Fact sheet: Capping—sediments

From: Public Services and Procurement Canada

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Description

In situ capping involves covering contaminated sediment with one or more layers of material such as sand, gravel, or geomembranes in an effort to chemically or physically isolate and immobilize contaminants. In situ capping allows the rapid reduction of exposure risks, yielding the shortest timeframe in comparison to Monitored Natural Recovery (MNR), dredging and excavation. By isolating the contaminated sediment and minimizing water transport processes within the sediment, in situ capping eliminates exposure pathways between the contaminants and the benthic community. This significantly reduces potential risks from exposure to contaminants in the sediment.

Caps may be permeable, semi-permeable, or impermeable. Options for overlay materials include clean sediment, silt, sand, or gravel, as well as impermeable materials such as clay. 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 material, generally composed of sand. 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 locations susceptible to erosion). 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 remediation objectives, amendments may be incorporated into the cap material, to enhance effectiveness. Various amendments have been shown to reduce permeability and flux through the cap, increase the sorption capacity of the cap layer, and enhance contaminant transformation and degradation. Amendments may be incorporated directly into the contaminated sediment (referred to as in situ treatment), or into the cap material (referred to as amended capping). Some examples include low-permeability clays, bentonite, organophilic clays (effective for the containment of non-aqueous phase liquids), activated carbon, nutrients and nutrient-rich media such as compost, and minerals such as apatite, zeolites, and zero-valent iron.

Capping is ideally suited to contaminated areas that are located in stable environments, where climate fluctuations (such as storms, floods, earthquakes) and future use activities (such as dredging, creating new infrastructure) are predictable and may be included in design consideration. Capping has been commonly used following dredging of harbour sediments, as a means of isolating the contaminants and/or managing residuals. Capping of lakes and rivers has increased in popularity in the U.S., but is still relatively uncommon in Canada.

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Implementation of the technology

The design goals of in situ capping include stabilization and chemical and physical isolation.

stabilization of contaminated sediments occurs when sediment is protected from erosive forces through armouring (placement of gravel or stone), in an effort to prevent re-suspension and transport of contaminants to other sites;

physical isolation of contaminated sediments reduces interaction and contact with the benthic community, the primary means for contaminant transport and trophic transfer;

chemical isolation of contaminated sediments reduces chemical reactions and transfer between the contaminants and the interstitial and overlaying water. This reduces the potential of contaminants flux into the biologically active zone.

Achieving successful results with sediment capping requires a well-thought-out engineering design, with carefully planned implementation and monitoring. Determining the extent of the cap and the materials to be used will depend on-site conditions, contaminants present, and sediment characteristics.

In general, the process of developing an in situ cap system may include:

  1. assessing the suitability of the site for capping (including consideration of current and future uses);
  2. designing the cap. Design considerations include:
    • the extent to which the underlying contaminated sediment is capable of physically supporting the cap design, while minimizing potential for site erosion;
    • how to minimize the bioturbation potential of native biota;
    • the resulting changes to water depth at the site;
    • improvements to the existing benthic community and protection of species at risk, and;
    • the use of vents to manage gaseous emissions in areas of significant organic decomposition, where gas build-up and bubbling may create cracks and fissures within the cap;
  3. preparing the site for installation of the cap (this may include dredging, in situ contaminant treatment, and habitat/species removal/relocation);
  4. determining the equipment required for cap placement. Equipment selection depends on-site specific conditions, and may include standard or bottom-dump barges, hoses, and pumps;
  5. placing the capping materials:
    • place the cap materials near the water surface and allow the material to gently settle through the water column. Mechanical or hydraulic placement may be necessary, to reduce the occurrence of sediment resuspension;
    • monitor uniformity and cap thickness once the layer has been allowed to settle;
    • continue laying cap materials until designed uniformity and thickness has been achieved;
    • place armoured layer (such as stones, geotextiles, armoured mats) on top of cap, if necessary;
  6. implementing a long-term monitoring plan to ensure ongoing performance success of the cap and ecosystem recovery.

Materials and Storage

Contractors may create stockpiles of cap material prior to placement. On-site stores typically consist of small amounts of fuel and lubricant (daily fuelling is often from a mobile tank) as well as miscellaneous construction site supplies.

Common capping materials consist of sand, silt, clay, gravel, armour stone, and geotextiles. These materials should be stockpiled and covered on-site, to minimize dust and protect them from precipitation.

When treatment components are incorporated into the cap, treatment materials may also be stored on-site. Some commonly used amendments are activated carbon, apatite, organophylic clay, zeolites, low-permeability clays, nutrients, and zero-valent iron.

Waste and Discharges

  • Site garbage is typical of a construction site, but in addition it may contain used/spent sorbent pads. Excess capping materials may be reused or disposed of according to local waste disposal guidelines.
  • Soluble and buoyant capping materials and amendments may enter the water column during placement, causing increased loading into the water column and potential migration downstream. Contaminant resuspension into the water column may occur during sediment cap placement, or if capping is combined with alternate in situ remediation technologies.
  • Surface water run-off from the unprotected materials can enter the water body, and may negatively affect the native biota. Stockpiled material should be covered/contained, and potential run-off should be captured and treated prior to discharge.
  • Vapour-phase discharges are typically limited to equipment exhaust, but may also occur through off-gassing. Biodegradation, particularly for hydrocarbons, may lead to off-gassing (such as carbon dioxide, ammonia and methane and/or hydrogen sulphide) under the in situ cap. To prevent gas releases to the environment, vents should be included in cap designs where the formation of potentially noxious gases is likely.

Recommended analyses for detailed characterization

Chemical analysis

  • Conductivity
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Non-aqueous phase liquids (NAPL) distribution (surface and subsurface)

Physical analysis

  • Soil granulometry

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
  • Characterization of hydrodynamic conditions includes:
    • current measurements
    • wave action
    • bed stability
    • etc.
  • Physical-chemical characterization of sediments and interstitial water
  • Bathymetry
  • Characterization of the physical environment includes:
    • the size of the water body
    • tidal influence
    • ice regime
    • aquatic vegetation
    • the presence of bridges
    • proximity to land
    • marine structures

Phase III

  • Conceptual site model with hydrogeological and geochemical inputs
  • Geotechnical characterization of sediment deposition
  • Characterization of the benthos
  • Surface water usage:
    • characterization of the present and proposed surface water usage and the water body in general (including the required draft for vessels)

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 oxidant into the overlying water.

Applications

Capping applies to both organic and inorganic contaminants and can be considered an option for contaminated site remediation under the following conditions:

  • the anticipated risk of human exposure to the sediment contamination is high and not addressed well by institutional controls;
  • the contamination covers a continuous area (in other words contaminants are not distributed over multiple spatially separated areas);
  • the waterways have low flow (no rapidly eroding streambeds), low groundwater upwelling or tidal pumping (velocities less than 1 cm/day), and limited wave effects;
  • contaminants are in the sediment solid phase or contaminants are adsorbed onto the solid phase. Under certain conditions capping may apply to contamination dissolved in interstitial water (in other words water between sediment particles), and to free phase contamination (such as dense non-aqueous phase liquid);
  • capping is optimal at depths of 1.5 m to 15 m. Below 1.5 m, there is insufficient headspace to install a cap without significantly altering the waterbody (such as a permanent restriction on navigation may be required). While conventional methods for placing sand caps are ineffective at depths greater than 15 m, it is possible with specialized equipment;
  • long-term risk reduction of the cap outweighs the immediate sedimentation and habitat disruption potentially caused by cap installation;
  • risk to cap integrity from human behaviour is manageable through institutional controls, such as restrictions on dredging;
  • capping is compatible with future site uses and expected infrastructure needs (piers, pilings, buried cables) and anticipated uses (navigation, flood control).

Applications to sites in northern regions

Remediation of sites in northern environments poses unique challenges. Sites are inherently remote and may be difficult to access. Much of the equipment required for site remediation must be transported by boat or plane, possibly from hundreds of kilometres away and at great cost. Climate restrictions (such as cold temperatures and ice conditions) and short seasonal windows to conduct work may limit remediation options.

Remediation may require the placement of restrictions or limitations on the human consumption of native organisms when contaminated sediment is present. Because local communities may rely on aquatic animals (such as seals and whales) and fish as important sources of food, these restrictions may have a significant impact on communities.

The location and suitability of cap materials, as well as the specialized equipment required for cap placement may be key limiting factors for cap installation in remote locations and northern environments. The long-term monitoring required for sediment capping may pose logistical challenges for remote regions and be very costly if frequent site visits are required.

Shallow coastal areas in northern environments are also commonly affected by ice scouring from icebergs and sea ice, which is a limitation for the feasibility of capping. The effects of climate change are of particular relevance for long-term management of caps in northern sites, as design conditions may be altered, affecting the life of the cap.

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
Does not exist
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
Does not apply
More than 5 years
Does not apply

Notes:

Comments: 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.

Long-term considerations (following remediation work)

The majority of failures in cap integrity occur within the first six months following cap placement (FCSAP, 2013). Post-placement of the cap, the natural environment will act on the sediment and the capping materials resulting in changes to the cap layers. Bioturbation, groundwater intrusion, contaminant migration, and erosion may all have an effect on the success of the cap. Periodic events, such as floods or water-level changes may also result in changes to the cap layers. Long-term success of contaminant capping is possible with continued monitoring and sampling of materials to evaluate cap effectiveness and integrity. Performance/maintenance monitoring programs should be funded for as long as the contaminant risk remains (this may be “in perpetuity”) to ensure that the following performance objectives are being achieved:

  • the chemical integrity of capping material is maintained over time and following disruptive events;
  • the physical integrity of the capping material is maintained over time and following disruptive events;
  • the hydrodynamic and sediment transport impacts on the cap are acceptable and predictable.

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. The US EPA (2005) recommends annual checks of the physical integrity, as well as a survey of the entire area every five years.

Secondary by-products and/or metabolites

Capping may induce anaerobic conditions in the uppermost layer of sediment resulting in the production of methane and sulphide gases beneath the cap. Anaerobic biodegradation of some compounds can generate hazardous by-products. Randall et al. (2013) found that methylmercury was produced under an in situ sediment cap containing metallic mercury. Other chemicals that have toxic metabolites include tetrachloroethene (PCE), a common substance used in dry cleaning, as well as dichlorodiphenyltrichloroethane (DDT), an insecticide commonly used prior to 1970. Metabolites may be a concern in terms of both the release of these gases to the environment and the potential for gas bubbles to form fissures or cracks that compromise the stability of the cap.

Limitations and Undesirable Effects of the Technology

The primary disadvantage of capping is that contaminants remain in place, resulting in an ongoing risk of contaminant loss, re-exposure, or disturbance of the contaminated sediment. Other limitations of using capping as a remedial strategy are as follows:

  • contamination in underlying sediments is not treated other than by natural attenuation;
  • there is a risk of contaminant migration through diffusion and advection, particularly when contaminants show the following characteristics: easily transported through interstitial water, low association with sediment grain size, and presence of NAPLs;
  • extreme weather events (such as storms, flooding, and earthquakes) as well as ice jams and ice scouring may compromise the stability of a sediment cap;
  • physical site conditions (such as groundwater upwelling, significant bioturbation potential, native sediment inability to support the cap weight, and required access to underwater structures) may restrict the use of capping;
  • access to cap materials and specialized equipment required for cap installation may limit the feasibility of this technology in certain locations;
  • cap placement may affect available navigable depth and flood-bearing capacity of a waterway. Institutional controls may be required to maintain cap integrity (such as ban on anchoring, drag-fishing, or direct hull contact);
  • local regulations may not allow capping in some areas;
  • long-term monitoring and maintenance of the cap is required.

Potential adverse effects include the following:

  • increased turbidity or suspended sediment within the water column (due to placement of capping materials);
  • resuspension of contaminated sediments, caused by disruption of contaminated sediment-water interface during cap placement;
  • permanent or temporary alteration of benthic habitat and may alter the use by native biota.

Complementary technologies that improve treatment effectiveness

Capping performance may be enhanced through in situ sediment pre-treatment, such as biodegradation or chemical oxidation (Palermo et al., 1998 and ITRC, 2014). Prior to capping, contaminant hot spots or highly mobile contamination may be removed through dredging or excavation.

An enhanced form of capping, deemed amended capping, involves incorporating sediment amendments and oxidizers into the cap material. Amended capping uses the same reagents used in biodegradation and chemical oxidation, and can provide similar enhancements associated with tandem in situ treatments. Example reagents include adsorbants, oxidizers, dechlorinaters, accelerators, biodegraders, and products that reduce the hydraulic conductivity of the capping material (such as bentonite) and minimize the release of contaminants to bioturbation zones or into the water column (ITRC 2014).

Required secondary treatments

No secondary treatment is required other than monitoring the stability of the capped area and the physical-chemical quality of overlying water and capping material.

Application examples

Examples of applications are available at these addresses:

Performance

The performance of in situ capping varies depending on its design, sediment consolidation, advection, the additives used, the specific characteristics of the target site, and the type of contaminants present. Because of this, cap effectiveness relies on the suitability of the design and quality of the installation. Life spans for capping projects are dependent on the type and quantity of contamination as well as contaminant fate and transport mechanisms operating within the cap. Past experience has demonstrated the predicted life span to be in the order of decades (Palermo et al., 1998). Models designed to assess long-term cap performance for the purposes of design or performance monitoring have also improved. Lampert et al. (2013) were able to demonstrate accurate long-term contaminant concentration levels in capped sites contaminated with PAHs, using a polydimethylsiloxane (PDMS)-based passive sampling device and comparing the results to measured pore-water concentrations.

Measures to improve sustainability or promote ecological remediation

  • Communicate with project stakeholders early on in the remediation planning process to ensure that potential institutional controls and ecological implications are identified and considered. Restrictions on navigational and commercial use of the area, and limitations on fishing and aquatic activities may be required until remedial measures have been completed.
  • Identify appropriate indicators (such as habitat-rebound or reduction of contaminant levels in fish tissue) to quantify the sustainability of the remediation project with respect to environmental, economic and social considerations and to monitor and report on the overall effectiveness of the cap for contaminant retention.
  • Identify opportunities to enhance the productive capacity of the habitat being remediated, through the use of natural materials.
  • Develop a site-specific plan that reduces consumption of energy, material, and water, minimizes harmful air emissions and waste generation, and protects ecosystems during cleanup.

Potential impacts of the application of the technology on human health

Aquatic Impacts

Potential effects on the benthic community include changes in sediment characteristics such as available nutrients and food supply, altered habitat structure and availability, and changes to salinity, dissolved oxygen, and temperature. These changes can lead to stranding, displacement, and mortality of benthic organisms. Mitigation measures may be considered to reduce benthic mortality and encourage redevelopment and habitat restoration. These include the use of native sediments to encourage redevelopment, as well as removal and replacement of endangered or sensitive organisms and habitats.

Activities and stressors associated with in situ capping of contaminated sediment that may lead to impacts to fish and fish habitat include:

  • placement of material or structures in water;
  • addition or removal of aquatic vegetation;
  • changes in the timing, duration and frequency of flow;
  • use of industrial equipment;
  • changes to fish passage;
  • organic debris management.

Adjustments to project timelines (such as timing activities to avoid significant periods for aquatic health), implementation of environmental protection measures (such as institutional controls to prevent human exposure, removal and replacement of species at risk), and waiting on-site conditions for optimal cap placement (such as cap placement during times of low energy or water flow) are all measures which may reduce the occurrence of environmental and human health impacts as a result of the cap placement.

Human Health Impacts

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 and Mitigation Approaches

In situ Treatment

Amendments (such as nutrients, microbes)

Skin contact, inhalation of particulates, incidental ingestion

Select nontoxic amendments, where possible. Educate staff on safety and provide appropriate personal protective equipment (PPE) and reactionary materials (such as sorbent pads), as necessary. Follow measures for safe storage and handling to minimize exposure, as outlined in MSDS sheets.

Stockpiling Cap Materials

Dust

Inhalation of particulates

Cover the piles and use water to suppress dust as necessary. Use PPE when handling material.

Sediment (run-off leading to sedimentation of surface water)

Ingestion of drinking water; direct contact while swimming

Cover piles to minimize generation of run-off. Monitor sediment loading at surface-water sources. Monitor visual indicators of sediment erosion (such as rills—shallow channels cut into the soil by erosive action). Use silt fences and other containment barriers if needed.

Installation of Cap

Dust

Inhalation of particulates

Avoid the use of very dry materials in cap formation. Apply material with minimal impact, to reduce dusting. Personnel should be equipped with PPE to prevent exposure.

Contact with concrete or asphalt

Skin contact (chemical burns, thermal burns)

Educate staff on safety and provide appropriate PPE and reactionary materials (such as sorbent pads), as necessary. Follow safe storage and handling to minimize exposure, as outlined in MSDS sheets.

Monitoring

Off-gassing of sediment under cap

Inhalation of chemicals and unpleasant odours

Monitor gas levels if vents have been installed in the cap. Monitor off-gassing for human health concerns, and put institutional controls in place as necessary. When feasible, consider gas-flaring (controlled combustion of flammable gas at the end of a flare stack, in an effort to reduce the volume of gas and control odours) as a method to reduce odours associated with certain compounds (such as methane, sulphides).

References

Author and update

Composed by : Bruno Vallée M.Sc, LVM Inc.

Latest update provided by : Bruno Vallée, M.SC., LVM inc. and Ashley Hosier, Ing., Royal military college

Updated Date : February 3, 2017

Version:
1.2.1