SITE INFORMATION AND CORRESPONDENCE

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• • Mr. James Barton Page 2 of 4 DEFINITION OF VERTICAL EXTENT OF GROUNDWATER CONTAMINATION We believe that the existing soil and groundwater sampling data are sufficient to provide a technically reasonable basis for concluding that the vertical extent of groundwater contamination is adequately defined. • The first water-bearing zone, sand between the average depths of 25-35 feet, is impacted with petroleum hydrocarbons within a very limited area. Groundwater samples collected only 30-50 feet downgradient of the source area, at locations SB-7, SB-8 and MW-1, were non-detect for petroleum hydrocarbons. This demonstrates the low mobility of the dissolved hydrocarbon contaminants in the horizontal plane, even in a relatively permeable media. • The logs of deeper borings SB-11 and SB-10 indicate a low-permeability barrier of clay and silt some 15-20 feet thick is present between the first and second water-bearing zones. The vertical mobility of dissolved petroleum hydrocarbons through this barrier would be expected to be several orders of magnitude less than the horizontal mobility through the first water-bearing sand. If the horizontal transport distance to date is less than 50 feet, the vertical transport distance should be much less. • The soil sample collected at a depth of 36 feet in boring SB-3 (located in the source area) was non-detect for all petroleum hydrocarbon analytes. Although low levels of dissolved petroleum hydrocarbons were detected in groundwater samples from borings SB-4 and SB-5, which borings did not have detectable soil contamination, the correlative conclusion is that dissolved contaminants at a depth of 36 feet in SB-3 would be of a similar low magnitude. Dissolved contaminants 10-12 feet deeper in the second water-bearing zone should be non-detectable. • No detectable petroleum hydrocarbons were found in the water sample collected from the second water-bearing zone at a depth of 49 feet in downgradient boring SB-11. There is no evidence of any unusual condition (like an overwhelming vertical groundwater gradient) which might accelerate vertical contaminant transport to the second aquifer zone. Even if impact does exist, the non-detect sample collected from downgradient boring SB-10 is testament to its limited extent. The conclusion that the second water-bearing zone, at a depth of approximately 50 feet, is not impacted to any significant degree is consistent with the hydrogeology of the site and with the analytical data. It is not necessary to physically verify this conclusion with additional drilling and sampling. SOIL VAPOR INTRUSION Contaminant vapor intrusion would not be expected to pose an unacceptable health risk to the occupants of the building. A comparison of contaminant concentrations with RWQCB Region 2 Environmental Screening Levels (ESLs) bears this out. ESLs are concentrations expected to result in an insignificant health risk (cancer risk less than one in one million, hazard quotient less than 0.2) under nearly any site condition. RWQCB derived the ESL concentrations by modeling vapor intrusion using the Johnson & Ettinger model. Physical parameters entered into the model such as building size, air exchange rate, soil permeability, etc. were intentionally very conservative which tends to maximize the calculated vapor flux and the resulting indoor air concentration. One of the most conservative features of the J&E modeling is the assumption that the soil beneath the entire building footprint is contaminated. ESLs, then, commonly overstate risk and contaminant concentrations that do not exceed ESLs can, in almost all circumstances, be regarded as posing no significant health risk. G:\GROUNDZEIGABBARDUepoRs\MONTB LY\Oc12008.doc