SITE INFORMATION AND CORRESPONDENCE

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if Ms.Lori Duncan October 26,2000 Page 2 of 3 Lincoln in July 1990. Well MW7 was installed on the southeast comer of Weber and Lincoln, i approximately 100 feet southeast of the Yacht Harbor USTs. The groundwater gradient in July i 1990 (wells MWl-MW7) was determined to be generally to the southeast with a more direct southerly component in the vicinity of well MW7. 1 • Between 1996 and 1999, the City of Stockton contracted Treadwell & Rollo to conduct additional investigations to further characterize soil and groundwater contamination associated with historical petroleum operations southwest of the site. Although specific reports were not made available for review, it appears that Treadwell & Rollo drilled installed, and sampled at least 26 additional soil borings and eight additional groundwater monitoring wells as part of their investigation. One well, MW8 was installed on the southwest comer of Lincoln and Weber, approximately 120 feet southwest (down-gradient to cross-gradient) of the subject site. Although specific reports were 'not made available for review, Mr. Jeff Wong of PHS/EHD indicated that a recent groundwater gradient map did not appear to be included in the Treadwell & Rollo report, which was located in his office. However, the report referenced historical 3 information that the groundwater';gradient was to the southeast. i I • In correspondence to the City of Stockton dated July 12, 2000 and July 27, 2000, respectively, both the Department of Toxic Substances Control(DTSC) and the CRWQCB document that the groundwater flow direction in the vicinity of the investigation is to the southeast. �. `In addition, a complete analysis of all potential petroleum hydrocarbon constituents was not performed on the collected sample. a i The groundwater sample was analyzed for TPHG/TPHD using EPA Method 8015 modified, and BTEX/MTBE using EPA method 8020. PHS/EHD personnel were present and observed virtually all of the field work performed. Analytical methods were discussed with and approved by PHS/EHD staff, with the agreement that a positive result in MTBE would be confirmed using I EPA method 8260. No MTBE was detected in the groundwater sample, nor in eight soil samples collected beneath the product lines that were analyzed by EPA method ,8020 and five soil samples collected beneath the gasoline UST that were analyzed by EPA method 8260 (other oxygenated gasoline additives werealso ND in these samples). All five soil samples analyzed by EPA method 8260 were in direct contact with groundwater. PHS/EHD and CVRWQCB agree that further investigation of the groundwater contamination at this site is warranted Monitoring wells are not needed at this time. " The groundwater collected from boring SBI, the only boring which yielded sufficient water to collect a water sample, contained 110 ppb TPHD. No TPHG, BTEX, or MTBE were detected in the sample. No State or Federal drinking water standards(MCLS) have been established for TPHD, although a taste and odor threshold'of 100 ppb has been suggested (CVRWQCB Water Quality Goals, August 2000). Two soil samples collected from boring SBI that were in direct contact with the groundwater were non-detect for TPHD and BTEX. The soil samples were not analyzed for TPHG or MTBE. None of the 21 samples collected from beneath the USTs and/or pipelines G:\Dam\GROUNDZE\W azedmnt\dancer I OOO.doc