SITE INFORMATION AND CORRESPONDENCE_FILE 2

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GEOMATRIX Ms. Mary Meays San Joaquin County Public Health Services 29 September 1993 Page 4 samples from the two upgradient wells, where detections would not be expected, were analyzed. In the locations where detections were likely, samples were not analyzed. - The data from boring B-10, located in the north-central portion of the site and designated as AW-9 on the figures, indicated 480 ppm of gasoline and 510 ppm of diesel at a depth of 12 feet. No samples were analyzed below this depth, therefore, the depth to which these high concentrations exist is unknown. - Up until this point, Alton presents the results of each soil investigation in the text of the report. When the most recent soil investigation is presented, which includes the collection and analysis of soil samples from the borings for three new on-site monitoring wells (AW-10, AW-11, and AW-12), the results are not presented in the text, but are referenced only in a table. These results indicate that up to 3700 ppm diesel, 590 ppm gasoline, and 6.6 ppm benzene were detected beneath the area of the site that was already excavated. If water levels continue to rise at the site, these contaminants will be spread into the clean fill above. Section 1.2.6.2, Pages 1-7 and 1-8 The screened intervals of the monitoring wells and how they relate to the site hydrogeology and the location of separate-phase hydrocarbons is not discussed in the report. This is important because misleading results are obtained when these factors are overlooked. The on-site wells AW -1 through AW-4 are screened between depths of 5 and 25 feet (Alton, 5 July 1991). On-site well AW-9 is screened between depths of 8 and 28 feet (Alton, 13 May 1992). The screened intervals for on--site wells AW- 10, AW-11, and AW-12 have not been reported, but are assumed to be similar to the five other on-site wells. Based on the site hydrogeology and soil chemistry data presented by Alton, it is clear that separate-phase hydrocarbons are present in the soil column within a fine-grained silt between depths of 10 and at least 12 feet, which represents the recent range in depth of the water table at the site. The base of this separate-phase zone has not been defined. The monitoring wells are screened across this interval; however, they extend down another 10 to 15 feet and are also screened across a coarser-grained sand unit below the silt unit. Because the wells are screened across both of these units, the water quality in the wells is representative of the chemistry in each zone in a proportion relative to the hydraulic conductivity of each zone. In other words, the silt unit, which contains the separate-