Fact sheet: Drawdown Pumping System for non-aqueous phase liquids (NAPL)

From: Public Services and Procurement Canada

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Description

The drawdown pumping system is a non-aqueous phase liquid (NAPL) recovery technique. This technique consists of creating a cone of depression (drawdown) around the extraction points with the help of a groundwater pumping system. The formation of the drawdown enhances the migration of NAPL towards the pumping well/trench, and reduces the impacts of piezometric fluctuations caused by the recharge and discharge activities of the groundwater. This technique is mostly used for light non-aqueous phase liquid (LNAPL) such as petroleum hydrocarbon contaminants including gasoline, diesel and heating oil.

The drawdown pumping technique can employ one or two pumps. The one-pump system mixes free phase and groundwater during pumping and thus requires a phase separation process to recover the free phase. The two-pump system allows for pumping of the free phase and the groundwater separately. In this configuration, the first pump is located within the groundwater to create a drawdown. If the pumped groundwater of this first pump does not meet site-applicable discharge conditions, it will require treatment. The second pump is located within the free phase and pumps almost exclusively free phase. The two-pump system is preferable in most contaminated sites because it reduces the volume and cost of pump water treatment.

Drawdown pumping systems are often installed at sites with recoverable NAPL to simultaneously recover NAPL, control groundwater pollution and/or remediate soils above the water table. Drawing down groundwater (by pumping) in the extraction well can increase the rate at which free product can be collected.

Drawdown pumping is an energy-intensive, expensive technology and is frequently discontinued in favour of other alternatives once recoverable free product has been exhausted. Residual contamination is thus extremely common at sites where drawdown pumping has been discontinued.

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Implementation of the technology

The systems may include:

  • Mobilization, site access and set-up of temporary facilities
  • Installation of pumping wells by drilling and/or of collection trench
  • Pump and conveyance pipe installation (typically in underground trenches designed for frost conditions and protected from traffic)
  • Pumping unit installation (in a container or small building). This includes holding tanks, pump(s), phase separation equipment (water and NAPL), water pumping and treatment equipment and controls
  • Accumulation tanks for non-aqueous phases (free product, recovered fuel, recovered solvent) for destruction or off-site disposal.
  • Groundwater treatment equipment is presented in the “pump and treat” factsheet.
  • Treated water discharge installations (for example, to ground, to injection wells, to infiltration or "leach" fields, to local stormwater system, to local sanitary sewer, or to surface water)
  • Pumping wells and treatment unit decommissioning

Materials and storage

  • Drawdown pumping systems rely on traditional water well, drainage, water works and utility construction methods and require commonly available equipment. Unless an extensive trench system is required, construction activities are typically low-impact, with minor on-site storage.
  • Treatment plants may be constructed on-site. Pre-assembled units in trailers, shipping containers or on skids are commonly available.
  • Pumping and conveyance require energy input and maintenance chemicals. For example, maintenance chemicals are needed to periodically clean out scaling or fouling.
  • Treatment systems have highly individual inputs, and may include a range of oxidants (such as hydrogen peroxide), biological substrates, sorbents, anti-foulants or anti-scaling compounds, emulsion breakers, auxiliary fuels, etc.
  • Pumping and treatment systems typically stock fresh reactants and process chemicals as well as collected residuals, such as collected free product and spent sorption media.

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 downhole equipment may be highly contaminated. 

Treatment systems may generate extensive solid and liquid residuals. Appropriate storage, management and disposal of treatment residuals are part of proper treatment system operation. Spent sorbent (for example, activated carbon) and accumulated solids (sludge) require collection and off-site transport, typically on a batch basis. The nature of treatment processes frequently concentrates contaminants in these solid residuals. Under specific conditions, solid residues may be flammable, corrosive and/or produce toxic leachate.

Ideally, treated groundwater meets all applicable criteria for release and does not constitute a high-risk discharge. However, inadequately treated discharge, discharge containing by-products, and discharge containing excess reactants or at unacceptable pH levels 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 implemented. 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

Chemical analysis

  • pH
  • Conductivity
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

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

Physical trials

  • Evaluation of the radius of influence

Hydrogeological trials

  • Permeability test
  • Pumping trials
  • NAPL recovery trials

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)

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 LNAPL in medium to high permeability aquifers.
  • Requires the installation of large pumping wells, at least 15 cm in diameter
  • Soils must be sufficiently permeable to allow the migration of NAPL towards the pumping wells. Soils with permeability greater than 10-4 cm/s are suitable for this type of treatment

Applications to sites in northern regions

Drawdown pumping systems may not be appropriate for remote northern sites without access to utilities or local operations and maintenance labour. Possible alternatives include source area excavation, passive skimming, passive reactive barriers, and 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 because ice has a larger volume than liquid water. The increased volume results in material transport through frost heave and related phenomena.

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
With restrictions
Chlorobenzenes
Does not apply
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
Does not apply
Petroleum hydrocarbons
Applies
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Does not apply

Notes:

This technology applies to NAPL  which include several compounds and may belong to different categories of contaminants.   

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
Does not apply

Notes:

Free product recovery rates typically decline quickly, in a matter of weeks to months. When further free product recovery becomes uneconomic, the system is often replaced with an alternative residual impact management strategy, such as monitored natural attenuation or groundwater pump and treat.

Long-term considerations (following remediation work)

As discussed above, drawdown pumping systems are commonly taken off-line and replaced once recoverable free product has been exhausted. At the point of replacement, environmental clean-up criteria typically have not been met.

Secondary by-products and/or metabolites

By-products are not inherent to drawdown pumping systems; however, in practice, a drawdown pumping system is coupled with a pump water phase separation unit and/or a water treatment unit and, depending on the contaminant(s) to be treated and on the treatment systems used, secondary by-products and/or metabolites may be produced. Management of treatment system by-products is a part of normal treatment system operation.

Limitations and Undesirable Effects of the Technology

  • Soil permeability must be greater than 10-4 cm/s
  • Requires a phase separator
  • The presence of impermeable sub-layers or preferential pathways can reduce performance and product recovery
  • Excessive drawdown of groundwater may promote contamination of the saturated zone as free phase product may become trapped within the saturated zone matrix
  • Groundwater recharge must be sufficient to stabilize the drawdown
  • Energy-intensive and expensive technology that is frequently discontinued in favour of other alternatives once recoverable product has been exhausted.
  • Significant pumping-induced changes to the hydraulic gradient may substantially alter the tension-saturated zone above the water table (capillary fringe) and may also “smear” free product across the vadose zone.
  • Major changes to groundwater flow have the potential to alter infiltration, transport and discharge, which, in turn, may change qualities such as pH and oxidation-reduction potential.
  • A large quantity of water must be managed when using the one pump system
  • 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.
  • Recovered free product typically contains contaminants at relatively high concentrations. Because of flammability and/or toxicity, it is usually handled, stored, transported, manifested, recycled, treated and/or destroyed as a hazardous material, hazardous waste, special waste or dangerous good.

Complementary technologies that improve treatment effectiveness

  • Drawdown pumping can also be used with enhanced Soil Vapour Extraction (SVE). Decreasing the groundwater level dewaters soils such that SVE becomes effective for the treatment of residual contamination located in the smearing zone (zone of water table seasonal fluctuations).
  • Thermal treatment reduces the viscosity of the free phase and increases the migration of residual contaminants within the vadose zone towards the pumping wells
  • Soil washing solutions can increase the recovery of NAPLs that are adsorbed onto soil particles
  • Soil fracturing may increase yield by creating new transport pathways.

Required secondary treatments

  • Requires a phase separation system. Phase separators include NAPL-water separators.
  • Groundwater treatment may be required.

Application examples

The following sites provide sample applications:

Performance

  • Allows the recovery of free phase
  • Permits recovery of residual contaminants within the vadose zone under certain conditions
  • Well established and tested technology

Measures to improve sustainability or promote ecological remediation

  • Pump type and size optimization
  • Schedule optimization for resource sharing and minimization of mobilization days
  • Use of renewable energy, such as geothermal or solar energy, and of energy efficient machinery for extraction
  • Wastewater process optimization to reduce wastes and consumables, such as activated carbon
  • Recycling of recovered free product for use as fuel
  • Optimization of water flow rates to reduce size of treatment equipment and energy consumption
  • Use of biofilters for water treatment
  • Implementation of cycling operation mode rather than continuous operation to improve recovery
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions

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 30, 2014

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

Updated Date : March 31, 2018

Version:
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