902 N BROADWAY BASE FILE 2025-03-27

Page 2 fueling system,focused removal of gasoline-contaminated soils, and installation/operation of an SVE system beginning in 2018. Site features are shown on Figure 1. In addition to continuing vadose-zone SVE operations as feasible, EES injected bioamendments in early 2021 and again in 2022, targeting gasoline-contaminated groundwater in both shallow perched and deeper water table zones.Treatment actions to date have reduced gasoline contaminant mass at the Site within both unsaturated and saturated-zone treatment areas and have reduced the extent and magnitude of known groundwater impacts, particularly regarding the deeper water table plume.Yet SVE performance data developed between 2021 and 2023 confirmed system inefficiencies and widespread leakage in SVE lateral lines based on multiple lines of evidence as previously reported. Subsurface characterization data generated in early 2023 indicate gasoline contaminants tend to be sorbed within the low-permeability, fine-grained perched zone soils,further reducing SVE effectiveness within the uppermost 25 feet of the treatment area. The residual gasoline contamination within the uppermost 25 feet of these soils provides a continuing source of contamination to perched groundwater within this shallow zone.As a result, the rate and overall effectiveness of hydrocarbon degradation within the perched groundwater zone (since active treatment measures began in 2021) has been diminished.This perched groundwater zone also represents a likely source of ongoing recontamination to the deeper water table,which may impede more successful remediation of that deeper zone. Although the current SVE system is unlikely to achieve remedial action objectives within the perched zone, operation of the SVE system does provide some benefits towards interim remedial objectives, including bioventing, which generally aids contaminant biodegradation. The SVE system also likely limits the migration of subsurface petroleum vapors identified at the Site. The full extent of vadose-zone contamination, particularly perched-zone soil and groundwater impacts and related vapor intrusion assessment, has not been characterized and represents a significant data gap that must be evaluated as part of a supplemental RI before an effective and permanent Site remedy can be developed. Based on recent findings, including information regarding historic fueling infrastructure locations dating to the 1950s, petroleum impacts likely extend further to the west and east than previously identified under the 2017 RI (see Figures 2 and 3).2 The following recommendations are provided to address identified key data gaps. Results of these proposed investigative tasks are intended to inform development of a permanent remedial action plan for this Site. z Figures 1 through 3 include locations of Shell-branded retail fueling/service station infrastructure in use at the Property between the 1950s and 1980s. EES ENVIRONMENTAL CONSULTING,INC. 2120-02 2024 SOW 20240408f