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SEP - 27 - 04 03 : 53 PM EF INC . 760 y-.--.. 8 8572 Pm02 r.� Glean Soils , Inc . 4359 Phelan Rvadt Phal ^ CA 42371 Phnne: 760168-8572 FoK: 760-868-8573 September 21 , 2004 Ms. Vicki McCartney Environmental Health Department, San Joaquin County 304 East Weber Avenue, 3`d Floor Stockton, California 95202=2708 Subject ; Addendum to Remedial Action Plan for Frontier Transportation Property located at 425 Larch Road, Tracy, California . Based on our conversations, Clean Soils, Inc. (CSI) submits the following changes to the Remedial Action Plan (RAP) dated June 17, 2004, These changes decrease the scope of work of the plan to expedite removal of the free phase gasoline source identified at A4' below grade surface (bgs) and 4' beneath the first water table. Removal of the significant free phase source will provide containment of the plume. The plume has migrated extensively and advectively in free phase, dissolved phase and vapor phase above, beneath and dissolved in the first groundwater. It is proposed that the Remedial Action Plan be limited to the following scope of work to provide necessary containment of the significantly expanding gasoline and MTBE plume. This expansion of the plume now identified at 260' west and hydraulically down gradient of the source identified 4 ' beneath the first groundwater in the excavation identified gasoline product concentrations well in the free phase range (TPH -gasoline at 2, 500 parts per million (ppm), BTEX at 1 ,307 ppm, and MTBE 26 at ppm). CSI shares the concerns expressed by Environmental Health Department staff that migration is ongoing in multiple permeable channels in the heterogeneous clayey soil. CSI proposes that excavation be performed expeditiously in the excavation source area intersected by monitoring well MW-3R. During excavation in May 2002 free phase (LNAPL) concentrations (greater than 200 parts per million) were identified at 14 ' bgs under a site wide first groundwater table at 10 ' bgs. This would effectively remove the free phase source inverted under first groundwater traveling westward under the pre- fabricated, steel, mechanic shop building at concentrations that would indicate free phase . (LNAPL) as well as dissolved and highly adsorbed gasoline, gasoline aromatic constituents and the oxygenate MTBE . Sample results tabulated in the June 2004 RAP indicate extensive lateral and unknown vertical migration of the MTBE. Excavation of the inverted LNAPL soils contamination beneath first groundwater the re-excavated tank removal area in its entirety must be performed. This addendum further proposes that water and soil vapor treatment be postponed until the extent of oxygenate NME and gasoline and aromatic constituent contamination is identified.