ARCHIVED REPORTS XR0011062 - Laserfiche WebLink

Home Browse Search

1 C7eotogica!rechnas Inc Page 2 Monta Additional Site Characterization Report Project No 724 2 Apnl 19,2004 ' soils from beneath the former USTs and product piping areas were excavated for disposal The resulting excavations were advanced to depths of 20-28 feet bgs, which generated ' approximately 750 cubic yards of soil for disposal Three monitoring wells (MW-1 through MW-3) were installed in September 1992 to determine if groundwater had been impacted by the release—see Figure 2 for well locations ' Groundwater samples obtained from the wells confirmed that BTEX and TPH-G were present and the contaminant concentrations exhibited minor fluctuations during 18 monitoring events conducted between 1992 and 1999 Two additional monitoring wells ' (MW-4 and MW-5) were installed in September 1999, and incorporated into the monitoring schedule Samples obtained in November 1999 and March 2000 from well MW-4 contained elevated levels of benzene and TPH-G for the two events ' On November 20 —22, 2000, five additional groundwater monitoring wells were installed to investigate the lateral and vertical extent of gasoline-range petroleum hydrocarbons in the groundwater MW-6, MW-7, MW-104, MW-106 and MW-107 (see Figure 2) Wells MW- 6 and MW-106 were placed on the adjacent parcel to the east of the site, approximately 60 feet south of MW-5 Wells MW-7 and MW-107 were placed approximately 15 feet east of the former UST field A discretely screened deep well, MW-104, was placed adjacent to the t former dispenser island on the western portion of the site The installation of these wells completed the definition of the soil plume, as soil samples from these bonngs did not contain detectable contamination It appears that the soil plume is defined by ❑ MW-3 and MW-5 to the north and northwest ❑ MW-2, MW-6 and MW-106 to the east ' ❑ MW-1 to the south ❑ MW-4 is interpreted to be on the edge of the soil plume on the west under Grant Street ' The shallow groundwater plume is evident in well MW-4 and defined laterally to the east (down gradient) by MW-5, MW-6 and MW-7 The presence of TPH-G contamination in intermediate wells MW-106 and MW-107 suggests that a submerged plume is being sourced by residual soil contamination beneath the present water table (at approximately 38 ' feet below surface in December 2003) It is noted that the screened intervals of these wells (55-60 feet bgs) are placed near the elevation of the groundwater table in 1992 — approximately 54 feet bgs ' In our March 18, 2002 "Risk Evaluation & Appendix B Closure Checklist" work plan, Geological Technics Inc (GTI) proposed performing contaminant mass estimate calculations and limited fate and transport calculations This work plan was submitted to ' the San Joaquin County Environmental Health Department (SJC/EHD) and is necessary to receive site closure concurrence from the Regional Water Quality Control Board In their letter correspondence dated April 24, 2002, the SJC/EHD denied GTI's work plan and ' directed that a work plan for additional plume definition be submitted Geological Technics Inc (GTI) submitted our June 27, 2002 "Additional Site ' Characterization Work Plan - Monitoring Well Installation"work plan to meet the SJC/EHD directive The work plan proposed the installation of four sets of off-site monitoring wells to define the lateral extent of the contamination