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Project No S8100-06-62 August 22, 1996 Page 2 Between March 1992 and January 1996, twelve monitoring wells (MW-1 through MW-12) have been installed at the site to evaluate groundwater quality and to attempt to delineate groundwater impacts In addition, two dual purpose vapor extraction/groundwater monitoring wells(VW-I and VW-2) have ' been installed in the vicinity of the former USTs in order to facilitate future hydrocarbon source removal through vapor extraction ' Historically, the highest TPHg and benzene concentrations in groundwater have been detected within and in the vicinity of the former UST excavation in wells VW-1 and VW-2 at concentrations of 16,000 micrograms per liter (ug/1) TPHg and 3,000 ug/1 benzene Concentrations of dissolved phase hydrocarbons in groundwater decrease to below test method detection limits in perimeter wells MW- 9, MW-10, and MW-12 However, TPHg, benzene, and methyl-tert butyl ether (MTBE) have been detected in the eastern most perimeter well(MW-11) ' FIELD ACTIVITIES ' Groundwater Level Measurements and Flow Direction A representative of Geocon measured the depths to groundwater in thirteen groundwater monitoring wells on July 1, 1996 Well MW-10 was buried under construction and maintenance materials and subsequently was not measured or sampled during this monitoring event Depth to groundwater measurements were obtained using a battery operated water level meter with measurements obtained from the top of each well casing ' Groundwater was encountered at depths ranging from 17 92 to 18 44 meters (58 81 to 60 51 feet) below the top of the well casings A review of groundwater elevations indicates that the groundwater elevation has risen approximately 0 34 meters (1 13 feet) since April 1996 A summary of the top of well casing elevations, groundwater level measurements and groundwater elevations is presented on Table 1 Based on the July 1, 1996 groundwater elevation data, the groundwater flow is directed towards the east at an approximate gradient of 0 001 ft/ft which is consistent with the previous monitoring event performed in April 1996 Groundwater elevation contours and gradient for the July 1996 monitoring event are depicted on the Site Plan, Figure 2 ' Well Purmna and Sam 1>n Well purging and groundwater sample collection was performed on July I and 2, 1996 Prior to groundwater sample collection, approximately three well volumes of groundwater were extracted from wells MW-1 through MW-9, MW-11, MW-12, VW-1, and VW-2 utilizing a submersible pump Groundwater extraction was performed to allow fresh formation water to infiltrate the wells During well purging activities, the groundwater was monitored for pH, electrical conductivity and temperature Groundwater recovery was monitored in order to ensure 80 percent well recovery prior to well sampling Monitoring well sampling data sheets are presented in Appendix A Purged 1 groundwater was discharged to the sanitary sewer system in accordance with a City of Stockton industrial waste water discharge authorization Following well recovery, a water sample was collected from each well using polyethylene disposable bailers and decanted into four 40-m1 volatile organic analyses vials and one one-liter amber bottle