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I n YVJ dV I <br /> Lateral advective migration of dissolved MTBE at concentrations greater than 1 ,000 <br /> ppm, extends approximatelyWfeet westward from the source in the first water. It is <br /> \, r osed that source removal be prioritized to help contain further migration of the <br /> plume and free product by removal of the majority mass of the significant source. <br /> As identified in site-wide cross sections A-A' and B-B ' , see June 2004 RAP, the soil <br /> lithology identified is predominately described as heterogeneous clay with hoarse=grained =; <br /> t' channeling. CSI proposes therefore that source removal be completed to contain the <br /> laterally spreading plume to provide cost effective long term clean up of the extensive <br /> `c gasoline plume dissolved in the first groundwater and in vapor phase in the unsaturated <br /> soil. <br /> Finally, the identification of the w9T.5enate MTBE at well *,m ion-ie�pt O in 10 z o ug I )t01 <br /> monitoring wells MW-3RD in the UST ie-excavated area, 70 p$Olt 20' bgs and.Xf ppb at <br /> ! rys1A' bgs in MW- 14, 120' west of the re-excavation support verticaTmigration of the <br /> MTBE to depths deeper than well advancement. a <br /> CSI proposes that two additional monitoring wells be drilled through the contaminated <br /> zone using conductor casing of 12"-diameter to 25 ' believed to be that depth beneath the <br /> free phase contamination layer. It is finther proposed that a direct push 2 '-diameter <br /> slotted monitoring well be advanced through the slurry backfilled conductor casing from <br /> 25 ' bgs to 40' bgs to assess potential contamination of a deeper aquifer. These wells are <br /> proposed to further define the vertical extent of the gasoline, BTEX and MTBE plumes, <br /> if any. Proposed boring locations are shown on the attached Site Layout Map. — No tP <br /> CSG/� ,y <br /> CSI proposes expedited implementation of this partial RAP to provide data necessary for " �'Q✓rof �' <br /> cost benefit analysis comparing groundwater sparging, monitoring intrinsic <br /> biodegradation or soil vapor extraction. These clean up options can not be appropriately <br /> addressed until the free phase source is removed to the maximum extent practicable. <br /> I <br /> I <br />