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

From: Public Services and Procurement Canada

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Description

In situ disposal generally follows dredging/excavation activities and is used as a means of containing contaminated sediment on-site. There are two main options for disposing of contaminated sediments in situ: confined aquatic disposal (CAD) cells and engineered containment facilities (ECF).

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. Man-made depressions are caused by historical mining or dredging activities, or created expressly for the purposes of the CAD cell.

ECFs are containment structures constructed within the sediment. They are designed to surround highly contaminated sediments with rigid impermeable walls. The contaminated sediment is shielded from disturbance that may cause resuspension into the water column; an occurrence with other in situ remediation options (such as CAD cells, Capping). These walls extend deep within the sediment to contain the contaminants and provide structural support. The walls extend up toward the surface, creating capacity for additional sediment. 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 similar materials as conventional capping (see fact sheet for Capping—Sediments). In general, ECFs offer a greater level of containment than capping does, but less environmental control compared to contaminant removal. An ECF may be designed to extend above water-level, providing additional area for building and work uses; see Randle Reef ECF (Hartman, 2012) under the heading “Application Examples.”

CAD cells and ECFs are typically located near the dredged/excavated area, reducing the need for transportation of contaminated sediments to off-site disposal facilities. Additionally, treating and disposing of contaminated sediment within the contaminated area negate the possibility that a pristine area may be affected by contaminant re-suspension associated with capping in an undisturbed location. Similar to other in situ sediment treatment options, there are effects to the benthic community due to habitat destruction and disturbance associated with sediment removal, burial, and construction of CAD cells/ECFs.

CAD cells and ECFs consolidate heavily contaminated materials, thereby reducing the overall footprint of a contaminated site and removing contaminant transformation and exposure pathways. A common situation where they are considered is in the removal and containment of “hot spots” (discrete locations of highly contaminated sediments) at sites where surrounding lower levels of sediment contamination may be treated in situ. Risks associated with CAD cells are similar to those associated with subaqueous capping (see Capping—Sediments fact sheet); however, there may be fewer restrictions on future site uses such as sailing or navigating a boat over the cell, as the elevation change with CAD cells is generally less than that of a subaqueous cap. 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 depth of cap and sediment beneath the cap. ECFs typically extend to near or above surface levels, increasing the footprint of the surrounding terrestrial area, but affecting the navigable depth.

Conditions within the CAD cell/ECF often become anaerobic or anoxic. The lack of oxygen may provide an avenue for the slow degradation of contaminants such as methyl mercury and PCBs, but may lead to gas buildup and release through the cap. When a significant amount of off-gassing is expected, vents may be incorporated into the cap design.

Monitoring is a significant portion of CAD and ECF implementation. Construction monitoring is carried out during the placement of materials and structures to ensure accuracy to the design specifications (such as berm thickness, structure location, volume available for sediment containment). Long-term monitoring of the integrity and performance of the containment structure is also required, as well as assessing remediation of contained contaminated sediments and benthic community recovery.

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Implementation of the technology

CAD process

Contaminated sediment is dredged and then placed within a natural or dredged depression and capped suitably. The process may include the following steps:

  • determine 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;
  • estimate the volume required for CAD cells, incorporating sediment bulking and necessary freeboard. This volume can then be used to design the size requirements for the CAD cell, as well as the volume of material required for the final cap;
  • complete a detailed site characterization of the bottom sediment including geotechnical parameters. The depth to bedrock, groundwater flow, and sediment shear strength (the ability of the sediment to resist a sliding fault) will all affect the physical capacity of a location to support sediments and effectively contain contaminants. In the case where sediment shear strength or flux through the native sediment precludes the ability to adequately contain the contaminated sediments, engineered structures (such as sheet metal side walls) may be incorporated to improve shear strength and reduce flux potential;
  • stockpile the clean sediment removed in the creation of the CAD depression on-site. The clean sediment 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 sediment and the material has consolidated sufficiently, apply the cell cap. Cap placement is similar to that of traditional capping, where material is sprinkled onto the water surface and allowed to settle through the water column. Material may be applied mechanically, either on shore or from a floating barge. Amendments such as activated carbon, and engineered materials (such as geotextiles) may be incorporated into the cap as a means of improving containment or contaminant degradation. See Capping—Sediments fact sheet for more information;
  • following cap placement, take several borehole samples to determine whether the cap thickness requirements have been met. Additional capping materials may be required;
  • once cap thickness has been reached, apply any necessary armouring materials;
  • conduct long-term monitoring to ensure integrity of contaminant containment and to quantify contaminant flux through the cap, thereby ensuring ongoing environmental safety. Institutional controls (such as restrictions on navigation) may be required to protect the cap from damage by human activities.

Modifications to Process for ECF

Contaminated sediment remains on-site and is isolated using containment structures. The process may include the following steps:

  • delineate the contamination, outlining contaminant “hot spots” and total volume of contaminated material;
  • determine optimal location for engineered containment, including overall size and depth;
  • complete a detailed site characterization using available hydrogeological information, including shear strength, sediment type, and pore size of the existing sediment. The site characterization will be used to provide information on the contaminant migratory patterns, and highlight whether the current sediment is capable of supporting the containment structure;
  • install containment walls into existing sediment. Installation may require site preparation, such as removal of rock debris. Walls may be installed through the use of a pile-driver, which may be anchored on the ground or on a stationary floating barge;  
  • remove water from the ECF, treating as necessary, and disposing back into the water body;
  • begin filling the ECF with dredged contaminated sediment. As sediments settle, remove, treat, and dispose of carrier water;
  • as needed, incorporate amendments, such as activated carbon, into the contaminated sediment to stimulate the remediation process;
  • once ECF is full, 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;
  • implement any necessary institutional controls to prevent human exposure and recontamination;
  • conduct long-term monitoring to ensure integrity of contaminant containment and to quantify the contaminant flux through the ECF, thereby ensuring ongoing environmental safety.

Materials and Storage

  • Materials stored on-site typically consist of small amounts of fuel and lubricant, as well as some construction supplies.
  • 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 sealed vessel to prevent contaminant transport through leaching into the ground, surface water runoff, and evaporation losses.
  • Wastewater associated with contaminated sediments may require separation and containment.
  • Depending on the project, CAD may be combined with other technologies, such as biodegradation or chemical oxidation. In this case, additional amendments, geotextiles, and engineered structures may also be stored on-site.

Waste and Discharges

  • Site waste is typically limited to small amounts of construction materials, including oil and lubricants, as well as plastic bags and containers, and may include used/spent sorbent pads.
  • Dredging and excavating activities may produce large amounts of sediment that requires repurposing and/or disposal. Contaminated sediment may need to have the bulk of the water removed prior to placement into the CAD cell or the ECF.
  • Any waste water removed from the contaminated sediments requires treatment prior to disposal back into the environment. Waste water may be treated on-site, or transported off-site to a treatment facility.
  • Stockpiles of clean sediments may be repurposed as capping material for use on—or off-site, or for some other beneficial reuse. For more information on waste and discharges associated with capping materials and amendments, please refer to the Capping—Sediments fact sheet.
  • Gas generation and buildup may occur under the cap, and create cracks and fissures as the gas tries to escape. Gas generation is particularly common in the anaerobic degradation of organic contaminants. Vents may be installed into the cap if gas buildup is a concern.
  • Contaminant release through sedimentation may occur during placement of sediments into the CAD depression. Fine sediments, or sediments placed in turbulent water, may become suspended in the water column. Alternatively, as the cell is filled, the force of adding additional material may cause already-filled sediment to escape the cell boundaries in a mud wave. Proper monitoring and meticulous filling of the cell will help reduce occurrences

Recommended analyses for detailed characterization

Biological analysis

  • Characterization of the benthic community

Chemical analysis

  • Total suspended solids
  • Non-aqueous phase liquids (NAPL) distribution (surface and subsurface)
  • Conductivity and total organic carbon content of the sediment and pore water

Physical analysis

  • Sediment water content
  • Sediment particle size distribution
  • Shear strength
  • Contaminant physical characteristics
  • Sediment load-bearing capacity
  • Bulk unit weight
  • Percentage of solids
  • Specific gravity

Hydrogeological Analysis

  • Bottom velocity
  • Bed stress
  • Gross sedimentation
  • Sediment core profiles
  • Sediment-water flux rates

Recommended trials for detailed characterization

None.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration
  • Physical-chemical characterization of sediments and interstitial water
  • Bathymetry
  • Detailed evaluation of biological conditions and ecological factors
  • Characterization of hydrodynamic conditions includes current measurements, wave action, bed stability, etc.

Phase III

  • Identification of preferential pathways for contaminant migration
  • Geotechnical characterization of sediment deposition

Notes:

Geotechnical laboratory testing is recommended to determine the behaviour of the sediment and capping material.

Knowledge of currents, wave action, and tidal patterns is required to estimate the potential loss of sediment and capping materials into the overlying water. Consider consulting a professional hydrologist, hydrogeologist, or environmental/water-resource engineer to develop an overview of site conditions.

Applications

In situ disposal options apply best in the following situations:

  • environmental dredging/excavation is planned or has occurred, and the sediment requires disposal;
  • off-site 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;
  • the improvements gained through contaminant remediation outweigh the short-term losses to the benthic community caused through creation of the CAD cell or ECF;
  • human activities are easily controlled through institutional controls, minimizing actions that may disrupt the cell cap or ECF;
  • natural hydrodynamic conditions are unlikely to compromise the CAD cell, are capable of supporting the ECF containment walls, or can be accommodated through engineering solutions;
  • contaminants have low rates of flux;
  • 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 use of an ECF compared with CAD

  • Removal of a portion of the contaminated sediment may cause undue harm to the water body, favouring confinement in situ.
  • Human activities would benefit from the increased useable space created through the closure and capping of the ECF.

Applications in Northern Environments

CAD cells and ECFs have limited applications in remote locations for a number of reasons. Constructing a CAD cell or an ECF requires for specialized equipment for dredging and cap placement, as well as access to necessary materials for cell/ECF structure and capping. The process of constructing, filling, and capping of a CAD cell or an ECF is also limited by the short work windows associated with Arctic environments. In addition, the CAD and ECF design must account for processes such as ice scour, which are common in coastal Arctic environments.

Monitoring of the CAD cell or ECF post-construction is also extensive, requiring regular testing of cell thickness and contaminant concentrations. Transportation of necessary equipment and manpower to conduct site monitoring will be logistically challenging and costly.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Does not exist
Control
Applies
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Does not exist
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
With restrictions
Metals
Applies
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Applies
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Applies
More than 5 years
Does not apply

Notes:

Comments: The time required to complete remediation activities will fluctuate with the scale of the project. The volume of contaminants, the transportation distance required (from dredge location to CAD location), and 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.

Similar to in situ capping, CAD cells rapidly reduce risks of contaminant exposure to ecological receptors while promoting long-term remediation of the confined contaminants. Frequent monitoring is required for the first six months, the time period in which cap failure is most likely to occur.

Long-term considerations (following remediation work)

As long as contaminants remain in place, continued monitoring is required to ensure the integrity of the cap/engineered structure. Monitoring parameters includes cell integrity, advective flow through the cell, and contaminant migration. The effectiveness of the CAD cell/ECF should be monitored until all remediation objectives have been met, with more frequent monitoring occurring immediately post-closure, followed by regular, infrequent monitoring (such as annually). Monitoring should include contaminant concentration and flux surrounding the CAD cell/ECF, and benthic uptake within the surrounding area. 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.

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 and/or recreational use.

Plans should be prepared and funded for the monitoring and maintenance of the CAD cell for as long as the contaminant risk remains (and possibly “in perpetuity”).

Secondary by-products and/or metabolites

Anaerobic conditions may occur within the confined sediments. Decomposition of organic compounds within the cell may lead to the production of sulphide gas and methane. The buildup and release of gases beneath the cap may cause fissures or cracks that could affect the integrity of the cap or containment facility.

Limitations and Undesirable Effects of the Technology

Limitations and Restrictions

  • Creation of a CAD cell, even within a natural depression, or an ECF causes destruction of the benthic community and associated habitats located on the site. Areas with species at risk and sensitive habitats may not be good candidates for CAD/ECF technologies.
  • 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 (see fact sheet CappingSediments for more information).
  • Areas with significant groundwater flow are not suitable, as upwelling may compromise the integrity of the CAD cell/ECF, causing unacceptable contaminant release.
  • CAD cells are not recommended for sediments with a low shear strength, as these sediments may not support the creation of a depression, and may continue to collapse inward during construction or filling. Sediment shear strength should also be sufficient to support containment walls for ECFs.
  • 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.
  • Large amounts of clean sediment are required for capping of CAD cells/ECFs.
  • Creation of a CAD cell or an ECF may place limitations on the future site use, the available navigable depth, and the flood-bearing capacity of a waterway.
  • Institutional controls, such as restrictions on navigation, are required to reduce potential disruption of the CAD cell/ECF by human activities (such as anchoring over the CAD cell, potentially rupturing the cap).
  • Sizing of the CAD cell/ECF is linked to the amount of material dredged. Dredging more material than originally estimated, or underestimating sediment bulking, may require redesign.
  • 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 should be monitored for any instances of contaminant surging.
  • Monitoring and maintenance of the cap “in perpetuity,” or for as long as the contaminant risk remains, are required.

Adverse Impacts

  • Contaminants remain in the aquatic environment, and may be subject to loss, re-exposure, or disturbance.
  • There is a risk of contaminant release/resuspension with the handling and placement of contaminated sediment. The release or resuspension of contaminants may create exposure pathways, leading to increased uptake by the benthic and aquatic communities.

Complementary technologies that improve treatment effectiveness

CAD cells may incorporate engineered structures, which improve physical strength and stability as well as containment ability. Similar to capping, CAD cell caps may be combined with biological, chemical, or immobilization treatment, which aids in the remediation of the contaminated sediment.

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) as a means of treating the contaminated sediment.

Required secondary treatments

No secondary treatment is required if the CAD cell or ECF achieves remediation objectives.

Application examples

Performance

CAD cells and ECFs have been demonstrated to successfully contain contaminated sediment and reduce occurrences of contaminants release into the water body. As CAD cells and ECFs primarily provide containment and disposal of contaminated sediments, as opposed to treatment, 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, propellers used near the CAD cell has shifted cap material in sites with poorly maintained institutional controls.

Measures to improve sustainability or promote ecological remediation

  • CAD cells and ECFs may be made more sustainable through the following:
  • consider opportunities to treat and/or reuse the contaminated sediments to reduce the volume of material requiring CAD or disposal in an ECF;
  • situate the disposal site near the location where sediments are removed, thereby reducing the distance required for in-water transportation of the material;
  • incorporate clean dredged sediment (for example, sediment removed in the creation of the CAD depression) into the capping layer. This reduces the cost of bringing in capping materials from off-site and transporting clean sediment off-site for reuse or disposal;
  • before applying the cap, allow for sufficient time to achieve consolidation of the contaminated sediment within the cell. Consolidation will reduce occurrences of cell material resuspension and pooling out of the cell;
  • place the cap materials gradually to reduce the amount of cap-fill mixing at the cell site;
  • plan cell dredging and filling activities to avoid significant seasons, such as spawning or commercial fishing and harvesting;
  • plan monitoring activities to reduce travel and mobilization of resources;
  • develop a site-specific plan that reduces energy, material, and water consumption; minimizes harmful air emissions, spillage, and waste generation; and protects ecosystems during cleanup. For example, to the extent possible:
    • use local services and service providers;
    • sequence work to minimize double-handling of sediments;
    • use energy-efficient equipment and optimize equipment performance to maximize efficiency;
    • use equipment with enhanced emission controls;
    • reduce idling of equipment;
    • Implement sustainable site-monitoring methods, such as passive sampling devices for site analytical techniques and telemetry (remote data collection).

Potential impacts of the application of the technology on human health

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

Mitigation of Aquatic Impacts

The location should be examined for species of special significance and sensitive habitats, which both may be removed and relocated prior to the start of construction. The scale of impact on benthic and aquatic organisms should be assessed, and methods of mitigating these impacts should 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).

Major Human Health Exposure Pathways

Exposure Pathway Triggers (Remediation stages)

Residency or Transport Media

Public Exposure Routes (On-site and Off-site)

Monitoring, Action Levels, & Mitigation Approaches

Stockpiling Cap Materials

Dust

Inhalation of particulates

Cover the piles. Use water to suppress dust, as necessary. Use Personal Protective Equipment (PPE) when handling material.

Sediment (runoff leading to sedimentation of surface water)

Ingestion of drinking water; direct contact while swimming

Cover the piles to minimize generation of runoff. Monitor visual indicators of sediment erosion (i.e., rills).

Filling CAD cell/ECF

Contaminated sediment

Skin contact, inhalation of volatile contaminants

Educate staff on safety and provide appropriate PPE and reactionary materials (such as sorbent pads), as necessary. Avoid handling very dry sediments to reduce the occurrences of inhalation of particulates.

Installation of ECF containment/CAD cap

Dust

Inhalation of particulates

Avoid the use of very dry materials in cap formation. Apply material with minimal impact, to reduce dusting. Equip personnel with PPE to prevent exposure.

References

Author and update

Composed by :

Latest update provided by : Ashley Hosier, Ing., Royal military college

Updated Date : December 12, 2016

Version:
1.2.1