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Vapor Intrusion Assessment Work Plan Project number:60653972 VOC impacts to the buildings and building occupants near areas of known soil and groundwater contamination and the Former Hazardous Material and Waste Storage Area. 3. Physical Setting This section presents the geologic and hydrogeologic setting for the project site. 3.1 Topography, Climate, and Setting The study area is located in San Joaquin County in the Great Central Valley of California and has an average ground surface elevation of approximately 20 feet above mean sea level(amsl).The topography of the Site is relatively flat and level. The climate at the Site is characterized by warm, dry summers and mild winter with heavy fog. Prevailing winds can trend north/northeast.The mean annual precipitation in the City of Stockton is approximately 14 inches(www.ncei.noaa.gov). 3.2 Geology and Hydrology The Site is underlain by predominantly fine-grained alluvial deposits(silty clay and clayey silt) (Parsons 1996, 1997, and 1998a). The relatively coarser-grained or permeable layers(e.g.,fine sand and sandy silt)appear to occur in thin,discontinuous zones. Predominantly fine-grained, low- permeability soils(silts and clays)have been encountered in most borings from the ground surface into the static groundwater table,which fluctuates seasonally. Thin, discontinuous sand and silty sand lenses are occasionally encountered within this depth interval. Historically,the Site groundwater elevations demonstrated a seasonal pattern of rises and falls in groundwater elevation,which appear to be related to recharge in the winter and local groundwater extraction in the summer.When the GWETS was in operation,the inferred flow direction was typically easterly with localized capture or depression zones near the groundwater extraction wells. Starting 2017 when the GWETS was not in operation,the depression zones disappeared, and the inferred flow direction was typically easterly during the spring and winter and turned slightly southeasterly in the summer and fall(AECOM 2016b,2017,and 2018). The hydraulic conductivity ranged from 1.1 x 10-3 to 5.3 x 10-3 centimeters per second(cm/sec)based on the results of slug tests and pumping tests(Parsons 1997 and 1998a).Using this site-specific range of hydraulic conductivities,the average horizontal hydraulic gradient across the Site was 0.002 feet per foot, and with an effective porosity of 0.20,the advective groundwater velocity was calculated to be between approximately 11 and 55 feet per year for the shallow groundwater system. 4. Sampling and Analysis Plan This section describes the planned methods of the field investigation. 4.1 Permits Prior to mobilization, necessary permits will be obtained from the appropriate regulatory agencies and/or municipalities. Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM