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Soil Suitability / Nitrate Loading Study <br /> 11065 North Alpine Road I Stockton, California r Terracon <br /> June 30, 2026 1 Terracon Project No. NA267068 <br /> 2.1.2. Mass Transport Soil Properties <br /> The longer the effluent remains near the surface, the more treatment is enhanced due to <br /> natural processes within the soil. Percolation testing previously conducted by Terracon <br /> on the site has revealed that the onsite soils have adequate percolation rate of 27.7 <br /> minutes per inch (B3 2019) at 21 feet bgs. Refer to Sections 1.6.6 and 1.6.7 for <br /> additional discussion. <br /> 2.2. Groundwater Analysis <br /> 2.2.1. Regional Groundwater <br /> A discussion of regional groundwater, including depth to groundwater and seasonal <br /> trends are included in Section 1.4.1. <br /> 2.2.2. Potential Ground Water Contamination Issues <br /> A discussion of issues related to potential groundwater contamination is included in <br /> Section 1.4.2. <br /> 2.2.3. Current and Future On-Site Ground Water Uses <br /> The proposed ADU is proposed to be connected to a new septic system. <br /> 2.3. Sources and Impact of Nitrate to Groundwater <br /> 2.3.1. Current Off-Site Sources of Nitrate <br /> Agricultural land use and septic systems in the area may be the most significant sources <br /> of nitrate in groundwater. Septic tank use in the vicinity is discussed in Section 1.3.5. <br /> 2.3.2. Contribution of Rainfall <br /> An estimate of the quantity and quality of rainfall was required for the nitrate loading <br /> analysis. Average precipitation and potential evapotranspiration (ETo) data was obtained <br /> from the California Irrigation Management Information System (CIMIS) from the Linden <br /> (262) station. The estimate first determines whether the average monthly precipitation <br /> exceeds average monthly evapotranspiration (ET). If so, then the difference is potential <br /> rainfall recharge. Due to the relatively flat nature of the site, runoff should be minimal. <br /> In February 2020, the EHD approved an adjustment to the evapotranspiration (ET) rate <br /> based on net rainfall recharge calculation utilizing University of California (UC) plant <br /> factors (PF) for established landscape plants. Terracon calculated the ET adjustment <br /> factor (actual ET) based on the proposed commercial and agricultural landscape. Refer <br /> to Appendix F Water Balance Recharge Analysis for monthly precipitation and ET <br /> averages. <br /> Facilities I Environmental I Geotechnical I Materials 16 <br />