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Ms.Vicki McCartney,SJCEHD August 29,2008 Work Plan for Bench-Scale Test and Injection Well Installation Project No.2046-3250-01 Parkinson's 1-5 ARCO Service Station,Stockton,California Page 4 approximately 50 feet bgs. Groundwater is first encountered within these sandy strata and monitoring wells MW-1 through MW-5 are screened in these sandy soils (screened from 10 to 25 feet bgs). Between approximately 50 and 90 feet bgs (maximum depth explored),,- predominately fine-grained soils have been encountered (by CPTs). Interbedded sandy intervals, approximately 2 to 7 feet bgs in apparent thickness and generally laterally continuous, were also encountered (by CPTs) between 50 and 90 feet bgs. Well MW-6 (continuously cored from 45 to 64 feet bgs) was screened within the uppermost of these sandy interbeds (from 57.5 to 62.5 feet bgs). Over the historic 8-year monitoring period, depth to groundwater beneath the site (in water table wells MW-1 through MW-5) has ranged from approximately 11.1 to 15.4 feet bgs (historic lows measured in Fall 2003 and historic high levels measured in July 2006). Seasonal fluctuations of the water table on the order of 2 to 3 feet are typical (Figure 4). Over the two-year historic monitoring period for deeper well MW-6, depth to groundwater has ranged from approximately 14.1 to 18.3 feet bgs (Figure 4). East and northeast groundwater flow directions, at relatively shallow hydraulic gradients, have consistently been reported. ��,, EXTENT GROUNDWATER IMPACT Upon initiation of groundwater sampling in September 2000, the chemicals of concern (COCs) at the site were GRO, BTEX, MTBE, tertiary butyl alcohol (TBA), and tertiary amyl methyl ether (TAME) (Tables 4 and 5). The highest historically reported concentrations of GRO and benzene were reported in well MW-1 (immediately downgradient of the USTs) at concentrations of 25,700 and 1,820 micrograms per liter (µg/L) in 2000. -The highest historically reported concentration of MTBE was reported in well l�V W-5 (due east of the fuel-dispensers) at a concentration of 41,000 l.tg/L (January 2003). Significant MTBE concentrations were also historically reported in CPT-4 (25 feet bgs; 12,000 µg/L) located about 150 feet northeast of the site across Hammer Lane. The highest historically reported concentration of TBA was reported in well MW-2 (immediately adjacent and upgradient of the USTs) at a concentration of 65,000 l.tg/L (April 2002). The highest historic concentration of TAME was reported in well MW-5 (downgradient of fuel-dispensers) at a concentration of 370 µg/L in 2003. Concentrations of the GRO and benzene, although relatively high in wells MW-1 and MW-3 in 2000/2001, have attenuated over the historic monitoring period (Figure 5). Currently, maximum concentrations of GRO and benzene of only 290 and 11 l.tg/L, respectively, are reported at samples collected from well MW-1 (July 2008). The GRO and benzene plumes appear to be decreasing/shrinking over time and are adequately characterized at this time. In addition, formerly elevated TAME concentrations in the wells have decreased over time and currently (July 2008) TAME is reported in only well MW-2 (1.3 µg/L). \ K:\Parkinsons Hammer 1-5 Arco\Work Plans\Work Plan for Ozone Bench Scale Test.doc