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4707 Pacific Ave. September 1997 Page 2 finds it difficult to make this conclusion given the monitoring capabilities available at the site. PHS/EHD requested additional monitoring wells be installed in correspondence dated March 27, 1996,(prior to the last 5 monitoring events)because of the changes in the depth to groundwater and the destruction of the boundary wells. PHS/EHD perhaps should have directed Unocal to install additional monitoring wells prior to March 1996 to more precisely evaluate the concentrations within the groundwater contamination plume;however,there was the availability of data which could be used to evaluate the off site migration of contamination. Please note that between October 18, 1990 and April 26, 1993 (12 monitoring events)groundwater samples were not collected for analysis because all the wells were dry. PHS/EHD,in efforts to direct actions in the most cost effective manner,approved the reduction of the sampling frequency of monitoring wells located within the contamination plume, in correspondence dated August 28, 1995 (prior to the last 7 monitoring events). A single monitoring well was proposed which will be insufficient to provide adequate monitoring capabilities for the site. Additional monitoring wells are required. The proposed construction for the monitoring well using 30 feet of screened interval is not approved. PHS/EHD has previously notified Unocal that the maximum amount of screened interval which will be allowed for monitoring wells will be 20 feet,so that samples obtained from the wells will be considered representative. Monitoring wells at this site have been screened with intervals as narrow as 3 feet, in the case of MW4. While the narrower the screened interval allows for more representative samples, PHS/EHD has typically approved screened intervals of up to 20 feet. Wells are generally constructed with 15 feet of screened interval below first water and 5 feet above to accommodate groundwater fluctuations. An addendum will be necessary which proposes a sufficient number of wells with construction details that will provide representative groundwater samples. PHS/EHD has requested a site meeting to evaluate the vapor extraction system and believes that an office meeting may be of benefit to further clarify regulatory requirements and site conditions. The addendum must be submitted no later than October 15,1997 and shall include a site map locating the required additional soil borings and monitoring wells, a description of the construction of the wells and depths of soil borings,a description of the ORC slurry,an evaluation of the cost effectiveness of ORC to actively remediate the dissolved groundwater plume,and PHS/EHD drilling permit applications and fees. Please provide information regarding the relationship of Unocal to Tosco. PHS/EHD does not have record that the property ownership has changed from Dayton Hudson Corp. According to Underground Storage Tank Regulations, unless there is evidence of a release following any change in operation,Tosco cannot be named as an additional responsible party. The information requested shall be reviewed so that PHS/EHD can properly identify all responsible parties and provide adequate communication to each. Au 13 1997 directive to submit a work plan with a minimum Thank you for your efforts to comply with PHS/EHD's gust of four additional monitoring wells. If you have any questions,comments or to schedule an office or a site meeting,please contact me at(209)468-0337. Donna Heran,REHS,Director Environmental Health Division eanio—r REHS Ma�Lagono,RE Site Mitigation Unit Supervisor cc: Elizabeth Thayer,Central Valley Regional Water Quality Control Board cc: David Irey,San Joaquin County Deputy District Attorney,Environmental Prosecutions Unit cc: Dayton Hudson Corp.,777 Nicollett Mall,Minneapolis MN 55402 cc: Greg Gurss, GeoStrategies