902 N BROADWAY BASE FILE 2025-03-27

Page 11 In order to facilitate future consideration of a broad range of cleanup alternatives, EES will also calculate "modified" site-specific Method B soil cleanup levels as an alternative to the default Method A criteria. The site-specific Method B soil cleanup level for total petroleum hydrocarbons (TPH) will be calculated based on the collection during Task 2 of EPH/VPH data and used for comparison to the default 1,500 mg/kg TPH cleanup level allowed by Ecology under the published Model Remedies(2017 Ecology Publication#16-09-057). In an effort to better delineate the extent of petroleum contamination, the following supplemental Site characterization work is proposed under Task 2: ■ Evaluate Shallow Conditions Under the Existing Building: In conjunction with an evaluation of sub-slab vapor conditions as described under Tasks 1A and 1B above, shallow under-building soil data will be collected under Task 2A. ■ Supplemental Perched-Zone Contaminant Delineation: To provide contaminant delineation elsewhere in uncharacterized areas of the Property as described above, the 2023 HRSC investigation results will be supplemented by Task 213 HRSC data that will again be collected using a direct-push membrane interface hydraulic profiling tool (MIHPT).The MIHPT device measures various chemical and physical parameters to aid in characterizing relative contaminant distribution and other soil/water characteristics.These MIHPT borings will be advanced to refusal, ideally approaching 40 feet bgs or as feasible, in order to help determine the lateral and vertical extent of contamination in these targeted areas of the Property. Discrete soil samples will also be collected and analyzed at selected locations to quantify contaminant conditions and in support of development of the modified, Site-specific Method B cleanup levels. TASK 2A: SUB-SLAB SOIL SAMPLING The historic Shell service station building was operated between 1955 and 1988,when it was demolished, and this area was subsequently redeveloped and covered by the existing commercial strip mall building(Figure 3). Available facility schematic maps indicate the location of this service station building, including UST vent lines beneath and affixed to the side of the building, but no demolition or facility decommissioning information has been identified by EES, and no subsurface investigation has occurred in this area to our knowledge. In addition to the UST vent lines,typical vehicle service activities in this area would likely have included infrastructure/chemicals of concern such as in-ground hydraulic vehicle hoists, floor drains, new and used chemical/lubricant storage and use, and possible sumps/drywell features. Persistent elevated gasoline vapor concentrations in soil vapor wells adjacent to the building (particularly near the former service building at SGW-4 and SGW-5) indicate a possibility of petroleum impacts associated with historic releases from Shell's former service building and/or UST vent lines.The remnants of this former infrastructure cannot be directly verified at this time, but sub-slab vapor (Task 1A) and soil data under Task 2A will help characterize potential impacts beneath the current and former building areas. Task 2A involves collecting four"grab" soil samples from fill and soil material under the current building floor slab, including the former service station building footprint, if possible.These limited-access indoor drilling/sampling locations require that borings be installed by hand or using a small hydraulic direct- EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f