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3.0 RATIONALE FOR SCOPE OF WORK
<br /> In accordance with WAC 173-340-900 Table 830-1 Required Testing for Petroleum Releases, the
<br /> potential constituents of concern (COCs) for the Site in soil and groundwater requiring consideration
<br /> are TPHo, benzene, toluene, ethyl benzene, and total xylenes (BTEX), ethylene dibromide (EDB),
<br /> ethylene dichloride (EDC), methyl tertiary butyl ether (MTBE), lead, polynuclear aromatic
<br /> hydrocarbons (PAHs), naphthalenes, polychlorinated biphenyls (PCBs), and halogenated volatile
<br /> organic compounds (HVOCs). All of the aforementioned analytes have been tested in soil and
<br /> groundwater, and TPHo was the only constituent exceeding established cleanup levels in both soil and
<br /> groundwater. Therefore, TPHo is the only Site COC requiring future management.
<br /> To achieve the objective of a NFA determination from Ecology, the following rationale has been used
<br /> in the development of the scope of work detailed in this Work Plan.
<br /> Soil Excavation: Based on results of the extensive soil sampling conducted between 1994 and 2011,
<br /> residual soil impacts are limited vertically and laterally, and do not extend beyond the footprint of the
<br /> current buildings. Excavation is the quickest method to remediate hydrocarbons in soil. With this
<br /> method, impacted soil is excavated, hauled off Site for disposal, and replaced with clean backfill
<br /> material. The three target areas for excavation are: (1) in the Quick Lube Service Bays within Building
<br /> 1 to a depth of approximately 14 ft bgs (HL-7-14 North, HL-8-10 North, and HL-8-10 South); (2) in
<br /> the Shop Service Bays within Building 1 in the vicinity of HL-10-10 South, HL-18-17, HL-16-9 North
<br /> and South, and 94172-B13-2 to a depth of approximately 10 ft bgs; and (3) in the Parts Warehouse
<br /> within Building 2 surrounding HL-2-5 West and HL-1-6 West to a depth of approximately 6 ft bgs. It
<br /> should be noted the approximate depths of excavation may change depending on field conditions. If
<br /> field conditions (elevated photoionization detector [PID] readings, staining, odors, etc.) indicate the
<br /> extent(s) of the excavation(s) should be extended, EEC will notify AHLLC prior to removing additional
<br /> impacted soil. The excavation is planned to remove and replace approximately 400 to 500 cubic yards
<br /> (cy) of soil. Excavation of the impacted soils will prevent human direct contact of the COC, human
<br /> ingestion of the COC, and further migration of the COC into Site groundwater, therefore, meeting the
<br /> requirements to adequately protect human health and the environment.
<br /> Test Pit: On August 20, 1999, a soil sample was collected west of the Building 1 Showroom (S-1) after
<br /> the removal of an in-ground hydraulic lift and the soil analytical results (3,930 mg/kg of TPHo)
<br /> indicated the presence of impacted soil at the depth of 9 ft bgs. On April 28, 2011, EEC collected soil
<br /> samples at the depth intervals of 5, 10, and 15 ft bgs within 10 feet of S-1 (SB-17). The analytical
<br /> results indicated all detected analytes were below the MTCA Method A screening levels. Based on the
<br /> previous analytical results of the soil samples collected from S-1, a test pit will be completed to the
<br /> depth of approximately 9 ft bgs at the location of S-1. The purpose of the test pit is to confirm soil at
<br /> this location is below MTCA Method A screening levels. If impacts are identified during completion of
<br /> the test pit, by either elevated PID readings and/or visual or olfactory evidence of impact(e.g. staining,
<br /> odors, etc.) the impacted soil will be removed and replaced, and confirmatory soil samples collected.
<br /> Groundwater Monitoring: Monitoring wells are appropriately positioned down-gradient of residual soil
<br /> impacts to determine whether groundwater is being adversely affected. Monitoring well MW-1 is the
<br /> only well which has contained any concentrations exceeding MTCA Method A screening levels.
<br /> Monitoring wells MW-7 and MW-8, located approximately 20 ft down-gradient of MW-1, do not
<br /> contain concentrations exceeding MTCA Method A screening levels; therefore impacted groundwater is
<br /> limited to a small area surrounding MW-1. Groundwater is not currently used as a drinking water
<br /> resource in the vicinity of the Site, and no water supply wells are present within a 1-mile radius of the
<br /> Site. Surface water is not present on the Site; however, storm water collected in catch basins on-Site is
<br /> 017386-174(123) 4 THIS DOCUMENT IS PRINTED ON RECYCLED PAPER
<br /> Encore Environmental Consortium.LLC
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