Fact sheet: Excavation (in dried soil) and off-site disposal – sediments

On this page

• Description
• Implementation of the technology
• Recommended analyses for detailed characterization
• Recommended trials for detailed characterization
• Other information recommended for detailed characterization
• Applications
• Applications to sites in northern regions
• Treatment type
• State of technology
• Target contaminants
• Treatment time
• Long-term considerations (following remediation work)
• Secondary by-products and/or metabolites
• Limitations and Undesirable Effects of the Technology
• Complementary technologies that improve treatment effectiveness
• Required secondary treatments
• Application examples
• Performance
• Measures to improve sustainability or promote ecological remediation
• Potential impacts of the application of the technology on human health
• References
• Author and update

Description

Excavation involves removing contaminated sediments from freshwater, estuarine or seawater bodies for remediation. Excavated contaminated sediments are transported to authorized (off-site) treatment and/or disposal facilities. Prior to excavation, and as opposed to dredging, a temporary relocation or redirection of the waterbody is required to remove the sediment using conventional excavating machinery for dewatered soils.

This technology involves isolating the excavation area for dewatering using containment structures. Subsequently, it may be necessary to pump residual water from the remediation area until only sufficiently dewatered sediments remain to allow excavation. Extracted contaminated sediments may require further dewatering and/or sieving before being transported off-site.

Sediments extracted by excavation can also be disposed of or managed on site by land, semi-water, or underwater capping, or in a confined aquatic disposal facility. See the fact sheets: Confined Aquatic Disposal and Engineered Containment Facilities and Capping for more information.

Internet links:

• ITRC. 2014. Contaminated Sediments Remediation: Remedy Selection for Contaminated Sites.
• U.S. EPA, 2005. The Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. Chapter 6: Dredging and Excavation.

Implementation of the technology

Implementation of this technology may include:

• Mobilization, site access and installation of temporary facilities.
• Installation of sediment collection structures such as sediment barriers and sediment retention fibre rolls. These structures will limit the migration and enhance the retention of clean and/or contaminated sediments within and/or outside the remediation area.
• Capture and relocation of aquatic organisms and fauna, if possible, located in the excavation area. See activities involving species at risk.
• Isolation and dewatering the area through removal or redirection of the waterbody. This will require the installation of water control structures such as dams and sheet pilings, earth berms, cofferdams, inflatable dams, etc.
• Installation of pumping and water removal equipment (to remove the build-up of groundwater infiltration as well as precipitation and runoff) in the excavation area. If applicable, water treatment and disposal may be required.
• Removal of large debris (rocks, vegetation, etc.) that can damage machinery. Debris should be put back post-remediation, if possible, to limit potential disturbance to aquatic fauna following remediation activities.
• Sediment excavation and stockpiling using mechanical equipment such as excavators, backhoes, and clamshell buckets.
• Sediment pretreatment phases such as dewatering or sediment size separation, as required.
• Water treatment systems, as required.
• Vapour and gas recovery and treatment systems, if required.
• Temporary storage of sediments in stockpiles on site or immediate loading for transport.
• Monitoring of the excavation area through a bathymetric survey or test samples to ensure that the remediation objectives are met.
• Trucking to off-site disposal or treatment site.
• Site restoration (grading, paving, revegetation, etc.).
• Short- or long-term monitoring after completion of the remediation activities to ensure natural restoration through the site’s living organisms.

Materials and Storage

On-site storage may include clean sediments, fuels, lubricants, amendments and other site materials required for operating the machinery and equipment for the implementation of the technology.

Temporary stockpiles of contaminated sediment may be stored on site, pending characterization and off-site transportation. They are usually covered to limit water infiltration and dust emission. It may be necessary to install impermeable membranes under the sediment stockpiles.

If water pumping is required during excavation and/or if drainage water from stored sediments needs to be captured or treated, tanks and a temporary water and/or vapour treatment unit may also be present on site.

Waste and Discharge

Water that comes in contact with the contaminated site may need to be treated prior to being discharged into the environment. This includes water from the dewatering of contaminated sediments, infiltrated groundwater and surface water flowing towards the site, as well as any water removed from the site before excavation work begins. Contaminated groundwater is either stored in tanks for off-site transportation and treatment or treated on site for discharge into the municipal sanitary or stormwater sewer system or into designated infiltration areas.

Dust may be emitted during construction activities, particularly from excavations, soil treatment areas or materials spread on the ground by equipment. Bulk sediments should be covered and protected to prevent dust generation and off-gassing of volatile contaminants, and to prevent precipitation from coming into contact with the sediment.

Vapour discharge may be released from equipment exhaust or from the volatilization of contaminants from freshly excavated faces or temporary sediment stockpiles. 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 excavation work during colder temperatures.

Resuspension of sediment and high turbidity in water are possible post-excavation, as water is allowed to return to the site. Resuspension may be mitigated by slowly allowing water to refill the area (i.e., preventing a surge of water to the area). In locations with very fine sediments, an armouring layer of gravel or rocks may be added to prevent sediment resuspension.

Recommended analyses for detailed characterization

Recommended trials for detailed characterization

Other information recommended for detailed characterization

Applications

In general, all sediment types can be excavated once the contaminated area is free from water and its walls are stable and safe to support the machinery. Excavation is best suited for shallow water bodies or littoral environments (streams, ponds, small rivers, small bays, etc.). It is effective in well-defined areas with high contamination (hot spots). This method is often selected for contaminated sediments with particle sizes different from the clean material, as it makes the segregation easier.

Applications to sites in northern regions

• The technology is applicable in northern environments, but remote sites have greater logistical challenges associated with mobilization, resulting in higher costs. In addition, equipment availability is limited and the seasonal windows to conduct work are short.
• Transportation of contaminated materials to off-site treatment facilities or treated materials for off-site reuse is often very costly or impossible in remote and northern areas. Rail or barge/ship transport may be feasible but not always economically viable.
• Monitoring and testing are limited by timely access to certified laboratories, and often necessitate the development of on-site testing and analysis of materials or the implementation of progressive interventions and/or implementation of a risk-based management approach.
• For on-site ex situ treatment, extremely cold temperatures can hamper, for example, biodegradation and volatilization. Treatment systems in northern environments require climate-appropriate design, including consideration for seasonal changes as well as long periods without human intervention, refuelling, etc.
• The technology may require the implementation of restrictions or limitations on the human consumption of native organisms when contaminated sediment is present. Because local people may rely on aquatic species as important sources of food, these restrictions may significantly impact communities.

Treatment type

State of technology

Target contaminants

Treatment time

Notes:

Notes:

The time required to meet all remedial needs depends on the volume and extent of contaminated sediment, the equipment used, the need for secondary treatment, and the means of transport for the excavated sediment. Completion time may be weeks to months when dealing with small, discrete volumes of contaminated sediment. However, it may require several years for large areas or volumes of contaminated sediment or when site characteristics (for example, infrastructure) pose significant logistical challenges.

Long-term considerations (following remediation work)

Excavation typically provides a high degree of accuracy with respect to contaminant removal, as it is conducted in dried conditions. In general, long-term monitoring is not required at excavation sites where all contaminated sediment has been completely removed and treated. Long-term monitoring may be required in cases where excavation is followed by other remedial technologies, or if an on-site sediment disposal method is used.

Excavation, however, remains a highly invasive remediation method, which can lead to long-term negative impacts on the benthic organism’s habitat in the absence of adequate control measures and management practices. Hence, it is recommended that the regeneration of the aquatic environment be monitored. If aquatic organisms were captured or confined, they should be reintroduced at the end of excavation work. It is essential to assess whether natural habitats can adapt to the new conditions and, if needed, provide adequate resources to avoid unnecessary disturbances.

Secondary by-products and/or metabolites

The excavation of sediments generates no secondary products or metabolites. Such products may be generated by the secondary treatment technology selected for processing excavated sediments. Excavation can theoretically introduce oxygen into anaerobic environments, causing localized changes to geochemistry of the area such as acidity (pH) or redox potential (ORP). However, few adverse effects have been documented.

Limitations and Undesirable Effects of the Technology

• Excavation is not appropriate in the following situations:
  • Locations where contamination is found in low concentrations over a large area, in thin layers, or in patchy distribution.
  • In areas where redirection of the waterbody is difficult or not feasible, such as deep-water bodies or locations with strong currents.
  • Sites with significant existing infrastructure and important utilities, such as submerged infrastructure, overhead restrictions, or significant debris and large rocks within the sediment.
  • Presence of unexploded explosive ordnances (UXOs), which pose a risk of unintentional detonation.
• Excavation is challenging under the following conditions:
  • Locations with a significant gradient or when sediment shear strength (i.e., ability to be compressed without sliding) is insufficient to support the weight of the excavation equipment and may lead to structural failure.
  • Contaminated sediment is in bedrock crevices, preventing removal with excavation equipment.
  • Locations with significant groundwater upwelling or frequent heavy rainfall.
  • Locations with possible cultural and archaeological resources.
  • Disposal facilities are far from the site and/or have limited capacity to deal with contaminated sediments.
• Excavation can have the following adverse effects:
  • Complete loss of the benthic community and recovery may take several years once the remediation work is completed.
  • Physical disturbances, such as cave-ins or slumping, caused by excavation and on-site transportation.
  • Diverting/redirecting waterbody flow around excavation activities may result in scouring and changes in channel morphology in the surrounding areas.
  • Excavation may alter the behaviour and usability of the affected waterway, including flood control abilities, aesthetic appeal, navigational and anchoring abilities.
  • Emission of dust or volatilization of certain contaminants during sediment handling.
  • Changes in the geochemical conditions of the area, which could favour undesirable reactions such as precipitation of certain metals or acid drainage through the oxidation of sulphide minerals.

Complementary technologies that improve treatment effectiveness

• Sediment dewatering
• Sediment sieving and segregation
• Sediment washing
• Monitored natural recovery (MNR) and enhanced natural recovery (ENR)
• Capping

Required secondary treatments

Multiple secondary treatment technologies may be used depending on the type of contaminants in sediments. Several technologies have been presented in the available fact sheets, including the following:

• Chemical oxidation
• Confined Aquatic Disposal and Engineered Containment Facilities
• Sequestration
• Biodegradation

Note:

The water removed from the excavated area, as well as water removed in dewatering efforts, may require treatment prior to disposal back into the environment.

Application examples

Application examples are available at these links:

• ITRC. 2014. Housatonic River, MA.
• Government of Canada. 2016. Completion of the Rock Bay Remediation Project.

Performance

Excavation is an effective method for removing contaminated sediments. However, the performance of this technology is dependent on the subsequent treatment of the sediments. It allows for a very precise extraction of contaminated sediments as the excavation area is visible and accessible with standard machinery. Resuspension concerns associated with remediation dredging can be minimized with excavation in a dried area by using sediment-containing controls that can be visually inspected for performance and adjusted as necessary.

Measures to improve sustainability or promote ecological remediation

• Using renewable energy and energy-efficient equipment for technology implementation.
• Reducing fuel consumption (and using renewable energy where available) for vehicles and heavy machinery.
• Optimizing the scheduling to promote resource sharing and reduce the number of mobilization days.
• Capturing and relocating the species at risk and sensitive habitats are likely to be affected by remediation work.
• Working during periods of low risk to fish and fish habitat.
• Identifying site-specific regulatory resources (for example, fishing licences), sensitivities, and appropriate avoidance/mitigation measures.
• Using engineered controls to prevent sediment resuspension (for example, silt curtains).
• Minimizing the removal of clean sediments through accurately identifying the extent of the impacted area.
• Minimizing the amount of sediment sent to disposal facilities by separating clean from contaminated sediments, using segregation and sieving methods, for example.
• Reducing the contaminant volume requiring disposal through chemical pretreatment, dewatering, or grain-size separation, where possible.
• Implementing mitigation measures to minimize potential impacts caused by dust emissions, soil erosion and uncontrolled waters.
• Checking sediment reuse possibilities before selecting a treatment or disposal method.
• Treating and disposing of sediment and wastewater on-site, if possible.

Potential impacts of the application of the technology on human health

Potential Human Health Impacts

The minor and major potential human health exposure pathways are presented in the following table.

Potential Aquatic Impacts

Sediment excavation and water diverting can significantly affect aquatic and terrestrial communities, habitats, nutritional sources, nutrient levels and oxygen concentrations. Full recovery may take months or even years to achieve.

Excavation has the potential to cause sediment resuspension, potentially releasing contaminants into the water column. Mitigation may require changes to project design and implementation (timing & phasing of the project). Silt fences and operational adjustments (for example, slower-moving buckets, work stoppages during high winds, etc.) should be considered as a means of reducing sediment resuspension into the water column.

Negative effects on the aquatic and benthic communities may be mitigated by conducting pilot studies or on-site studies in advance. Pre-excavation studies will determine whether excavation will likely pose unacceptable risks to communities and whether species at risk are present on the site. The presence of species at risk may preclude the use of excavation as a remedial option, or these communities may require collection and relocation during excavation activities or mitigation and/or compensation measures may need to be implemented.

Short- and long-term monitoring must be performed following remediation work to ensure that the site is restored. Monitoring should include residual and clean sediment toxicity, benthic community recovery, the presence of bioaccumulating contaminants, and contaminant concentrations in fish tissue. In addition, aquatic and benthic recovery may be improved by replacing large debris and boulders. This will provide cover for aquatic organisms and encourage rebound of the site after excavation is complete.

References

• Canada Gazette. 2013. Species at risk act. Permits Authorizing an Activity Affecting Listed Wildlife Species Regulations. Gatineau, QC
• Transport Canada. 2016. Completion of the Rock Bay Remediation Project. Government of Canada.
• Interstate Technology & Regulatory Council (ITRC). 2014. Contaminated sediments remediation: remedy selection for contaminated sediments (CS-2). Interstate Technology & Regulatory Council, Contaminated Sediments Team. Washington, D.C. 525 p
• Interstate Technology & Regulatory Council (ITRC). 2014. Contaminated Sediments Remediation: Remedy Selection for Contaminated Sediments, Housatonic River, Massachusetts.
• United States Environmental Protection Agency (U.S. EPA). 2005. Contaminated Sediment Remediation Guidance for Hazardous Wastes. EPA-540-R-05-0012. Office of Solid Waste and Emergency Response. 236 p.

Author and update

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

Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada

Updated Date : November 24, 2016

Latest update provided by : Frédérick de Oliveira, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.

Latest update date :March 31, 2024