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(VYATC A S S O C I A T E S I N C QUARTERLY MONITORING ACTIVITIES ATC performed quarterly groundwater monitoring and sampling on March 20, 1998 Field work included measuring the depth-to-water in wells MW1 through MW6 and purging and sampling the water from wells MW2 through MW6 and the nearby Thornton No 2 Municipal Supply Well (Supply Well) The samples were analyzed for the presence of dissolved petroleum hydrocarbons Additionally, at the request of PHS/EHD staff, fluid level measurements were collected from the i Supply Well with an electronic oil/water interface probe and a groundwater grab sample was also collected from this well with a peristaltic pump In addition, San Joaquin County Utility Maintenance Division personnel measured the bottom of the Supply Well which was determined to be 235 feet bgs Depth to water measurements were collected on March 20, 1998 from monitoring wells MW1 through MW6 The water table elevation has risen an average of 6 58 feet since the last monitoring event in December 1997 Groundwater beneath the site generally flows in a northerly direction at an average gradient of 0 001 ft/ft, or approximately 5 4 feet/mile, with apparent mounding in vicinity of MW3 Groundwater monitoring data are summarized in Table 1 A groundwater gradient map for the March 20, 1998 monitoring episode is shown on Figure 2 An electronic oil/water interface probe was utilized to measure depth to water within the Supply • Well to determine if lubricating oil from the well's pump was present at the water table Depth to water was measured at 11 51 from to top of the sounding tube (or approximately 8 5 feet below ground surface) This corresponds to an average 5 39 foot rise in the potentiometric surface since the previous December 1997 monitoring event Groundwater samples were collected from wells MW2, MW3, MW4, MW5, and MW6 after purging and stabilization of physical parameters (pH, electrical conductivity, and temperature) The monitoring wells were purged and sampled according to RESNA Industries Quality Assurance/Quality Control Plan, on file with PHS/EHD Well purge logs are included in Attachment 1 In response to PHS/EHD requests made during the December 8, 1997 project meeting and briefly discussed in PHS/EHD correspondence dated January 6, 1998, ATC collected a groundwater grab sample the Supply Well This was accomplished by employing a peristaltic pump along with sanitized vinyl tubing which was lowered through the well's sounding tube The tubing was initially lowered in the well to a depth of approximately 30 feet bgs Approximately 30 gallons of water was purged from the well prior to sampling Collected groundwater samples were placed on ice and transported under chain-of-custody to State-certified American Environmental Network, Inc (AEN) Samples were analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX) by EPA method 8020, TPHg by EPA method modified 8015, and/or methyl tertiary butyl ether (MTBE) by EPA method 8020 Additionally, the water sample collected from the Supply Well by analyzed for fuel oxygenate additives by EPA method 8260 w 1943261cW1y11gr-1998&c 3