Laserfiche WebLink
January 11, 2022 <br />BGG No. G130.09B <br />Table 2 – Weighted Average Concentration Calculation <br />Use Average Daily Flow Estimated Waste <br />Water Quality <br />Weighted Average <br />Product <br />Residents <br />(full-time) <br />250 gpd 35 mg/L-N 8,750 gpd-mg/L-N <br />Totals 250 gpd 8,750 gpd-mg/L-N <br />Weighted Average Concentration: 8,750 gpd-mg/L-N / 250 gpd = 35 mg/L-N <br /> <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 (N2) 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 />20 percent was estimated for the site due to the silty and sandy soils, in accordance with SJCEHD <br />OWTS Standards 2017. <br /> <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 /> <br />Table 3 – Assumptions for Nitrate Loading Calculation <br />Variable Value Units Description <br />Q 250 gpd Effluent flow rate <br />Nw 35 mg/L-N Effluent stream <br />concentration <br />A 0.74 acres Site area <br />Nb 1.00 mg/L-N Concentration of rain <br />D 0.20 constant Denitrification factor <br />R 9.94 inches per year Deep percolation of <br />rainwater <br />W 0.9 inches per year Uniform wastewater <br />loading <br /> <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 /> <br />Table 4 – Results of Nitrate Loading Calculation <br />Variable Value Units Description <br />Nr 9.467 mg/L-N Estimated long-term <br />average concentration <br />of percolating effluent <br />