Fact sheet: Dragage et élimination hors site (Ex situ)— Sédiments

From: Public Services and Procurement Canada

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Description

The terms dredging, environmental dredging, or underwater excavation refer to the removal of contaminated sediments from a water body for purposes of sediment remediation. Dredging, followed by treatment and disposal of the contaminated material, has become one of the most commonly implemented methods for sediment remediation in North America. The key advantages of dredging over other remediation options are that dredging typically requires shorter timelines to complete and results in greater certainty about the long-term effectiveness of remediation activities. In addition, fewer limitations are placed on future site uses, as compared to in situ remediation where contaminated sediments remain in place. However, dredging is a highly invasive remediation option, and can have long-term negative impacts on aquatic habitats and benthic species if appropriate control measures and best management practices are not used (ITRC, 2014; Manap and Voulvoulis, 2014). All associated risks must be carefully considered during the design phase to develop a successful plan for mitigating risks and reversing short-term disturbances.

Two types of sediment dredging technologies are considered in remediation projects: mechanical and hydraulic. Mechanical dredging involves removal of the sediment using a bucket attached to a crane. On-site treatment (such as dewatering) of the sediment is often required, although water content of dredged sediments is typically lower with mechanical dredging than with hydraulic dredging techniques. As such, mechanical dredging is optimal in locations where sediment disturbance doesn’t lead to significant re-suspension into the water column (for example, in locations with coarse-grained and sandy sediments).

Hydraulic dredging uses a dredge head and hydraulic pump to fluidize the contaminated sediment in place, and then pump the slurry to a handling location. The slurry is typically composed of 10% to 15% (by weight) solids, and may contain as little as 1% to 2% solids. Hydraulic dredging requires large amounts of water to transport the sediment (carrier water), consequently dewatering and water treatment are major design considerations. Hydraulic dredging is best suited for wet, fine-grained sediment.

Secondary sediment treatment may also be required if, post dredging, residual contamination remains above regulatory criteria for a human and/or environmental health hazard. Capping—Sediments and Monitored Natural Recovery (MNR)—Sediments are two technologies commonly used to treat residual contamination, as they are well suited to treating low levels of widespread contaminants (ITRC, 2014).

Internet links:

ITRC, 2014. Contaminated Sediments Remediation: Remedy Selection for Contaminated Sites.

US EPA, 2005. The Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Chapter 6: Dredging and Excavation.

Implementation of the technology

Multiple steps are required in sediment dredging operations and project complexity is highly site-specific. Sediment characteristics, contaminant types and concentrations, and the physical and hydrodynamic environments all play a role in the complexity of a dredging project.

Dredging projects may include:

  • detailed site characterization, focusing on contaminant “hot spots” (discrete areas with high concentrations of contaminants) and biological inventory to identify any sensitive or endangered species;
  • relocation of sensitive or endangered species, if possible, when contact with the project site is likely. Fish salvage or invasive work should be scheduled to avoid spawning periods. In Canada, a Species at Risk Act (SARA) permit may be required for the capture and handling of threatened and endangered species during fish salvage;
  • identification of the optimal process (such as mechanical or hydraulic) and necessary equipment based on water depth, distance to the shoreline, sediment characteristics, and available technology;
  • equipment placing and staging, which may occur on the shoreline or on a barge, dependent on the proximity of the dredge operations to the shoreline;
  • removal of contaminated sediment while minimizing removal of clean sediment. This involves:
    • removal of sunken debris, logs, wrecks, and refuses that damage machinery and cause resuspension of sediment;
    • odelineation of paths and depth of dredge cut lines;
    • removal of bulk sediment following a predetermined dredge path;
    • implementation of controls to minimize resuspension of sediment and release of contaminants to the water column. Controls may include the use of silt curtains, cycle time of clamshell buckets, multiple dredge passes, or speciality equipment.
    • implementation of best-practice resources to dredge with accuracy and minimize deviation from planned path. Resources may include global positioning to accurately locate the dredge head, over-dredging to capture contaminants and reduce residuals, and timing of dredging to accommodate currents;
    • monitoring cut lines post-dredge, in an effort to determine the quantity of residual contamination:
      • where residual contamination is more significant than anticipated, additional dredge passes to remove more material may be considered;
      • when residual contamination is widespread, or caused as a result of re-suspended contaminants settling back to the site, secondary treatment options may be considered (such as Capping—Sediments);
  • transferrals of sediment to containment for dewatering and/or pre-treatment:
    • particle-size separation is a common pretreatment method when contaminants are strongly linked to grain size;
    • dewatering equipment (such as settling ponds, sand screws, clarifiers) may be required;
    • carrier water may require treatment prior to discharge;
  • transportation of sediment to the final disposal and/or treatment facility;
    • options for disposing of dredged material include:
      • sanitary or hazardous waste landfill;
      • confined disposal facility (CDF): an area specifically designed for the containment of contaminated dredged material (see fact sheets for Capping—Sediments and Confined Aquatic Disposal and Engineered Containment Facilities—Sediments);
      • confined aquatic disposal (CAD): capping the sediment within a subaqueous depression (see fact sheet for Confined Aquatic Disposal and Engineered Containment Facilities—Sediments), and;
      • reuse at an approved location.

Hydraulic dredges, which use pumps to move the sediment-water slurry, may use long pipelines (up to several kilometres) and booster pumps to move sediment from the dredging site to a final disposal or intermediate handling site. Hydraulic dredges collect relatively large amounts of water per unit of dredged sediment (about 5 to10 times the volume of sediments) compared with mechanical dredges. The amount of water required is determined by the native sediment water content, sediment grain size, and required pumping distance.

Dredged material may require dewatering, to prepare it for disposal or reuse/repurposing. Dewatering will improve handling properties, reduce the overall volume of sediment, and reduce costs associated with transportation and disposal. When the liquid fraction is expected to be deposited back into the waterbody, dewatering is most efficiently done near the source.

Disposal of the contaminated sediment is a significant and costly requirement of dredging. The composition of the sediment (i.e., sand, shell, silt, clay, fines, or sludge) may restrict land disposal, or require treatment in order to meet disposal requirements. In coastal environments, salt content of materials may also place restrictions on land disposal (BC Ministry of the Environment, 2009). Particle-size separation may allow project managers to separate out sediment fractions with lower contaminant levels, potentially reducing overall disposal costs.

Materials and Storage

Materials stored on-site are typically limited to small amounts of fuel and lubricant as well as miscellaneous construction supplies. Contractors may create temporary stockpiles of contaminated materials on barges or on shore pending treatment and disposal. Sediments should be stored and transported in liquid-tight containers and covered to prevent loss of dredged liquids and volatilization.

In addition to contaminated sediment, amendments used in the treatment of carrier water and sediments may also be stored on-site. Sediment treatment amendments will vary with contaminant types, but may include activated carbon, organophilic clay, fly ash, and quicklime. Typical wastewater treatment products such as alum, quicklime, ferric chloride, and chitosan may be used to treat carrier water. Amendments should be stored on-site, and covered to protect from dust production and from runoff caused by precipitation.

Waste and Discharges

Dredging typically doesn’t produce significant solid waste, outside of the sediment that is bulk-hauled for treatment or disposal. Site garbage is generally similar to what is produced on a construction site, but in addition may contain used/spent sorbent pads and/or water treatment media.

There is a risk of accidental discharges (solid, liquid, and/or gas) when conducting dredging activities. Resuspended particles that are distributed during dredging activities may be redeposited at the dredging site or, if not controlled, transported to downstream locations in the water body (Wasserman et al., 2014; Manap and Voulvoulis, 2014). The release of contaminated sediment can occur during material transport, handling, and treatment. High concentrations of dissolved and suspended contaminants may be released from untreated liquid discharges. Sediment loss is a function of dredge size and production rate, and can be estimated in the design phase. Operational factors (such as propeller wash and grounding of barges) may increase sediment resuspension. These factors can be mitiga

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Chemical analysis of each particle size fraction
  • Dissolved salt concentration in water
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Dissolved methane concentration
  • Temperature
  • Soil granulometry
  • Suspended solids concentrations
  • Type and concentration of mineral salts in contaminated matrix
  • Sediment water content

Recommended trials for detailed characterization

Chemical trials

  • Soil washing/flushing trials

Physical trials

  • Gas permeability trials
  • Vapour survey

Hydrogeological trials

  • Pumping trials

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
  • Characterization of the present and proposed surface water usage and the water body in general (including the required draft for vessels)

Phase III

  • Identification of preferential pathways for contaminant migration
  • Volume of contaminated material to treat
  • Geotechnical characterization of sediment deposition

Applications

Remediation dredging may be suitable for contaminated sites that meet the following conditions:

  • the long-term risk reduction associated with contaminant removal outweighs the disturbance and habitat disruption that will be caused;
  • contaminants are found in high concentrations, over a discrete area;
  • sediments underlying the contaminated sediments are clean or much cleaner;
  • the risk of human exposure to contamination is high and not well controlled by institutional controls (such as restrictions on swimming and fishing);
  • the water depth is sufficient to accommodate the dredge equipment, yet not so deep as to make dredging infeasible. For example, mechanical dredging becomes difficult when water depths approach the length of the crane boom arm, and hydraulic dredging success is reduced at depths greater than 25 m;
  • sediment handling and storage equipment for mechanical dredging are located in close proximity, either on a floating barge or on shore;
  • a suitable disposal area is available within a practical distance;
  • local infrastructure can accommodate dredging equipment and needs;
  • the amount of debris (such as logs, boulders) is minimal, and can be removed prior to dredging;
  • contaminants are highly correlated with grain size, allowing separation of contaminated sediment from clean sediment;
  • velocity of the water current is low or can be reduced to minimize resuspension and downstream transport.

Applications to sites in northern regions

Dredging is constrained in northern environments by the high costs associated with mobilizing equipment and personnel, limited local availability of equipment, limitations to site access, and short seasonal work windows. In addition to importing much of the dredge equipment, Arctic environments may require icebreaker assistance as well as monitoring and alerting of ice conditions, significantly inflating operational costs and organizational requirements. Transport of dredged materials off site to existing disposal facilities is often cost-prohibitive. On-site sediment and wastewater treatment may reduce costs associated with disposal; however, transportation of the necessary dewatering and mixing equipment, as well as treatment amendments (such as activated carbon) may be costly and logistically challenging.

Northern communities often rely on local biota and aquatic organisms as important sources of food. Disruption to aquatic biota resulting from dredging operations and required institutional controls, even for a short duration, may hinder community acceptance of dredging as a remedial solution in some northern environments.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Does not exist
Chemical
Does not exist
Control
Applies
Dissolved contamination
Does not exist
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
Does not 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
Applies
Non metalic inorganic compounds
Applies
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
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
Applies

Notes:

Comments: The time required to meet all remedial needs is dependent on the volume and extent of contaminated sediment, and the need for secondary treatment. Completion time may be weeks to months when the dredge volume is small, or the goal is to remove discrete volumes of contamination. However, dredging projects may require multiple years to reach completion when addressing a large area or volume of contamination, if secondary treatment is necessary, and where the site characteristics (such as infrastructure) pose significant logistical challenges.

Long-term considerations (following remediation work)

All dredge sites experience some level of residual contamination, as sediments are easily transported and re-suspended into the water column when disturbed. Residual contamination can be estimated and accounted for during the design phase. Residual contaminants may require a secondary treatment (such as MNR, enhanced monitored natural recovery [EMNR], or capping) when dredging operations are unable to meet remediation goals.

Removal of sediment in dredging operations causes habitat destruction for benthic organisms. Recovery times for benthic communities vary greatly between habitat types, ranging from weeks to years. Recovery may be less than one year in areas of high sediment mobility or in the presence of opportunistic species. By comparison, in areas where benthic communities are dominated by long-lived species, full recovery may take many years.

Post-construction monitoring may be a regulatory requirement, to ensure success of sediment removal and to demonstrate biological rehabilitation. In general, however, long-term monitoring is not required at dredging sites where all contaminated sediment has been completely removed and treated, and where confirmatory sampling has identified that the remediation goals have been met. Long-term monitoring may be required in cases where dredging is combined with other remedial technologies, such as combining dredging in areas of high contaminant concentrations with the use of MNR in surrounding areas of lower contaminant concentrations.

Secondary by-products and/or metabolites

Secondary by-products are not expected through the use of dredging, as contaminants are physically removed from the site.

Limitations and Undesirable Effects of the Technology

  • Dredging is a very disruptive technology that is not suitable:
    • for locations with underwater or in-water infrastructure (such as pipelines, cables, piers);
    • for locations with sensitive benthic organisms or aquatic habitats;
    • at depths sufficiently great as to hinder equipment effectiveness: mechanical dredging and silt curtains typically have a limit of 8 m and hydraulic dredging becomes difficult after 25 m (ITRC, 2014), and;
    • for heavily used areas with a significant amount of existing infrastructure.
  • Dredging requires large area for stockpiling and dewatering or pre-processing when necessary.
  • If a disposal facility is not located in close proximity, the transport and disposal of contaminated sediments following removal is expensive.
  • Dredging operations may limit site use because of the need to erect staging equipment on shore or on a floating barge, thereby restricting access to the shore or navigable waterways.
  • Dredging is not efficient where contamination is found in low-levels, is distributed over a large area, or must be completely removed. These conditions require multiple dredge passes to achieve success, resulting in the removal of significant quantities of clean material.

Potential Adverse Effects

  • Dredging causes unavoidable disruption to the benthic environment. Project design may incorporate measures to promote recolonization and recovery of bottom habitat. However, complete recovery of the biotic community may take several years.
  • Resuspension, residuals, and release of contaminants may lead to an increase in contamination levels in ecological receptor tissues compared with pre-dredge levels, and a need for additional site management. NRC (2007) has shown that up to 10% of dredged sediment may be lost to resuspension releases, which may be exacerbated by site conditions (such as high-energy environments, deep water, moisture content in sediment). Resuspended particulate matter may be redeposited at the dredging site or, transported to downstream locations in the water body.
  • Community concerns typically include noise, odour, and lights (at night). Noise complaints from waterfront residents are common, as sound is easily carried over water.
  • Dredging projects are often complicated by weather, changes in water energy (wave action, currents, and tides), and marine traffic. These factors may change the bottom contour in a matter of hours. Rapid and aggressive bottom contour change may affect the integrity of engineered controls, such as silt curtains, located at depth.
  • Excavating may expose anoxic sediment to oxygen, causing major short-term changes in pore-water chemistry with the potential to influence fate and transport of some contaminants, for example methyl mercury generation.

Complementary technologies that improve treatment effectiveness

The remediation benefits of dredging can be improved by combining it with a secondary technology to overcome the problem of residual contamination. A common technique is to focus dredging on contaminant “hot spots” and follow up with an in situ remediation, such as MNR or backfilling, to treat residual contaminants. Residual contamination may be a result of low-level, widespread contamination, or may be due to the resuspension and sedimentation of contaminated sediments during dredge removal efforts.

Dredging typically disturbs and causes resuspension of contaminated material, which may be redeposited as a loose, difficult-to-capture layer. The risk posed by this material may be reduced post-dredge by installing a thin-layer sand cap to contain the contamination. The cap also provides an immediate physical barrier to stimulate natural recovery through bioturbation and dilution.

Required secondary treatments

The dredged sediment requires treatment of the solid and/or liquid fraction. Treatment may occur on-site or at a secondary treatment and disposal facility. Sediment treatment may include biological treatment, chemical treatment, extraction or washing, immobilization, solidification or stabilization, incineration, and thermal destruction (U.S. EPA, 2005).

Water from the dredged material requires treatment. A mobile water treatment plant may be brought onto the site. This type of system often includes a flash mixer, flocculent tank, clarifier, sand filters, and additional treatment equipment (such as activated carbon).

When remediation objectives are sufficiently low, dredging may not be capable of removing the required amount of contaminant in one dredge pass. In these cases, combining dredging with a complementary treatment option is more efficient and sustainable than conducting additional dredge passes. Utilizing secondary treatment technologies, such as Monitored Natural Recovery (MNR)—Sediments or Capping—Sediments, has been shown to reduce the amount of sediment being handled, thereby reducing associated costs and leading to more frequent project success.

Application examples

Performance

Dredging has become one of the most commonly used remedial technologies in North America (ITRC, 2014). NRC (2007) conducted a review of U.S. EPA Superfund sites that use dredging as the remediation pathway and found a 50% rate of success in attaining original remediation objectives. This success rate is based on key measures of long-term risk reduction within a predicted magnitude and timeline. Performance effectiveness is primarily limited by the resuspension and release of contaminants during dredging, making it difficult to meet risk-based goals. Key factors limiting dredging performance and success include:

  • sediment properties (particle size, cohesion and bulk density);
  • site conditions, such as water depth and energy, that effect operational control;
  • obstacles within the sediment or water column (such as debris, cables, bedrock, piers),
  • presence of non-aqueous phase liquids (NAPLs), and;
  • operational design and experience factors, including equipment type and operator skill level.

Enhanced planning and operation efficiency have been shown to improve removal efficiency and reduce resuspension into the water body. Significant efforts placed on the planning and design phase have improved the outcome of dredging projects. Monitoring is recommended during remediation, and should be conducted after each dredge pass. This will allow for assessment of dredge effectiveness in meeting cleanup requirements.

Measures to improve sustainability or promote ecological remediation

Sustainable remediation is the enhancement of a remediation project’s environmental, social, and economic benefits through varying approaches and technological implementation. Activities that can be undertaken to improve the sustainability of a dredging project include the following:

  • where possible, avoid dredging during sensitive periods of the year (for example during fish spawning and commercial fishing seasons), and complete the dredging in the shortest possible time frame;
  • conduct sediment dredging in a manner to minimize the residual contamination. For example, consider the use of post-dredge institutional controls (such as silt curtains) to further reduce sedimentation and protect sensitive environments. (Palermo et al., 2008; Manap and Voulvoulis, 2014);
  • if possible, perform on-site dewatering and water collection and treatment to reduce the cost and amount of greenhouse gas emissions associated with transportation of contaminated water to a treatment facility;
  • conduct dredging in a manner that produces a dredgate (the matrix collected by dredge operations) with a high solid content, thereby reducing the amount of dewatering and return water treatment that is required.

Potential impacts of the application of the technology on human health

Aquatic Impacts

Dredging causes immediate and irreparable effects to the benthic community, including alteration of habitat and access to habitat, removal of food supply, changes to temperature and gas pressure, as well as fluctuations in sediment and water concentrations of nutrients, contaminants, salinity, and dissolved oxygen. The changes associated with dredging will lead to mortality of benthic organisms, and the benthic community may take months to years to rebound.

Dredging activities cause significant disruption to aquatic organisms, and may directly cause mortality of anything located within or around the dredge area. Dredging drastically alters the profile of the sediment bed, uprooting habitats and vegetation. The resulting erosion and suspension of sediment into the water column may be taken up by aquatic and benthic organisms or transported downstream from the site.

Impact from dredging activities may be minimized by focusing on reduction of erosion and resuspension into the overlaying water. Improved planning and operational efficiency have been shown to reduce resuspension and release of contaminants caused by dredge activities. Methods shown to limit sediment resuspension include selecting equipment based on the dredge design, dredging more slowly (as compared to navigational dredging, for example), dredging in waters with low energy, and using geospatial equipment to increase the accuracy of dredge movements. Contaminant removal has been shown to be optimized when the dredge head (mechanical bucket or hydraulic dredge head) is able to dig down into clean sediment to collect the contaminated sediments. Obstructions, such as bedrock and hardpan, debris, or underwater structures, cause deviations in the dredge path and limit the ability to control resuspension. Water depth and energy levels will reduce operator control over the dredge head and may affect resuspension and residual levels.

A detailed review of aquatic and benthic organisms within and surrounding the dredge area should be undertaken prior to commencing dredging activities. If species at risk are found, mitigation measures may include harvest and removal of the organisms, or consideration of alternate treatment technologies.

Human Health Impacts

Potential human health concerns associated with worker and community exposure to dredging operations, as well as recommended mitigation approaches, are presented in the table below.

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

Sediment Removal

Contaminated and clean sediments

Skin contact, inhalation of particulates, incidental ingestion

Educate staff on safety and provide appropriate personal protective equipment (PPE) and reactionary materials (such as sorbent pads), as necessary.

Storage of Dewatered Sediment

Dust

Inhalation of particulates

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

Sediment (runoff leading to sedimentation of surface water)

Ingestion of drinking water; direct contact while swimming

Enclose the sediment to minimize generation of runoff and loss of material due to wind. Monitor sediment loading at surface-water sources.

Wastewater Treatment and Disposal

Water treatment amendments

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.

References

Author and update

Composed by :

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

Updated Date : November 24, 2016

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