In situ capping of subaqueous waste

It is very important to evaluate the site and goals of a specific project to determine if ISC is the right technique to use. First, it is important to find out if ISC will satisfy all of the desired remedial objectives. To determine if ISC will satisfy the remedial objectives it is important to look at the three primary functions previously listed for ISC. For the first function it is important to realize that “the ability of an ISC to isolate aquatic organisms from the sediment contaminants is dependent upon” the deposition of new sediment contaminants being deposited on the cap. If contaminated sediment is deposited back on top of the cap then a cap was built to separate to contaminated layers. Thus, “ISC should only be considered if source control has been implemented." Stabilization of the contaminated sediment could be a design function if the goal of remediation is to prevent negative environmental impacts due to “resuspension, transport and redeposition” of the contaminated sediments to other remote areas. Furthermore, if a remedial objective is desired, then the purpose of the ISC could be to isolate the contaminated soil from the surrounding environment, thus controlling the environment of the contaminated soil and causing possible degradation of the contaminate.[1]

On site evaluation to see if ISC is a good remediation technique is based on several criteria: the surrounding physical environment, current and long-term hydrodynamic conditions, the geotechnical and geological conditions, hydrogeological conditions, on-site sediment characterization, and current and long-term waterway uses.[2]

Many of the physical properties of the surrounding area where the cap would be placed are important. 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”.[1] It is best if the area surrounding the ISC is flat for ease of installation.

The hydrodynamic conditions are of equal importance. It is best if in-situ capping projects are performed in low-energy waterways such as harbors, low flow streams, or estuaries.[2] High energy and high flow environments can affect the long-term stability of the cap and cause plausible erosion over time. Currents are also important. Currents vary along a water column and placement of the ISC can be negatively affected by changing currents. It is important to take into consideration the long-term impacts of episodic events such as tidal flow on bottom current velocities. Modeling must be done to determine if placement of the in-situ cap will alter existing hydrodynamic conditions.[1]

A study of the geotechnical and geological conditions must be made before the placement of the in-situ cap because of potential settling underneath the cap. If settling is predicted to be significant, the cap design may have to be designed thicker than originally projected to allow the settling to not alter the integrity of the cap.[1]

Hydrogeological conditions are important to consider before placement. It is important to locate areas of discharge, which are areas where the groundwater flow path has an upward component.[5] This discharge can cause the in-situ cap to become displaced or cause containments to be transported to the surface water, thus causing decreased effectiveness of the in-situ cap.[1]

Typical sediment characterization is needed before construction and design of the ISC can be implemented. 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)”.[1]

It is important to realize what the current waterway uses are and how they may be affected with the placement of an in-situ cap. Some waterway uses that may be affected by the construction of an in-situ cap include but are not limited to “navigation, flood control, recreation, water supply, storm water or effluent discharge, waterfront development, and utility crossing." Since the construction of an in-situ cap may limit some of these activities due to the importance that the caps integrity be maintained over an extended period of time, any use that may cause displacement of the cap should be limited. 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. These limitations on the waterway may also have social and economic impacts that must be considered.[1]

It is important to know all of the regulatory standards in place for the desired location of the ISC. 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 six general steps for in-situ cap design, provided by Palermo et al. are listed below:[1]

1. Identify candidate capping materials and compatibility with contaminated sediment at the site.
2. Assess the bioturbation potential of indigenous benthos and design a cap component to physically isolate sediment contaminants from the benthic environment.
3. Evaluate potential erosion at the capping site due to currents, waves, propeller wash, and design a cap component to stabilize the contaminated sediments and other cap components.
4. Evaluate the potential flux of sediment contaminants and design a cap component to reduce the flux of dissolved contaminants into the water column.
5. Evaluate potential interactions and compatibility among cap components, including consolidation of compressible materials.
6. Evaluate operational considerations and determine restrictions or additional protective measure needed to assure cap integrity.

Identifying the materials should be assessed at the beginning of the project because they typically represent the largest cost to the project. Thus, if the materials needed cost too much the project may not be feasible at all.[1] Granular materials are used in most cases. These can include but are not limited to “quarry sand, naturally occurring sediments or soil materials”.[1] Studies have shown that fine-grained materials and sandy materials can be effective in the construction of an in-situ cap.[6] Furthermore, fine grain materials have been shown to act as better chemical barriers than sand caps.[7] Thus a fine grain material is a better capping component than factory-washed sand. It is important to have control the amount of organic material within the cap because the benthic organisms have shown interest in burrowing within any unconsolidated fine grained sediments containing organic matter.[1] Increased levels of organic matter in sands have shown an increase in the retardation of hydrophobic organic contaminants through the cap and encourage degradation of contaminant.[3] Thus a careful balance of organics is necessary.

Geomembranes can serve numerous purposes in a cap design, including “provide a bioturbation barrier; stabilize the cap; reduce contaminant flux; prevent mixing of cap materials with underlying sediments; promote uniform consolidation, and; reduce erosion of capping 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.[8] Although geomembranes seem to have great benefits, the problem of uplift and ballooning has arisen and not much research has gone into assessing what causes the lift of the geomembranes off of the surface.[9] Further research is needed to determine the overall effectiveness of geosynthetics for chemical isolation.[1]

Armoring stone, which is any stone that is used to "shield" the rest of the in-situ cap, can be used for resistance to erosion and should be considered in cap design.[1][3] The long-term ability of the cap to perform depends primarily on its ability to withstand external forces, mostly hydraulic forces.[3] There are three basic approaches that may be used to have long-term cap stability:

1. The cap layer needs to be armored sufficiently to hold up under the various hydraulic forces.[3]
2. Cap at a deeper layer since hydraulic forces typically decrease with decreasing depth.[3]
3. Try to control the hydraulic forces to limit their effect on the cap layer with breakwaters, dams, navigational controls, etc.[3]

Bioturbation is defined as the disturbance and mixing of sediments by benthic organisms. Many aquatic organisms live on or in the bottom sediments and can greatly increase the “migration of sediment contaminants through the direct movement of sediment particles, increasing the surface area of sediments exposed to the water column, and as a food for epibenthic or pelagic organism grazing on the benthos." The depth of bioturbation in marine environments is greater than that in fresh water environments. To prevent and reduce the impact of bioturbation on the cap, the cap should be designed with a sacrificial layer, typically only a few centimeters thick (5–10 cm). 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. The thickness of the sacrificial layer should be based on a study of the local organisms and their behavior in the surrounding sediment near the area of the cap construction, since some benthic organisms have been known to burrow at depths of 1m or more. The presence of armor stone has been known to limit the colonization by deep burrowing benthic creatures. Another method of preventing benthic organisms from destroying the integrity of the cap design is to pick a granular media that the local benthic organism find unattractive and are not known to readily colonize on that surface, thus limiting the chance a benthic organism will grown on the cap.

The consolidation of the in-situ cap must be considered, provided that the selected material for the cap “is fine-grained granular material." The consolidation of the underlying material should be taken into account due to “advection of pore water upward into the cap during consolidation."[1]

Erosion should be carefully considered. To determine the level of protection against erosion it is important to look at “the potential severity of the environmental impacts associated with cap erosion and potential dispersion of the sediment contaminants in an extreme event” (such as a 100-year event). An under-designed in-situ cap could be compromised by erosion resulting in the release of contaminants. An over-designed cap would result in extremely high costs.[1][3]

Fredette et al. outlines five steps for the development of a physical/biological monitoring program for ISC projects:[10]

1. Designating site-specific monitoring objectives
2. Identifying elements of the monitoring plan
3. Predicting responses and developing testable hypotheses
4. Designating sampling design and methods
5. Designating management options

Thus it is important a monitoring program be put into place at the onset of construction. A short-term monitoring program should be used to monitor the in-situ cap during construction and immediately following construction. This monitoring program should include frequent testing so real-time data is provided to allow quick adjustments to the overall cap design. A long-term monitoring program should be established to provide data about the overall effectiveness of the cap design and to make sure the cap is meeting all of its required regulations and that the cap is not excessively eroded. 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.[1]

During monitoring, it is important to schedule routine maintenance. This may include placement of material equal to the predicted amount of material removed due to erosion.[1]

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). These areas were capped, an approximate area of 75,000 square feet (7,000 m²), with a three-layer ISC composed of 6 inches of sand, 6 inches of gravel and 6 inches of armor stone.[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. This was done in shallow regions were direct placement was possible.[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. The cap system constructed over the contaminated waste involved a geotextile working mat, a GCL, a scrim-reinforced polypropylene liner, a geotextile cushion, and a gabion mat.[2]