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Ms.Amber Ginorio,RWQCB January 30,2023 Addendum to Sampling and Analysis Plan 2478 Wilcox Road,Stockton,California Page 3 Proposed Modifications to SAP Scope of Work Based on the information that has been gathered to date, and discussions with RWQCB personnel, Stratus is proposing modifications to the original scope of work. The primary changes to the scope of work presented in the SAP are summarized below. • Reposition two of the three groundwater monitoring wells to new locations in order to assess the potential groundwater impacts presumedly downgradient of the septic system and to establish upgradient groundwater quality at a closer proximity to the septic system than was originally proposed. The new locations also provide better triangulation and spacing from which the groundwater flow direction and gradient can be calculated (see Figure 2). • Reduce the number of soil borings from nine(9)to six(6). Given that the structural integrity of the septic tank appears to be sound, the four soil borings proposed immediately east and west of the septic tank(two borings on either side of the tank) may not be warranted at this time. • Relocate the soil borings to focus on assessing the potential impact to soil in the area containing the seepage pits, as well as directly beneath the damaged outlet lateral running from the septic tank to pit#1 (Figures 2 and 3). • Increase the boring depths to first encountered groundwater. Given the total depth of the deepest seepage pit (approximately 29 feet bgs) and the anticipated depth to groundwater (approximately 60 feet bgs), it is important to assess the potential impact to soil within this 29 to 60-foot interval, as well as to identify the lithology and strata that could influence vertical migration. • Collect continuous cores in soil boings from 5 feet bgs to total depths (near groundwater interface) to improve identification of sediment horizons and lateral continuity. This will also aid in selecting target depths for sample collection and laboratory analysis. • Advance one soil boring (located adjacent to the damaged outlet lateral) into the first encountered water bearing zone and collecting two "grab" groundwater samples via Hydropunch (see Figure 3). The samples will be collected just below the water table surface and at approximately 15 feet below the first sample. • Include a contingency for installation of a fourth groundwater monitoring well north-northeast of the septic system, in the event that collection of a grab groundwater sample is unsuccessful (e.g., fine sediments such as silts/clays may not produce adequate volume for Hydropunch collection). T:\Stockton Propeller\Addendum to SAP\SM_Addendum to Sampling and Analysis Plan.docx STRATUS