It is an option when pump and treat becomes too expensive and the area surrounding the site is a low energy system.
The design of the cap and the characterization of the surrounding areas are of equal importance and drive the feasibility of the entire project.
Some things to consider when constructing a cap would be “waterway dimensions, water depths, tidal patterns, ice formations, aquatic vegetation, bridge crossings and proximity of lands or marine structures”.
It is best if in-situ capping projects are performed in low-energy waterways such as harbors, low flow streams, or estuaries.
It is important to take into consideration the long-term impacts of episodic events such as tidal flow on bottom current velocities.
These tests on the sediments include: “visual classification, natural water content/solids concentrations, plasticity indices (Atterberg limits), total organic carbon (TOC) content, grain size distribution, specific gravity, and Unified Soil Classification System (USCS)”.
Furthermore, the construction of an in-situ cap will cause a drop in water depth thus limiting the size of ships that may cross the area.
All ISC must comply with the requirements in the Resource Conservation and Recovery Act (RCRA) and the Toxic Substances Control Act (TSCA), although the ability of in-situ capping to meet those standards in the long term has not been successfully researched and studied enough due to lack of data.
The cap designs “must be compatible with available construction and placement techniques” along with meeting the three previously mentioned criteria above.
Presently, laboratory tests and models of the various processes involved (advection, diffusion, bioturbation, consolidation, erosion), limited field experience, and monitoring data drive cap design.
These can include but are not limited to “quarry sand, naturally occurring sediments or soil materials”.
[1] Geomembranes have been used for stabilization in two projects along with granular media for the ISC constructed at Sheboygan River and in Eitrheim Bay, Norway.
[3] There are three basic approaches that may be used to have long-term cap stability: Bioturbation is defined as the disturbance and mixing of sediments by benthic organisms.
This layer will be assumed to be completely mixed with the environment and should prevent benthic organism from descending further into the in-situ cap.
This monitoring program should include frequent testing so real-time data is provided to allow quick adjustments to the overall cap design.
This long-term monitoring need only be assessed on a yearly to bi-yearly basis unless a problem is discovered; then more frequent testing will be required.
In Massena, New York, at the General Motors Superfund site, PCB-contaminated soils were dredged repeatedly but some areas still had high levels of contaminant (>10ppm).
[1] In Manistique River, Michigan, PCB-contaminated sediments were capped with a 40mm thick plastic liner over an area of 20,000 square feet (1,900 m2) with varying depths of up to 15 ft.[1] In Sheboygan River, Wisconsin, PCB-contaminated sediments were capped with a sand layer and armor stone layer.
[11] In Cold Spring, New York, in the Hudson River, sediment was contaminated with cadmium and nickel from a battery manufacturing facility.
A Geosynthetic clay liner (GCL) and a 12-inch covering of sandy loam was planted on top of the contaminated area.
[2] In Elkton, Maryland, contaminated sediment was discovered with excess amounts of volatile organic components and dense non-aqueous phase liquids, resulting is severe discharge.