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A S S O C I A T E S I N C monitoring wells since the last monitoring event in June 1998 Groundwater beneath the site generally flows in a easterly direction at an average gradient 0 002 ft/ft, or approximately 10 6 feet/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 September 29, 1998 monitoring episode is shown on Figure 2 Groundwater samples were collected from wells MW 1 through MW6 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 91210) Samples were analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX) by EPA method 8020, and TPHg by EPA method modified 8015 Additionally, the water samples collected from Monitoring wells MW 1 through MW6 were analyzed for dissolved oxygen by EPA method 360 1 • 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 1 parts per billion (ppb) and 5 ppb, respectively This is above the 10 ppb California Primary Maximum Contaminant Level (MCL) for drinking water Groundwater samples collected from wells MW1, MW2, MW5, MW6, and the Supply Well contained no detectable concentrations of BTEX or TPHg These analytical results are generally consistent with historical levels Dissolved oxygen was detected in the sample collected from well MW 1 at the laboratory method detection limit (MDL) of 1,000 ppb Dissolved oxygen was not detected above the MDL in water samples collected from the remaining monitoring wells Analytical results are summarized in Table 1 Laboratory reports and chain-of-custody documentation are presented in Attachment 2 • w W43261quaAIyl2gr-1998 doc 3