ARCHIVED REPORTS XR0010073 - Laserfiche WebLink

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Reconnaissance groundwater data support the previously mapped plume location The deeper groundwater samples showed reduced TPHG and greatly reduced Benzene concentrations The April 2000 data showed higher concentrations overall, however, these samples were collected somewhat shallower and flowing conditions may have released contaminants from desorbtion by the flowing The deeper samples were collected well below the capillary fringe, and lower concentrations would be expected in deeper portions of the aquifer When compared to the most recent quarterly data, these reconnaissance sample plumes overlay each other in the area of the historic plume location Data from PB-20 and PB-21 show the dissolved plume present to the west of the trailers at very low concentrations, which are attributed to dispersion over time Cleanup Options PHS-EHD requested that Wright propose two possible cleanup options to address cleanup approach for these residual contaminants The UST sources have been removed in 1992- 1993 The geophysical survey showed that there were no indications of a UST remaining in the area investigated Wright concludes that there are no other USTs present on the basis of this data, however, there are residual soil sources which can contribute too the dissolved plume Figure 5 shows the proposed areas of cleanup to remove residuals of concern to PHS-EHD . © The former UST pit and the area immediately downgradient of the former pit 0 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 downgradient of the former pit comprises an area of about 50 by 50 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 the following three cleanup approaches to address these sources Option 1 Perform soil excavation in the areas shown in Figure 5 The soil would be excavated and clean soil segregated from contaminated soil As the site is too constrained to allow soil treatment on-site, the contaminant soil will be properly disposed off-site Soil would be removed to a depth of 10- to 12-feet or as deep as practicable, then replaced with clean fill and compacted Two constraints are present, the presence of flowing conditions, and the presence of numerous sewer, water and gas utilities serving the trailers, tree and the large clay pipe irrigation pipeline Excavation will be advanced to the nearest safest point to these constraints and modified in the field as necessary It is anticipated that the excavation and soil hauling to proper disposal would take up to two weeks, with backfilling and • compaction immediately following the ultimate limit of excavation The site would need to be fenced and monitored during the period where excavations are open Page 8