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�- iN March 21 , 2005 Ms. Vicki McCartney Environmental Health Department, San Joaquin County 304 East Weber Avenue, P Floor Stockton, California 95202-2708 Subject : Addendum to Remedial Action Plan for Frontier Transportation Property located at 425 Larch Road, Tracy, California. Based on site findings and regulatory negotiations, Biophysics Environmental Assessents, Inc. (BEA) submits the following changes to the Remedial Action Plan (RAP) dated June 17, 2004. These changes decrease the scope of work of the plan for technically achievable and cost-effective removal of the free phase gasoline source identified at 14' below grade surface (bgs) and 4' beneath the first water table. Removal of then significant free phase source will provide containment of the plume, which has migrated extensively advectively in free phase, dissolved phase and vapor phase above, beneath and dissolved in the first groundwater. ? r 1t% � r� It is proposed that an interim Remedial Action Plan be limited to the following scope of work to provide additional delineation of the significant]X4panding gasoline and MTBE plume. This expansion of the plume is now identified atwest and hydraulically down gradient of the source. The source has been identified 4' beneath the first so>fbe groundwater in the excavation. Gasoline product concentrations wer� identified as high Bu+' W NS as the free phase range (TPH -gasoline at 2, 500 parts per million tpVmj, BTEX at +,�/ zo '1 044'b, ppm, and MTBE 26 kppm) . BEA shares the concerns expressed by Environmental Health Department staff that migration is ongoing in multiple permeable channels in the heterogeneous clayey soil . BEA proposes that excavation be performed in the excavation source area intersected by monitoring well MW-3R after completion of delineation. During excavation in May 2002 tree 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 7pt BlF Excavation of the inverted LNAPL soils contamination beneath first groundwater�t e re- r 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 MTBE and gasoline and aromatic constituent contamination is identified.