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September 30, 1999 47491 1 Harding Lawson Associates Margaret Lagono, Supervising REHS ' San Joaquin County Public Health Services Environmental Health Division Page 3 ' the remaining impacted sods do not contain the COCs associated with# six fuel that would typically pose a potential threat to health or safety ' • The soluble data from the courtyard samples, and the additional soluble data from the six UST-removal samples, indicate that the impacted soils remaining on site do not pose a threat to groundwater quality ' • The risk based corrective action (RBCA) assessment shows that the Site does not pose a threat to human health or safety 210,000-gallon UST Removal Project A detailed discussion of UST removal activities was presented in HLA's December 22, 1999 report entitled ' Closure Report, 210,000-gallon UST Removal Project The following is a brief summary of the investigation, and its results and conclusions The Number 6 fuel UST was located in the southeast corner of the Site (Plate 2) The UST was cylindrical in shape, approximately 70 feet in diameter, constructed of reinforced concrete ' sidewalls and bottom that was approximately 7 25 feet bgs The closure of this UST was by complete removal of the concrete sides and bottom of the UST Removal activities were performed between June 15 and July 27, 1998, under the direction of HLA, in accordance with the June 15, 1998 UST Removal Permit issued by PHS/EHD On July 9, 1998 during removal activities, a nm of impacted soil was encountered and staff from PHS/EHD indicated that a work plan for over-excavation must be approved by the PHS/EHD Site Mitigation group prior any over-excavation HLA's work plan (BLA, 1998a)was approved and two phases of over-excavation and confirmation sampling were performed, with two areas of impacted soil remaining in place Based on the findings of the removal activities,HLA presented conclusions that included amongst others the following Impacted soils were restricted to a narrow rim located adjacent and outside of the concrete sidewalls of ' the UST and these impacted soils were excavated and stockpiled temporarily for chemical profiling prior to offsite disposal • Impacted soils were properly transported by Don Beste Transportation Inc , a state-registered hauler, and disposed of at Altamont Landfill, a licensed TSD facility in Altamont, California • The excavation was appropriately backfilled and compacted ' • Impacted soils remain in place on the northwest and northeast sides of the excavation • The lateral and vertical extent of the soils have been determined ' • The remaining impacted soils do not contain detectable concentrations of BTEX • Soluble analyses indicate that these soils do not contain detectable concentrations of soluble TPHd or TPHmo, and therefore do not pose a potential threat to groundwater quality Following the completion of the UST removal project, quarterly groundwater monitoring of the four wells around the UST was performed for two additional quarters Quarterly monitoring was performed on July 28, 1998, a few days after removal activities, and on October 14, 1998, approximately 3 months after removal ' r,