Fact sheet: Soil vapour extraction

From: Public Services and Procurement Canada

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Description

Soil Vapor Extraction (SVE) is an in situ technology that removes residual or free phase contaminant(s) from the vadose zone. SVE applies to volatile and semi-volatile organic compounds (VOCs and SVOCs, respectively), and light to medium petroleum hydrocarbons such as gasoline, diesel, heating oil, and kerosene.

The treatment principle consists of applying a vacuum within the soil vadose zone to create a movement of air through the soil pores and induce a transfer of the contaminant, by advection and diffusion, to the airstream. Contaminated vapours are extracted from the soil through horizontal or vertical extraction wells and are treated prior to release into the atmosphere. A geomembrane, asphalt or bentonite can be used to cover the soil surface to prevent vapour migration into the atmosphere and to increase the radius of influence of the extraction wells.

SVE systems are usually combined with other in situ technologies targeting volatile and semi-volatile contaminants, such as air sparging, thermal desorption and chemical oxidation. The circulation of air within the soil vadose zone when applying vacuum increases the oxygen concentration and stimulates biodegradation.

Internet links:

4.8 Soil Vapor Extraction—FRTR Remediation Technologies Screening Matrix and Reference Guide, Version 4.0

Implementation of the technology

Soil vapour extraction systems use relatively large amounts of energy to create relatively high subsurface vacuums and gas flow rates, stripping volatile contaminants out of the soil. Wells, trenches, permeable drains or other structures are used to extract air. Extracted air is treated prior to being released into the environment.

On-site pilot tests are necessary to determine the type of wells, the radius of influence and spacing of the extraction wells and the system operation settings (flow, pressure, intermittent extraction cycle, etc.).

SVE systems may include:

Mobilization, site access and temporary facilities

Installation of extraction wells;

Installation of an air extraction system consisting of a pipe network and an extraction and treatment unit. The unit normally includes a vacuum pump (or blower), an air-water separator (“knockout drums”) and a treatment system, such as activated carbon filters, biofilters or a thermal oxidizer Monitoring of air emissions

Decommissioning of extraction wells and system is also required

Materials and storage

Air extraction and treatment units must be stored on site. Otherwise, SVE typically require very little on-site storage. Storage could include auxiliary fuel for thermal oxidizers and new and contaminated air treatment media such as granular activated carbon. 

Waste and Residues

There are few to no residual generated, depending, on the air emissions controls used.

Spent air treatment sorbent (for example, activated carbon) could require periodic off-site transport and regeneration or disposal.

Extraction systems may require periodic draining and off-site disposal of contaminated water from the air/water separator.

Catalytic oxidation of chlorinated organics produces acid gasses. These are typically managed with a caustic scrubber. In such a case, scrubber water would then require periodic neutralization and disposal.

Recommended analyses for detailed characterization

Physical analysis

  • Soil granulometry
  • Measurement of NAPL surface tension under site conditions
  • Contaminant physical characteristics

Recommended trials for detailed characterization

Physical trials

  • Gas permeability trials
  • Vapour survey
  • Evaluation of the radius of influence
  • Airflow rate
  • Evaluation of operating pressure/vacuum

Hydrogeological trials

  • Measurements of seasonal water level fluctuations

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

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

Targeted contaminant groups for SVE are primarily VOCs but SVOCs could also be targeted, and certain residual or free phase petroleum hydrocarbons within the vadose zone

Generally applies to contaminants with a vapour pressure above 0.5 mm Hg and to soils with a minimal air permeability.

Certain complementary technologies increase the efficiency of total contaminant removal.

Vapour emission migration is controlled by the SVE system, which reduces the risk of inhalation and explosion.

Low installation costs.

This technology is well accepted by regulators and the public in general when combined with an air treatment system.

Applications to sites in northern regions

Intensive soil vapour extraction may not be appropriate for remote northern sites without access to utilities or local operations & maintenance labour. Extreme cold can hamper volatilization in shallow material, but deep soil temperatures are relatively constant over the course of the year. Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in saturation and air permeability and long periods without operator intervention, fuel supply or sorbent removal.

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
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
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
Does not apply
Petroleum hydrocarbons
With restrictions
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Does not apply

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:

Treatment time may take months to years. SVE is prone to “tailing” behaviour (where extracted vapour concentrations fall over time but still remain above desired levels) very similar to that observed with groundwater pumps and treat systems. 

Long-term considerations (following remediation work)

None. SVE does not affect soil properties and structure.

Secondary by-products and/or metabolites

The application of the SVE system enhances the volatilization of the contaminant(s) and the vapours produced during treatment must be collected and treated. Management of treatment system by-products is a part of normal treatment system operation.

SVE treatment can stimulate biodegradation of certain contaminants within the vadose zone through enhanced air circulation and aeration. The biodegradation of monocyclic and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively) and petroleum hydrocarbons does not produce toxic by-products. The biodegradation of certain aliphatic chlorinated hydrocarbons can generate toxic metabolites (e.g. the biological transformation of dichloroethene produces vinyl chloride).

Limitations and Undesirable Effects of the Technology

  • The SVE technique does not apply to dense oil products, PCBs, and metals.
  • High organic matter and moisture contents in soil reduce the performance of the SVE system.
  • Not efficient within the groundwater capillary fringe because of a too high moisture content.
  • Not suitable for soils with permeability lower than 10-4 cm/s, for heterogeneous soil, stratified soil, or soil with significant preferential pathways.
  • The treatment times may vary from short—to long-term.
  • Very high applied vacuum can cause groundwater upwelling (a “groundwater mound”). Failure to plan for mounding can result in intermittently submerged vapour extraction well screens, reducing extraction capacity or system availability.
  • Handling, for example, fuel vapours at levels near the lower explosive limit (LEL) and/or using supplemental fuel or reactive oxidation catalysts may pose a fire/explosion risk. Designers typically specify special “intrinsically safe” equipment in areas where flammable vapours are handled and incorporate ventilation, alarm, control interlock and fire suppression measures.
  • Mobilization of naturally occurring radon is theoretically possible (but is generally not an issue of significance).

Complementary technologies that improve treatment effectiveness

In situ biological treatment (bioventilation, biosparging,...)

Thermal treatments (electromagnetic, electrical resistance, hot air injection or vapour injection) enhance the volatilization of less volatile contaminant(s) and accelerate the biodegradation process when the temperature is kept below 40oC. It allows the less volatile compounds to be volatilized.

Decreasing the water table level can increase the depth of application of the SVE technology, and improve the treatment of the soil capillary fringe. It is not a recommended technology when light NAPL is present.

Geomembranes or bentonite covering the soil surface prevents short circuiting and increase the radius of influence of the extraction wells.

Air injection technologies facilitate the volatilization and extraction of deep contamination, contamination in low permeability soils and contamination in the saturated zone (air sparging).

Increasing soil fractures with a mechanic or pneumatic system improves soil permeability.

Required secondary treatments

Extraction systems typically require treatment of extracted vapours before discharge to air. Extracted soil vapour is first directed through “knockout drums” (air/water separators) before treatment in an air emissions control systems using either adsorption (granular activated carbon), biofiltation or thermal oxidation (with or without a catalyst). Those thermal oxidation systems, if improperly configured, can discharge products of incomplete combustion in the atmosphere. 

Application examples

Several pilot studies and full-scale application examples of the SVE technology are available on the US EPA website. There are also several private companies that offer the SVE technology as a remediation solution.

Some application examples are available at these addresses:

Soil Vapor Extraction and In Situ Chemical Oxidation at Swift Cleaners, Jacksonville, Florida (case 404)—Costperformance.org—FRTR

Air Sparging and Soil Vapor Extraction at Landfill 4, Fort Lewis, Washington (case 84)—Costperformance.org—FRTR

Abstracts of Remediation Case Studies Volume 4—FRTR—US EPA pdf

In Situ Bioremediation and Soil Vapor Extraction at the Former Beaches Laundry & Cleaners—FRTR

Performance

The SVE technology is a low cost, well tested and known technology.

SVE is effective only where gas flow in soil can be induced. To extend its effectiveness below the water table, it may be enhanced by dewatering or by air sparging (the injection of air below the water table, which oxygenates the groundwater and/or strip contaminants out of the saturated zone and transports them to the unsaturated zone with a stream of bubbles). 

Measures to improve sustainability or promote ecological remediation

  • Pump size optimization to lower energy consumption.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Use of renewable energy and energy-efficient equipment (e.g., geothermal or solar energy for extraction).
  • Favour the biotreament and possibly the use of passive or pulsed bioventing.
  • Use of a biofilter for air treatment.
  • Allow longer treatment time to avoid operations in winter conditions (avoid having to winterize the system) and thus lower energy requirements.
  • On-site treatment of condensate and purged water from air conduits.
  • Minimizing site visits by the use of telemetry for remote monitoring of site equipment.

Potential impacts of the application of the technology on human health

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References

Author and update

Composed by : Josée Thibodeau, M.Sc, National Research Council

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

Updated Date : March 1, 2008

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1