Engineering and Expeditionary Warfare Center

Sediment Remediation

Remediation Technologies

 

Sediments often present unique challenges for remediation. This page summarizes the various types of remediation technologies applied to clean up contaminated sediments.

 

The table below lists the general response actions, technologies, and process options for remediation of contaminated sediments. The term "technology" refers to general categories of technologies, such as capping or dredging. The term "process option" refers to specific processes within each technology family, such as using sand for an isolation cap or mechanical equipment for dredging. A general response action may be accomplished by several types of remedial technologies and process options. The screening process eliminates remedial action technologies and process options that are inappropriate or infeasible for the sediment at a particular site. Process options that are retained after screening are combined into potential remedial alternatives for the site.

 

List of Examples of General Response Actions, Technologies, and Process Options for Contaminated Sediments

Click on a Bold General Response Action or Technology  for more information

General Response Actions

Technologies

Process Options

 

No Further Action None None

Institutional Controls

Land or water use restrictions

Deed restrictions on groundwater use, excavation, and future use

No dredge zones in public waterway

No anchor zones

Swimming restrictions

Physical barriers

Perimeter fence

Warning buoys of restricted zones

Breakwater to restrict vessel traffic

Consumption advisories

Fish consumption or health advisory

Seafood consumption or health advisory

Monitored Natural Recovery

Natural deposition of native sediment

Natural deposition areas

Artificial feature to increase deposition

Natural biological degradation

Natural biological activity

Containment

Isolation cap

Sand/silt cap

Geosynthetic membrane cap

Bentonite blanket cap

Reactive cap

Organic soil or sediment

Activated carbon in cap

Wetland cap

Thick cap to raise ground elevation

Wetland soil for plant growth

Biodegradable fabric

Wetland planting

Habitat Enhancement

Thin-layer cap to improve habitat and accelerate natural recovery

Shoreline stabilization

Rock rip-rap

Biodegradable fabric

Vegetation

Sediment Removal

Dredging (subaqueous excavation)

Mechanical

Hydraulic

Hybrid

Pneumatic

Excavation (after lowering water level)

Cofferdam dewatering (either circular or double-wall earth filled)

Single sheet pile wall

Fabric dam

Earth dam

Dredged Material Transport

Mechanical Transport

Floating barge

Amphibious vehicle

Wheeled vehicle

Rail

Conveyor

Hydraulic transport

Pipeline slurry direct from dredge

Pipeline slurry from hybrid dredge or mixing basin

Dredged Material Dewatering

Passive dewatering

Gravity separation

Air drying

Mechanical dewatering

Belt filter press

Plate and frame press

Centrifuge

Dredged Material Disposal

Confined Aquatic Disposal (CAD)

Excavate subaqueous pit

Place material in natural depression

Nearshore Confined Disposal Facility (CDF)

Permeable dikes and cap

Impermeable dike and/or cap

Base liner and leachate collection

Soil gas collection system

Upland CDF

Monofill for dredged material

Multi-user facility for dredged material

Solid waste landfill

Dredged Material Beneficial Use

Upland fill in brownfields

Place dredged material on brownfield site and cap

 

Subaqueous fill for habitat improvement

Place dredged material to raise elevation and cap

Dredged Material Treatment

Solidification

Mix with cement, lime or fly ash

 

Stabilization

Mix with cement, lime or fly ash

Mix with custom-designed stabilization agents (i.e., apatite)

Thermal treatment

Incineration (high-temperature destruction)

Thermal desorption (low-temperature separation)

Biological treatment

Aerobic degradation

Anaerobic degradation

Chemical treatment

Solvent extraction

Chemical reaction

 

This section provides a brief description of the various remediation technologies used for contaminated sediments.

 

No Further Action

Under a no further action (NFA) response action, no remedial actions would be performed and there would not be any long-term monitoring. The NFA response represents the existing site conditions and is retained in the screening process to serve as a baseline for comparison with other response actions. NFA may be appropriate if the site does not pose a potential threat to human health or the environment, or if previous remedial actions eliminated the need for further actions.

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Institutional Controls

 

Institutional controls (ICs) are legal or administrative restrictions that limit exposure to chemicals of concern or otherwise procedurally mitigate risks. ICs for contaminated sediment sites include land or water use restrictions and health advisories.

Land or water use restrictions consist of restricting actions that could result in contamination being released into the environment or could result in contact with contaminated sediment (or porewater or surface water). Such restrictions could be used to prohibit future use of surface water or sediment, prohibit future excavation or dredging that could lead to the release of contaminated sediment, and restrict recreational uses such as boating, swimming, or fishing. Health advisories could be used to restrict behaviors that could lead to unacceptable risks. One such advisory is a fish consumption advisory, which would limit the number of fish consumed or preclude the consumption of fish from a contaminated area. These advisories would protect human health by limiting potential exposure to contaminated fish or seafood. In some cases, warning buoys could be placed in the water to warn of potential risks, for instance by warning boaters to stay out of the area or anglers not to fish in the area.

 

Monitored Natural Attenuation

 

Monitored Natural Attenuation (MNA) on natural processes to decrease the concentration of chemicals of concern in sediment over time. The most common mechanisms (essentially equivalent to technologies) are natural deposition of clean sediment, natural bioturbation that mixes clean sediment with contaminated sediment, and natural biological degradation. In stable depositional areas, clean sediments naturally cover contaminated sediments after upland sources are reduced or eliminated.

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Containment

 

Sediment containment would reduce the potential exposure to human or ecological receptors by preventing direct contact with contaminated sediment and by reducing chemical migration from the sediment to the environment. The most common technology to accomplish containment is an isolation cap, where clean material is placed over contaminated sediment to prevent direct contact, to prevent sediment erosion, and to mitigate migration in porewater.

A reactive cap involves placing reactive materials in the cap to increase adsorption, chemically react with contaminants, or accelerate biological degradation. In some locations, placement of cap materials will raise the existing sediment elevation, which can result in a change in habitat type. In this situation, the top layer of the cap can be modified to support habitats such as wetland plantings (referred to as a wetland cap). In depositional areas, a thin-layer cap, otherwise known as a habitat enhancement cap, may be used to improve habitat substrate and to accelerate natural recovery. Where contamination exists along a shoreline, caps can be placed in the zone between subaqueous areas and upland areas. Shoreline stabilization caps may be placed on relatively steep slopes and can be designed to resist erosion by waves, wind, and storm flows.

 

Isolation Cap

 

The primary objectives of isolation caps are described as follows:

  • Physical isolation of the contaminated material from the benthic environment
  • Stabilization of contaminated material, preventing resuspension and transport to other sites
  • Reduction of the flux of dissolved contaminants into the cap and overlying water column

Isolation caps are commonly made from sand or clean dredged material and range from one foot to several feet thick. Other materials utilized for isolation caps include clay and organoclay formulations. The isolation layer may be supplemented by the additional layers for various purposes such as providing habitat or erosion control. Isolation capping is an effective remedy under the right conditions.

To evaluate the effectiveness of capping, the following factors must be considered:

  • Potential Water Column Impacts During Placement: Cap material must be placed through the water column to cover contaminated sediment. During placement, there is the potential for the cap material to be released into the water column. There is also the potential for soft, unconsolidated sediment to be re-suspended into the water column as the cap material is placed onto the sediment surface.

 

  • Ability to Place The Cap Materials: The implementability of cap placement depends on the availability of cap materials, the ability to transport the materials to the site, and the ability to place the materials. Caps have been placed in water depths of up to 100 feet.

 

  • Long Term Stability: The cap must be stable in the long term to provide an effective remedy. The evaluation must consider the potential for long-term erosion by currents, waves, or other forces such as anchors and boat traffic, potential bioturbation and long-term contaminant migration due to advection or diffusion. Caps are most effective when the bottom slope is close to horizontal.

 

  • Site Stability: The site must provide stable, reliable, long-term support for the cap. The site should be in areas where sediment naturally deposits and in relatively level areas.

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Wetland Cap

 

The difference between wetland caps and isolation caps is that the top layer of cap material in a wetland cap is designed to support wetland vegetation and the cap elevation must be at the appropriate elevation relative to surface water elevations to promote vegetative growth. In order to provide environmental protection, a wetland cap should be made from two layers. The lower layer would be designed as the chemical isolation layer. The upper layer would be designed with the soil types best suited to support the target vegetation types. The thickness of the upper layer would be based on plant root depths and potential bioturbation depth.

 

 

Habitat Enhancement Cap

 

The habitat enhancement cap technology is a method of enhancing or accelerating natural recovery. These caps have also been called thin-layer caps. Habitat enhancement caps are not designed to provide complete chemical isolation, therefore, they do not need to be designed to prevent chemical migration by diffusion or advection. The cap would be designed to provide physical isolation of the contaminated sediment from the benthic environment and to prevent re-suspension or transport of contaminated sediment particles. The material type selected for a habitat enhancement cap depends on factors such as hydrodynamic energy and the type of habitat desired.

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Shoreline Stabilization Cap

 

In areas of potential erosion, such as shorelines and intertidal zones, caps must be designed to prevent erosion. In this situation, the upper layer of the cap can be made from armor stone designed to be stable in a storm or flood.

The armor layer would be designed using established methods for shoreline protection. The armor layer would be placed over the base cap or over an intermediate layer of coarse sand intended to protect the underlying cap. The intermediate layer may be necessary to provide a stable base for the armor to prevent fine-grained particles in the underlying cap layer from migrating into the void spaces in the armor layer. A separate bioturbation layer is generally not needed because the armor would prevent burrowing organisms from reaching the isolation layer.

 

 

 

Sediment Removal

 

Sediment removal involves moving sediment from its current location to a new location, reducing the potential mobility and exposure to humans or the environment. Dredging involves removing sediment from under water. Excavation involves removing the overlying water and then removing sediment. The sediment removal process involves a variety of possible methods to physically remove, dewater, and then dispose of the sediment following specific state and federal requirements.

The ERB dredging technology webpage provides more information and detailed descriptions of a variety of dredging, dewatering, transport, and disposal processes and options to consider.

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Dredged Material Beneficial Use

 

After dewatering and debris removal, dredged material is composed predominantly of inorganic particles that have properties similar to upland soils. Dewatered sediment with low levels of chemicals of concern can be viewed as a resource, rather than a waste, and used in conjunction with redevelopment or for habitat improvement.

In areas where a thick cap is needed to improve habitat, contaminated dredged material could be used in the lower portion of the cap and then covered with clean cap material. This type of beneficial use would require perimeter containment dikes. The contaminated dredged material would be covered with capping material to provide a chemical isolation layer and bioturbation layer. The top layer of the cover would be covered with soil or sediment necessary to provide the type of habitat desired.

 

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Dredged Material Treatment

 

Dredged material can be treated to remove or transform the chemicals of concern and to create material that can be reused or disposed of properly following state and federal regulations.

Solidification/Stabilization (S/S) is a technology used to transform the chemicals of concern in sediment and make them inert with considerably less mobility.

Other technologies include:

  • Thermal Treatment can be done by either “low-temperature” desorption or by “high-temperature” incineration. The low-temperature systems heat the sediment to several hundred degrees, so that organic contaminants volatilize and are then collected in the vapor released from the sediment. In high-temperature systems, the sediment and vapor are heated to the temperature required to break chemical bonds in the contaminants.

 

  • Aerobic or Anaerobic Biological Treatment where the sediment is mixed with the appropriate nutrients and then the temperature and oxygen conditions are adjusted to accelerate degradation of the contaminants.

 

 

 

  • Chemical Treatment where chemicals are mixed into the sediment to change the form of the contaminant to a less toxic or less mobile form or to separate the contaminants from the sediment

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