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i <br /> 1i <br /> i <br /> I <br /> ARCADIS Ms. Lori Duncan <br /> October 8, 2008 <br /> !�f j <br /> Geophysical Survey <br /> On June 11, 2008 ARCADIS supervised Spectrum Geophysics (Spectrum)during <br /> geophysical survey activities used to evaluate probable locations of closed <br /> (abandoned in-place) USTs or other remaining facilities. The geophysical survey was <br /> performed using the two techniques described below: <br /> An electromagnetic(EM)survey was performed first as a rapid screening tool to <br /> identify potential USTs and other subsurface structures. Results of the EM survey <br /> were used to focus subsequent ground-penetrating radar(GPR)survey activities. <br /> The EM survey data was collected using vertical dipole orientation with both <br /> quadrature(apparent conductivity)and in-phase (metal sensitivity) modes. Two <br /> anomalies were identified during the E-M survey(Attachment 3, Figure 3). These <br /> anomalies were attributed to window glass and scrap metal waste piles. <br /> The GPR system transmits high-frequency electromagnetic waves into the ground <br /> and detects the energy reflected back to the surface. Energy is reflected back along <br /> boundaries or subsurface interfaces that possess different electrical properties. <br /> Reflections typically occur at stratigraphic contacts, or changes in subsurface <br /> materials having high electrical contrasts; including metal objects such as USTs, <br /> drums, and utility pipes. These reflections are detected by an antenna and processed <br /> into an electrical signal, which is used to create an image of the subsurface features. <br /> Three anomalies were identified during the GPR survey(Attachment 3, Figure 4). <br /> These anomalies were attributed to either former conduit trenching or existing <br /> electrical conduits. <br /> Upon review of all EM-61 and GPR.profiles, no definitive UST-like signatures were <br /> observed. <br /> Groundwater Monitoring Wells andSoil Boring Completion <br /> Between June 16 and 23, 2008, Cascade Drilling Incorporated (Cascade),.under the <br /> supervision of ARCADIS, installed four groundwater monitoring wells (MW-2, MW-3, <br /> MW-4, and MW-5) and advanced one-boring (B-3)to evaluate the potential extent of <br /> petroleum hydrocarbon impacts to groundwater near the former UST complex. <br /> Page: <br /> XUa=ch-usloftiredata%SmmmWm CA\EMVACHEVRONM907516 Deliverables%ReportsNssessmen1309075Assessmenl RepM_181810950.4. 3/10 <br />