From: Public Services and Procurement Canada
Capping is a containment technology that isolates contaminated sediments from the surrounding aquatic environment using clean layers of geological material and/or synthetic liners. By isolating contaminated sediments and significantly reducing the process of sediment transport by water, capping rapidly eliminates exposure pathways between contaminants and the natural environment, thereby preventing contamination of the latter.
Conventional or in situ capping refers to capping materials that act as a passive barrier to sediments and water. Caps may be permeable, semipermeable or impermeable. The surface of the capping layer can also be designed to improve the ecological value of the substrate by providing habitat for native flora and fauna.
When capping is insufficient to manage the remediation objectives, amendments may be incorporated into the capping materials to improve their effectiveness, known as capping with amendments.
The capping approach is based on sediment immobilization, physical isolation between the sediments and the aquatic environment, and chemical isolation, which reduces chemical reactions and transfers between contaminants, pore water, and overlying water.
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Implementation of this technology may include:
Caps may be permeable, semipermeable, or impermeable. Options for overlay materials include clean sediments, silt, sand, or gravel, as well as impermeable materials such as clay. On-site storage may include these materials, as well as fuels, lubricants, amendments, and other site materials required for operating the machinery and equipment to implement the technology.
Specialized synthetic layers (such as geomembranes) can be used in combination with natural materials to increase cap effectiveness.
Geosynthetics are sometimes deployed over low-dispersion sediments to support the overlain capping materials. A protective armouring layer, often consisting of coarse material (gravel, stone) can be installed over the capping material (sand) to prevent the suspension and erosion of the capping layer in unstable areas (such as those vulnerable to erosion). These materials should be stockpiled and covered on-site to minimize dust and protect them from precipitation.
When vapour and/or water treatment components and treatment systems are incorporated into the cap, they may also be stored on-site.
According to local waste disposal guidelines, excess capping materials can be reused or disposed of.
Soluble and buoyant capping materials and amendments may enter the water column during placement, causing increased loading in the water column and potential downstream migration. Resuspension of contaminants in the water column may occur during placement of the sediment cover. The timing of the remediation work should be selected to minimize migration.
Surface water run-off from unprotected materials may enter the waterbody (if storage has not been properly organized). Stockpiled material should be covered/contained, and potential run-off should be captured and treated prior to discharge.
Biodegradation of contaminants or organic matter, for example, may lead to off-gassing (such as carbon dioxide, ammonia and methane and/or hydrogen sulphide) under the cap. Vents should be included in cap designs to prevent gas releases into the environment, where potentially noxious gases are likely formed.
In addition, vapour discharge may be released from equipment exhaust or from the volatilization of contaminants from contaminated sediments in place prior to capping. Also, gaseous emissions from sediments containing high levels of organic matter content can cause odour issues. Workers who find themselves in the potential presence of odours or volatile compounds must take precautions to prevent gas emanations by monitoring concentrations, using adequate personal protective equipment and/or by conducting sediment capping work during colder temperatures.
Capping applies to both organic and inorganic contaminants. This method has a high potential for success when the extent of contamination is well defined and at depths between 1.5 and 15 metres. Typically, capping is used when contaminants are present in the solid phase of sediments or when contaminants are adsorbed onto the solid phase. Capping could also be applied to control dissolved contamination in pore water and free-phase contamination (such as dense non-aqueous phase liquid). The demonstration work required for some types of contaminants can be complex and expensive.
Periodic inspections of the cap conditions and long-term monitoring are required to ensure the cap’s integrity and effectiveness.
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The timeframe for installation typically ranges from one to four months. Frequent inspections are required for the first six months, the period in which cap failure is most likely to occur.
Capping has relatively few adverse effects on the site, so there will be little or no long-term considerations. In addition, because contaminated sediments are left in place, capping does not require the treatment, transportation, and disposal of sediments, which is a significant advantage that reduces the environmental impacts of capping (such as greenhouse gases and energy requirements).
Most cap integrity failures occur within the first six months following cap placement. Post-placement, the natural environment will act on sediments and the capping materials, resulting in changes to the cap layers. Bioturbation, groundwater intrusion, contaminant migration, and erosion may all influence the success of the cap. Periodic events, such as floods or water-level changes, may also result in changes to the cap layers.
Implementing institutional controls (such as bans on dropping anchors or trawling within the capped area) may be necessary for capping success in locations where human activity is expected. Therefore, it is recommended to carry out annual checks of the physical integrity and a survey of the entire area every five years.
Capping may induce anaerobic conditions in the sediments’ uppermost layer, resulting in methane and sulphide gas production beneath the cap. Anaerobic biodegradation of some compounds can generate hazardous by-products.
If sediments contain mercury, the capping conditions could transform it into methylmercury (Randall et al. 2013). Other chemicals contain hazardous metabolites and degradation by-products, such as tetrachloroethene (PCE), a common dry-cleaning substance, and dichlorodiphenyltrichloroethane (DDT), an insecticide commonly used before 1970.
One of the issues associated with gas production in sediments beneath the cap is the risk that this gas production will increase pore pressures and affect the integrity of the cap. As a result, cracks or fissures could form, compromising the stability of the cap and the release of contaminants into the environment.
Capping with amendments is considered as a modified capping technology. It involves incorporating biological and/or chemical amendments into the overlay material. Modified capping employs the same reagents as biodegradation, sequestration and chemical oxidation, and can deliver improvements similar to those associated with in situ treatments. Examples of reagents include adsorbents (such as activated carbon), oxidants, reducers (such as zerovalent iron), nutrients or products that reduce the hydraulic conductivity of the capping material (such as bentonite or organophilic clays) and minimize contaminant transfer to bioturbation zones or the water column (ITRC 2014).
Gas may be generated under the cap. Long-term secondary treatment may be considered to control gas emissions into the environment.
Applications examples are available at these links:
The performance of capping varies depending on its design, sediment consolidation, advection, the additives used (if any), the specific characteristics of the site, and the type of contaminants present. Because of this, cap effectiveness relies on the suitability of the design and quality of the installation. Lifespans for capping projects are dependent on the type and quantity of contamination as well as contaminant fate and transport mechanisms operating within the cap. Experience has demonstrated that the predicted lifespan is in the order of decades.
The minor and major potential human health exposure pathways are presented in the following table.
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site & Off-Site)
Monitoring
Mitigation Measures according to Residency or Transport Media
Site Preparation
Dust
Inhalation of Dust
Dust Monitoring
Vapours
Sediments
Surface Water
Groundwater
Animals and Plants
Inhalation of Vapours
Air Quality Monitoring
Capping
Stockpiling
Emissions Monitoring and Critical Thresholds
(contaminated by sediments)
Ingestion of Drinking Water
Country Foods Consumption
Groundwater Monitoring
Dermal Contact
Wastewater Monitoring
Surface Water Monitoring
Capping Material Inspection
Incidental Ingestion
(including fish, shellfish and wildlife)
Animal and Plant Tissue Monitoring
Monitoring and Testing of Sediments
Vapour Collection and Treatment
(Gas emission underneath capping)
(Capping Integrity/contaminant transport)
Sediment capping can lead to changes in nutrient availability and habitat conditions for aquatic species. Changes in geochemical conditions (salinity, dissolved oxygen, and temperature) can also occur. These changes can negatively impact aquatic species and result in their displacement or increase in mortality.
Activities and stressors associated with capping contaminated sediments that may impact fish and fish habitats include placing material or aquatic structures, removing aquatic vegetation, modifying flow, altering fish passage and managing organic debris.
Mitigation measures may be considered to reduce benthic mortality and encourage redevelopment and habitat restoration. These include the use of sediments with similar composition and characteristics as native sediments to promote redevelopment, as well as the removal and replacement of endangered or sensitive organisms and habitats.
Following remediation work, short- and long-term monitoring must be carried out to ensure site restoration.
Composed by : Bruno Vallée M.Sc, LVM Inc.
Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada
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
