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i i Chevron Chevron 8 JAN' 2(6 PPI 1 09 Chevron Products Company January 21, 1998 6001 Bollinger Canyon Road Building L San Ramon,CA 94583 P.O.Box 6004 San Ramon,CA 94583-0904 Mr.. Ron Rowe Marketing—Sales West San Joaquin County Public Health Services Phone 510 842-9500 304 E. Weber Ave., Third Floor Stockton, CA 95202 Re: Chevron Station#9-1452, 334 E. Main St.,Ripon, CA Dear Mr. Rowe: Chevron's consultant,Pacific Environmental Group, Inc. (Pacific), submitted a Case Closure Report for the subject site to your agency on June 16, 1997. In anticipation of eminent closure, Chevron instructed Pacific to proceed with well destruction permitting during September, 1997. On September 30, 1997 comments were received from your agency regarding Pacific's well destruction permit and Case Closure Report. Those comments were: 1)Insufficient groundwater data exists for area west of former and current UST complexes; 2) Workplan requested to sample groundwater from one location west of UST complexes; 3)Analyze groundwater for oxygenates at all existing monitoring wells by GUMS analytical method; 4) Pending favorable results from additional investigation and well sampling, case closure would likely be recommended;and 5) Well destruction denied until case closure issued. In response to the comments received on September 30, 1997, Chevron performed oxygenate sampling at all site-related wells on October 31, 1997. The results were submitted in Blaine Tech Services' Fourth Quarter Monitoring Report dated December 11, 1997. No workplan for further groundwater assessment has been submitted. Instead, Chevron requests that your agency give further consideration to case closure. To assist you in that effort, the following information is provided: The groundwater sampling event performed on October 31, 1997 did not detect any groundwater contamination, including oxygenates. These results confirmed the trends in groundwater quality that had already been established by the previous five events. The measured direction of groundwater flow during October, 1997 was generally toward the west. Of the other five monitoring events at the site, two measured groundwater flow in a southwesterly direction, two measured groundwater flow in a northwesterly direction, and one measured groundwater flow toward the east-northeast. Of greater importance is the relatively low hydraulic gradient measured across the site during all events. The highest gradient measured was 0.003 ft/ft toward the northwest during February, 1997. The average hydraulic gradient measured over one hydrologic cycle(2/96- 11/96)was 0.001 ft/ft. The significance of this low gradient is that it produces very low advective forces that are responsible for transporting groundwater contaminants. This, in combination with inconsistent flow directions, would result in a localized contaminant plume spread around a source instead of a narrow plume moving directly away from a source. With this in mind and with any appreciable source of contamination at the UST complex, groundwater containination would most likely be detected in monitoring well MW-3. The fact that it has not suggests there is no appreciable source of groundwater contamination beneath the former UST complex. This is further supported by soil sample results obtained during the UST removal in 1995.