SITE INFORMATION AND CORRESPONDENCE

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Karen Petryna L 1'0 Shell Oil Products 880 E. Victor Rd., Stockton, CA Page 2 of 5 EHD comments as follows: The plume of impacted groundwater, commonly first encountered in silty sand to poorly sorted sand between approximately 50 feet and 60 to 63 feet below surface grade (bsg) is reasonably delineated toward the northeast by MW-6, toward north by MW-5, toward the west northwest and southwest by MW-8 and MW-9, and toward the south by MW-10. The analytical data from groundwater samples from these wells does appear to indicate plume stability and minimal lateral migration in the 45- to 60-foot depth interval (locally to 63 feet bsg), except for the monitoring gap toward the east and beyond the impacted MW-7 toward the southeast. Boring logs from wells near CPT locations on this site show that the CPT logs can underestimate grain size, causing fine sand to be classified as silt or stiff fine-grained soil of uncertain composition, an observation made on other sites in San Joaquin County. EHD believes that MW-11 soil composition data indicates that the interval between 65 and 90 feet bsg may, and probably does, contain several thin permeable coarse-grained units masked in the CPT logs by interbedded fine-grained units. EHD is concerned that there may be unidentified, more highly permeable units underlying the monitored interval that can facilitate lateral migration of contaminants. A gravelly sand unit, identified and sampled in CPT-2 between 61 and 65 feet bsg, occurs just below the monitored interval, and is impacted by MTBE and TBA. The orientation of this probable channel bed deposit is not well defined at this time. This interval, being gravelly, may be a unit of much higher hydraulic conductivity and may be acting as a significant linear or curvilinear migration pathway that is currently not characterized or monitored. Cambria has concluded that the plume of impacted groundwater is not migrating away from the site; EHD agrees that there is no direct indication from the monitored interval that the plume is actively migrating, except possibly toward the southeast as indicated by MW-7. EHD's uncertainty with the 'no migration' characterization lies in the potential for unencountered and unrecognized coarser-grained channel deposits, such as the gravelly sand described above, occurring in the sand-rich subsurface of this site and acting as undetected contaminant migration pathways. Cambria concludes that MWA 1 demonstrates the vertical extent of impacted groundwater. EHD is not certain of this as review of historical groundwater elevation maps indicates that there is a fairly persistent trough or depression of the groundwater surface that trends from the UST area toward the southeast, putting the point selected for MW-11 in an up-gradient position from the inferred source area. The validity of MW-11, located approximately 30 feet from the