SITE INFORMATION AND CORRESPONDENCE

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Shell No. 204-7524-4404 2 - • 5 December 2006 3011 W. Benjamin Holt Drivlockton Release #1: One 550-gallon waste oil UST was removed in June 1987. Subsequently, two 8,000-gallon gasoline USTs and a second 550—gallon waste oil UST were removed in October 1996. Maximum soil confirmation sample results from the USTs excavation in 1996 were: total petroleum hydrocarbons as gasoline (TPHg), 41 milligrams per kilogram (mg/kg); benzene, 0.064 mg/kg; and Methyl tert-Butyl Ether (MtBE), 1.1 mg/kg. After the 1.996 (Gen 1) USTs were removed, two new 15,000-gallon gasoline USTs and one 550-gallon waste oil UST (Gen 2) were installed and the Shell station continued operations. The MtBE impacts to groundwater increased several orders of magnitude from November 1996 to February 1997, after the Gen 1 USTs were removed and the Gen 2 USTs were installed. Release #2: In 1999 during Site remodeling for street construction, a piping leak was observed near a product dispenser and confirmed by a single soil sampling result (MtBE, 0.0404 mg/kg). Subsequently, Shell terminated their ownership of the Site, and in November 2004 the two 15,000-gallon gasoline Gen 2 USTs, one 550-gallon waste oil Gen 2 UST, dispensers and piping were removed (Figure 2). The USTs system was replaced in 2004 (Gen 3) and the Site reopened as the currently operating Shell-branded (not Shell-owned) station. Confirmation soil samples obtained from the 2004 Gen 2 UST excavation pit did not contain detectable contamination; however, petroleum hydrocarbons were detected in a grab groundwater sample from the Gen 2 USTs excavation pit (TPHg, 220 ug/L; MtBE, 9.2 ug/L). Groundwater monitoring sample results from a series of Site wells (MW-1, MW-2, MW-5, MW-11, and MW-12; Figure 2), conducted from November of 1996 through the present, show a series of increases and decreases in concentrations (peaks) that correlate to a MtBE plume moving in the downgradient direction. Monitoring well MW-3, located 10 feet from the Gen 1 USTs, peaked at 920 ug/L in November 1996 and declined to non-detect (<1.0 ug/L) by April 2004. Monitoring well MW-2, 10 feet (closest downgradient well) from the Gen 2 USTs, showed a significant increase in MtBE concentration (3 orders of magnitude) between November 1996 (<2.5 ug/L) and February 1997 (7,600 ug/L). By June 1997, the MtBE concentration in MW-2 increased another order of magnitude to a maximum 29,000 ug/L, and then began declining. The next Gen 2 USTs downgradient well, MW-1 (70 feet from MW-2) began detecting MtBE in November 1997 (28 ug/L, six months after MW-2); and increased quarterly until August 1999 when the MW-1 MtBE peak concentration of 3,120 ug/L was reached. MW-5, located 70 feet downgradient from MW-1, also showed a similar order of magnitude increasing trend starting in December 2001 (99 ug/L), with a peak MtBE concentration of 790 ug/L in July 2003. Two additional wells, MW-11 (70 feet downgradient of MW-5) and MW-12 (110 feet downgradient of MW-5) showed similar MtBE concentration trends, increasing an order of magnitude in 2004 then generally decreasing through March 2006. However, the declining trends in concentration have not been consistent, and have shown several minor increases/decreases to the present. In April 2006, analysis of MW-11 groundwater reported an increase to 86 pg/L. My comments are as follows: 1 . The data show that the MtBE groundwater plume has moved under Interstate 5 and may be continuing towards sensitive receptors (irrigation wells) on the opposite side of Interstate 5 (Figure 1).