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Ms.Amber Ginorio,RWQCB January 30,2023 Addendum to Sampling and Analysis Plan 2478 Wilcox Road,Stockton,California Page 2 findings that were recently discovered, and represents the basis for modifying specific portions of the SAP's scope of work. Details associated with activities leading to these findings, along with the results of the septic system evaluation (lab results, diagrams, photos,and video footage,etc.)and the subsequent soil and groundwater investigation,will be presented in the Site Investigation Report. Preliminary Findings_ of Septic System • The septic tank receives waste from both SPI and Formurex facilities (based on the plumber's report detailing the tracing of lateral piping from facility lavatories to the septic tank). • The laboratory results from aqueous samples collected from the septic tank showed minimal concentrations of constituents analyzed. • The septic tank appears to be structurally sound, having no noticeable cracks or separations along construction seams (based on visual inspection, including down- hole camera following removal of septic tank waste and pressure washing tank walls and floor). • Five vertical seepage pits have been identified immediately adjacent to the septic tank (Figure 3). The diameter of each pit (likely containing backfilled drain rock) is currently unknown. • The total depth of the seepage pits ranges from 17 to 29 feet below ground surface (bgs), each containing 4-inch diameter perforated high density plastic pipe extending vertically. Each pit is laterally connected by 4-inch diameter PVC piping to adjacent pit laterals, all of which appear to tie into the lateral running directly from the septic tank outlet to seepage pit #1, located approximately 7 feet south- southeast of the septic tank(see Figure 3). • The lateral pipe from the septic tank to seepage pit #1 appears to be damaged (location shown on Figure 3), allowing surrounding backfill material (3/4-inch gravel)to enter the pipe(observed via down-hole camera). Outflow from the septic tank does not travel past the damaged section of pipe, and therefore, does not reach any of the seepage pits (based on injecting water at approximately 5 gallons per minute [gpm] into the tank outlet lateral, and not observing any flow or evidence of water in the lateral on the opposite side of the damaged section). • The lateral pipe connecting seepage pit #2 to the lateral running between seepage pit#1 and the septic tank outlet is perforated(observed via down-hole camera). T:\Stockton Propeller\Addendum to SAP\SM_Addendum to Sampling and Analysis Plan.docx STRATUS