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1-1b-1999 12:03PM FROM HORIZON ENVIRONMENTL 916 939 2172 P.2 Beacon Station No.474 December 28, 1998 3440 East Main StreM Stockton,California Amodment To Work Plan samples will be collected from this depth and analyzed as described above for boring B-7 in Task 2.1. If soil boring B-8 encounters hydrocarbon-impacted soil in the vadose zone and hydrocarbon-impacted soil or groundwater at or near the historical low groundwater level, a 2-inch diameter PVC well (MW-13) with 5 feet of 0.020- I inch screen will be installed across the lowest impacted interval. if well MW- 13 is constructed across the lowest impacted interval, an adjoining boring (B- 8A)will be drilled within 5 feet of boring;B-8 to facilitate the installation of the deep groundwater well. Because of the proximity of boring $-8A to B-8, this boring will be drilled without the collection of soil samples to the approximate depth of 55 feet or less. Based on anallrtical data, one or possibly two 2-inch diameter PVC vapor extraction wells with 0.020-inch factory-machined slots will then be installed as a single or dual completion in boring 13-8A. If soil boring B-8 encounters no hydrocarbon-impacted soil in the vadose zone, Md encounters hydrocarbon-impacted soil or groundwater at or near the historical low groundwater level, a 2-inch diameter PVC well (MW-13) with 5 feet of 0.020-inch screen will be installed across the deepest impacted interval in the 8-inch diameter boring. If soil-boring B-8 encounters no hydrocarbon-impacted soil in the vadose zone and no hydrocarbon-impacted soil or groundwater at the historical low groundwater level, the boring will be co-mpleted as a vapor-extraction well for monitoring or improving lateral vapor-eictraction effectiveness. This well will be screened from approximately 40 to 50 feet bsg. Task 2.5: If soil borings B-7 and 8-8 do not encounter hydrocarbon-impacted soil or j groundwater at or near the historical low groundwater level, it will not be necessary to drill and sample boring B-9. i If soil-boring B-8 encounters hydrocarbon-impacted soil or groundwater at or near the historical low groundwater level, the 8-inch diameter soil boring B-9 will be drilled and sampled east of the dispenser islands (Figure2). If the discreet groundwater samples collected at the historic low water level in boring B-9 are impacted, a 2-inch diameter PVC well (MW-14) with 5 feet of 0.020- inch screen will be installed across the deepest impacted interval. Based on subsurface conditions encountered, it may be prudent to drill and convert B-9 into a vapor-extraction well for monitoring or improving; vapor-extraction effectiveness. This option will be discussed and approved prior to drilling. Tasks 3 and 4 are unchanged from the original work plan. i 1474-22.awp 3 HORIZON ENVIRONFAMTAL INC.