SITE INFORMATION AND CORRESPONDENCE

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Ms. Laurie Cotulla L `,,o, s -September 23, 1992 Page 2 The request for a work plan for additional site characterization was based on a number of factors as stated in the County letter. The items are addressed here in the same order in which they appear in the August 28 letter. Item 1 The 1987 soil vapor study conducted by Western Geo-Engineers did indicate that hydro- carbon-affected soil was present with the highest concentrations occurring on the western portion of the site. At the time this study was conducted, groundwater was present at a depth of approximately 10 to 12 feet below the ground surface. The soil vapor data collected from 10 feet below the ground surface is more than likely indicative of the concentration of dissolved hydrocarbons in the groundwater than representative of hydrocarbon concentrations in the soil. Item 2 The results of the soil investigation conducted by BCC in May 1987 indicated that hydrocarbons were present in the soil. However, hydrocarbons were identified above laboratory detection limits only in the soil samples collected at a depth of 10 feet below the ground surface. This would have been within the capillary fringe because at the time the soil samples were collected, the static groundwater surface was approximately 10 to 12 feet below the ground surface. The hydrocarbons identified in the soil during this investigation were most probably residual hydrocarbons from the groundwater. Therefore, we feel strongly that the lateral extent of hydrocarbon-affected soil has been characterized at this site. Item 3 It is true that unlined underground utilities could provide a path for migration of contaminants. The County states in the letter that the septic system line that existed on the south side of the station should be investigated as it could be a migratory pathway for contaminants. The septic system line would not have been buried at a depth greater than 4 to 5 feet. Hydrocarbons were not identified in soil samples collected from a depth of 5 feet in boreholes AS-6 and AS-7 during BCC's 1987 investigation. Borehole AS-7 is approximately 18 feet from the location of the sewer line and borehole AS-6 is approximately 28 feet from the line. Based on this information, we do not feel it is necessary to investigate this area further. Item 4 Lithologically, this site is well defined. Fine-grained sediments are prevalent beneath the site to a depth of 10 to 15 feet below ground surface. The fine-grained sediments in the upper 10 to 15 feet retard the migration of hydrocarbons in the soil. Sands and gravels are present below the fine-grained zone and currently encompass most, if not all, of the capillary fringe. Brown and Caldwell 6746\CORRESP674"8.13 Consultants