SITE INFORMATION AND CORRESPONDENCE FILE 1

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,low 139 South Center Page 2 migrating. Chevron's most recent correspondence dated April 28, 1997 stated that there is insufficient information regarding the transport mechanisms and the downgradient extent of the groundwater plume, and yet Chevron is able to make certain inferences about plume migration rate(3 feet per year)and dimensions. PHS/EHD understands that the station has been in operation since 1969 and that the leaking tanks were removed in 1990. During the investigation, contamination had migrated to MWS, located approximately 200 feet from the former tank pit. PHS/EHD questions how the plume migration rate was calculated given the plume size currently known and the dates of operation. Generally, groundwater velocities are determined after hydrologic studies have been completed. If aquifer testing has been performed at this site,the results should be submitted immediately. Please note that the map that PHS/EHD received,with permits, included the location of the boring, SB 10,that Chevron has requested. The"Addendum to Work Plan for Well Installation"dated October 17, 1996, indicated that selected soil samples would be analyzed for water content, bulk density, porosity, and fractional organic carbon content. The location of this proposed upgradient soil boring is approximately 25 feet southeast of SB3 and 15 feet northeast of SB4. SB3 and SB4 were drilled and sampled at the direction of PHS/EHD to determine the extent of soil contamination associated with the former underground storage tanks. Soil contamination of 2.0 ppm TPH-gas and 0.18 ppm benzene was detected only in soil samples collected from S133 at 40 feet below ground surface(bgs). However, significant contamination of up to 15,000 ppm TPH-gas and 160 ppm benzene was evidenced in the soil sample collected from S134 at 43 feet bgs. At the time that S133 and S134 were drilled,May 13, 1994,the depth to water was approximately 42 feet bgs. The vertical extent of soil contamination in the former tank pit area is unknown. PHS/EHD suggests that if contamination is detected during the installation of SB 10 that there be careful evaluation of total organic carbon results and that TCLP analysis is performed. Given that groundwater was most recently reported at 25 feet bgs and that all of the on site wells are screened between 35 and 60 feet bgs and have submerged screened intervals, and that PHS/EHD has requested the installation of wells to evaluate the feasibility of remedial alternatives,PHS/EHD hereby directs you to convert this proposed soil boring into a monitoring/remedial testing well and that PHS/EHD be contacted following the completion of drilling activities to discuss the soil samples that will undergo analysis. Additional remedial testing wells will be necessary,but it would not be cost effective to drill and grout a soil boring given the site conditions. Intrinsic Bioremediation Parameters and Groundwater Monitoring Requirements The groundwater samples that were collected on February 27, 1997 were analyzed for ferrous iron(Iron II), nitrate,phosphate, sulfate and sulfite. Anaerobic biodegradation can be indicated by changes in the concentrations of Iron II that indicates anaerobic biodegradation and by concentrations of nitrate and sulfate, that act as an electron acceptors for anaerobic respiration. These initial analytical results were not included in the summary table. When PHS/EHD tabulated the results in terms of contaminated verses uncontaminated wells, no clear correlation could be made regarding the biogradation potential at the site. It is unlikely that another single sampling and analysis event will provide adequate information to evaluate the biogradation potential at the site.