SITE INFORMATION AND CORRESPONDENCE_FILE 1

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page 2, 4075 Main Street The CPT borings will be advanced to a minimum depth of 100 feet bsg. One boring will be located on site for vertical assessment of the plume near the source area. The other two borings will be located offsite in the down gradient direction for lateral assessment of the plume, both shallow and deep. Soil and grab groundwater samples must be collected during this phase of the investigation and analyzed for total petroleum hydrocarbons as gas, benzene, toluene, ethylbenzene and xylenes, fuel oxygenates, 1,2-dichloroethane (1,2-DCA) and ethylene dibromide (EDB) by appropriate EPA approved methods. All detected peaks must be reported. Immediately following receipt of the lithological and analytical data from the CPT investigation, monitoring wells for assessment of the lateral extent of the plume will be installed. Please note that it may be necessary to install both shallow and deep-screened wells aVthe offsite locations, as the vertical and lateral extent of impacted groundwater must be delineated. GHH proposes to install a groundwater extraction well with 70 feet of screened casing, and to use this well for both a step-drawdown test and a soil vapor extraction test. SJC/EHD has some concerns with this proposal. First, 70 feet of screened casing in a well is not appropriate. Fifty feet of screen open in the vadose zone extending 20 feet into the saturated zone will create a potential vertical conduit for contaminants to reach groundwater. The CPT boring proposed for completion onsite will provide lithological information that can direct the proper construction of the groundwater extraction well. Cross-sectional diagrams of the geohydrologic units and the contaminant distribution should be prepared to select an appropriate screen interval. Second, in order to accurately conduct a soil vapor extraction test, a soil vapor extraction well would need to be installed. The vapor extraction test, as proposed, may yield a qualitative assessment of SVE as a remediation method by demonstrating adequate removal of hydrocarbons from the vadose zone, but SJC/EHD does not believe the SVE test design is likely to produce quantitative information needed for remedial system design regarding radius of influence due to the limited screened intervals in the proposed observation wells, MW-1, MW-2 and MW-4. The proposed test will extract soil gas from approximately 50 feet of vadose zone, and will monitor via approximately 5 to 6 feet of open screen in the monitoring wells. The open screen in MW-1, the closest well, is in clay; MW-4 is open in silt, and MW-2, located approximately 70 feet from the proposed extraction well, is open in sand. It appears that the sand addressed by MW-2 may not be in direct communication with the sand units likely to be addressed by the proposed extraction well. The design may present some interpretational challenges. SJC/EHD suggests, and will approve a work plan addendum for installation of two vapor extraction wells having similarly screened intervals. Both wells should be located in the area of significantly impacted vadose zone soil and should not extend to the saturated zone. SJC/EHD conditionally approves the work plan as follows: 1. The CPT investigation followed by the installation of two or three groundwater monitoring wells for lateral plume definition is approved. 2. Installation of one groundwater extraction well is approved, with an appropriate length of screen interval. 3. Performance of a step-drawdown test is approved. Per the work plan, data collected from this test will be evaluated by the Bouwer & Rice Model to determine hydraulic conductivity and sustained yield.