Fact sheet: Excavation and treatment technologies

From: Public Services and Procurement Canada

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Description

Contaminated soil excavation and treatment technologies are the oldest of all site remediation technologies. They apply to all types of contaminants within the soil vadose zone. Excavated soils may be treated or disposed of at an appropriate site. The contaminated soil treatments are ex situ, and may be performed directly at the excavation site or at an off-site treatment facility.

The contaminated material, once excavated, can be treated by biological, thermal, chemical, or physical remediation techniques. The goal when treating the excavated soil is to meet the required contaminant concentrations before disposing of the material. There are several types of disposal methods for treated or non-treated soils, such as burial, used as fill material, or as capping material in landfills.

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Implementation of the technology

Conventional excavation equipment is used to physically remove shallow contaminated soil, which is transported to an approved off-site facility for treatment or disposal. Once the contaminated materials are removed, the site is backfilled or otherwise prepared for future use. The method relies on traditional/commonly available civil/earthworks construction equipment and methods. The process may include:

  • Mobilization, site access and temporary facilities;
  • Clearing/grubbing/demolition;
  • Topsoil stripping and temporary stockpiling;
  • Excavation dewatering (sumps, wells, well points, cut-off walls, etc.);
  • Slope stability controls (cutbacks, benches, piling, soilcrete, controlled low strength materials, etc.);
  • Foundation protection for structures to be preserved (shoring, underpinning, etc.);
  • Excavation (typically with a hydraulic excavator or backhoe; less frequently with a clam shell, drag line, dozer, loader, bucket auger or other);
  • Temporary on-site stockpiles or direct loading into a transport vehicle;
  • Transport by truck (most common) or barge/rail/other means of transportation for disposal or treatment at pre-existing off-site facility;
  • Backfilling with imported non-contaminated fill material or with on-site clean excavated material;
  • Surface restoration (grading, paving, hydro seeding or planting).

Materials and Storage

  • On-site storage is typically limited to small amounts of fuel and lubricant (daily fuelling is often done from a mobile tank) as well as miscellaneous construction site supplies;
  • Contractors may create temporary stockpiles of contaminated materials pending characterization and/or off-site shipment; 
  • Sites with water treatment systems (seepage into excavations and/or stormwater run-off) may also store water treatment system materials on-site;
  • Source area removal projects intending to leave low-level contamination in place may also handle materials such as oxygen release compounds used during backfilling operations, which may be stored on-site;
  • Projects with on-site ex situ soil treatment may store additional substances (such as nutrients for example, used in aerobic biopile) for treatment;
  • On a project specific basis, additional substances, such as lime, calcium chloride, road salt, etc. may also be stored on-site.

Waste and Discharges

  • There is no significant solid wastes other than material that is transported off-site for treatment, recycling or disposal (i.e. contaminated soils, demolition debris [concrete, asphalt, etc.]);  
  • Site garbage is typical of a construction site, but may also include used/spent sorbent pads and/or water treatment media;
  • Windblown dust originating from the excavation face, vehicles (track-out) or stockpiles may also deposit directly on downwind surfaces;
  • Diverted and collected/treated stormwater is typically passed into the local stormwater system;
  • Contaminated groundwater is either stored in tanks for off-site shipment and treatment or is treated on-site and discharged to the municipal sanitary sewer, the local storm system or to infiltration areas;
  • Contaminated material stockpiles are typically covered to limit the infiltration of water (which generates contaminated leachate) and windblown dust. Many sites also require low permeability lining under stockpiles, with the collection and treatment of waste water (leachate) generated from the pile;
  • Vapour phase discharges are typically limited to equipment exhaust and volatilization of contaminants from soil excavation activities, fresh excavation faces and/or soil stockpiles. In specific cases, naturally occurring radon may also be mobilized;
  • Natural biodegradation, particularly for hydrocarbons, may also lead to off gases such as carbon dioxide (CO2), ammonia (NH3), methane (CH4) and/or hydrogen sulphide (H2S);
  • Contaminant volatilization is a function of vapour pressure, concentration, temperature and exposed area. Suppression techniques include pre-treatment, limiting the exposed areas, spray-on foam, etc. In extreme cases, for example, related to very high vinyl chloride levels in ambient air, excavations may be contained within a temporary structure equipped with a vapour control system. Soil stockpiles are typically covered, which limits dust generation and off-gassing.

Recommended analyses for detailed characterization

Chemical analysis

  • Oxidation reduction potential (Eh)
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Recommended trials for detailed characterization

None.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)

Phase III

  • Volume of contaminated material to treat

Applications

  • Suitable for all types of contaminants;
  • Applicable to residual contamination within the vadose zone;
  • Suitable for non-aqueous phase liquid (NAPL).

Applications to sites in northern regions

  • Remote and northern sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short seasonal work windows;
  • Difficulties in procuring timely analytical results may necessitate reliance on field screening, staged interventions and/or risk management;
  • In less populated areas, risk management might require less intensive monitoring and control measures that those generally required in more densely populated areas; 
  • For on-site ex situ treatment, extreme cold can hamper, for example, biodegradation and volatilization. Northern treatment systems require climate-appropriate design, including consideration of seasonal changes and long periods without operator intervention, fuel supply, etc.;
  • Due to permafrost, only very shallow soils may be viable for excavation (active layer);
  • Road transportation of contaminated material to existing disposal sites at remote and northern sites is often cost-prohibitive and/or not feasible. On-site excavation and treatment options should be given prime consideration (including land farming). Train or barge/boat haul may be economically feasible, but is typically not viable. If multi-mode transportation is used, accumulation, transfer and re-handling sites are subject to dust, vapour, odour, noise, track-out and stockpile leaching considerations.

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
Applies
Resorption
Applies
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
Applies
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
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Short treatment time, a few weeks to a few months.

Long-term considerations (following remediation work)

  • Isolated cases exist where uncontrolled backfill material was later demonstrated to itself exceed applicable standards for site use;
  • Poorly engineered backfill (poor material choice, placed frozen, not compacted, etc.) can create geotechnical stability problems or differential settlement.

Secondary by-products and/or metabolites

There is few to no by-product production from the excavation of contaminated soil.

Limitations and Undesirable Effects of the Technology

  • No longer the first remediation option because of the manipulation of contaminated material and the fact that the problem is only transported elsewhere. In situ treatment solutions are now the first consideration;
  • Risks of contaminants volatilization;
  • Dust production during soil manipulation is a significant issue particularly with fine-grained material in dry and windy conditions. Typically, water spray is used for dust suppression. A number of alternate (more expensive) approaches were used for certain site-specific reasons including polymer sprays, calcium chloride solutions, lignosulfonates, foaming agents, etc. Biodegradable fabric ground cover can be used for controlling both dust and erosion as vegetation re-establishes;
  • Transportation of contaminated material increases the cost of their treatment;
  • Transportation of contaminated material is not well accepted in residential areas.
  • Contamination depth limits the application of the technology;
  • Excavations can theoretically make anoxic/anaerobic systems oxic/aerobic (and iron staining is commonly observed in seepage faces at hydrocarbon sites), but few adverse effects have been documented. Similarly, localized changes to pH and ionic strength are possible. Changes in Oxidation-Reduction Potential (ORP), pH, ionic strength, and/or soil organic content can affect transport of metals such as cadmium, copper, zinc or chromium;
  • The physical disruption of excavation is significant:  cave-ins, slumping and related damage to nearby structures are possible if geotechnical and civil engineering is inadequate. Dramatic, although short-lived, changes to site-scale hydrology and hydrogeology are common. Large excavations below the water table typically require either groundwater cut-off walls or extensive pumping, both of which alter local flow paths;
  • Accidents are unlikely to affect off-site receptors. Typical scenarios include slope failures, equipment collisions, small spills of fuel or hydraulic fluid, damage to utilities, damage to adjacent structures or breathing air quality issues in low-lying areas, such as the excavation pit itself;
  • Excavation activities and use of heavy machinery may result in neighbours/stakeholders concerns such as dust, noise, odour, lights (at night) and traffic;
  • Where blasting or ripping activities generates waste rock high in sulphide mineralization, exposure of waste rock to moisture and air can lead to sulphide oxidation, low pH, high sulfate concentration and high dissolved metals in leachate. This constitutes acid mine drainage (AMD) but is typically not a major concern with short time-scale remedial actions.

Complementary technologies that improve treatment effectiveness

Often called pre-treatment, there are several complementary technologies which can reduce the quantity of material to be treated or prepare the contaminated soil for treatment. Here is a list of some pre-treatments:

  • Soil drying;
  • Screening;
  • Washing;
  • Magnetic or gravimetric separation of soil particles.

Required secondary treatments

Depending on the contaminants present in the soil matrix, there is a wide variety of secondary treatments that can be employed in the decontamination of excavated soils. These technologies are called ex situ treatments, and several have been reviewed in the following technical sheets:

Biological Treatment

  • Bioreactor treatment
  • Biopile
  • Wetlands
  • Land farming

Chemical Treatment

  • Chemical oxidation
  • Soil washing
  • Soil mixing and chemical treatment

Physical Treatment

  • Adsorption
  • Physical separation
  • Solidification/stabilization

Thermal Treatment

  • Thermal desorption
  • Hot gas decontamination
  • Pyrolysis
  • Vitrification
  • Incineration

Application examples

Application examples are available at these links:

Performance

  • Most established method for unsaturated soil remediation (in vadose zone).
  • Excavation costs rise quickly when the depth of the contamination is below 10 metres below ground surface.
  • Transportation and disposal costs depend on the availability of commodities in the area of the site as well as the nature and concentration of the contaminants.

Measures to improve sustainability or promote ecological remediation

  • Mitigation measures to minimize the impacts of possible soil erosion and uncontrolled stormwater run-off.
  • On-site or nearby treatment options, as opposed to disposal/treatment facilities that involve long distances.
  • Use of on-site or nearby sources to backfill or re-use of excavated and treated material on-site.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Use of native plants for revegetation, if applicable, to reduce irrigation.
  • Minimizing fuel consumption (and use of renewable energy where available) for vehicles and heavy machinery.
  • Use of equipment and vehicles with alternate or cleaner fuels; minimization of equipment idling.         

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

Composed by : Martin Désilets, B.Sc., National Research Council

Updated by : Martin Désilets, B.Sc., National Research Council

Updated Date : March 1, 2008

Latest update provided by : Daniel Charette, P.Eng., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017

Version:
1.2.1