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(VATC A S S O C I A T E S I N C IRW QUARTERLY MONITORING ACTIVITIES ATC performed quarterly groundwater monitoring and sampling on December 23, 1998 Field work included measuring the depth-to-water in wells MW1 through MW6 and purging and sampling the water from wells MW2, MW3, MW4 in addition to the nearby Thornton No 2 Municipal Supply Well (Supply Well) The samples were analyzed for the presence of dissolved petroleum hydrocarbons Depth to water measurements were collected on December 23, 1998 from monitoring wells MW through MW6 The water table elevation has risen an average of 1 05 feet in the monitoring wells since the last monitoring event in September 1998 Groundwater beneath the site generally flows in a westerly direction at an average gradient 0 0008 ft/ft, or approximately 4 foot/mile, with apparent mounding in vicinity of MW3 Historically groundwater beneath the site generally flows in a westerly direction Groundwater monitoring data are summarized in Table 1 A groundwater gradient map for the December 23, 1998 monitoring episode is shown on Figure 2 Groundwater samples were collected from wells MW2, MW3, and MW4 after purging and stabilization of physical parameters (pH, electrical conductivity, and temperature) The monitoring wells were purged and sampled according to ATC's Field Procedures on file with PHS/EHD Monitoring well purge logs are included in Attachment 1 Furthermore, ATC coordinated with San Joaquin County Utility Maintenance Division personnel and collected a water sample from the sampling port of the Supply Well Collected groundwater samples were placed on ice and transported under chain-of-custody to State-certified Sequoia Analytical laboratory (FLAP #1210) Samples were analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX) by EPA method 8020, TPHg by EPA method modified 8015, and methyl tertiary butyl ether (MTBE) by EPA method 8020 RESULTS Laboratory analyses detected concentrations of one or more petroleum hydrocarbon constituents in the samples collected from wells MW3 and MW4 The water samples collected from these wells reported benzene at 1 0 parts per billion (ppb) and 12 ppb, respectively These are at or above the 1 0 ppb California Primary Maximum Contaminant Level (MCL) for drinking water Samples collected from wells MW3 and MW4 also contained TPHg, toluene, ethylbenzene and/or xylenes Groundwater samples collected from well MW2 and the nearby Supply Well contained no detectable concentrations of BTEX, TPHg, or MTBE These analytical results are generally consistent with historical levels Analytical results are summarized in Table 1 Laboratory reports and chain-of-custody documentation are presented in Attachment 2 w 1625821quad1y14gr-1998 doe 4