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SUPPLEMENTAL SITE INVESTIGATION WORK PLAN South Pointe Property Stockton, California Conceptual Site Model 3.0 CONCEPTUAL SITE MODEL The following sections describe the conceptual site model including historical chemical source areas, groundwater and soil vapor conditions, historical soil disturbance areas at the site, and the extent of chemicals of concern in soil. 3.1 HISTORICAL CHEMICAL SOURCE AREAS As summarized in the 2020 Phase I ESA, the property has a lengthy industrial history dating to the late 1800s. Historical uses include grain storage and milling, fertilizer mixing and handling, and materials loading along two rail alignments that were historically located along the centerline of the site. Based on historical investigations, concentrations of OCPs and lead in soil appear to be concentrated along these rail lines, likely attributable to historical materials handling practices. DTSC previously requested that Stantec review historical documents such as aerial photographs and Sanborn insurance maps to identify any specific features referenced in the Phase I that could represent historical chemical release areas such as above-ground storage tanks, boilers, chemical handling and/or mixing areas, etc. Stantec reviewed historical aerial photos from 1937 through 2016. In general, it is difficult to discern specific potential chemical handling and/or process areas. During the timeframe reviewed, the site is generally built out with large warehouse buildings on either side of the two rail lines that bisected the property from east to west. A group of vertical silo-type structures are visible in the majority of the aerial photos in the southeast portion of the property; these are identified on Sanborn maps as 'grain tanks'. Review of historical Sanborn fire insurance maps revealed several details that could represent historical chemical source areas. Stantec reviewed Sanborn maps for the years 1895, 1917, 1950, and 1972 and identified the following features, the locations of which are illustrated on Figure 2. • A feature identified as a 'cleaner' on the 1917 Sanborn map was identified as a '40- gallon chemical tank on wheels' on the 1850 and 1972 Sanborn maps. This feature was located in the northwest portion of the site inside a 'grain and feed warehouse'. • A boiler feature was identified in the central portion of the site on the 1950 and 1972 Sanborn maps, apparently attached to the adjacent California Transportation Company facility. • Two outbuildings, one marked as 'oil', were identified in the central portion of the site on the 1917 Sanborn map. Based on previous investigations documenting extensive soil disturbance and movement at the site since demolition of the site structures (detailed in the following sections), it seems unlikely that shallow, near-surface chemical impacts attributable to historical features would still be in 3.1