From: Public Services and Procurement Canada
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.”
The system may include:
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.
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.
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.
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.
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.
Pump and Treat system for DNAPL is not anymore used as a remediation technology. Recent examples and applications are very limited.
Unavailable for this fact sheet
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
