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J 1� Ms. Margaret Lagorio,SJCEHD February 2,2009 Work Plan for Collection of Soil Gas Data Joe's Travel Plaza, 15600 S.Harlan Road Lathrop,California Page 3 (MW-13A/B) is screened from approximately 11 to 31 feet across both Sand A and underlying Sand B. Four groundwater monitoring wells (MW-8, MWO-lx, MW-1413, and MW-1513) have been discretely screened to depths of up to approximately 40 feet bgs to monitor groundwater occurrence and quality in Sand B. Two groundwater monitoring wells (MW-12C and MW-13C) have been discretely screened from 60 to 70 and 65 to 75 feet bgs, respectively, to monitor groundwater occurrence and quality in Sand C. Depth to groundwater in the wells (over the historic monitoring period) has been as shallow as 6.3 feet bgs to as deep as 12.4 feet bgs, with a historical average (Sands A and B) of approximately 9.0 feet bgs. Historically, groundwater flow beneath the site property and surrounding vicinity was in Sand A, predominantly towards the northeast; however, in early 2006, a shift of the flow direction towards the northwest was noted. Historically, groundwater flow direction calculated using the four Sand B wells was toward the north and northwest. CURRENT EXTENT OF IMPACT Sand A Analytical results of samples collected during the most recent quarterly groundwater monitoring and sampling event (December 29, 2008) indicate the only chemical of concern (COC) in Sand A wells is methyl tert-butyl ether (MTBE). No concentrations of benzene, toluene, ethylbenzene, and total xylenes (BTEX) were reported in any of the wells; only low concentrations (<230 micrograms per liter [µg/L]) of gasoline range organics (GRO) and/or (laboratory- flagged) diesel range organics (DRO) were reported in three wells (MW-13A/B, MW-14A, and MW-4). The current approximated extent of MTBE in Sand A wells is illustrated on Figure 5. Currently, the dissolved-phase MTBE plume in Sand A (exceeding the secondary maximum contaminant level [MCL] of 5.0 pg/L) extends from the site (wells MW-2 and MW-7) towards the northeast to the vicinity of wells MW-9, MWO-2, and MWO-3, and to the north-northwest to the vicinity of well MW-13A/B. The maximum concentration of MTBE (100 µg/L) was reported in well MW-13A/B. Historical analytical results clearly demonstrate the natural attenuation of the plume over time in all Sand A wells, with the exception of well MW-13A/B (in which insufficient data has been gathered). Currently, the MTBE plume is adequately (and/or practically) assessed to the secondary MCL in all directions in Sand A. Sand B Analytical results of samples collected during the most recent quarterly groundwater monitoring and sampling event (December 29, 2008) indicate the only COC in Sand B wells is also MTBE. No concentrations of GRO or BTEX were reported in any of the wells screened in Sand B, and only relatively low concentrations of (laboratory-flagged) DRO were reported in wells MW-8 K:Voe's Track Plaza\Work Plans\Work Plan for Collection of Soil Gas Data 2-2-09.doc