ARCHIVED REPORTS XR0009987 - Laserfiche WebLink

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Figure 5 shows the proposed areas of cleanup to remove residuals of concern to PHS- EHD I • The former UST pit and the area immediately down gradient of the former pit • The area in the near vicinity of MW-9 interpreted to reside in strata paralleling the irrigation pipeline The area of the former tank pit and area down gradient of the former pit comprises an area of about 68 by 62 feet The area around MW-9 comprises an area of about 10 by 30 feet It is Wright's opinion that these soil sources are a contributing cause to the elevated dissolved concentrations observed in quarterly monitoring Wright has reviewed and selected the following three cleanup approaches to address these sources - Option 1 — Soil Excavation and Removal A soil excavation would be performed in the areas shown in Figure 5 The contaminated soil would be excavated and clean soil segregated from contaminated soil As the site is too constrained to allow above ground, ex-situ soil treatment on-site, the contaminant soil will be properly disposed off-site The targeted soil of interest is that in the capillary fringe at depths of about 9 to 14 feet depending upon location (see Geologic Cross Sections Figure 2) These areas are located in the vicinity of the former UST pit and MW- 3 and at MW-9 The area around MW-9 is interpreted to be confined to small, sandy strata that is the apparent source of the residual contaminants Soil would be removed to a depth of 10- to 12-feet or as deep as practicable, there replaced with clean fill and/or overburden soil and compacted to an engineering specification There are three constraints to this approach, the presence of the inhabited trailers, presence of possible flowing conditions in the upper portions of the aquifer, and the presence of numerous sewer, domestic water and gas utilities serving the trailers, a large (possible "heritage") tree, large clay pipe irrigation pipeline and trunk sewer lines Excavation would be advanced to the nearest safest point to these constraints and modified in the field as necessary The presence of possible flowing conditions indicates that an engineer with experience in shoring would need to review the site and conditions and prepare shoring prior to excavation work starting It is anticipated that the excavation and soil hauling to proper disposal would take up to two weeks, with backfilling and compaction immediately fallowing the ultimate limit of excavation The site would need to be fenced with 24-hour security and monitored during the period where excavations are open and several trailers and their occupants would require temporary relocation In addition, utilities for other trailers and their occupants would be interrupted for several days during the excavation and site restoration phase Page li