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Revised Workplan-Additional Site Investigation for PFAS BSK Project E20-016-01F Stockton Metropolitan Airport(SCK) March 1, 2023 Stockton, California Page 2 2.2 Site Operational History Overview Prior to its current use as a commercial airfield, SCK was operated as both the former Stockton Army Airfield (AAF) and the former Sharpe Army Depot. Stockton AAF dates back to at least 1940, when Stockton Field served as a training installation. In 1946,the City of Stockton and the County of San Joaquin resumed operating the former Stockton Municipal Airport under a joint(interim)license. In 1948,the City of Stockton and the County of San Joaquin jointly assumed administration over the airport. In 1957 the City of Stockton transferred half of its interest in the Stockton Municipal Airport by Grant Deed, and San Joaquin County assumed administration over the airport. By July 1962,the Army Material Command was established with several sub-commands,including the Supply and Maintenance Command. This brought about another name change when Sharpe General Deport became Sharpe Army Depot Field Annex. The Sharpe Army Depot Field Annex was decommissioned in 1964 and the Stockton Municipal Airport was renamed Stockton Metropolitan Airport. The California Army National Guard maintains a presence at the airport. 2.3 Prior Environmental Investigations Between August 1987 and June 1997,twenty(20) underground storage tanks (USTs)were removed from the former"fuel farm"area of the Site (GeoTracker,2019). The former fuel farm was located immediately northwest of the intersection of C.E. Dixon Street and Lockheed Court. Soil and groundwater investigations were performed in 1996 and 2007 by Ramage Environmental, Inc. of Reno, Nevada. The 1996 investigation included installation of soil borings and collection of soil and groundwater samples. Eight groundwater monitoring wells were installed in the former tank farm area. Monitoring wells were constructed of 2-inch polyvinyl chloride (PVC) with depths ranging from 60 to 65 ft below ground surface (bgs). The 2007 investigation included drilling of a deep soil boring to 104 ft bgs using a Cone Penetrometer Test (CPT) drilling rig and installation of five additional monitoring wells. The CPT drilling log indicated Site soils at the former tank farm consisted of clay and silts with thin, discontinuous sand intervals with thicknesses of up to 5 ft to a depth of 105 ft bgs. A 2014 report prepared by Ramage Environmental, Inc.(Ramage,2014)indicated that groundwater in the area of the former tank farm occurred at a depth of 57 ft bgs during drilling in 2003 and that the static water level was approximately 46 to 47 ft bgs. Groundwater depth in 2014 ranged from 33.91 to 37.32 ft bgs. The groundwater flow direction was determined to be to the northeast. Groundwater gradient was calculated to be 0.0012 ft/ft toward the northeast. Total Petroleum Hydrocarbons as Gasoline (TPH-g) was detected in groundwater at a concentration of 13,800 µg/L. The Leaking Underground Storage Tank(LUST)Site was remediated in 2015 and the monitoring wells were destroyed in 2015. The LUST Case was closed by the CVWB on January 6, 2016. PFAS compounds were not investigated at this Site prior to BSK's 2020 investigation.