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CLEARWATER G R Q U P, I N C 7.4 Selection of Remedial Approach and Conceptual Remedial Design Based on the information presented above, Clearwater recommends a remedial ' approach consisting of dual-phase soil vapor and groundwater extraction Vapor should be treated during the first several months of operation by a rental IC engine, until influent TPHg concentrations in vapors drop below approximately 1,000 ppmv After that, vapor-phase GAC should be utilized to treat the vapors extracted by an electric regenerative blower Soil vapor should be extracted from vapor wells VW-1, VW-2, VW-3, RW-1 and proposed RW-2 Soil vapor extraction from these wells should exert a combined effective radius of influence large enough to cover the areal extent of soil contamination (Figure 12) Controllerless, pneumatic, total-fluids pumps should be used to extract groundwater from recovery wells RW-1 and proposed RW-2 Combined water extraction from these wells at flowrates of 5 gpm each should create a zone of capture great enough to affect the entire dissolved hydrocarbon plume and SPH (Figure 13) Groundwater treatment could consist of either air stripping or aqueous-phase GAC A spreadsheet comparing the costs of the two water treatment alternatives is presented on Table 7 Although both are technically suitable treatment options and nearly equal in capital an basic maintenance costs, Clearwater recommends using aqueous-phase GAC over an air stripper to treat the groundwater, based on cost savings which will be realized in the long term by less complex permitting and personnel maintenance time A conceptual remediation process diagram is shown on Figure 14 7 4 1 Remediation Compound A remedial compound should be constructed adjacent to the building on its south side near the eastern limit of the property This location will provide maximum distance from the residential portion of the on-site structure, and is also located just beneath a power pole from which an electrical drop can be obtained A standard 115- volt weather proof power outlet should be installed in the remediation compound in addition to the needed power drops for remedial equipment, so that maintenance personnel will have a power supply for light electric tools The compound should ' be approximately 20 feet by 20 feet in order to accommodate proposed remedial equipment It should be designed such that a berm short (>12 inches high) constructed around the perimeter of the compound will have a volume equal to ' 120% of the total water volume in the treatment system at any time An automatic electric pump should be placed in a sump constructed in a corner of the compound and plumed into the water line after the oil/water separator with a check valve in 1 order to clear the compound floor of any standing water resulting from rain or unexpected system leaks The compound should be surrounded with an 8 foot high cyclone fence with swinging gate in the front to provide vehicle access As an alternative to a common pad/fence compound, an enclosed pre-fabricated structure with secondary containment should also be considered Bollards should be placed at ' D-107,PAR/RAP 21 February 21, 1996