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Background 1997 correspondence to TVG (PHS/EHD, 1997), PHS/EHD entitled Scheduling of Field work and approved the proposed work, and made additional Work Plan Addendum Number 2, Proposed comments and requests On January 7, 1998 a Supplemental Site Investigation, (HLA, 1998b) meeting of representatives of PHS/EHD,TVG, This letter also presented the required written and HLA was held to discuss issues raised in the notice that the field work was scheduled for March PHS/EHD letter regarding the proposed SSI 26, 1998 During the meeting it was collectively agreed that the Site appeared to qualify as a low-risk Preliminary Results SSI — Phase One groundwater site and the purpose of the SSI was to collect the remaining site data required to submit a Following completion of phase one of the SSI, closure request in accordance with the criteria HLA informally communicated the analytical outlined in Appendix B to the Tri Regional Board results of soil and groundwater sampling to Staff Recommendations(RWQCB, 1996) As PHS/EHD in a telephone conversation in June described below in the Field Investigation SSI— 1998 While some of the objectives of the SSI had Phase One Section, the approved scope of work been achieved(definition of vertical and lateral 1 consisted largely of that presented in TVG's extent of impacted soil),the Hydropunch original work plan(TVG, 1995) The following groundwater analytical results suggested that additional items were agreed on dissolved hydrocarbon concentrations increased with depth PHS/EHD was concerned that the • Installation of a proposed monitoring well data suggested the presence of a newly-discovered MW-7 within the center of the former UST deeper groundwater plume with concentrations location greater than in the shallow groundwater plume On July 24, 1998 a conference call between • During the proposed SSI one soil and one representatives of TVG, PHS/EHD, and HLA groundwater sample will be analyzed for the occurred to discuss an additional scope of work five oxygenates and 1,2-dichloroethane (DCA) (SSI—Phase Two)to evaluate the extent of the by EPA Method 8260B to satisfy RWQCB putative deeper groundwater plume PHS/EHD closure requirements indicated that because the existing shallow monitoring wells at the site were not deep enough All wells will be resampled once more a to provide constraints on the lateral extent of the minimum of 30 days following completion of deeper plume, additional borings and deep ' the SSI Hydropunch data would be required The above modifications and requests were HLA submitted to PHS/EHD and the RWQCB, documented in HLA's January 14, 1998 Work Plan Addendum Number 3,dated December correspondence to PHS/EHD entitled Summary of 12, 1998(HLA, 1998c) The addendum included Meeting Agreements and Work Plan Addendum, a boring permit application and a scope of work Proposed Supplemental Site Investigation, (HLA, designed to evaluate the lateral extent of the 1998) putative deep plume PHS/EHD responded in correspondence to TVG dated January 14, 1999 PHS/EHD contacted HLA on January 15, 1998 to (PHS/EHD, 1999)with several additional requests indicate that PHS/EHD felt it was not necessary to and concerns On February 17, 1999 a meeting of install and then shortly afterwards (site closure) representatives of TVG, PHS/EHD, RWQCB and destroy the proposed well MW-7 PHS/EHD HLA took place to discuss the issues raised in indicated that field and analytical data from the PHS/EHD's letter to TVG It was collectively Hydropunch boring would provide sufficient agreed that there were two potential explanations information to show that free product was not for the Hydropunch analytical data that suggested present and to calculate the remaining mass of a deeper plume of groundwater may be present at dissolved hydrocarbons Following discussions the Site One explanation was that vertical with TVG, HLA documented their concurrence in groundwater gradients at the Site had entrained HLA's March 19, 1998 correspondence to hydrocarbons from the shallow depths of the L 1300001387371WP138737Ni9 Harding Lawson Associates 5