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Ms. Lori Duncan January 11, 2005 Page 4 of 7 levels below the secondary California Maximum Contaminant Level (MCL) of 5.0 µg/L. Groundwater samples collected since that time have shown a slightly increasing trend of MTBE concentrations in well MW5, with the maximum concentration of 14 µg/L detected in September 2004. The most recent sample collected in December 2004, however, contained MTBE at 4.9 µg/L. Analytical results of soil and groundwater samples collected at the site are summarized in Tables 1 and 2, respectively. Extent of Subsurface Contamination During the last four sampling rounds, benzene was detected only once in a single well (MW2) at 1.1 µg/L., and no toluene, ethylbenzene, or xylenes have exceeded MCLs. MTBE has been detected in wells MW I, MW2, MW3, and MW5 ranging from 1.9 to 14 µg/L during the same period. No MTBE has been detected in wells MW4, MW6, or MW7. Wells MW1, MW2, MW3, and MW5 have contained average concentrations of MTBE of 5.1 µg/L, <2.0 µg/L, 11 µg/L, and 6.3 µg/L, respectively, over the last four sampling rounds, and their combined average over the same period is 6.0 gg/L. Estimates of the volume of dissolved MTBE in the shallow aquifer beneath the site over the past year have ranged from a high of approximately 2.5 teaspoons in September 2004 to a low of less than 1 teaspoon in December 2004. Analytical results of soil and groundwater from the subsurface investigations are summarized in Tables I and 2, respectively. The depth to groundwater in site monitoring wells has ranged from approximately 5 to 7.5 feet below ground surface (bgs). Groundwater elevation data is summarized in Table 3. Figures 3 and 4 present a groundwater gradient map and MTBE isopleth map, respectively, for data collected in December 2004. Subsurface investigations conducted at the site since 1999 have determined the following: • Vertical migration of soil contamination related to the former UST is limited to less than 15 feet bgs; • No MTBE was detected in soil beneath the former UST; • Shallow groundwater beneath the site occurs at approximately 5-7.5 feet bgs and generally flows northwest; • Groundwater contamination is primarily limited to dissolved MTBE in the shallow aquifer beneath the site between the depths of 5 and 25 feet bgs; • The dissolved contaminant plume contains between'/4 and 2 %2 teaspoons of MTBE; • The dissolved MTBE plume is contained entirely within site boundaries, with the nearest downgradient property boundary approximately 400 feet from the plume; • No water supply wells have been impacted from the low-level contamination and the nearest downgradient well is used only as an emergency fire suppression well. PROPOSED WORK PLAN In a letter dated September 8, 2004, EHD agreed that vertical migration of the plume near the source area has been limited, but stated that "further investigation at depth further from the source G:\GROUNDZE\TuffBoy\Reports\Workplans\WP 1204,doc