WORK PLANS - Laserfiche WebLink

Home Browse Search

ij 10 By October 1987, BC had obtained permission from Mervyn's to install two downgradient wells on their property. The County approved BC's work plan for these two additional off-site monitoring wells_ In November 1987, BC installed monitoring wells W-7 and W-8 (Figure 3). No hydrocarbon constituents were identified in soil;; samples collected from boreholes W-7 and W-8. BTY and TPH were again identified in groundwater samples collected from monitoring wells W-1, W-2, and W-3. Minor BTX concentrations were identified in groundwater samples collected from monitoring wells W- 4, W-5, and W-6. No hydrocarbons were identifiet in groundwater samples collected from monitoring wells W-7 and W-8 (Table 2). Groundwater flow direction was to the south under a hydraulic gradient of approximately 10 feet per mile. Depth to groundwater was approximately 54 feet below ground surface. Following the November 1987 work, BC ft alized their remediation plant for the hydrocarbon-contaminated soil and groundwater beneath Station 434. To confirm that downgradient migration of hydrocarbon constituents in groundwater retrained low, BC sampled all off-site monitoring wells in February 1988. Minor BTX constituents were identified in groundwater samples collected from monitoring well W-5. No hydrocarbon constituents were identified in groundwater samples collected from monitoring wells W-4, W-6, W-7, and W-8 (Table 2). Groundwater flow direction was to the south- :southeast under a gradient of approximately 10 feet per mile. Depth to groundwater was approximately 52 feet below ground surface. A Second Addendum to the 'Problem Assessment Report describing the results of the November 1987 work and February 1988 groundwater sampling, was issued on February 23, 1988. The Second Addendum also contained BC's soil and groundwater remediation plan, as Appendix 3. Part of the remediation plan involved install tion of a groundwater extraction well next to monitoring well W-2. In March 1988, BC submitted a work plan for a .100-foot- deep groundwater extraction well to the County. e County referred the work plan to the Regional Board for approval. The Regional Bard verbally approved the w6rk plan. During a 24-hour pump test of Municipal Well 12 (screened between 210 and 500 feet below ground surface), water levels in the much shallower on-site and off-site monitoring wells were drawn down. Due to the hydraulic communication between the shallow and deeper parts of the aquifer, BC requested the Regional Board to approve deepening the extraction well to 140 feet. The Regional Board withdrew their approval of any extraction system due to concerns regarding the adequacy f information on the vertical extent of hydrocarbons in the groundwater. In an April 988 meeting between ARCO,, BC, the Regional Board, the County, and the City, it was agreed that installation of the extraction system and implementation of the remediation plan would wait on depth-specific groundwater sampling. The depth-specific sampling would be accomplished through the installation of depth-specific monitoring wells. In May 1988, BC submitted a work plan f r two on-site depth-specific monitoring wells to the County and the Regional Board. In une 1988, the Regional Board-approved the work plan. BC installed monitoring wells W-2A and W-213 (Figures 2 and 3). Monitoring well construction details are shown on Figure 4. TPH was identified in composite soil samples of the drill cuttings from boreholes W-2A and W-2B (see'Table 1). Groundwater samples were collected for laboratory analysis from monitoring wells W-2,