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| Resource Library > Technology Transfer > Programs and Initiatives > Source Zone Treatment > Background > Groundwater Extraction Treament |
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Groundwater Extraction and Treatment
Groundwater extraction is a component of many pump-and-treat processes, which are some of the most commonly used groundwater remediation technologies at sites where groundwater is contaminated. Possible objectives of groundwater pumping include removal of dissolved contaminant mass from the subsurface, and containment of contaminated groundwater to prevent continued migration to downgradient locations.
Schematic of Groundwater Extraction System Operated to Achieve Containment
Prior to design or installation of a groundwater extraction and treatment system, it is necessary to determine whether cleanup or containment will be the most appropriate remedial action. If cleanup is selected, the cleanup objectives must be established. If containment is chosen, groundwater extraction is used to establish a hydraulic barrier to prevent continued migration of contaminant plumes.
After groundwater is extracted from the subsurface, it may be treated to remove entrained contaminants prior to discharge or disposal of the extracted water. Actual treatment may include a process or a train of processes such as gravity segregation, air strippers, or carbon filtration systems tailored to remove specific contaminants. Any groundwater extraction remedy also must incorporate a monitoring program to verify the performance and effectiveness of the remedy.
The final requirement in the design of a groundwater extraction remedy is to determine the point at which operation of the system can be terminated. Termination requirements are based on the cleanup objectives defined in the initial stages of remedial design. Termination criteria also are dependent on site-specific characteristics of the subsurface and the remedy, identified during remedial operations.
Extraction systems occasionally may be used to remove NAPL and groundwater from recovery wells or trenches. Pumping removes water and lowers the water table near the extraction area to create a cone of depression. The cone of depression in the vicinity of the extraction well produces a gravity head that induces NAPL to move toward the well and increases the thickness of the NAPL layer in the well. In most cases, establishing a cone of depression will increase NAPL recovery rates.
Schematic Diagram of NAPL Extraction System
NAPL extraction may be accomplished with one or two pumps in a single well. In a single-pump configuration, one pump withdraws both water and NAPL. The dual-pump configuration uses one pump located below the water table to withdraw groundwater and a second pump located in the NAPL layer to recover NAPL. Capital and operating costs are reduced with a single-pump system, which also allows simpler control systems and operation, but produces a stream of mixed water and NAPL that must be separated above ground surface prior to treatment and discharge.
The dual-phase extraction (DPE) process for addressing undissolved liquid-phase organic liquids (also known as "free-product recovery") is used primarily in cases where a light non-aqueous liquid (LNAPL) lens more than 8 inches thick is located at the groundwater table. The free product generally is extracted and drawn up to the surface by a pumping system. Following recovery, it can be disposed of, re-used directly in an operation not requiring high-purity materials, or purified prior to re-use. Systems may be designed to recover only product, mixed product and water, or separate streams of product and water.
Groundwater extraction and dual-phase extraction are standard, full-scale technologies. Nearly any soluble contaminant can be addressed to some degree using groundwater extraction; and mobile NAPL can sometimes be removed using DPE. Criteria for wellfield design, pumping system, and treatment are dependent on site-specific physical characteristics and contaminant type(s). The duration of operation and maintenance required for a groundwater extraction system or DPE system to achieve remediation objectives typically is long-term.
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