Storage

  • The installation of equipment is carried out using traditional / current equipment readily available.
  • The refrigeration unit must be pre-assembled and transported to the site in a container or on a closed trailer.
  • Operation of the refrigeration unit requires energy and must include an emergency power source.

Residues and Discharges

The installation and the operation of a frozen barrier generate few residues and discharges with the exception of drill cuttings resulting from the development of probes or wells for the implementation of piping.

Recommended analyses for detailed characterization

Physical analysis

  • Soil water content
  • Soil granulometry
  • Soil thermal conductivity
  • Air temperature
  • Contaminant freezing point

Recommended trials for detailed characterization

Hydrogeological 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
  • Regional climatic conditions (precipitation, temperature, etc.)

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • 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:

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.

Applications

  • Allows the containment and limitation of the migration of contamination in the saturated zone, but must be treated in conjunction with a groundwater collection system.
  • Allows the containment of contaminated groundwater under and around reservoirs and infrastructures.

Applications to sites in northern regions

The establishment of a frozen barrier is possible in northern regions but requires a good power supply and constant monitoring of the ground temperatures to validate the integrity of the barrier. This type of environment favours the use of thermosiphon passively because of the cold temperatures of the ambient air. These devices do not require power supply.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Does not exist
Control
Applies
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Does not exist
Resorption
Does not exist
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:

Allows control of the dissolved contamination only.

Treatment time

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

Notes:

Allows control of the dissolved contamination only.

Long-term considerations (following remediation work)

During barrier operation, monitoring of ground temperatures and groundwater levels are required upstream and in barrier position. In addition, monitoring the quality of groundwater downstream of the barrier must also be done.

System maintenance and the presence of a back-up power source in case of power failure are also recommended especially if the contamination is very mobile.

Secondary by-products and/or metabolites

The frozen barrier does not produce any by-products since this technology only allows the control of contaminant migration rather than the treatment of the contamination.

Limitations and Undesirable Effects of the Technology

  • Risk of the leak of refrigerant fluid in the environment is possible.
  • Very porous and dry soils could limit the instalment of a frozen barrier.
  • Complex hydrogeological conditions and the presence of underground infrastructures could limit the integrity of the barrier.
  • High energy consumption.
  • In cases where the contaminant plume is highly mobile, or the site is remote, additional measures must be implemented to ensure that an adequate and uninterrupted power supply is available.
  • Cryogenic materials that produce temperatures lower than -150 °C may become necessary in the case of an area contaminated with certain compounds (example: trichlorethylene). The utilization of these materials increases the cost of application of the technology.

Complementary technologies that improve treatment effectiveness

A frozen barrier does not treat groundwater, but only limit its migration. Thus, the dissolved contamination upstream of the barrier must be pumped and treated by a groundwater abstraction and treatment system.

Required Secondary Treatments

Does not apply.

Required secondary treatments

There is currently no information regarding required secondary treatment.

Application examples

Frozen barrier technology has been field-tested, certified and documented by US DOE Innovative Technology (DOE/EM-0273).

The following sites provide application examples:

Performance

The frozen wall technology was efficient in the containment of contaminated groundwater at the Oak Ridge site, Tennessee (U.S. EPA, 1997). To maintain the frozen wall, the system required 288 kilowatts of electricity per hour.

Measures to improve sustainability or promote ecological remediation

  • The number of field visits could be limited by using telemetry for remote monitoring of site conditions.
  • The use of renewable energy and low-energy equipment would be preferred.
  • The use of thermosiphon is to be evaluated if conditions allow.

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Does Not Apply

Monitoring and Mitigation

Dust

Does not apply

N/A

Atmospheric / Steam Emissions—Point Sources or Chimneys

Don't apply

N/A

Atmospheric / Steam Emissions—Non-point Sources

Don't apply

N/A

Air / steam—by-products

Does not apply

N/A

Runoff

Does not apply

N/A

Groundwater—displacement

Applies

Modelling the effects of the required barrier and monitoring using pressure sensors.

Groundwater—chemical/ geochemical mobilization

Applies

Modelling the effects of the required barrier and monitoring using pressure sensors.

Groundwater—by-product

Does not apply

N/A

Accident/Failure—damage to public services

Applies

File checks and obtaining pre-drilling licences, development of drilling 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/explosion

Does not apply

N/A

Other—Management of drill cuttings

Applies for the management of drill cuttings resulting from the realization of wells for the installation of piping.

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

References

Author and update

Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR

Latest update provided by : Karine Drouin, M.Sc., National Research Council

Updated Date : February 1, 2008

Version:
1.2.1