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e� Clean Soils , Inc . o R 4359 Phelan Road; Phelan, CA 92371 '� Phone; 760-868-8572 Fax: 760-868-8573 5 ' i NOV 0 8 2004 ENVIRONMENT HEALTH November 3, 2004 PERMIT/SERVICES Attn: Ms. Vicky McCartney Environmental Health Department San Joaquin County 304 East Weber Avenue, 3`d Floor Stockton, California 95202-2708 Re: Addendum to proposed Remedial Action Plan for contamination identified in 2002 during excavation of diesel and gasoline contaminated soil at the Frontier Trucking Facility, 425 Larch Road, Tracy, California. We thank your office for the prompt answer to the RAP and for your patience in discussing the potential needs for cost effective and technically feasible remediation of this site. As a result of groundwater monitoring and soil testing performed since fuel tank removal at the site, our staff feels that we have identified the migration pathway of the gasoline oxygenate, MTBE component of the discharged gasoline. In monitoring well MW-13 soil sample analysis at 10' bgs identified MTBE at 25,000 ' ppb attenuating to 140 ppb at 30' bgs. (1 F at lul )VP lr ib i Soil sample analysis at 10' bgs in monitoring wells MW- 14, MW- 15 , MW- 16 and MW- 18 identified MTBE concentrations of 1 ,020 'ppb, 1 ,310 ppb, 131 ppb, and 957 ppb, respectively. These wells are screened at 30' to 40' and have no detectable dissolved MTBE when monitored at that depth. Lesser dissolved concentrations of MTBE were identified in MW- 17, screened at shallow depth, at 15 ' bg and 20' bgs at 101 ppb and 22 j ,} S v �v A ` ppb, ti1PO) A � � � �� dq'� However, MW4 , approximately 50' west-northwest of the pre-fabricated building and screened at shallow depth, identified dissolved MTBE levels in monitoring performed in March, June and September 2004 of 1 ,300 ppb, 1 ,200 ppb and 8, 100 ppb. MW47, approximately 60 ' south of MW- 1 , also screened in the shallow water identified MTBE at lower levels, 99 ppb, 130 ppb and 3 . 1 ppb in monitoring performed in March, June and September 2004. I Our staff believes that a dissolved MTBE plume has been identified migrating in a west- northwesterly direction from the underwater source identified in the removed tank excavation. Migration of the oxygenate MTBE appears to have occurred at groundwater table depth as identified by the 10' bgs soil sample analysis described above.