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CVV28 . A S S O C I A T E S I N C 3.0 GROUNDWATER MONITORING Following the monitoring well installation, three groundwater monitoring events conducted by Advanced GeoEnvironmental Inc and one groundwater monitoring event conducted by ATC have occurred The analytical results show a maximum benzene concentration of 11 micrograms per liter (gg/L) on July 11, 1996 (EPA Method 8020) The maximum TPHg concentration was 3,500 pg/L on July 11, 1996 The maximum methyl tertiary butyl ether (MTBE) concentration was 16 pg/L on July 11, 1996 (EPA Method 8020) During subsequent monitoring events the concentrations of the constituents analyzed steadily decreased to where benzene had not been detected for the final three monitoring events, and MTBE had not been detected for the final three monitoring events The last monitoring event, groundwater was analyzed for MTBE by EPA Method 8260 and MTBE was not detected Furthermore, the latest three groundwater sampling events show concentrations of TPHg at fairly constant levels (Figure 5) Groundwater monitoring data and analytical results are included in Tables 1 and 3, respectively Concentrations of TPHg in groundwater are shown on Figures 7 and 8 Groundwater flow direction has consistently been to the north-northwest Figure 6 shows the most recent groundwater contour map as measured in March 2000 • 4.0 WELL SURVEY ATC reviewed San Joaquin County groundwater well construction logs to identify any groundwater wells located within a 2,000-foot radius of the site The search revealed five potential wells located within approximately 2,000 feet of the site The approximate well locations are shown on Figure 3 The wells were located on the map according to "well location" description provided on the well construction logs Available well construction details are included in Table 4 Well logs are included in Attachment 5 In order to assess hydraulic characteristics beneath the site, ATC reviewed data collected during previous groundwater monitoring events conducted by Advanced GeoEnvironmental Inc and ATC's latest quarterly monitoring report The direction of groundwater flow has been consistently to the north-northwest According to USGS 7 5-minute topographic map titled Union Island, Calf 1978 there is one point located approximately 650 feet north of the site on Holly Drive identified as Well 26 Boring logs were not found for this well On March 19, 2001, ATC performed a site vicinity survey to try and locate this well Some piping and associated valves were observed in the vicinity where Well 26 is located on the map According to City of Tracy, Department of Engineering, this piping is not associated with a well but is associated with a concrete irrigation • ditch W 192054 00021closure-addendum doe 3