Fact sheet: Confined Aquatic Disposal and Engineered Containment Facilities—Sediments

On this page

• Description
• Implementation of the technology
• Recommended analyses for detailed characterization
• Recommended trials for detailed characterization
• Other information recommended for detailed characterization
• Applications
• Applications to sites in northern regions
• Treatment type
• State of technology
• Target contaminants
• Treatment time
• Long-term considerations (following remediation work)
• Secondary by-products and/or metabolites
• Limitations and Undesirable Effects of the Technology
• Complementary technologies that improve treatment effectiveness
• Required secondary treatments
• Application examples
• Performance
• Measures to improve sustainability or promote ecological remediation
• Potential impacts of the application of the technology on human health
• References
• Author and update

Description

This technology includes contaminated sediment containment structures to limit contaminants spreading in the environment. There are two main options for disposing of contaminated sediments in situ:

• Confined aquatic disposal (CAD) cells:
  • CAD cells refer to the subaqueous capping of dredged or excavated sediments when placed within a natural or man-made depression and overlain with clean sediments, geotextiles, and/or engineered structures.
  • CAD cells are designed to be set into the existing bottom profile of the aquatic environment, while subaqueous capping elevates the bottom profile by the total thickness of the cap and sediments beneath the cap.
• Engineered containment facilities (ECF):
  • ECFs are containment structures that are not entirely subaquatic. Contaminated sediments remain on-site and are isolated using containment structures.
  • ECFs can be built in sediments on upstream sites, in the intertidal zone with one or more faces in the water, or as an insular containment area.
  • ECFs are designed to surround highly contaminated sediments with rigid, impermeable walls. These walls extend deep within sediments to contain the contaminants and provide structural support. The walls extend up toward the surface, creating capacity for additional sediments.
  • Materials used in ECFs typically consist of impermeable walls such as sheet piling, and may involve multiple barriers (such as double walls, armouring, sand fill) to prevent the release of contaminants.
  • Once filled, ECFs are capped using materials similar to conventional capping (See Capping fact sheet).

CAD cells and ECFs consolidate heavily contaminated materials, thereby reducing a contaminated site’s overall footprint and removing contaminant transformation and exposure pathways.

Internet links:

• Fredette, T. J. 2006. Why Confined Aquatic Disposal Cells Often Make Sense.
• Rollings, M. P. 2000. Geotechnical Considerations for Contained Aquatic Disposal Design. U.S. Army Engineer Research and Development Center.
• U.S. EPA. 1999. Subsurface Containment and Monitoring Systems: Barriers and Beyond.

Implementation of the technology

Implementation of the CAD technology may include:

• Mobilization, site access and installation of temporary facilities.
• Capture and relocation of aquatic organisms and fauna, if possible, located in the rehabilitation area. See activities involving species at risk.
• Delineation of the contamination, identifying contaminant “hot spots” and total volume of contaminated material.
• Identification of requirements and desired location for the CAD. Where a natural depression does not exist, standard dredging or excavating equipment may be used to create a depression.
• Removal of sediments, generally using dredging or excavation.
• Water treatment of the dredging slurry or contact water.
• Stockpile clean sediments removed in the construction of the CAD depression on-site. Clean sediments may be used as capping material once the cell has been filled.
• Deposit contaminated sediments into the CAD and let them settle and condense.
• Once the CAD cell has been filled with sediments and the material has consolidated sufficiently, apply the cell cap. Cap placement is similar to that of traditional capping, where the material is sprinkled onto the water surface and allowed to settle through the water column. Material may be applied mechanically, either from the shore or from a floating barge. Amendments such as activated carbon and engineered materials (such as geotextiles) may be incorporated into the cap to improve containment or contaminant degradation.
• Collection of several samples using boreholes to determine whether the cap thickness requirements are met.
• Application of all necessary impermeable wall materials.
• Implementation of any necessary institutional controls to prevent human exposure and recontamination.
• Conduct monitoring of the area through a bathymetric survey and/or sediment sampling to ensure the contaminated area is completely covered.
• Implementation of long-term monitoring to ensure the integrity of contaminant containment and to quantify contaminant flux through the cap, thereby ensuring ongoing environmental protection.

Implementation of the ECF technology may include:

• Mobilization, site access and installation of temporary facilities.
• Capture and relocation of aquatic organisms and fauna, if possible, located in the rehabilitation area. See activities involving species at risk.
• Delineation of the contamination, identifying contaminant “hot spots” and total volume of contaminated material.
• Optimal location selection for engineered containment, including overall size and depth.
• Site preparation, such as removal of rock debris.
• Installation of containment walls into existing sediments.
• Removal of water from the ECF, treatment as necessary, and disposal into the waterbody.
• Incorporation of amendments, such as activated carbon or nutrients, into the contaminated sediments to stimulate the remediation process or increase retention.
• Once ECF is filled, it may be left open, covered, or repurposed into a usable surface. If repurposing the facility, the internal material will require consolidation and compaction, to provide sufficient shear strength, depending on the planned usage.
• Site restoration of the temporary storage area (grading, paving, revegetation, etc.).
• Implementation of any necessary institutional controls to prevent human exposure and recontamination.
• Supervision of construction activities during the placement of materials and structures to ensure the accuracy of design specifications (such as thickness of berms, location of structure, volume available for sediment containment).
• Implementation of long-term monitoring to ensure the integrity of contaminant containment and to quantify the contaminant flux through the ECF, thereby ensuring ongoing environmental protection.

Materials and Storage

Materials stored on-site typically consist of small amounts of fuel and lubricant, as well as some construction supplies. In the case of ECF, construction materials are also temporarily stored on-site (for example, sheet piling).

Large amounts of clean and contaminated sediments, as well as sand and gravel, are often involved with CAD/ECF projects and may require temporary storage on-site. Clean sediments, sand, and gravel may be stockpiled and covered to control dust and prevent sedimentation loss from exposure to wind and precipitation.

Contaminated sediments should be stored in a watertight container to prevent contaminant transport through leaching into the ground, surface water runoff, and evaporation losses. Stockpiles of contaminated sediments can be stored on the ground, usually covered, to limit water infiltration and dust emission. It may be necessary to install impermeable membranes under stockpiles of stored sediments.

Wastewater associated with contaminated sediments may require separation, treatment and containment.

Depending on the project, amendments may be used to enhance the remediation process, in which case they will be stored on-site (for example, activated carbon). CAD may be combined with other technologies, such as biodegradation or chemical oxidation. In this case, additional amendments, such as nutrients, oxidants, etc., may also be stored on-site.

Waste and Discharge

Water from the dredged sediments needs to be removed prior to placement into the CAD cell or the ECF. Any contact water removed from the contaminated sediments requires treatment prior to disposal into the environment. Contact water may be treated on-site or transported offsite to a treatment facility.

Gas generation and buildup may occur under the cap, and fissures may be created as the gas tries to escape. Gas generation is particularly common in the anaerobic degradation of organic contaminants. Vents or an active gas collection system may be installed into the cap if the gas buildup is a concern.

Fine sediments, or sediments placed in turbulent water, may become suspended in the water column during the placement of sediments into the CAD depression. As the cell is filled, the force of adding additional material may cause already-filled sediments to escape the cell boundaries in a mud wave. Proper monitoring and meticulous filling of the cell will help reduce occurrences of contaminant sedimentation. Placement of material at the sediment surface through the use of mechanical or hydraulic equipment may reduce sedimentation

Recommended analyses for detailed characterization

Recommended trials for detailed characterization

Other information recommended for detailed characterization

Notes:

Knowledge of currents, wave action, and tidal patterns is required to estimate the potential loss of sediment and capping materials into the overlying water.

Applications

Generally, CAD and ECF are the most suitable in situ disposal options when dredging or excavation activities are planned or have been completed, and contaminated sediments need to be disposed of. This technology applies best in the following situations:

• Offsite disposal facilities are not a feasible consideration based on facility limitations, location, or cost.
• On-site terrestrial or semiaquatic/subaquatic capping is restricted or not feasible (such as when alterations to navigable depths are restricted).
• A suitable natural depression exists near the required dredge/excavation site, or a suitable location exists to create a subaquatic depression or an ECF.
• Future site use (navigation, flood control) and expected infrastructure planning (piers, pilings, buried cables) are compatible with the CAD cell or ECF.

Conditions that favour the use of an ECF, as opposed to CAD, are the following:

• Removal of a portion of the contaminated sediments may cause undue harm to the waterbody, favouring in situ confinement.
• Human activities would benefit from the increased usable space created through the closure and capping of the ECF. However, institutional control mechanisms (such as navigation restrictions) may be necessary to prevent damage to the cap from human activities.

Applications to sites in northern regions

• The technology is applicable in northern environments, but remote sites have greater logistical challenges associated with mobilization, resulting in higher costs. In addition, equipment availability is limited and the seasonal windows to conduct work are short.
• The construction of a CAD cell or ECF requires access to the materials needed for the structure and capping, which are more limited in northern environments.
• Arctic environments may require the assistance of an icebreaker, as well as monitoring and reporting of ice conditions, which considerably increases operational costs and organizational requirements.
• Monitoring and testing are limited by timely access to certified laboratories and often necessitate the development of on-site testing and analysis of materials, the implementation of progressive interventions and/or the implementation of a risk-based management approach.
• The design of the CAD or ECF must consider processes such as ice erosion, which are common in Arctic coastal environments.
• Water treatment systems in northern environments require climate-appropriate design, including consideration for seasonal changes as well as long periods without human intervention, refuelling, etc.

• The technology may require the placement of restrictions or limitations on the human consumption of native organisms when contaminated sediments are present. Because local people may rely on aquatic species as important food sources, these restrictions may significantly impact communities.
• Monitoring of the CAD or ECF cell after the construction phase is also extensive, requiring regular testing of cell thickness and contaminant concentrations. Transporting the equipment and workers needed to monitor the site is costly and presents logistical difficulties.

Treatment type

State of technology

Target contaminants

Notes:

Conditions within the CAD cell/ECF often become anaerobic or anoxic. The lack of oxygen may allow contaminants such as methyl mercury and PCBs to slowly degrade.

Treatment time

Notes:

The time required to complete remediation activities will fluctuate with the project’s scale. The volume of contaminants, the transportation distance required (from dredge location to CAD location), and the use of an existing or creation of a new underwater depression will all influence the time required. ECFs require significant effort for the design and construction phase, and the structure may take years to construct and fill.

Long-term considerations (following remediation work)

As for any in situ technology, continuous monitoring is required to ensure the integrity of the cap and/or active structure. Monitoring parameters include cell integrity, advective flow through the cell, and contaminant migration. Monitoring may also include benthic recovery, where measurements are taken to assess the abundance of key taxonomic groups and the diversity of the benthic community. The effectiveness of the CAD cell/ECF must be monitored until all remediation objectives have been met. Frequent monitoring must be implemented for at least the first six months after closure when capping failure is most likely to occur. Afterward, long-term monitoring (such as annual) must be implemented. Plans must be prepared and funded for the monitoring and maintenance of the CAD cell or ECF for as long as the contaminant risk remains.

Institutional controls will be required to limit the effects of human activities within the area. These controls may include limitations on infrastructure development and navigational or recreational use.

Secondary by-products and/or metabolites

Anaerobic conditions may occur within the confined sediments of a CAD cell or ECF. Decomposition of organic compounds within the cell may lead to the production of sulphide gas, methane or other gases. The buildup and release of gases beneath the cap may cause fissures or cracks that could affect the cap’s integrity or containment facility. If a gas generation is expected, vents or an active gas collection system may be integrated into the capping design.

Secondary by-products may be generated by the treatment technology selected for the carrier water.

Limitations and Undesirable Effects of the Technology

• CAD and ECF are not appropriate in the following situations:
  • Water depth and energy (such as current) must not be so great as to prevent controlled placement of sediment and capping materials. Depth limitations are similar to in situ capping (inefficient at depths greater than 15 m).
  • Areas with significant groundwater flow are not suitable, as upwelling may compromise the integrity of the CAD cell/ECF, causing unacceptable contaminant release.
  • Where larger than feasible amounts of clean sediments are required for the capping of CAD cells/ECFs.
  • Sediments with a low shear strength as they may not support the creation of a depression and may continue to collapse inward during construction or filling. Sediment shear strength must also be sufficient to support containment walls for ECFs.
  • Presence of unexploded explosive ordnances (UXOs), which pose a risk of unintentional detonation.
• CAD and ECF are challenging under the following conditions:
  • Locations where the construction of a CAD or ECF could destroy the benthic community and associated habitats present on-site.
  • Extreme events (storms, floods and earthquakes), as well as ice jams and ice scouring, can compromise the stability of a sediment cap. Therefore, there is potential for reintroducing contaminants (after a major disturbance of the cover structure).
  • Depth to bedrock may limit the volume capable of being stored within a CAD cell and may limit the placement of containment walls associated with an ECF.
  • Narrow channels may limit the depth available for a CAD design, requiring steep cell walls.
  • Initial design of CAD cell or ECF is not sufficient to contain the full volume of sediments, requiring redesign.
  • Gas production below the cap requiring additional management measures.
• CAD and ECF can have the following adverse effects:
  • Loss, re-exposure or disturbance of contaminants.
  • Risk of contaminant release/resuspension with the handling and placement of contaminated sediments. The release or resuspension of contaminants may create exposure pathways, leading to increased uptake by the benthic and aquatic communities.
  • Restrictions to future site uses and to the navigable depth. ECFs typically extend to near or above surface levels, increasing the footprint of the surrounding terrestrial area but affecting the navigable depth.
  • Limitation to the flood-bearing capacity of a waterway.
  • As the CAD cell/ECF approaches capacity, surging of contaminated material out of the cell is possible, which may release contaminants into the water column. The water column must be monitored for any instances of contaminant surging.

Complementary technologies that improve treatment effectiveness

CAD cells may incorporate engineered structures, which improve physical strength and stability as well as containment ability. ECFs may incorporate structural and mechanical options, such as geotextiles and armouring, to improve containment and structural stability. Contained sediments may be combined with other in situ treatment techniques, such as:

• Biodegradation
• Chemical Oxidation
• Sequestration

Required secondary treatments

No secondary treatment is required if the CAD cell or ECF achieves remediation objectives. However, gas may be generated under the cap following the implementation of the technology. Long-term secondary treatment may be considered to control gas emissions into the environment.

Application examples

Application examples are available at these links:

• City of Hamilton. 2023. Randle Reef sediment remediation project.
• U.S. EPA. 2023. New Bedford Harbor Cleanup Plans, Technical Documents and Environmental Data.  
• Port of Baltimore. 2021. Confined Aquatic Disposal (CAD).
• Birkelo, C. & Malone, J. 2013. A Unique Partnership for Contaminated Sediment Management: The Port of Hueneme Confined Aquatic Disposal Project.
• U.S. Army Corps of Engineers. 2003. Great Lakes Confined Disposal Facilities.

Performance

CAD cells and ECFs have been demonstrated to successfully contain contaminated sediments and reduce occurrences of contaminant release into the waterbody. Long-term performance is related to sustained containment and ensured through continued monitoring. Similar to in situ capping, instances of failure are more common within the first six months after closure. In certain instances, performance has been affected by human interference. For example, propeller use near the CAD cell has shifted cap material in sites with poorly maintained institutional controls.

Measures to improve sustainability or promote ecological remediation

• Using renewable energy and energy-efficient equipment for technology implementation.
• Reducing fuel consumption (and using renewable energy where available) for vehicles and heavy machinery.
• Optimizing the scheduling to promote resource sharing and reduce the number of mobilization days.
• Capturing and relocating the species at risk and sensitive habitats likely affected by remediation work.
• Working during periods of low risk to fish and fish habitat.
• Identifying site-specific regulatory resources (for example, fishing licences), sensitivities, and appropriate avoidance/mitigation measures.
• Using local services and service providers.
• Designing the cap to reduce the potential production of greenhouse gases in sediments underneath the cover.
• Designing the cap to improve aquatic habitat quality (increase ecological value of the environment).
• Using materials with the same composition and with similar characteristics as the native sediments, to promote habitat recovery by living organisms.
• Using customized mechanisms to prevent sediment resuspension.
• Locating the CAD cell near the location where sediments are removed, thereby reducing the distance required for in-water transportation of the material.
• Incorporating clean dredged sediments (for example, sediments removed in the creation of the CAD depression) into the capping layer. This reduces the cost of bringing in capping materials from offsite and transporting clean sediments offsite for reuse or disposal.
• Consolidating contaminated sediments into the cell to reduce occurrences of cell material resuspension and pooling out of the cell.
• Consider opportunities to treat and/or reuse the contaminated sediments to reduce the volume of material requiring CAD or disposal in an ECF.
• Placing the cap materials gradually to reduce mixing between layers.

Potential impacts of the application of the technology on human health

Potential Human Health Impacts

The minor and major potential human health exposure pathways are presented in the following table.

Potential Aquatic Impacts

Construction of a CAD cell or an ECF requires significant disruption of the existing benthic community, so locating the CAD cell/ECF in an area of sensitive habitat must be avoided, if at all possible. Organisms and habitats located on the CAD/ECF site must be removed prior to the start of construction. Construction activities may lead to a change in the food supply, temperature, and chemical makeup of the surrounding environment, which will inevitably lead to some level of benthic mortality and loss of habitat.

The scale of impact on benthic and aquatic organisms must be assessed, and methods of mitigating these impacts must be considered. Examples of mitigation efforts include changes to the project plan (such as the timeline and rate of sediment removal) and the project design (such as types of materials used and placement methods).

Short- and long-term monitoring plans should be developed for ongoing site maintenance. These activities will assess the recovery of the benthic community, the presence of bioaccumulative contaminants and contaminant concentrations in fish tissue.

Improved planning and operational efficiency have been shown to reduce resuspension and release of contaminants caused by dredging activities. Methods shown to limit sediment resuspension include selecting equipment based on the dredge design, slower dredging (as compared to navigational dredging, for example), dredging in waters with low energy, and using geospatial equipment to increase the accuracy of dredge movements.

In addition, to limit the amount of suspended solids during cell capping, the capping layer materials must be placed gradually and slowly.

References

• Aquatic Bioassay and Consulting Laboratories. 2004. Los Angeles Contaminated Task Force Confined Aquatic Disposal Site Long-Term Monitoring Program, 2002–2003, 112 p. (PDF)
• Birkelo, C. & Malone. J. 2013. A Unique Partnership for Contaminated Sediment Management: The Port of Hueneme Confined Aquatic Disposal Project. (PDF)
• City of Hamilton. 2023. Randle Reef sediment remediation project
• Environment and Climate Change Canada. 2022. Evaluation of Project Designs for Contaminated Sediment Management, 200p. (PDF)
• Fredette, T. J. 2006. Why Confined Aquatic Disposal Cells Often Make Sense. Integrated Environmental Assessment Management 2(1). p.35-38.
• Government of Canada. 2013. Federal Contaminated Sites Action Plan (FCSAP): FCASP Long-Term Monitoring Planning Guidance. 307 p. (PDF)
• The Randle Reef Sediment Remediation Project Technical Task Group AECOM. 2012. Randle Reef Sediment Remediation Project. Environment Canada. 457 p. (PDF)
• Pearlman L. 1999. Subsurface Containment and Monitoring Systems: Barriers and Beyond. U.S. Environmental Protection Agency. 66 p. (PDF)
• Port of Baltimore. 2021. Confined Aquatic Disposal (CAD).
• Rollings, M. P. 2000. Geotechnical considerations in contained aquatic disposal design. DOER Technical Notes Collection (ERDC TN-DOER-N5), U.S. Army Engineer Research and Development Center, Vicksburg, MS. (PDF)
• United States Environmental Protection Agency (U.S. EPA). 2023. New Bedford Harbor Cleanup Plans, Technical Documents and Environmental Data.
• Unites States Army Corps of Engineers. 2001. Monitoring of Boston Harbor Confined Aquatic Disposal Cells. 114 p. (PDF)
• United States Army Corps of Engineers. 2003. Great Lakes Confined Disposal Facilities (PDF)
• Waters, R., Lee, D. 2002. Regional Background Report: A Review of Confined Aquatic Facilities. Environment Canada. No. PR02-02. 25 p. (PDF)

Author and update

Composed by : Ashley Hosier, P.Eng. Royal Military College of Canada

Updated by : Frédérick de Oliveira, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.

Updated Date : February 3, 2017

Latest update provided by : Juliette Primard, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.

Latest update date :March 31, 2024