SITE INFORMATION AND CORRESPONDENCE

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r ENVIRONMENTAL HEALTHOEPARTMENT °U N SAN JOAQUIN COUNTY Donna K. Heran, R.E.H.S. Unit Supervisors < Director 304 East Weber Avenue,Third Floor Carl Borgman,R.E.H.S. Al Olsen, R.E.H.S. Stockton,California 95202-2708 Mike Huggins, R.E.H.S.,R.D.I. • cq` .N`P . Program Manager Telephone: (209)468-3420 Douglas W.Wilson, R.E.H.S. FOR Laurie A. Cotulla, R.E.H.S. Margaret Lagorio,R.E.H.S. Program Manager Fax: (209)464-0138 Robert McClellon, R.E.H.S. Website:www.sjgov.org/ehd/ Jeff Carruesco, R.E.H.S. PAUL SUPPLE AUG 112005 ARCO PRODUCTS COMPANY P O BOX 6549 MORAGA CA 94570 RE: ARCO #6100 SITE CODE: 1708 25775 Patterson Pass Road Tracy, CA San Joaquin County Environmental Health Department (EHD) has received and reviewed Quarterly Groundwater Monitoring Report, Fourth Quarter 2004, dated 26 January 2005, and Quarterly Groundwater Monitoring Report, dated 20 April 2005, both prepared for the above-referenced site by Stratus Environmental, Inc. Both reports note the presence of sheen or free product in four wells, VW-1, VW- 4, MW-1 and MW-15. There are no wells with water in the area between these four wells, which cover an area of approximately 4200 square feet. The groundwater elevation on which the sheen occurs in the vapor wells varies by approximately 15 feet and varies between the vapor wells and the monitoring wells by approximately 50 feet. The groundwater encountered on this site appears to occur as several thin, perched bodies migrating at the base of sand units. At the present time, free product and groundwater is being passively removed through periodic bailing of the impacted wells. The total volume of water removed at any one time is not great enough to create a significant cone of depression. There is a distance of approximately 45 feet between the free product-impacted monitoring wells, potentially allowing a significant volume of free product to migrate between the wells plus whatever may be migrating outboard of the area between the wells. Groundwater in the general down-gradient direction, in well MW-12, is intensely impacted by the contaminants of concern. It is not clear that MW-12 delineates the down-gradient extent of free product or is merely on a lateral margin of it if the main plume has shifted more easterly or westerly at this distal point. The sand units have been reported to dip by at least 5 degrees and the local gradient has been reported at 0.09, which is a high gradient. The potential for migration of large volumes of free product and impacted groundwater, even if perched, is of great concern to EHD.