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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 23 • Indications of contamination, if any. • Other observations as needed. The following approximate depth intervals are proposed for collection of subsurface soil samples for chemical analyses in the monitoring wells, and vertical groundwater definition borings: • 1.0 ft bgs or directly beneath pavement, if present • 5.0 ft bgs • 10.0 ft bgs Samples for chemical analysis would be labeled and placed into a chilled cooler immediately following collection. Samples would be transported under chain of custody protocol to an ELAP approved laboratory for analysis. 6.3.2.2 Vertical Definition Groundwater Sampling Prior to sampling groundwater, a licensed C-57 drilling contractor(e.g.., MR Drill Co.)will advance borings to first encountered groundwater using an HSA drilling rig. Advancement of these borings are two-fold; first, they will be advanced to obtain groundwater samples for vertical definition of groundwater in or in the vicinity of PRA 1 and PRA 2 below SCK property,and second;the data collected will be used to confirm the pre-determined placement of four (4) additional monitoring wells in the vicinity of PRA 1 and PRA 2. Advancement of said borings will not take place until the second phase of investigation has commenced. Once first encountered groundwater is reached, groundwater will be sampled using the following procedures. Groundwater samples will be collected during the second phase of investigation,while advancing borings for vertical definition of groundwater using BESST INC's, SimulProbe sampler, or similar sampling device. The SimulProbe sampler obtains a groundwater sample once the desired sample depth is reached by: 1. Pressurizing the SimulProbe sampler using Nitrogen gas or a compressed air source. 2. Next,the sampler is deployed through the center of the HSA where the SimulProbe is driven into the formation.The screen of the SimulProbe is exposed by pulling back on the sampler. 3. After the screen is exposed,the pressure is slowly released,creating a vacuum which draws water from the formation, into the SimulProbe sampler. 4. Once groundwater is collected into the sampling device, the SimulProbe is pressurized again to hold groundwater in the sample chamber and is retrieved to the surface. The bottom half of the sampler is disassembled, and groundwater is collected by inserting a small piece of tubing into the bottom of the SimulProbe, allowing the water to drain into the appropriate sample container(s).