SITE INFORMATION AND CORRESPONDENCE

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San Joaquin County DIRECTOR Donna Heran, RENS Environmental Health Department I" ASSISTANT DIRECTOR 2:. 600 East Main Street Laurie Cotulla,REHS s :{ Stockton, California 95202-3029 PROGRAM COORDINATORS • Carl Borgman,REHS Mike Huggins, REHS,RDI C9�%F0 R�;P Website: www.Sjgov,org/ehd Margaret Lagorio, REHS Phone: (209)468-3420 Robert McClellon,REHS Fax: (209)464-0138 .teff Carruesco, REHS,RDI Kasey Foley, REHS June 6, 2008 Denis L. Brown Shell Oil Products 20945 S. Wilmington Carson, CA 90810-1039 Subject: Shell Station 4315 Waterloo Road Stockton, CA The San Joaquin County Environmental Health Department (EHD) has reviewed Groundwater Monitoring Report — First Quarter 2008 dated May 9, 2008, prepared by Conestoga-Rovers & Associates and our file information for the above subject site and provides the following comments. The concentrations of tertiary butanol (TBA) and 1,2 dichlorethane (1,2 DCA) in the groundwater samples from monitoring wells MW-1 and MW-5 are of concern to the EHD and the lateral and vertical extent of contamination in the soil and groundwater is not defined. High concentrations of total petroleum hydrocarbons as gasoline remain in the soil beneath the dispensers as well as low concentrations of other contaminants. The receptor survey conducted in 2001 for this site identified 12 domestic or agricultural use wells within 2000 feet of the site. The Shell site also had a well located on the south side of the parcel by Waterloo Road that provided water for the facility until it was destroyed on May 8, 1990. Many of the wells identified in the well survey have also been destroyed. There are still some wells in use to the east of the site. The groundwater flow has been to the northwest, northeast, east and southeast. Operating wells may be influencing the groundwater flow. A rose diagram depicting past and current groundwater flow should be prepared for this site and included in future quarterly reports. Since MW5 has the highest concentrations of 1,2 DCA and methyl tert-butyl ether(MTBE) the predominant groundwater flow direction may be to the east. According to our records, the underground storage tanks (USTs) that were removed in 1994 from the site were installed in 1966. Therefore, leaded gasoline with 1,2 DCA would have been stored in these USTs and is considered the source of the 1,2 DCA in the groundwater. Water samples from MW-1, MW-2, MW-3 and MW4 are not being analyzed for 1,2 DCA. Analysis for 1,2 DCA on the water samples from these wells is to be conducted annually. In addition, a deeper screened monitoring well should be placed in the vicinity of MW-5 to investigate the vertical extent of the groundwater contamination in this direction and shallow screened monitoring wells should be placed east of MW-5 and in the southeast corner of the site to investigate the lateral extent of the groundwater contamination in these directions.