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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 2.3.6. Impact of Nitrate Loading to Nearby Water Sources The nearest surface water source is Bear Creek, located approximately 2,000 feet northwest of the site. Based on the distance of the septic system and position relative to the river, the river is not anticipated to be affected by percolating effluent. 2.4. Discussion of Nitrate Loading Results 2.4.1.Total Nitrate Impact Based on the nitrate loading calculations, nitrate levels as N for the exiting four bedroom residence and proposed ADU is 12.7 mg/L-N which is above the MCL of 10 mg/L-N. 2.4.2. Methods of Mitigation If the EHD requires mitigation for the project, mitigation may consist of the utilization of an advanced water treatment product, which are specifically designed to lower nitrates at point of discharge or other nitrate mitigation options. 3.CONCLUSIONS AND RECOMMENDATIONS Three percolation tests (B1 at 25 feet, B2 at 15 feet and B3 at 3.5 feet bgs) were conducted on the site to fulfill the requirements of the Soil Suitability Study. Soils encountered in the percolation test borings consisted of primarily silts and sandy-silt mixtures. Percolation test borings B1, B2, and B3 demonstrated unsatisfactory percolation rates of 62.5, 50, and 0, minutes per inch, respectively. Four percolation tests were previously conducted under county oversight in 2019 by Terracon. Percolation test borings B4-2019 (16 ft bgs), B5-2019 (18 ft bgs), and B6- 2019 (18 ft bgs) demonstrated unsatisfactory percolation rates of 0 minutes per inch, while B3-2019 (21 ft bgs) demonstrated a satisfactory percolation rate of 27.7 minutes per inch. One groundwater sample was collected from the on-site well and analyzed for nitrate as N and 1,2-dibromo-3-chloropropane (DBCP). Nitrate as N was reported at a concentration of 12 mg/L, which is above the California maximum contaminant level (MCL) of 10 mg/L for safe drinking water. Terracon recommends that the well be considered for closure, or water treatment options should be considered to reduce nitrates in accordance with local agency requirements. DBCP was not reported above the laboratory reporting limit (RL) of 0.010 micrograms per liter (pg/L). Groundwater analytical reports are presented in Appendix G. Facilities I Environmental I Geotechnical I Materials 19