Mobile Dual Phase Extraction
Mobile Dual Phase Extraction (MDPE) is a process of combining Soil Vapor Extraction (SVE) with groundwater depression with an internal combustion engine and vacuum pump creating the induced well vacuum, and an in-well pump creating the groundwater depression and induced hydraulic gradient. During the process of increasing the SVE induced vacuum in an extraction well screened across the static groundwater level, the groundwater will rise in the extraction well reducing the available well screen area. Also, if NAPL is present in the extraction well, the rising groundwater will result in decreasing the NAPL thickness.
MDPE is a methodology whereby high vacuums can be induced in the capillary fringe (smear zone) and below zones without raising the groundwater level in the extraction well over these exposed zones. Unlike other methods that use a "stinger" or drop tube, with MDPE the groundwater depression is the result of an in-well pump that creates a deep enough depression to overcome the groundwater rising effect as a result of high induced vacuums (up to 320"H20, 23.5”Hg). A specially designed well manifold is required to allow the pump cord and discharge lines to enter the well without the loss of induced vacuum. A data logger (pressure transducer) is installed in the extraction well to determine the level of groundwater depression. The recovered groundwater/NAPL is pumped through a volume meter and collected in a standby tank or truck.
The advantages of a MDPE process are:
The total induced vacuum is directed to SVE as compared to the MDPE stinger or drop tube process where a substantial percentage of the total induced vacuum is used to raise the groundwater/NAPL through the drop tube. This can substantially reduce the induced vacuum in the exposed screened area.
Vacuum and groundwater flow can be independently controlled. This provides substantially higher quality data recorded from the outer observation wells.
Generally, a higher percentage of NAPL recovery is also accomplished. In addition, non-diluted vapor samples can be taken from the influent vapors.
The DPE technology can be used in conjunction with a Pilot Test where the vacuum radius of influence and the induced hydraulic gradient is calculated or with the pump out event program. The DPE technology has also been very effective in recovering crude oil.
Dual Phase Extraction Event
Dual Phase Events consist of SVE with groundwater extraction from one or more extraction wells. It is also referred to as Mobile DPE. The event process is a form of remediation that has been adopted for certain sites where the installation of a permanent remediation system is not cost effective or feasible. The events are usually scheduled once a month and can vary from 8 to 24 hours, to seven consecutive days. With the in-well groundwater pump, the MDP event can be conducted in depths of up to 300 ft and provides accurate comparative data regarding the liquid and vapor phase hydrocarbons that are recovered.
As events are performed over an extended period of time, the results of the current event are compared to the data from the previous events to measure the progress made. AcuVac has a mobile unit that is completely self-contained and has a computerized fuel controller that can run extended events for several weeks at a time with little intervention.
The photo demonstrates AcuVac’s ability to get its equipment where it is needed, when it is needed. The project depicted in the photo is a storage tank release of diesel. AcuVac was called to perform a Mobile Dual Phase Extraction Pilot Test and follow-up events as soon as the recovery wells were completed to prevent the product release from spreading, and thus limiting the liability. At one point, Air Sparging was being used to prevent the underground migration of the diesel while SVE was performed on one well and DPE was being run on another well. AcuVac’s ability to respond quickly, enabled recovery of a significant amount of the diesel. Based on the work performed by AcuVac, a permanent system was installed to remove the remaining diesel.