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i PROBLEM ASSESSMENT REPORT 1 605 South EI Dor.do Street Stockton California Based on water levels observed during drilling, perforated casing was set from 40 to 60 feet bs in wells MW-4, MW-5 and MW-7, and from 35 to 55 feet bs in well MW-6 Each of the wells was capped with a locking wellhead cap and secured by a water- tight, traffic-rated wellhead vault The top-of-casing elevation, above mean sea level, of each well was surveyed based on a City of Stockton benchmark In the vapor extraction wells, perforated casing was set from 5 to 15 feet bs in VEW-1 , within the former UST backfill, and 15 to 35 feet bs in VEW-2 Each of the wells was capped with a locking wellhead cap and secured by a water-tight, traffic- rated wellhead vault The groundwater monitoring wells were surged and developed with a 10-foot-long baler on the drilling rig prior to placing the bentonite seal The wells were developed by purging each of approximately 55 gallons, until the water ran relatively clear Well development water was stored temporarily on site in DOT-approved, 55-gallon drums, pending laboratory analysis of the groundwater samples 3 4 Groundwater Levels On June 21 , 1993, Aegis personnel collected depth-to-groundwater measurements from all wells associated with the site Based on data from the previously-existing wells (MW-1 through MW-3), water levels have risen approximately 1 4 feet since March 25, 1993 (Figure 6) The June 1993 measurements indicated depth-to- groundwater ranged from 42 15 to 44 76 feet bs, corresponding to groundwater Ielevations ranging from 31 27 to 31 43 feet below mean sea level Figure 7 is a potentiometric surface map of the shallow water-bearing zone on ' June 21 , 1993 Based on this map, groundwater is estimated to flow to the east- northeast at an average gradient of approximately 0 001 ft/ft The March 1993 measurements indicated an average groundwater gradient of 0 001 ft/ft to the east Previous groundwater levels are summarized in Table 5 All groundwater elevation measurements were made from the referenced wellhead elevations, measured to the nearest 0 01 foot, and conducted according to the Aegis SOP included as Appendix A 3 5 Groundwater Sampling and Analysis On June 21 1993 Aegis personnel collected groundwater samples from all wells (MW-1 through MW-7) associated with the site The samples were delivered under chain-of-custody to PACE The samples were analyzed for concentrations of • TPH, as gasoline by EPA Method 8015 • BTEX by EPA Method 8020 91 044A PAR 7 i