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Mr Harlin Knoll San Joaquin County Environmental Health Division IF, 10 N 24 October 2000 . Page 3 Approximately 3 casing volumes were purged from the wells prior to sampling Throughout purging and dust before sampling the wells, pH, specific conductivity, temperature, and turbidity of the evacuated groundwater were measured and recorded These measurements were made to confirm that the well was purged sufficiently Water samples were collected after the measurements stabilized to within 10 percent of the previous readings or a baseline was established Purging and sampling logs are provided in Attachment 1 Purging and sampling was performed using a 2-inch-diameter Teflon and disposable polyethylene bailers suspended from a nylon line The Teflon bailer was decontaminated between each well use The bailers were equipped with a bottom- release device Water samples were collected from the wells in new 40-milliliter glass bottles with Teflon-lined caps provided by the analytical laboratory These samples were collected so that no headspace was present in the bottle A duplicate sample was collected from MW-2 for lab quality control, a field blank was not collected because the wells were sampled using disposable bailers The samples were delivered to Positive Lab Service of Los Angeles, California, a State—certified laboratory The samples were analyzed for TPHg using modified EPA Method 8015, for BTEX, MTBE, and 1 ,2-dichloroethane (DCA) using EPA Method 8260, and 1 ,2-dibromoethane (EDB) using EPA Method 504 1 Analytical laboratory reports and chain-of-custody forms for the August 2000 (third round) sampling activities are included in Attachment 2 In August 2000, groundwater samples from well MW-1 (installed near the former source area) contained no detectable amounts of TPHg or BTEX A small amount of MTBE was detected in MW-1 at a concentration of 7 7 µg/I Upgradient well MW-2 contained TPHg concentrations of 430 µg/1 The samples from cross-gradient well MW-3 did not contain detectable concentrations of TPHg, BTEX, and MTBE DCA and EDB were not detected in the groundwater samples from each well Analytical results from this and the two previous rounds of groundwater sampling are summarized in Table 2 Details of Ithe groundwater sampling procedures are included in Attachment 3 Total Residual Mass Calculations The volume and mass of impacted soil and groundwater were calculated to determine the amount of residual hydrocarbon as TPHg The approach included overestimating the area of impact and then calculating volumes of soil and groundwater in the vicinity of well MW-2, the only location with measurable TPH concentrations Analytical data used included TPHg concentrations from Orion's soil samples collected during the installation of MW-2 and the results from Orion's recent groundwater sampling of MW- 2 The calculations are detailed below IS\BFS\STOCKTON\LTR\LTR005 DOC 10/24/00