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Site Inspection UFP-QAPP Addendum Stockton AASF, Stockton, California No recent groundwater monitoring data are available for the AASF; however, the CAARNG has been monitoring groundwater at the adjoining FMS #24 site since 2010. Depth to groundwater has been generally encountered from 30 to 35 ft bgs, and the hydraulic gradient was measured at approximately 0.0015 ft/ft in 2013 (Adanta-ECM Joint Venture, 2018). Groundwater levels measured in nine monitoring wells and two remediation wells generally flows to the northeast (ECM Consultants, 2018). However, regional unconfined groundwater flows generally southwesterly toward the valley (Adanta-ECM Joint Venture, 2018). Groundwater features are presented on Figure 10-2. Hydrology The nearest surface water feature is Rydberg Creek, which wraps around the southern and western boundary of the Stockton AASF facility, flows northwest, and connects with French Camp Slough. According to the National Wetlands Inventory, there are no wetland areas within the AASF property (EDRTM, 2019). The western portion of the AASF is in the Federal Emergency Management Agency (FEMA) 100-year flood zone. Stockton AASF lies within the Walker Slough-French Camp Slough Watershed. Storm water is diverted to storm drains inlets that are located on and around the AASF. The storm drains then discharge offsite through two discharge points into the city of the Stockton storm sewer system, which eventually leads to the San Joaquin River. The western discharge point, however, has not been observed to have any flow. Surface water that is not captured in storm drains is drained south into an adjoining ditch canal (Rydberg Creek), which is a tributary to the San Joaquin River. The closest surface water intake is along the San Joaquin River, approximately 18 miles to the northwest. Based on UCMR3 data, there were no detections of PFAS in surface water intakes for the cities of Stockton and Lathrop. The AASF has no water treatment system, and the oil water separator associated with the wash rack is connected to the Stockton sanitary sewer system (HazCon, 2017). Surface water features are presented on Figure 10-3. Climate Stockton AASF is in a semi-arid, Mediterranean climate zone characterized by warm, dry summers and mild winters. The average annual rainfall is approximately 13.95 inches, with the majority of the rainfall occurring between late fall and early spring. Summer temperatures peak in July, with an average high of 94 degrees Fahrenheit (°F) and an average low of 61 OF. Winter temperatures are lowest in December to January and range from 53 OF during the day to 37 OF at night. Prevailing wind speeds are westerly or northwesterly for nine months out of the year and southeasterly for three months out of the year (National Oceanic and Atmospheric Administration [NOAA], n.d.). Current and Future Land Use The Stockton AASF serves as a CAARNG aviation maintenance facility for helicopters. The AASF includes a maintenance hangar, various storage buildings, and related infrastructure including parking lots, aircraft parking areas, wash rack, and refueling pads. The AASF is categorized as a small-quantity hazardous waste generator because it manages a variety of hazardous materials. Operating hours are from 0700 hours to 1630 hours Monday through Friday, and the AASF is staffed by approximately 95 CAARNG personnel (HazCon, 2017). Reasonably anticipated future land use is not expected to change from the current land use described above. Areas of Interest and Conceptual Site Models PFAS-containing materials were potentially released to soil and groundwater within the boundary of Stockton AASF through fire training exercises and during AFFF storage and handling. Three AECOM QAPP Worksheet #10 Page 3 of 10