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January 21, 2021 <br />BGG Job No. G130.09 <br />Denitrification Reductions <br />Denitrification is a process that occurs in the septic system drain field whereby certain bacteria <br />oxidize the nitrate (NO3) in the wastewater and release nitrogen gas (Na) to the atmosphere. Due to <br />the biologic processes in the shallow soils, nitrogen in the drain field may undergo a reduction <br />between 10 percent to 25 percent. Soils with higher clay content, moist soil conditions, high pH, and <br />organic material denitrify by about 25 percent, and soils with high sand content and fast percolation <br />rates tend to denitrify at a lower rate of about 10 percent. A typical denitrification factor of <br />25 percent was estimated for the site due to the presence of clay soils at the site, in accordance with <br />SJCEHD OWTS Standards 2017. <br />Nitrate Loading Estimate <br />A nitrate loading scenario was employed to determine the impact from the proposed residence to <br />groundwater. The scenario utilizes the variables discussed previously, which represent the estimated <br />conditions and uses associated with this project. The following table presents a summary of the <br />assumptions for the nitrate loading calculation for the site. <br />Table 3 — Assumptions for Nitrate Loading Calculation <br />Variable <br />Value <br />Units <br />Description <br />Q <br />200 <br />gpd <br />Effluent flow rate <br />NW <br />35 <br />mg/L-N <br />Effluent stream <br />concentration <br />A <br />0.57 <br />acres <br />Site area <br />Nb <br />1.00 <br />m /L -N <br />Concentration of rain <br />D <br />0.25 <br />constant <br />Denitrification factor <br />R <br />10.3 <br />inches per year <br />Deep percolation of <br />rainwater <br />W <br />0.9 <br />inches per year <br />Uniform wastewater <br />loading <br />The details of the nitrate loading calculation are included in Plate 3. The results of the calculation are <br />presented in the following table. <br />Table 4 — Results of Nitrate Loading Calculation <br />Variable Value Units <br />Description <br />Nr 8.9 mg/L-N <br />Estimated long-term <br />average concentration <br />of percolating effluent <br />The US EPA recommended MCL for nitrate in drinking water is 10 mg/L-N. The expected worst- <br />case scenario would result in a calculated maximum buildup of nitrates in the aquifer of 8.9 mg/L-N <br />from the use of the proposed onsite septic system. The result of the nitrate loading estimate is below <br />the maximum drinking water standard. <br />BAEZ GEOTECHNICAL GROUP Soil Suitability Study and Nitrate Loading Study - Valdovinos Residence Page 8 <br />