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1 ' for site conditions This alternative is generally. g e lly among the most cost-effective of the ' alternatives • Dual-phase extraction. Dual-phase extraction (DPE) is a technology that uses high vacuum ' to remove liquids and vapors from wells placed in the source area of a site Typically a blower or liquid ring pump is used to generate a vacuum of at least 20 inches of mercury The vacuum is applied to dip-tubes that are placed in the extraction wells The end of the ' dip-tubes are usually placed below the water level and are used to depress the water level while at the same time extracting vapors through the newly formed vadose zone This technology is effective at remediating smear zones that are less than 10 feet in depth During operation, soil vapor, groundwater, and LPH (if present) are all extracted from the extraction ' wells through the dip-tubes ' Application of DPE at the ExxonMobil site would require some modifications due to the fact that LPH and soils containing hydrocarbons at residual saturation are located as much as 20 feet below the current water level Although DPE is able to extract groundwater from the ' subsurface, extracting large amounts of groundwater significantly reduces the volume of soil vapor the pump can extract For the conditions at the ExxonMobil site, a groundwater pump and treat system was considered to lower the water level in the source area This would ' allow the DPE system to more efficiently remove soil vapor from the submerged soils, but may promote migration of hydrocarbons and MTBE from offsite sources A groundwater flow model was developed for the site using MODFLOW Simulation results show that 5 ' wells screened between 25 and 50 feet bgs and operating at 4 gallons per minute (gpm) each could potentially lower the groundwater level 18 to 20 feet Simulated groundwater elevation contours for such a system are shown in Figure 13 Application of this system is complicated by the amount of extracted water that would have to be discharged An estimated 30 gpm of water would be generated, 20 gpm from the groundwater extraction wells and 10 gpm from the DPE wells The local POTW indicated that a discharge greater than 10 gpm would be problematic The purveyor of the local storm water system was contacted but was not able to determine if the local system can handle the ' proposed discharge volume Application of a DPE system at the site would be problematic at best and is not proposed for the following reasons Extraction of groundwater and vapor would occur through the more permeable sediments and Iwould reduce the dewatering effect in the finer-grained sediments This, in turn, would restrict vapor flow through the finer-grained sediments which likely contain the bulk of the hydrocarbons As can be seen in Figure 13, dewatering of the source area would be incomplete Incomplete I dewatering would likely leave LPH and soils containing high concentrations of hydrocarbons in place after asymptotic extraction rates are reached I GVPrq"c 73N21MASTM\WPVWP03031WPtxdm 19