SITE INFORMATION AND CORRESPONDENCE FILE 2

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q t Rodger Liston Darpetro Inc. 749 E. Charter Way, Stockton, California Page 3 of 7 The seepage rate, or the rate at which ground water moves across the site is dependant on the hydraulic conductivity, the gradient and effective porosity (as opposed to total porosity). As previously noted by EHD by letter dated 05 September 2003 , EHD believes the hydraulic conductivity determined by CGG may be somewhat overestimated due to the test conditions . The hydraulic conductivity was derived from a slug test performed by adding water to the well when the water table was within the screen interval , which may cause an overestimation of the hydraulic conductivity (Driscoll , 1986). Therefore, the estimated seepage rate of 6 . 3 feet per quarter is probably high and should be refined , but will serve as a working estimate. The amended SCM still makes much of the water sample that could not be collected at 45 feet below surface grade (bsg) in HP-2 , but was later collected at 55 feet bsg at the same location , thereby inferring that the lithology at 45 feet bsg could not transmit water, forcing the ground water to migrate around and under the tight soil . EHD has previously commented on this point by letter dated 05 September 2003; the same points still apply. CGG's belief that the rise of ground water elevation caused the ground water flow direction on the 701 Charter Way site to shift toward the south while ground water below 55 feet bsg continued to flow toward the northeast is unsupported by any observational data, such as water level measurements in wells screened separately above and below 55 feet bsg. The SCM-A noted that dissolved methyl tert-butyl ether (MTBE) concentrations increased from undetectable concentrations at 39 feet bsg to 2. 1 micrograms per liter (µg/1) at 50 feet bsg and to 16 .5 µg/I at 60 feet bsg in boring B-3 . CGG interprets these data as indicating northeastward flow of impacted ground water at 55 feet bsg and inferred that the known UST releases to the west were potentially the sources of the contaminants. Increasing contaminant concentrations with depth does not necessarily indicate an off-site source; it could just as easily be due to modest separation from an onsite source. Indeed, some sites exhibit higher concentrations at greater depth than at shallow zones in areas of the known source. Concerning the interpretation of the HP-2 and B-3 data, EHD notes again that several factors can influence the success of obtaining grab groundwater samples and that a number of factors can also influence contaminant distribution , i.e. , soil composition , vadose vs. saturated zone, proximity of source, etc. Contaminant concentration can also be influenced , often negatively, -by sample collection and handling techniques. EHD concurs with CGG's belief that the sand interval commonly encountered at 50 feet bsg could be a major conduit for contaminant migration and should be carefully assessed and monitored . Other sand intervals at greater depth identified on nearby sites may be present on the subject site too and are also of potential concern . EHD is unsure what CGG's statement in point 7 above means ("TPH and MTBE concentrations from the 641 E . Charter Way boring is toward our well MW-11 ") ; however, EHD notes that the data presented on the cross section was collected over a period of time measured in years and that grab ground water data was mixed with ground water monitoring well data ; therefore a consistent trend is not demonstrated. CGG reported that the concentrations of MTBE in monitoring well MW-1 on the 701 E.