Contaminated Sediments Program
Table of Contents
Sediment Assessment and Remediation Report
Guidance for In-Situ Subaqueous Capping of
This document presents technical guidance for planning and design of in-situ subaqueous capping projects. The guidance is summarized as follows:
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.