Dense Nonaqueous Phase Liquids (DNAPLs)
- Policy and Guidance
- Chemistry and Behavior
- Environmental Occurrence
- Detection and Site Characterization
- Treatment Technologies
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Direct and Multiphase Recovery
DNAPL source areas can be divided into two major categories—those that contain flowable material and those that contain residual material. Residual materials are those that cannot move as a mass because they cannot overcome the capillary pressure of the matrix they are in. Residual DNAPL may be induced to move with technologies, such as heat and surfactant/cosolvents.
Flowable masses of DNAPL are generally addressed by placing an interception trench in front of them if they are still moving or by placing an extraction well into the mass. The flowable material enters the trench where it moves to a sump and is recovered by bailing or pumping. In the case of a well, the removal of the DNAPL in the well creates an induced gradient in the DNAPL that causes the flowable mass around the well to move into it. Pumping or bailing are used to remove the DNAPL depending upon how much DNAPL is present and how fast it will flow into the well.
The Army Corps of Engineers' Multiphase extraction (MPE) engineering and design manual provides the following description of the technology:
Multiphase extraction comprises a generic category of in-situ remediation technologies that simultaneously extract more than one fluid phase from wells or trenches. These phases generally include air (i.e., gaseous phase including organic vapor) and water (i.e., aqueous phase including dissolved constituents), and may include NAPL. The terminology presented by EPA (1997), which distinguishes between dual-phase and two-phase extraction technologies, is as follows:
(1) In dual-phase extraction (DPE), soil gas and/or separate phase liquids are conveyed from the extraction well to the surface in separate conduits by separate pumps or blowers. A common ï¿½pipe within a pipeï¿½ configuration is a submersible pump suspended within the well casing that extracts liquid, which may be NAPL and/or groundwater, and delivers it through a water extraction pipe to an aboveground treatment and disposal system. Soil gas is simultaneously extracted by applying a vacuum at the well head. The extracted gas is, in turn, conveyed to a gas-liquid separator prior to gas phase treatment. DPE is in essence a rather straightforward enhancement of SVE [soil vapor extraction], with groundwater recovery being carried out within the SVE well. Other DPE configurations are also common, such as use of suction (e.g., exerted by a double-diaphragm pump at the ground surface) to remove liquids from the well, rather than a submersible pump. A line-shaft turbine pump could also be employed to remove liquids from the well, provided the water table is shallow enough.
(2) In two-phase extraction (TPE), soil gas and liquid are conveyed from the extraction well to the surface within the same conduit, which has been referred to with various names including drop tube, slurp tube, stinger, lance, or suction pipe. A single vacuum source (vacuum pump or blower) is used to extract both liquid and gaseous phases. A common configuration has the suction pipe suspended within the well casing (typically to the well's base) that can extract a combination of NAPL and/or groundwater, and soil gas. These phases are conveyed to an aboveground gas-liquid separator. If extraction of NAPL is anticipated, an oil-water separator may be installed downstream of the gas liquid separator.
MPE is most often associated with residual DNAPL remediation in the unsaturated and shallow saturated zone. If a groundwater pump is employed it can lower the water table in the vicinity of the well and potentially expose more residual for capture by the SVE system. In fine grained soils (silts and clays) the two-phase system is often employed because it can dewater the well and keep it dewatered while a high vacuum SVE system can potentially reach residual at some depth below the water table as well as the unsaturated zone. A system using a pump in fined grained soil will be much more difficult to operate since the pump can only run when there is pumpable water.
Two case studies are presented in the halogenated alkene section. One case study involves the use of a two-phase system to recover both vapor and free DNAPL. Another case study describes the use of a pump in a less permeable subsurface to draw down water and expose the formerly saturated soils to vacuum recovery. In this system the extraction wells have 65 ft screens, and sufficient groundwater entry exists over the saturated zone to allow the pump to operate without completely dewatering the well.
Analysis of Selected Enhancements for Soil Vapor Extraction
U.S. EPA, EPA 542-R-97-007, 1997
This report provides an engineering analysis of, and status report on, selected enhancements for the following SVE treatment technologies: air sparging, dual-phase extraction, directional drilling, pneumatic and hydraulic fracturing, and thermal enhancement. It also offers an evaluation of each technology's applicability to various site conditions, cost and performance information, a list of vendors specializing in the technologies, a discussion of relative strengths and limitations of the technologies, recommendations to keep in mind when considering the enhancements, and extensive references.
Engineering and Design: Multiphase Extraction
U.S. Army Corps of Engineers. EM 1110-1-4010, 1999, 285 pp
This engineering manual provides practical guidance for evaluating the feasibility and applicability of MPE for remediating contaminated soil and groundwater, and it describes the design and operational considerations for MPE systems.
Guidance for Design, Installation and Operation of Groundwater Extraction and Product Recovery Systems
Wisconsin Department of Natural Resources, PUB-RR-183, 65 pp, 2003
This technical guidance for groundwater and free-product recovery systems is intended for consultants and DNR staff.