Ground freezing is an innovative technology that is used to form subsurface frozen soil barriers that halt the spread of hazardous and radioactive contaminants in soils and groundwater. Refrigerant piping, known as thermoprobes, is installed vertically at equal intervals around the perimeter of a known source of contaminants in the subsurface. It is used to freeze the soil pore water, forming a frozen soil barrier that may provide complete containment. Frozen barriers can be repaired in place by injecting water into the leakage area. One of the key differences between this and other barriers is that the barrier can be easily removed after the remediation or removal of the source is completed.
The above-ground refrigeration units are standard commercial machines that function to condense carbon dioxide vapor on the interior walls of the thermoprobes. The heat energy removed from the condensing carbon dioxide is transferred to the refrigeration units and expelled. Insulation prevents heat gain at the ground surface and a membrane inhibits infiltration of rain water into the isolated zone. Barriers are often used when the waste mass is too large for practical treatment and where soluble and mobile constituents pose an imminent threat to a drinking water source. Barriers can be positioned to maximum depths of 1,000 feet and do not require excavation for installation.
Limitations and Concerns
Although the design states that the refrigerant is environmentally friendly, there is concern about possible leaks of refrigerant and the toxicity of the compound. Many refrigerants, if released to the environment, have serious environmental consequences.
Applications for arid environments or where the subsurface is very porous (sandy) are limited unless methods are developed to add and retain moisture in the soils.
This approach may be limited because of soil movement.
Careful installation of the piping is necessary to ensure complete barrier formation.
Barrier verification technologies need to be developed to detect leaks. Verification of the integrity of the barrier may be problematic due to complex hydrology and pre-existing conditions (i.e., presence of an abandoned buried pipeline).
To maintain the barrier, energy use will be high. Remote sites will require electrical power and utility installation. Backup power will be required at all sites if the plume is highly mobile.
More expensive cryogenics (e.g. liquid nitrogen) may be required to form frozen soil barriers in areas where plumes of low-freezing-point contaminants (TCE, etc.) exist.
Long-term maintenance of the frozen barrier at remotes sites requires electrical power and utility installation. Backup power is required at all sites if the plume is highly mobile. The effectiveness of the frozen soil technology for containment of contaminants in dry soils has not been demonstrated. Further development of suitable methods of homogeneously adding and retaining moisture in arid soils is needed.
This technology may be used to isolate and control the migration of radioactive material, metals, and organic contaminants that are found in the saturated soil matrix. Frozen soil barrier technology may be competitive with other subsurface flow-control technologies such as liners, slurry walls, sheet piling, and grouting. Unlike other barriers, frozen soil barriers require application of electrical power for the life of the barrier. Therefore, use of these barriers is best restricted to short to medium duration (20 years or less).
Technology Development Status
Formation of frozen soil barriers in saturated, fine-grained soils has been demonstrated in the field.
Other Resources and Demonstrations
Frozen Soil Barrier technology was tested at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee from September, 1996 through September, 1998. The demonstration site was a former earthen impoundment used from 1958 through 1961 for retention/settling of liquid radioactive wastes generated from the operation of the Homogeneous Reactor Experiment (HRE) (Figure 2). In 1986 it was estimated that approximately 75 Curies (Ci) of strontium 90 and 16 Ci of cesium 137 were contained in the buried sediments of the impoundment. Verification monitoring was conducted by the Environmental Protection Agency’s (EPA) Superfund Innovative Technology Evaluations (SITE) Program.
See http://www.crrel.usace.army.mil/techpub/CRREL_Reports/reports/TR00-19.pdf, Frozen Soil Barriers for Explosives Containment, September, 2000, ERDC/CRREL TR-00-19. The U.S. Army Corps of Engineers produced this report. The objectives were to test the efficacy of frozen barriers to contain royal demolition explosive (RDX), trinitrotoluene (TNT), and picric acid through soils and to test the concept of leaching contaminated soils above a frozen barrier as a method of soil cleanup.