Frozen Barriers
Description
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.
Tests with cesium-137 showed no detectable diffusion
through the barrier although sorption on the
soil grains may have been responsible for the immobility.
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.
Applicability
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.
Web Links
http://www.containment.fsu.edu/cd/content/pdf/305.pdf
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 AgencyÕs
(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.