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C\v/ATC I . A S S O C I A T E S I N C not anticipated the change of scope will exceed the amount previously pre-approved by the Fund on November 1997 QUARTERLY MONITORING ACTIVITIES ATC performed quarterly groundwater monitoring and sampling on December 28, 1999 Field work included measuring the depth-to-water in wells MW 1 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 28, 1999 from monitoring wells MW 1 through MW6 The water table elevation has risen an average of 0 83 feet in the monitoring wells since the last monitoring event in October 1999 Groundwater beneath the site generally flows in a westerly direction at an average gradient 0 001 ft/ft, or approximately 5 28 feet/inile, 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 28, 1999 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, and TPHg by EPA method modified 8015 RESULTS Laboratory analyses detected only concentrations of TPHg at 4,920 parts per billion (ppb), ethylbenzene at 84 7 ppb, and xylenes at 102 ppb in the sample collected from well MW4 The water sample collected from MW4 reported benzene was below the laboratory detection limit of 10 ppb It should be noted however that the laboratory detection limit for sample MW4 was multiplied by a factor 20 due to the elevated concentration of TPHg Groundwater samples collected from wells MW2, MW3, and the nearby Supply Well contained no detectable concentrations of BTEX or TPHg These analytical results are generally consistent with historical levels w 1625821quart1}\4W-1999 doc 4