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A S S O C I A T E S I N C QUARTERLY MONITORING ACTIVITIES ATC performed quarterly groundwater monitoring and sampling on June 30, 1998 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 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 Supply Well with an electronic oil/water interface probe and a groundwater grab sample was also collected from this well with a peristaltic pump Depth to water measurements were collected on June 30, 1998 from monitoring wells MW1 through MW6 The water table elevation has decreased an average of 2 02 feet in the monitoring wells since the last monitoring event in March 1998 Groundwater beneath the site generally flows in a westerly direction at an average gradient 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 June 30, 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 21 92 from to top of the sounding tube (or approximately 18 9 feet below ground surface) This corresponds to an average 10 41 foot drop in the potentiometric surface since the previous March 1998 monitoring event assuming hydrostatic conditions prevail 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 included in Attachment 1 Well purge logs are included in Attachment 2 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 attempted to collect a groundwater grab sample from the Supply Well utilizing dedicated 1/2-inch plastic tubing and a centrifugal pump The tubing was lowered approximately 40 feet down the well's sounding tube and fitted with an in-line "T" valve The valve was positioned in the tubing prior to pump connection In theory this would allow the water to be sampled prior to the entering the pump's impellers However, this methodology proved unsuccessful, as the pump's vacuum was repeatably "lost" when the sampling valve was opened in order to collect a water sample ATC subsequently collected a water sample from the well's existing sampling port Collected groundwater samples were placed on ice and transported under chain-of-custody to State-certified Sequoia Analytical laboratory (FLAP #1271) 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, w 1943261quaA1y12gr-1998 doc 3