From: Public Services and Procurement Canada
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.
Contaminated sediment is dredged and then placed within a natural or dredged depression and capped suitably. The process may include the following steps:
Contaminated sediment remains on-site and is isolated using containment structures. The process may include the following steps:
None.
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.
In situ disposal options apply best in the following situations:
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.
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.
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”).
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.
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.
No secondary treatment is required if the CAD cell or ECF achieves remediation objectives.
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.
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.
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).
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
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.
Composed by :
Latest update provided by : Ashley Hosier, Ing., Royal military college
Updated Date : December 12, 2016
