From: Public Services and Procurement Canada
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.
The systems may include:
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.
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.
Notes:
This technology applies to NAPL which include several compounds and may belong to different categories of contaminants.
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.
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.
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.
The following sites provide sample applications:
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
