Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 405

so that the contaminants will not bypass the barrier. The materials of the barrier must be
compatible with the contaminants (USEPA, 1989). Slurry materials can include Portland
cement, soil-bentonite, and cement-bentonite mixtures that are pumped into an exca-
vated trench by a backhoe. Grout curtains are made of materials such as Portland cement,
sodium silicate, or consist of polymers or bitumen. The materials are injected through a
series of drilled holes. The spacing of the holes is dependent on the soil permeability and
type of grout used. Sheet piling involves driving steel or concrete sheets. The sheets are
connected through sealing or interlocking. This method is often used during removal of
underground storage tanks (UST) or for prevention of erosion.
Capping can be used to reduce the iniltration of surface water to provide stability over
the contaminated site, prevent mobilization of contaminants or to improve the appear-
ance of the site. Capping can be simple or consist of multiple layers and can be temporary
or permanent. Ditches and berms can be used to manage surface water runoff. Synthetic
membranes can be used by installing sheets that are overlapped and seamed together.
They can be made of polymers, fabrics, rubber, and other materials. The installation must
be done properly without puncturing or tearing. Weathering and root penetration over
time can be problematic. Other materials for capping include low permeability (10 −6 to
10 −7 cm/s) soils and clays in the compacted state. A clean soil can then be added over the
cap as a topsoil to allow vegetative growth on the site. Liming, fertilization, and seeding
of plants such as grasses without the potential for deep roots will prevent erosion and not
penetrate the cap. For landills, multilayer caps as previously discussed can be employed.
A groundwater extraction system may be used to reduce the level of the water table. The
barriers can also be placed either upstream or downstream or totally around the con-
taminated area. Horizontal barriers through horizontal drilling and grout injection can be
used to restrict downward movement.
11.2.2 Confined Disposal
In the case of contaminated soil or sediment, in situ/ex situ remediation can be performed.
If remediation is not possible, the excavated or dredged material can be disposed of in
a conined land-disposal facility designed and constructed to contain the contaminants.
In the case of sediments, they must be previously dewatered such as in a contained dis-
posal facility since landill facilities cannot handle slurries. Large volumes cannot usu-
ally be accommodated since landills do not have the capacity. Potential mechanisms for
contaminant release are due to leachates, runoff, efluents, volatilization, uptake by plants,
and ingestion by animals. Therefore, pretreatment by stabilization/solidiication may be
necessary. Secure containment facilities (not landills) can be used for storage, dewatering,
and pretreatment for other processes. These costs are usually less than those for landill.
Contained aquatic disposal, such as the placement of material in a conined aquatic area
is called a conined disposal facility (CDF). These areas can be strategically placed in depres-
sions and conined by dikes. This technique can be used for disposal of contaminated soils
or sediments. Clean material can be placed above and at the edges. A good accounting of
the use of CDFs for dredging project in the Great Lakes can be found in the review pub-
lished by the U.S. Army Corps of Engineers (USACE) and U.S. Environmental Protection
Agency (USEPA, 2003).
Conined aquatic disposal (CAD) is used for placement of contaminated materials in a
natural or excavated depression. It has been used mainly for navigational purposes such
as in Boston Harbor, but not for disposal of contaminated material. It may be appropriate if
landill disposal or in situ capping is not possible. Maintenance costs are low and there can