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Former 76 Station 4409 S E C ® R January 27, 2006 Page 2 • Following removal of the former station canopy, canopy footings, and kiosk, additional over- excavation was completed in the dispenser area Over-excavation was terminated when obviously impacted soil was removed, the structural integrity of the sidewalk was 4 r endangered, or groundwater was encountered A total of approximately 1,707 tons of soil was transported offsite for disposal at the Forward Landfill in Manteca, California On December 9 and 10, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed in the A monitoring zone to investigate the extent of petroleum hydrocarbons in soil and groundwater beneath the site (Attachment A) A vertical assessment by ; geoprobe was conducted in July 2000, and soil and groundwater impacts were encountered at depths of 60 feet bgs The San Joaquin County Environmental Health Department (SJCEHD) subsequently requested a work plan for further delineation of soil and groundwater impacts Three on- site B monitoring zone groundwater monitoring wells (MW-5 through MW-7, Attachment A) were installed in November 2000 These wells identified B monitoring zone groundwater petroleum hydrocarbon impacts In August and September 2001, three additional B monitoring zone groundwater monitoring wells (MW-8 through MW-10, Attachment A) were installed onsite to evaluate the extent of B monitoring zone groundwater impacts In June and July 2003, four off-site B monitoring zone groundwater monitoring wells (MW-11 through MW-14, Attachment A) were installed to further delineate the lateral extent of • dissolved petroleum hydrocarbon plume within the B monitoring zone a In November 2004, SECOR installed one on-site C monitoring zone groundwater monitoring well (MW-15) In April 2005, SECOR installed two C monitoring zone groundwater monitoring wells (MW-16 and MW-17) These wells were installed to further delineate the vertical and lateral extent of dissolved petroleum hydrocarbons within the C monitoring zone Soil samples from MW-15 and MW-16 were not collected since adequate data has been collected from assessments conducted in the immediate vicinity of those two 3 wells No detectable concentrations of TPHg, benzene, methyl tertiary butyl ether (MtBE), or 1,2-dichloroethane (1,2-DCA) were reported in soil samples collected from the boring for { MW-17 to a depth of 120 feet bgs Total xylenes were detected at a concentration of 0 017 mglkg in the soil sample collected at 120 feet bgs from the boring for MW-17 TPHg and benzene were reported in the groundwater samples collected from all three wells following well development in April 2005 1,2-DCA was reported in two of the three wells at a maximum concentration of 1 0 micrograms per liter (Ng/L) in MW-15 MtBE was not reported in any of the wells Additional data will be needed to determine groundwater concentration trends in the C zone wells The results of the November 2004 and April 2005 assessments are presented in the Results of Vertical Assessment and Updated Site Conceptual Model(SECOR, July 28, 2005) SENSITIVE RECEPTORS According to IT Corporation's Site Assessment Report, dated February 25, 2000, a water supply well search was performed to locate any water supply wells within 2,000 feet of the y„ subject site Review of well logs at the California Department of Water Resources (DWR) indicated the presence of one water supply well within the 2,000-foot search radius, located • at 1521 North Hunter Street, downgradient (northeasterly) of the site Well construction13 4409 402005 QSR(CR)doc SECOR Intemational incorporated v r