SITE INFORMATION AND CORRESPONDENCE

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Shell No. 204-7524-4404 - 3 - 28 August 2008 3011 W. Benjamin Holt Drive&ockton The Model conclusions stated no sensitive receptors were located within 2,500 feet of the MTBE plume, and that the MTBE plume would 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. Staff reviewed the Model and commented in a 1 February 2008 letter that the enclosed Model output figure showed, worst case with no degradation, that the MTBE concentration will be 0.47 ug/L in the year 2042 at the maximum run distance of 850 feet from the source (within the right of way of 1-5). Staff was able to acquire a working copy of the Model and duplicated the Model results. However, the USEPA warned that Bioscreen may encounter problems with short run time and high dispersivity coefficient input values (10 or more). Based on USEPA input, staff reran the model by increasing the run time to 120 years and reduced the Model dispersivity value from the Model 12.8 to 9.8. Staff results showed that between 100 and 108 years under a No Degradation scenario, the MTBE plume concentration will drop below the 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, staff agreed that the MTBE plume should remain under 1-5 for the life of the plume and not reach sensitive receptors. According to Caltrans bridge records, the 1-5 overpass at Benjamin Holt Drive has been in existence for nearly 40 years (1970). Land use is unlikely to change in the next 100+ years. MTBE is known to degrade biologically; therefore, the worst case scenario stated above, No Degradation (without degradation), may be overly conservative with the 100+ years to restore water quality timeframe, although not impossible under the right conditions. Also, the site is located next to a freeway off-ramp and is suitable only for a commercial use, particularly as a service station, repair shop, or convenience store. Additional remediation onsite for the residual MTBE concentration (24 ug/L) is neither cost effective nor necessary, as the MTBE plume continues to exhibit a declining concentration trend. Off-site investigation or remediation under 1-5 is not practical due to 1-5 access conditions. On 9 June 2008, C-R&A submitted the Environmental Screening Level Reassessment and Closure Request (Report). The Report evaluated historical soil and groundwater data with the November 2007 San Francisco Bay Regional Board Environmental Screening Levels (ESLs), and concluded that there was no threat to sensitive receptors from vapor intrusion from petroleum hydrocarbons contamination left in place. See attached tables (Table 1, Table 2, Table 3, and Well Concentrations) for all soil and groundwater data. The site is no longer operating as a gasoline station, although an automotive service bay and convenience store wer@ operating in the building in 2007. The fate of the Gen 3 tanks is unknown. Staff concurs to the findings that natural attenuation is occurring and reducing petroleum hydrocarbons pollution, that the Model and Report predict that sensitive receptors will not be impacted as a result of contaminant transport onsite and under 1-5, and that land use at the Site and 1-5 will not change in the foreseeable future. All required public participation (comment period) has been conducted, with no adverse comments. Staff concurs with C-R&A and Shell that case closure is appropriate and recommends consideration of a low risk Site closure. SJCEHD has no objections to case closure. Enclosure