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Soil Suitability/ Nitrate Loading Study Addendum Irerracon <br /> Arriaga Property Stockton, California <br /> August 31, 2020 Terracon Project No. NA207046 <br /> 1.6.8 Discussion of Septic System Sizing <br /> In general, septic system construction should follow the San Joaquin County On-Site Wastewater <br /> Treatment Standards (OWTS) where applicable. <br /> 2.0 NITRATE LOADING STUDY ADDENDUM <br /> Terracon previously prepared a Nitrate Loading Study (NLS)for the Arriaga property, dated April <br /> 26, 2019 (Terracon Project No. NA187059). In February 2020, the EHD approved a revised <br /> calculation method for long term average discharge of waste water and recommended 100 <br /> gallons for the first bedroom and 50 gallons for each additional bedroom. The EHD additionally <br /> approved an adjustment to the evapotranspiration (ET) rate based on net rainfall recharge <br /> calculation utilizing University of California (UC) plant factors (PF) for established landscape <br /> plants. The below sections are an addendum to Terracon's 2019 NLS which reflect the revised <br /> calculation methods. <br /> 2.1 Chemical and Physical Properties of Soil <br /> Refer to Terracon's prior NLS report dated April 26, 2019 (Terracon Project No. NA187059). <br /> 2.2 Groundwater Analysis <br /> Nitrate as N (5.9 mg/L)was identified in a groundwater sample collected from the onsite domestic <br /> well below the MCL of 10 mg/L-N. A second groundwater sample reported below laboratory <br /> detection limits for DBCP. Refer to Terracon's prior NLS report dated April 26, 2019 (Terracon <br /> Project No. NA187059)for further discussion. <br /> 2.3 Sources and Impact of Nitrate to Groundwater <br /> 2.3.1 Current Off-Site Sources of Nitrate <br /> Agriculture land use and septic systems in the area may be the most significant sources of nitrate <br /> in groundwater. Septic tank use in the vicinity is discussed in Section 1.3.5. <br /> 2.3.2 Contribution of Rainfall <br /> As indicated in our prior 2019 NLS report (Terracon Project No. NA187059), the average deep <br /> percolation of rain for Stockton area was estimated to be 2.64 inches/year. <br /> In February 2020, the EHD approved an adjustment to the evapotranspiration (ET) rate based on <br /> net rainfall recharge calculation utilizing University of California (UC) plant factors (PF) for <br /> established landscape plants. Terracon calculated the ET adjustment factor(actual ET) based on <br /> Responsive ■ Resourceful ■ Reliable 12 <br />