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SEP - 27 - 04 03 : 54 PM BF -' INC . 760 r '-g 8572 P . 04 <br /> Lateral advective migration of dissolved MTBE at concentrations greater than 1 ,000 <br /> ppm, extends approximately 260 feet westward from the source in the first water. It is <br /> proposed that source removal be prioritized to help contain further migration <br /> 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 coarse-grained <br /> 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 /> gasoline plume dissolved in the first groundwater and in vapor phase in the unsaturated <br /> sail . <br /> Finally, the identification of the oxygenate MTBE at well termination depths in <br /> monitoring wells MW-3R in the UST re-excavated area, 170 ppb at 20' bgs and I 1 ppb at <br /> 40' bgs in MW44, 120' west of the re-excavation support vertical migration of the <br /> MTBE to depths deeper than well advancement. <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 further 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, , /t' <br /> if any . Proposed boring locations are shown on the attached Site Layout Map . — Ndt JAl / r � <br /> CSI proposes expedited implementation of this partial RAP to provide data necessary for It <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 /> 9 <br /> 1 <br />