Fact sheet: Pump and Treat for Dense Non-Aqueous Phase Liquids (DNAPL)

From: Public Services and Procurement Canada

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Description

Pump and treat for dense non-aqueous phase liquid contamination (DNAPL) involves pumping free phase product or a mixture of free phase products and groundwater from the subsurface to be treated above ground. The movement of free-phase NAPL contamination within the groundwater can also be controlled with the installation of pumping wells downgrading from the contaminated plume. The pump and treat method produces large quantities of waste water to be treated and is therefore a relatively costly remediation method. More information about the pump and treat method in general is provided in the technical sheet, “Pump and Treat.”

In the 1980s and 1990s, this technology was one of the most commonly used technologies for the remediation of DNAPL. However, with time, it did not prove successful in achieving cleanup targets.  Pump and treat for DNAPL is now primarily used as containment approach or in combination with other remediation techniques.

Pumping and treatment of DNAPL requires a thorough knowledge of the physical properties of the contaminant(s) and of the geological and hydrogeological site conditions, including the precise location and extent of contamination. This information is important for the design of the pumping system and the location of the pumping wells.

To facilitate the movement of DNAPL toward the pumping system, surfactants or organic solvents can be injected into the contaminated area. More details on techniques to increase the mobility of DNAPLs are given in the technical sheet “Soil Washing, leaching, or chemical extraction-in situ.”

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Implementation of the technology

The system may include:

  • Detailed environmental, hydrological and geological site investigation
  • Mobilization, preparation of the site and site access, and set-up of temporary facilities
  • Installation of pumping wells and observation wells
  • Pumps, typically using electric or pneumatic pumps, and conveyance pipe installation (often underground in trenches engineered for frost and traffic protection)
  • Groundwater treatment system installation; this may require a small building or container; treatment varies depending on the contaminant but may include filtration or coagulation/precipitation, activated carbon sorption, air stripping, chemical and/or UV oxidation, biological treatment, ion exchange or reverse osmosis, evaporation, dissolved air floatation, skimming/oil-water separation, electro coagulation, phytoremediation (treatment wetlands), steam stripping, and liquid/liquid extraction
  • If vapours are generated (by air stripping, for example), vapour/off-gas treatment which may include thermal oxidation, catalytic oxidation, biofiltration or granular activated carbon sorption
  • Discharge system installation, such as discharge to existing pipelines, new surface water outfall, re-injection into the ground, to injection wells, infiltration field or infiltration pond, to local stormwater or sanitary sewer systems or to surface water
  • Pumping well and treatment plant decommissioning
  • Long-term monitoring is required to ensure that groundwater concentrations are below regulatory levels after the shutdown

Pump and treat require in-depth hydrogeological knowledge of the site and is often implemented based on a number of field tests such as slug tests and pumping tests, followed by a groundwater modelling study.  Once the system is online, pumping data may allow for more detailed analysis of the capture zone, which in turn may lead to modifications to the pumping system aimed at improving its effectiveness in capturing or containing groundwater contamination. 

Materials and storage

  • Pump and treat installations for DNAPL rely on traditional water well, drainage, water works and utility construction methods and require commonly available construction equipment. Treatment plants may be constructed on-site. Pre-assembled units in trailers, shipping containers or on skids, are commonly available. Extraction and conveyance require energy input. Maintenance chemicals are also required, for example, to periodically clean out scaling or fouling
  • Apart from sites requiring extensive trench systems, construction activities are typically low-impact, with minor on-site storage
  • Treatment systems typically stock fresh reactants and process chemicals as well as collected residuals, such as concentrated brines, spent sorption media or collected sludge. These may include a range of oxidants (such as hydrogen peroxide or chlorine), reductants (such as polysulphides), biological substrates, sorbents, regenerant brines, anti-foulants, anti-scaling compounds (including phosphonates, polyphosphates, sulphamic acid), emulsion breakers (solvents, non-ionic surfactants, various amines, specialty polymers), antifoaming agents, etc.

Waste and Discharges

  • System installation typically requires drilling or excavating in contaminated areas, resulting in the handling and disposal of contaminated soils, which are typically containerized and disposed of off-site. Drill cuttings and down-hole equipment may be highly contaminated
  • Treatment systems may generate extensive solid and liquid residuals, which vary depending on the nature of the contamination present and the treatment methods used.  Spent sorbent such as activated carbon and collected solids (sludge) require collection and off-site transport. Under specific conditions, solid residuals may be flammable, corrosive or produce toxic leachate. The appropriate storage, management and disposal of treatment residuals are part of proper treatment system operation. See also “Pump and Treat fact sheet”
  • Ideally, treated groundwater meets all applicable criteria for release and doesn’t constitute a high-risk discharge. Inadequately treated discharge containing by-products or excess reactants may constitute a hazard to downstream receptors
  • Free product (NAPL) is typically batched in drums or tanks for eventual shipping off-site. On-site use or destruction (typically by incineration or co-firing with other fuels) may be applied. Note that waste fuel, dirty fuel and/or waste oil combustion in unspecialized equipment may cause deleterious emissions to air

Recommended analyses for detailed characterization

Physical analysis

  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Measurement of NAPL surface tension under site conditions
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Hydrogeological trials

  • Permeability test
  • 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

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

  • Allows for the recovery of free phase and dissolved DNAPL contamination
  • Soils must be sufficiently permeable to permit movement of the contaminant(s) toward the pumping wells. In general, soils with permeability greater than 10-4 cm/s permit sufficient circulation
  • DNAPL viscosity must be low
  • A minimum quantity of free phase must be present in the system to allow for pumping of the contamination

Applications to sites in northern regions

Intensive pumping may not be appropriate for remote northern sites without access to utilities or local operations & maintenance labour. Possible alternatives include source area excavation, passive skimming, passive reactive barriers, and/or bioventing.  Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in ground conditions and long periods without operator intervention, fuel supply or collected product removal.

In cold climates, freeze-thaw cycles can cause the remobilization of residual NAPL. As wet soil freezes, its volume increases. The increased volume results in material transport through frost heave and related phenomena. The increase in viscosity associated with cold temperatures may also limit DNAPL recovery rates.  

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
Applies
Dissolved contamination
Applies
Free Phase
Applies
Physical
Applies
Residual contamination
Does not exist
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
With restrictions
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
With restrictions
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
With restrictions

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
Applies
More than 5 years
Applies

Notes:

DNAPL pump and treat systems which are designed primarily for the recovery and treatment of free products from the subsurface may operate for shorter periods. Large volumes of DNAPL must be present to be effective and once the recoverable DNAPL has been pumped, the system may be modified or replaced with a less energy- and cost-intensive technology to attain long-term rehabilitation goals.

Pump and treat systems designed for containment and treatment of groundwater contaminated with DNAPL typically operate for longer periods, years to decades.  Like pump and treat systems in general, they are prone to “tailing,” a phenomenon in which contaminant concentrations in groundwater asymptotically approach a steady state value above the remediation criteria, necessitating the indefinite operation of the collection and treatment system.

Long-term considerations (following remediation work)

Pump and treat systems are generally ineffective at remediating residual contamination. A long-term strategy for residual contamination management may be necessary.

When a system is initially taken off-line, “rebound” is often observed. “Rebound” refers to the increase in contaminant concentrations in groundwater which is sometimes observed after flushing/dilution by the pump-and-treat system ends. Long-term monitoring is required to ensure that post-shutdown groundwater concentrations stay below levels of concern.

Secondary by-products and/or metabolites

In general, the pumping procedure doesn’t produce by-products in the subsoil. However, incomplete reaction within the treatment system may result in hazardous degradation products. In rare cases where chlorination is employed, disinfection by-products such as trichloromethane may also be formed. Management of treatment system by-products is a part of normal treatment system operation. 

Limitations and Undesirable Effects of the Technology

  • Efficient recovery is heavily dependent on precise knowledge of the location and extent of DNAPL plumes
  • Significant treatment costs
  • Treatment times can be long and indefinite
  • Complete recovery of the DNAPL by pumping is almost never possible without the application of a complementary remediation technology such as multiphase extraction or a source remediation technology (thermal or chemical oxidation technologies as examples)
  • Pump and treat alone has limited capacity to remediate residual contamination;
  • Soil permeability must be greater than 10-4 cm/s
  • A large quantity of water may need to be managed if the system is designed for high permeability aquifers
  • Presence of impermeable sub-layers or preferential pathways can reduce the recovery efficiency
  • Clogging or biofouling (excessive growth of microorganisms) of the extraction wells and associated treatment equipment may occur and will require higher maintenance requirements
  • Inadequate or inappropriate treatment may expose receptors downstream of the system discharge point to contaminants or by-products
  • Free product and its vapours can create serious fire or explosion hazards. Free product typically contains contaminants at relatively high concentrations. Because of flammability and/or toxicity, it is usually handled, stored, transported, disposed as a hazardous material

Complementary technologies that improve treatment effectiveness

  • Thermal treatment system (conduction, electrical resistance or vapour injection heating) may increase the movement of the contaminant(s) toward the pumping wells
  • The injection of washing (surfactant) solutions may increase the recovery of DNAPL by pumping
  • Soil fracturing maybe used to create new transport pathways and increase groundwater yields

Required secondary treatments

  • Requires phase separation of the pumped liquids. Phase separators include “knockout drums” and oil-water separators
  • Treatment of pumped groundwater is also required.  The type of treatment system depends on the nature of contamination present.  Additional details about groundwater treatment processes typically used in pump and treat systems is available in the fact sheet “Pump and Treat”

Application examples

Pump and Treat system for DNAPL is not anymore used as a remediation technology. Recent examples and applications are very limited.

Performance

  • Use of pump and treat to remediate DNAPL contamination is no longer common
  • Pump and treat is a long-term treatment and is often costly
  • Complete remediation of a site is not always possible using only pump and treat systems

Measures to improve sustainability or promote ecological remediation

  • Ensure pump size is appropriate and pump is energy efficient
  • Optimize scheduling to reduce resources and minimize mobilization days
  • Use of renewable energy and energy efficient machinery (such as solar energy for pumps, geothermal or solar energy for treatment plant)
  • Optimize pumping flow rates in both the pumping system and the water treatment unit to reduce energy
  • Ensure water treatment process is efficient and reduce wastes and consumables such as activated carbon
  • Recycling of recovered DNAPL
  • Use biofilters for water treatment
  • Operate in cyclical rather than continuous mode to improve recovery
  • Limit the number of site visits by using telemetry to monitor site conditions remotely

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 : Jennifer Holdner, M.Sc., Public Works Government Services Canada

Updated Date : April 29, 2014

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

Updated Date : March 31, 2018

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