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rom a dispenser or a line leak. The two additional downgradient monitoring wells installed in April appear to have defined the downgradient extent of the groundwater plume resulting from the tank leak based on two sampling events. An upgradient well was not proposed in the workplan, because at that time an upgradient well location would have been located south of the former showroom, which would be too far from the source to provide useful data. A review of gradient information has now provided enough data to locate an upgradient well to the south of the former sales office Quarterly groundwater monitoring data obtained in June and presented in the July report unfortunately does not confirm the surprisingly rapid decrease in contaminant concentrations that occurred from December 1995 to March of 1997. This raises the possibility that natural attenuation may not be occurring at the site and active remediation may be necessary. The well survey reported the nearest wells as a domestic well 650 feet to the southeast (upgradient)with a total depth of 217 feet, and a well which may be agricultural or domestic use 700 feet to the northeast(downgradient)with a total depth of 427 feet . Considering current groundwater data from the monitoring wells this site poses no risk to these existing wells. The site assessment is not complete at this time. The lateral and vertical extent of soil contamination have not been determined. The upgradient extent of the groundwater plume has not been determined. There is a possibility that a downgradient component of the groundwater-plume extends beyond the western side of the building. In order to comply with Article 11 of the California Code of Regulations a Corrective Action Plan (CAP) shall be submitted once the investigation is complete. The CAP shall include an assessment of the hydro eolo is conditions of the site, the extent of contamination, determine the feasibility of at least two altematives for mitigating the release, propose the most cost effective corrective action and propose numerical cleanup objectives. l` In conclusion PHS/EHD requires that the quarterly reporting requirements remain in effect and that an EPA method 8260 (with full disclosure of fuel additives) be added to the required list of analyses. PHSIEHD recommends that you submit a workplan to complete the site assessment by October 15, 1997. For additional information please call Steven Sasson at(209)468-3459. Donna Heran, REHS, Director Envir 'nmental Health Division t res for RENS Margar !-agorio,AREHY Site Mitigation Unit Supervisor SS c: Elizabeth Thayer-CVRWQCB-UST section Mel Jacobo - SWRCB c: Gary Barker- Horizon Environmental, Inc.