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Soil Suitability / Nitrate Loading Study 11065 North Alpine Road I Stockton, California r rerracon June 30, 2026 1 Terracon Project No. NA267068 1.6.7. Discussion of Percolation Test Results Based on the 2026 percolation test results, the percolation test borings B1 (25 ft bgs), B2 (15 ft bgs), and B3 (3.5 ft bgs) demonstrated unsatisfactory percolation rates. Based on the 2019 percolation test results, the percolation test borings B4-2019 (16 ft bgs), B5-2019 (18 ft bgs), and B6-2019 (7 ft bgs) demonstrated unsatisfactory percolation rates, while B3-2019 (21 ft bgs) demonstrated a satisfactory percolation rate. According to the OWTS Standards Table 1.10.2, for percolation rates between 5 and 30 minutes per inch, a minimum of 8 feet of separation is required from the bottom of the dispersal system to groundwater. Groundwater was not encountered at a maximum explored depth of 25 feet bgs in boring B1 (2026) and is mapped between 100 to 110 feet bgs, which is likely to meet the 8-foot separation requirement from the proposed bottom of the septic leach line. 1.6.8. Discussion of Septic System Sizing In general, septic system construction should follow the San Joaquin County On-Site Wastewater Disposal Standards (OWTS) where applicable. 2. NITRATE LOADING STUDY Because of concern regarding nitrate leaching into groundwater from septic systems, a nitrate loading analysis is required by the EHD for the proposed development to determine the potential concentration of percolating effluent and rain fall over the long term. Where percolating effluent leaches into the ground, groundwater quality may be impacted to some degree should the effluent reach groundwater. The method utilized for the nitrate loading analysis is based on a simple mass balance formula, discussed in a published article by Hantzsche and Finnemore (1992), which provides a convenient and simplified method to estimate long-term groundwater nitrate impacts on an area-wide basis. Hantzsche and Finnemore found that over a long period of time, water quality in the upper saturated groundwater zone closely approximates the quality of percolating recharge waters. They believe that their method is a good planning tool and provides useful information to planners and local officials but with some real limitations. In their article published in Ground Water, Hantzsche and Finnemore mention that "...the equation ignores dispersion, lateral flow, and mixing with groundwater flow from up- gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen concentrations in groundwater to the extent that the nitrate-nitrogen concentration of groundwater flow from up-gradient areas is lower." Facilities I Environmental I Geotechnical I Materials 14