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Shell Station No. 204-75204 - 2 - • 1 February 2008 3011 Ben Holt Drive, Stock Table 1 summarizes the Model No Degradation and First Order Decay results. Source of Date No Degradation (worst case) 1st Order Decay (biodegradation) Data & Initial MTBE Concentration & MTBE Concentration & Concentration Model MTBE Plume Length Model MTBE Plume Length 2Q2007 2017 19 ug/L at 0 feet 19 ug/L at 0 feet 25 ug/L 18 ug/L at 85 feet 3 ug/L at 85 feet (MW-2) 7 ug/L at 170 feet 0 ug/L at 170 feet 0 u /L at 255 feet 2Q1997 2042 0 ug/L at 0 feet 0 ug/L at 0 feet 29,000 ug/L 5.877 ug/L at 680 feet 0 u9/L at 680 feet MW_2 0.47 u /L at 850 feet The Model conclusions state no sensitive receptors are located within 2,500 feet of the MTBE plume, and that the MTBE plume will migrate no more than 680 feet to the northeast from the source area, which is under the median dividing the northbound and southbound lanes of 1-5. However the enclosed Model output figure shows, worst case with no degradation, that in the year 2042, the MTBE concentration will be 0.47 ug/L, which is an order of magnitude below the WQG, at the maximum run distance of 850 feet from the source (within the right of way of 1-5). The Model requests site closure. Comments: I was able to acquire a working copy of the US Environmental Protection Agency Bioscreen version 1.4 (Bioscreen) used in the Model, and to run, compare and duplicate the Model results. However, the USEPA webpage warns that Bioscreen may encounter problems with short run time and high dispersivity coefficient input values. I reran the model by increasing the run time to 120 years, and contacted the USEPA by email for further instructions on an appropriate run time and dispersivity coefficient. Their email response was to limit the dispersivity value to less than 10, and they considered my run time as long enough to avoid problems. I reduced the dispersivity value from 12.8 to 9.8 and reran Bioscreen to 120 years. My results (enclosed figures) show that between 100 and 108 years under a No Degradation scenario, the plume concentration will drop below the Regional Water Quality Control Board's Water Quality Goal (WQG) of 5 ug/L, at a distance between 1,440 and 1 ,600 feet from the source. While 100+ years is obviously not a reasonable time to reach WQGs, the MTBE plume should remain under 1-5 for the life of the plume. According to Caltrans bridge records, the 1-5 overpass at Benjamin Holt Drive (and 1-5) has been in existence for nearly 40 years (1970), and land use is unlikely to change in the next 100+ years. Also, the site is located next to a freeway off-ramp and is suitable only for a commercial use, particularly as a service station. Additional remediation onsite at the current MtBE concentration (25 ug/L) is neither cost effective nor necessary, as the MTBE plume continues to exhibit a declining concentration trend. Off-site remediation under 1-5 is not practical due to 1-5 access conditions. I concur that the Model predicts that no sensitive receptors will be impacted. Prior to seeking closure, I request that you compare the site soil results to the latest (November 2007) San Francisco Bay Regional Board Environmental Screening Levels (ESLs), which may be downloaded at the following address: http://www.waterboards.ca.gov/sanfranciscobay/RBSL/esl 107/esl.pdf