Fact sheet: Excavation (in the dry) and off-site disposal – sediments

From: Public Services and Procurement Canada

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Description

Excavation involves the removal of contaminated sediment from fresh water, estuarine, or marine water bodies for purposes of sediment remediation. It differs from dredging in that excavation requires temporarily displacing or redirecting the water body, in order to remove the sediment using conventional dry land excavating equipment. As water removal may be onerous, excavation is best suited for shallow or near-shore areas (for example, streams, ponds, small rivers, and small bays). Excavation is best employed on sites with discrete locations of high contamination. It is not well suited to sites with widespread contamination unless combined with other remedial approaches. Excavation of sites with widespread contamination is costly, and modern excavation equipment lacks the necessary agility and precision.

As with dredging, a key advantage of this technology is that it typically requires shorter time frames to achieve remediation goals and results in a high degree of certainty about the long-term effectiveness in sustaining clean-up goals. As contaminants are removed in a relatively short time frame, institutional controls (for example, restrictions on navigation and water use) are not commonly required, permitting a wide range of future site uses. Excavation, however, is also a highly invasive remediation option, and can have long-term negative impacts on aquatic habitats if appropriate control measures and best management practices are not used.

In preparation for excavation, water must be diverted from the excavation site. Once the area is isolated, water within the excavation site is pumped out, leaving behind the dry sediment. Contaminated sediment is removed from the site and dewatered prior to disposal or treatment. Excavated materials typically contain less water than dredged sediments, but may still require dewatering or solidification. The water fraction should be treated and disposed of on-site. Contaminated sediments may be treated and/or disposed of, or repurposed with or without treatment. Many options for repurposing contaminated sediment will not apply without treatment, and may require regulatory approval.

Excavated sediment may be disposed of off-site, at a hazardous waste facility or landfill, or on-site, through the use of a terrestrial or subaquatic/semiaquatic cap or a confined aquatic disposal (CAD) facility. See fact sheets for Capping – Sediment and Confined Aquatic Disposal and Engineered Containment Facilities – Sediments, for more information.

Internet links:

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

http://www.itrcweb.org/contseds_remedy-selection/

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

https://semspub.epa.gov/work/HQ/174460.pdf

Implementation of the technology

The process of planning and carrying out contaminated sediment excavation and disposal may involve some or all of the following:

  • detailed site characterization to determine the area to be excavated and total volume of sediment to be removed
  • determining the optimal location for the installation of water-containment devices and the volume of water requiring displacement
  • removal and relocation of aquatic organisms and water fowl, if possible, located in the excavation area. If relocation of endangered or threatened species is necessary, regulatory permits may be required
  • isolation and dewatering the area through removal or redirection of the water body. This will require the installation of water control structures such as dams and sheet pilings, followed by removal of water within the excavation site
  • installation of sediment capture measures (for example, silt fencing), if necessary. These structures will reduce the amount of clean sediment entering the site, as well as reduce the occurrences of sediment redistribution (i.e., through wind action) during the operation
  • 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. Water treatment and disposal may be required
  • removal of large debris likely to hinder excavation equipment. Debris should be replaced post-excavation, if possible/as necessary, to provide cover for fish
  • sediment excavation and stockpiling using mechanical equipment such as excavators, backhoes, and clamshell buckets
  • sediment pretreatment such as dewatering and/or sediment size separation, as necessary.
  • excavated sediment should be treated and/or disposed of or reused, based on available site options
  • monitoring of the excavation area through a bathymetric survey or test samples, to ensure that the target depth or contaminant removal has been met
  • treatment and disposal of the water fraction post sediment dewatering

Sediments with contaminant levels above regulatory thresholds must be disposed of in an acceptable manner, treated and deposited back into the environment, or repurposed with or without treatment. Sediment disposal options will vary based on-site conditions, contaminant types and concentrations, and proximity to disposal facilities.

Where possible, sediment reuse and repurposing should be considered and may result in lower costs, as treatment and disposal may not be required. Reuse options include landfill cover, construction fill, mined lands restoration, subgrade cap or restoration material (where material will be overlain with clean sediment), building materials, and beach nourishment.

In general, the quantity of sediment handled will directly affect the project scope, time, and cost, and should be minimized to the extent possible while still maintaining human and ecological health standards.

Materials and Storage

  • Materials brought on-site are typically limited to heavy equipment and construction supplies, as well as small amounts of fuels, lubricants, and packaging. If machine fuelling takes place on-site, a mobile tank may also be stored on-site
  • Contaminated sediment may be stored in bulk or stockpiled on-site, along with clean sediment and amendments when secondary treatment is utilized. Contaminated sediment stockpiles should be sealed, to prevent leachate from volatilizing or leaching into the groundwater. Coverings also prevent dust and particles from becoming airborne and precipitation from contacting the contaminants
  • Stockpiles of chemicals and amendments used in secondary treatment (such as biodegradation or capping) may be stored on-site
  • Wastewater treatment materials (flocculants, clarifiers, etc.) as well as sediment treatment materials may be stored on-site, when on-site dewatering or treatment of sediment is planned

Waste and Discharge

  • Site waste is typical of construction sites, and may include spent sorbent pads. Bulk sediments removed from the site must be treated and disposed of, likely requiring transportation to a disposal site (for example, landfill, confined disposal facility)
  • Water that has come into contact with the contaminated site may require treatment prior to being discharged into the environment. This includes carrier water from dewatering contaminated sediment, groundwater infiltration and surface water runoff into the site, as well as any water removed from the site prior to excavation activities
  • Vapour discharges are site and contaminant specific, where volatile contaminants may be released from freshly excavated faces or sediment stockpiles. If naturally occurring radon is present at the site, it may be mobilized and volatilized by excavation activities. Additionally, off-gassing of sediments containing a high organic content may cause odour issues. Workers at sites where odours or volatile compounds are likely should take precautions, such as strategically timing removal operations at night and in colder temperatures, to prevent contaminant off-gassing
  • Resuspension of sediment is 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
  • Dust control may become a concern with bulk storage of dry sediment. Bulk sediments should be covered to prevent dust generation and off-gassing of volatile contaminants, and to prevent precipitation from contacting the sediment. Dust suppression may be required when evaporation and dust cause an unacceptable risk to human and ecological health
  • Recommended analyses for detailed characterization

    Chemical analysis

    • pH
    • Chemical analysis of each particle size fraction
    • 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

    Physical trials

    • Gas permeability trials
    • Vapour survey
    • Sediment washing/flushing trials

    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
    • 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
    • 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

    Excavation may be suitable for contaminated sites where:

    • the long-term risk reduction in contaminant removal outweighs the immediate destruction to the benthic community
    • the site doesn’t contain sensitive or endangered organisms and habitats, or if sensitive organisms are present, they may be captured, contained, and reintroduced post-excavation with minimal disruption. Native habitats should be adaptable to an increase in water depth, which naturally occurs with excavation. Where habitats are not adaptable, and excavation is still desired, clean replacement sediment or fill should be introduced to return sediment to pre-disturbance elevation
    • contamination exists in high concentrations over discrete areas (i.e., hot spots), making removal through excavation practical. Sediments with a high potential for resuspension losses as well as sediment containing highly mobile, free phase material (i.e., non-aqueous phase liquid (NAPL)) are poor candidates for dredging and capping, but can more easily be controlled through excavation
    • redirecting or removing the water overlying the contaminated area is practical and feasible
    • suitable locations for staging equipment and for sediment disposal are available on-site or within a reasonable distance. Shorelines are accessible with stable slopes, capable of supporting excavation equipment and sediment storage containers
    • site access is not significantly impaired by local structures (for example, piers, buried cables). Debris, such as logs and boulders, is minimal and can be removed prior to excavation.
    • contaminants are strongly associated with a particular grain size and can be easily separated from clean sediment.

    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, typically from hundreds of kilometres away and at great cost. Climate restrictions (for example, cold temperatures and ice conditions) and short seasonal windows to conduct work may limit remediation options.

    Excavation may require the placement of restrictions or limitations on the human consumption of native organisms when contaminated sediment is present. Because local people may rely on aquatic animals (for example, seals and whales) and fish as important sources of food, these restrictions may have a significant impact on communities. All of these factors may change the approach and remediation options for remote regions.

    The use of excavation 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.

    Monitoring and testing are limited by timely access to certified laboratories, and often necessitate the development of on-site testing and analysis of materials.

    Costs and logistics associated with transportation of contaminated sediment to existing disposal sites are often prohibitive, and may necessitate on-site treatment and disposal. On-site treatment often involves a high cost and high level of uncertainty, and may need to incorporate design considerations specific to northern environments such as intensive freeze-thaw dynamics and ice scouring.

    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
    Does not exist
    Dissolved contamination
    Does not exist
    Free Phase
    Applies
    Physical
    Applies
    Residual contamination
    Does not exist
    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
    Does not apply
    Metals
    Applies
    Monocyclic aromatic hydrocarbons
    Applies
    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
    Does not apply
    1 to 3 years
    Does not apply
    3 to 5 years
    Does not apply
    More than 5 years
    Does not apply

    Notes:

    The time required to meet all remedial needs is dependent on the volume and extent of contaminated sediment, the equipment used, the need for secondary treatment, and the means of transporting the excavated sediment. Completion time may be weeks to months when dealing with small, discreet volumes of contaminated sediment. However, it may require several years for large areas or volume 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 dry 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.

    Secondary by-products and/or metabolites

    Few secondary metabolites are involved in excavation treatment, as contaminated sediment is removed, treated, and/or disposed. Excavation poses a risk for loss of volatile contaminants to the air, and associated inhalation risks to workers and the community. Excavations can theoretically introduce oxygen into anaerobic environments, causing localized changes to pH; however, few adverse effects have been documented.

    Limitations and Undesirable Effects of the Technology

    Limitations

    • 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 distributions.
      • In areas where removal or redirection of the water body is difficult or not feasible, such as deep-water bodies and locations with strong currents.
      • Sites with significant existing infrastructure and utilities, such as submerged infrastructure, overhead restrictions, or significant debris and large rocks within the sediment.
      • Presence of unexploded ordnance, which poses 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 excavation equipment and may lead to structural failure.
      • Contaminants with high volatility may lead to an increase in air inhalation hazards for workers and the community.
      • Contaminated sediment is located in bedrock crevices, preventing capture 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.

    Adverse Impacts

    • Excavation results in the complete loss of the benthic community and recovery may take several years. Habitat restoration may require addition of a fill layer, regrading, revegetation, installation of structures to improve the aquatic habitat, and bioengineering mechanisms.
    • Other physical disruption is common, such as cave-ins and slumping, which may cause damage to excavation equipment and nearby structures, and may impact the safety of employees. Geotechnical design must be robust to adequately determine the degree of consolidation and sheer strength of the sediment, as well as predict the resulting weakening of geological conditions in and around the site.
    • Diverting/redirecting waterbody flow around excavation activities may result in scouring and changes in channel morphology in the surrounding areas.
    • Excavation may alter behaviour and usability of the affected waterway, including flood control abilities, aesthetic appeal, navigational and anchoring abilities, and benthic health. Engineered or operational controls may be required to minimize the effect of these alterations. Significant alterations may also indicate that a different remediation method would be preferable. Regulatory permits (for example, from a transportation authority) may be required to alter a waterway. Dust control is a concern with dry excavation and bulk storage of sediment. Bulk sediments should be covered and protected from dust generation, off-gassing, and rainfall contamination. Sites containing volatile contaminants where evaporation and dust cause unacceptable risk to human and ecological health may use amendments to suppress dust. Examples of amendments used include polymer sprays, calcium chloride solutions, lignosulfonates, and foaming agents.
    • Excavation of boulders and bedrock with high contents of sulphide mineralization may increase vulnerability to sulphide oxidation or acid rock drainage, caused by sulphide exposure to air and moisture.

    Complementary technologies that improve treatment effectiveness

    Excavation is not effective in achieving complete removal of low-level, widespread contamination. Because of this, it is most commonly used to remove discrete locations of high-level, high-risk contamination. Complementary technologies such as Monitored Natural Recovery (MNR) – Sediments are employed to address surrounding low-level contamination.

    Contaminant residuals following excavation may be addressed through the addition of a thin-layer cap on the site, prior to re-establishing the water body. The cap isolates residual contaminants from contact and transport into the water column.

    Required secondary treatments

    Secondary treatment includes the disposal, treatment and disposal, or reuse with or without treatment, of the excavated sediment. Sediment with contaminant levels above regulatory thresholds requires treatment and/or disposal. Sediment may be treated on-site or transported to a secondary treatment and disposal site. Secondary treatment technologies may include ex situ chemical oxidation, ex situ capping, and confined aquatic disposal (CAD). Detailed fact sheets may be found explaining these technologies in detail. In addition, the water removed from the excavated area, as well as water removed in dewatering efforts, will require treatment prior to disposal back into the environment.

    Application examples

    US EPA. Housatonic River, MA. PCB and NAPL removal using excavation.

    http://www.itrcweb.org/contseds_remedy-selection/Content/Appendix%20A/A%2027Case%20Study%20Housatonic.htm

    Victoria, B.C. Removal using excavation of contaminated sediments at Rock Bay, Victoria.

    http://www.qmenv.com/Case-Studies/Rock-Bay

    Performance

    Excavation provides a high degree of accuracy in contaminant removal, as the site is visible and accessible with common construction equipment. Resuspension concerns associated with remediation dredging can be minimized with excavation “in the dry” by using sediment-containing controls that can be visually inspected for performance and adjusted as necessary.

    Work in the dry is costly, and costs may be difficult to control. Costs may be reduced when sediment handling and disposal volume is minimized, and when the amount of water requiring removal/rerouting is minimized.

    Measures to improve sustainability or promote ecological remediation

    Excavation may be made more sustainable by:

    • removal and replacement of species at risk and sensitive habitats likely to be impacted by excavation activities.
    • use of engineered controls to prevent sediment resuspension (for example, silt curtains).
    • Completion of excavation activities outside of sensitive periods (for example, spawning, commercial fishing).
    • identification of site-specific regulatory resources (for example, fisheries licences), sensitivities, and appropriate avoidance/mitigation measures.
    • minimizing the removal of clean sediment through improved accuracy of bucket position and depth.
    • minimizing the amount of material sent to disposal facilities. Where possible, the following options should be considered:
      • search out opportunities for sediment reuse prior to settling on treatment and/or disposal options. Reuse options include landfill cover, construction fill, mined lands restoration, subgrade cap or restoration material, building materials, and beach nourishment.
      • treat and dispose of sediment and wastewater on-site.
      • reduce the contaminant volume requiring disposal through chemical pretreatment, dewatering, or grain-size separation.
      • where possible, reduce the transport distances and handling requirements. Treat low-volume contamination with a passive remediation technology, such as MNR.

    Potential impacts of the application of the technology on human health

    Aquatic impacts

    Excavation operations can significantly alter the sediment profile and cause complete loss of the benthic community. Full recovery may take months to years to achieve, as excavation removes existing habitats and food supply in the area and may alter the nutrient and oxygen levels within the sediment.

    Negative effects to the aquatic and benthic communities may be mitigated by conducting pilot studies or site reviews in advance. Pre-excavation studies will determine whether excavation is likely to pose unacceptable risks to the aquatic and benthic 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 remediation option, or these species may require collection and relocation during excavation activities.

    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 bucket, work stoppage during high winds) should be considered as a means of reducing sediment resuspension into the water column.

    Monitoring the site is an important step to ensure targets are met. Post-remediation monitoring will improve residuals management and allow timely implementation of risk mitigation measures. Risk mitigation may involve additional excavation or implementation of secondary treatment (for example, Capping – Sediments). In addition to immediate post-remediation monitoring, short and long-term monitoring plans should be developed for ongoing maintenance at the site. Monitoring should include sediment toxicity, benthic community recovery, presence of bioaccumulating contaminants, and contaminant concentrations in fish tissue. In addition, aquatic and benthic recovery may be improved through replacement of large debris and boulders. This will provide cover for aquatic organisms and encourage rebound of the site after excavation is complete.

    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
    Water Body Redirection Dissolved Contaminants Skin contact, accidental ingestion Provide staff with appropriate personal protective equipment (PPE) and reactionary materials (for example, eyewash station), as necessary.
    Sediment Removal Contaminated and clean sediments Skin contact, inhalation of particulates, accidental ingestion Educate staff on safety and provide appropriate PPE and reactionary materials (for example, sorbent pads), as necessary.
    Storage of Dewatered Sediment Dust Inhalation of particulates Cover the sediment and use water to suppress dust, as necessary. Use PPE when handling material.
    Wastewater Treatment and Disposal Water treatment amendments Skin contact (chemical burns, thermal burns) Educate staff on safety and provide appropriate PPE and reactionary materials (for example, sorbent pads), as necessary. Follow safe storage and handling to minimize exposure, as outlined in MSDS sheets.

    References

    Author and update

    Composed by : Ashley Hosier, Ing., Royal military college

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

    Updated Date : November 24, 2016

    Version:
    1.2.1