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PRQM <br /> • 1997.1 15:48 #661 P.04/06 <br /> PACIFIC <br /> ENVIRONMENTAL <br /> GROUP INC. <br /> October 14, 1997 <br /> Project 311-165.7A <br /> A1r. Ed Ralston <br /> Tosco Marketing Company <br /> 2000 Crow Canyon Place, Suite 400 <br /> San Ramon, California 94583 <br /> Re: Unocal Service Station 6981 <br /> 4707 pacific Avenue <br /> Stockton, California <br /> Dear Mr. Ralston: <br /> At your request, Pacific Environmental Group- Inc. (PACIFIC)has reviewed historic <br /> technical documents and regulatory correspondence related to the site referenced above. <br /> The purpose ofPACIFIC's review was to provide recommendations to Tosco Marketing <br /> Company(Tosco) regarding future environmental activities at the facility. Tosco <br /> manages the environmental projects related to active Unocal service stations. - <br /> BACKGROUND <br /> Approximately 12 borings have been drilled and 13 monitoring wells installed on and <br /> adjacent to the site. four perimeter monitoring wells have recently been abandoned due - <br /> to road work adjacent to the property. A disagreement has arisen between Tosco and <br /> San Joaquin County Public Health Services,Environmental Health Division(PHS/EHD) <br /> regarding the need to replace these four wells and several other interior wells currently <br /> with screened intervals below the water table. <br /> A soil vapor extraction(SVE) system has been operated at the site since 1993. Over <br /> time the effectiveness of the system has declined. PHS/EHD has requested that multiple <br /> Wrings be drilled to a depth of at least 50 feet to monitor the effectiveness of the SVE <br /> system. Currently, the depth to groundwater beneath the site is approximately 35 feet <br /> below ground surface. Tosco has questioned the need for these deep borings_ <br /> In a letter dated September 12, 1997 to PHS/EHD, Tosco stated that it was their obser- <br /> vation that the dissolved hydrocarbon plume was stable with decreasing groundwater <br /> concentrations and that the site should be considered for closure as a low-risk ground- <br /> 20:!5 Gateway Place,5uile 440,San lose,California 95:10 (408)441-7500 FAX(408)441-7539 <br />