In-Situ Capping (ISC) refers to placement of a covering or cap over an
in-situ deposit of contaminated sediment. The cap may be constructed of clean sediments,
sand, gravel, or may involve a more complex design with geotextiles, liners and multiple
layers.
ISC is one of many options for the remediation contaminated sediments,
which should be considered using the full suite of guidance development under the ARCS
Program.
An ISC operation must be treated as an engineered project with carefully
considered design, construction, and monitoring to ensure that the design is adequate.
There is a strong interdependence between all components of the design
for a capping project. By following an efficient sequence of activities for design,
unnecessary data collection and evaluations can be avoided and a fully integrated design
is obtained.
The basic criteria for a successful capping operation is simply that the
cap components required to isolate the contaminated material from the environment be
successfully placed and maintained.
The contaminated sediment to be capped must be characterized as part of
the project design. The capping materials (granular sediments or other materials) must
also be characterized.
The evaluation of the site is a critical requirement for an ISC capping
design. Bathymetry, currents, water depths, waterway uses, bottom sediment
characteristics, potential groundwater flow, and operational requirements must be
evaluated.
A number of different equipment types and placement techniques can be
considered for ISC operations. Conventional discharge of granular capping material from
barges and hydraulically dredged material from hopper dredges or pipelines can be
considered as well as use of diffusers, tremies, and other equipment needed for submerged
discharge. Controlled discharge and movement of barges and use of spreader plates or boxes
with hydraulic pipelines can be considered for spreading a capping layer over a larger
area. Specialized equipment may be needed for placement of geotextile or membrane
components. Armor stone may be placed using conventional placement methods for riprap.
Compatibility between equipment and placement technique for contaminated and capping
material is essential for any capping operation.
Accurate navigation and precise positioning during material placement
are required for capping operations. State-of-the-art equipment and techniques must be
employed to assure accurate placement to the extent deemed necessary. Diligent inspection
of operations to insure compliance with specifications is essential.
The composition and dimensions (thickness) of the components of a cap
can be referred to as the cap design. This design must perform one or more of the three
functions of a cap (physical isolation, stabilize sediment, and reduce flux of dissolved
contaminants). The design must also be compatible with available construction and
placement techniques.
Caps composed of multiple layers of granular materials as well as other
materials such as armor stone or geotextiles are often considered for ISC projects, and
the in-situ cap design cannot always be developed in terms of cap material thickness
alone.
Monitoring of capped sites is required during and following placement of
the contaminated and capping material to insure that an effective cap has been constructed
and to insure that the cap as constructed is effective in isolating the contaminants and
that long term integrity of the cap is maintained. Design of monitoring programs must be
logically developed, prospective in nature, and tiered with each tier having its own
thresholds, null hypotheses, sampling design, and management responses based on exceedance
of predetermined thresholds.
Management of an ISC requires the routine maintenance of the cap,
protecting cap integrity through enforcement of waterway use restrictions, repair and
modification of the cap as needed to address changing conditions or design deficiencies
indicated by monitoring data.
