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400l Mr.James Barton,RWQCB May 14,2008 Work Plan for Additional Site Assessment Former Dolly Madison Bakery,Stockton,California Page 6 boring MW-9 sample collected from 32 feet bgs, and methyl tertiary butyl ether (MTBE; 0.0063 mg/Kg) was reported in the sample collected from boring MW-9 at 47 feet bgs. Samples collected below 50 feet bgs at borings MW-9 and MW-13 did not contain petroleum hydrocarbon or fuel additive concentrations above laboratory method detection limits. Petroleum hydrocarbons or fuel additives were not detected in samples collected from borings VW-2, MW-5, MW-7,MW-8,MW-10, and MW-11. Petroleum Hydrocarbon Impact to Groundwater GRO, BTEX compounds, MTBE, tertiary butyl alcohol (TBA), tertiary amyl methyl ether (TAME), and 1,2-dichloroethane (1,2-DCA) have historically been reported in groundwater beneath the site. Shallow groundwater has been monitored at the site since 1997, with limited well sampling between May 2004 and June 2007. Discussions relating to impact to deeper water bearing intervals at the site is largely based on the results of the well sampling event completed during the first quarter 2008. Shallow Groundwater Impact GRO, benzene, and MTBE iso-concentration contour maps for the shallow water bearing interval are presented as Figures 9 through 11, respectively. GRO and BTEX impact to groundwater appears limited to the area immediately surrounding the former UST complex. At the time of the first quarter 2008 well sampling event, GRO (44,000 micrograms per Liter [µg/L]) and benzene (140 µg/L) were reported in the sample collected at well VW-1. GRO was detected in the samples collected from wells MW-1 (260 µg/L) and MW-2 (76 µg/L), and benzene was detected in the MW-1 sample (1.1 µg/L). GRO and benzene were not detected in the samples collected from newly completed wells MW-3 through MW-7. MTBE concentrations in shallow groundwater are relatively low, ranging from 3.8 µg/L (MW-1) to 46 µg/L (MW-2). 1,2-DCA was detected in the samples collected from shallow screened wells MW-2 (5.2 µg/L), MW-5 (3.9 µg/L), and MW-7 (19 µg/L). No petroleum hydrocarbons or fuel additives were detected in the well MW-6 sample. The lateral extent of MTBE impact to shallow groundwater is not fully characterized in the northern portion of the site, north of wells MW-2, MW-3, and MW-7. MTBE impact to shallow groundwater is also not fully assessed south of well MW-1. Mid-Depth Interval Groundwater Impact GRO, benzene, and MTBE iso-concentration contour maps for the mid-depth water bearing interval (approximately 55 to 70 feet bgs) are presented as Figures 12 through 14, respectively. The highest concentrations of petroleum hydrocarbons were observed in well MW-9, located in close proximity to the former UST complex. GRO (220 µg/L), benzene (49 µg/L), MTBE (39 µg/L), and TBA (17 µg/L) were reported in the well MW-9 sample. KATimothy Kong Property\Work Plans\Knng-may03workp1an.doc