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Soil Suitability / Nitrate Loading Study <br />ALL STAR HEAVY HAUL & TOWING INCORPORATED Tracy, California <br />July 1, 2022 Terracon Project No. NA217024 <br />2.3 Sources and Impact of Nitrate to Groundwater <br />2.3.1 Current Off -Site Sources of Nitrate <br />Irerracon <br />Agricultural 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 />An estimate of the quantity and quality of rainfall was required for the nitrate loading analysis. <br />Average precipitation and potential evapotranspiration (ETo) data was obtained from the <br />California Irrigation Management Information System (CIMIS) from the Manteca (70), and Ripon <br />(249) stations. The estimate first determines whether the average monthly precipitation exceeds <br />average monthly evapotranspiration (ET). If so, then the difference is potential rainfall recharge. <br />Due to the relatively fiat nature of the site runoff should be minimal. In February 2020, the EHD <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. Terracon calculated the ET adjustment factor (actual ET) based on the proposed <br />commercial landscape. Refer to Appendix F `v"Jater Balance Recharge Analysis for monthly <br />precipitation and ET averages. <br />Table 9. <br />Net Rainfall Recharge <br />CIMIS Station <br />Average Precipitation <br />Potential ETO <br />ET <br />Actual ET <br />Net Rainfall <br />Number / Locationear <br />inl <br />( y ) <br />(in/year) <br />Adjustment <br />(in/month) <br />Recharge <br />Factor <br />(in/year) <br />70/Manteca <br />15.46 <br />50.50 <br />0.4 <br />20.20 <br />i 8.55 <br />249/Ripon, <br />I <br />I <br />Based on this method, the average net rainfall recharge for the site is estimated to be 8.55 <br />inches/year. <br />The nitrate concentration of rainfall is estimated to be 1 mg/L-N (Hantzsche and Finnemore, <br />1992). <br />2.3.3 Current On -Site Sources of Nitrate <br />The current on-site sources of nitrate include the existing septic systems and natural processes. <br />2.3.4 Future On -Site Sources of Nitrate <br />Future on-site sources of nitrate include the existing septic systems, proposed septic systems, <br />and natural processes. <br />Responsive ■ Resourceful ■ Reliable 16 <br />