WORK PLANS - Laserfiche WebLink

Home Browse Search

Geo1o#cj1 'edurks1= Page 2 L Additional Site Characterization Work Plan Project No.724.2 June 27,2002 Lgroundwater: MW-6, MW-7, MW-104, MW-106 and MW-107 (Figure 2). Wells MW-6 and MW-106 were placed on the adjacent parcel to the east of the site, approximately 60 Lfeet 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 former dispenser island on the western portion of the site. The installation of these wells Lcompleted the definition of the soil plume, as soil samples from these borings did not contain detectable contamination. LIt appears that the soil plume is defined by: o MW-3 and MW-5 to the north and northwest ✓•LLf ,to the south-6 and MW-106 to the east T/ © MW-Ito 63f�1y f�v� ❑ MW-4 is interpreted to be on the edge of the soil plume on the west under Grant Street LThe 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 L 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 35 feet below surface in December 2001). It is noted that the screened intervals of these wells (55-60 feet L bgs) are placed near the elevation of the groundwater table in 1992 —approximately 54 feet bgs. LIn our March 18, 2002 "Risk Evaluation & Appendix B Closure Checklist" work plan, Geological Technics Inc. (GTI) proposed performing contaminant mass estimate L 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 L 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. This work plan is intended to meet the requirements of the SJC/EHD April 24, 2002 directive. L 2.0 PROPOSED INVESTIGATION LGroundwater depth averaged 33 feet below grade during the March 2002 monitoring event and this is an increase of about 21 feet since the October 1992 event. The screened intervals Lof wells MW-106 and MW-107 (55-60 feet bgs) are placed near the elevation of the groundwater table in 1992 — approximately 54 feet bgs. The presence of TPH-G contamination in these intermediate wells suggests that a submerged plume is being sourced Lby residual soil contamination beneath the present water table. L L