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Addendum Revised Soil Suitability Nitrate Loading Study <br />Manteca Unified School District <br />241 4th Street, French Camp, CA <br />Page 9 <br /> <br /> <br /> <br />2.3.4.1 Denitrification Estimate <br />Denitrification is an important process which occurs within the leach field whereby certain bacteria utilize <br />the oxygen in the nitrate-NO3 and release nitrogen gas (N2) to the atmosphere. Denitrification is expressed <br />as a percentage, and the normally accepted range is 10 percent to 35 percent. Soils with higher clay content, <br />moist soil conditions, high pH, and organic material denitrify by about 35 percent and soils with high sand <br />content and fast percolation rates are estimated to denitrify at a lower rate, at about 10 percent to 15 percent. <br /> <br />A typical denitrification estimate of 35 percent was chosen based on the predominantly clay soils for the <br />Site. <br /> <br />2.3.5 Calculated Mass Balance of Nitrogen Loading Considering all Potential Sources Identified <br />A nitrate loading scenario was performed to assess the impact from the proposed project using the <br />Hantzsche and Finnemore formula/model. The scenario utilizes the variables discussed previously which <br />represent the estimated conditions and uses associated with this project. Table 4, below, presents a summary <br />of the nitrate loading calculation for the school site, which consists of multiple contiguous parcels. A copy <br />of the nitrate loading calculation sheet is included in Appendix F. <br /> <br />Table 4 <br />Summary of Nitrate Mass Balance Analysis <br />Result <br />(Nc) * <br />(mg/L-N) <br />Acres <br />(A) <br />Average Daily Flow <br />(Q) <br />(gpd) <br />Effluent <br />Concentration <br />(Nw) <br />(mg/L-N) <br />Leachfield <br />Denitrification <br />(d) <br />Deep <br />Percolation of <br />Rainfall <br />(R) <br />(inches) <br />9.9 13.91 1,509 100 35% 8.99 <br />* Maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L-N; <br />mg/L = milligrams per liter; gpd = gallons per day. <br /> <br />2.3.6 Calculated Impact of Nitrogen Loading from Site Project to Down Gradient Wells, Rivers, <br />Lakes, Etc. <br />Based on the relatively flat topography of the Site, we do not foresee percolating effluent affecting any <br />surface water resources. <br /> <br />2.4 DISCUSSION OF NITRATE LOADING RESULTS RELATIVE TO PROPOSED <br />METHOD OF WASTEWATER DISPOSAL <br />2.4.1 Total Nitrate Impact from Proposed Project <br />The nitrate MCL set by the US EPA is 45 mg/L-NO3 or 10 mg/L-N. This report concludes that the <br />modernization project is not expected to result in increased attendance at this time. The estimated usage <br />was calculated at 1,509 gallons per day effluent into the proposed septic system, based on a population of <br />644 students/faculty, at 5 gpd per person for 180 days, assuming 95% attendance. The proposed <br />development may result in an estimated nitrate load of 9.9 mg/N-L when calculated over the 13.91 acres <br />that comprise the school site, which is below the drinking water MCL of 10 mg/L-N. <br /> <br />One must consider that nitrate-NO3 concentrations in groundwater inflow from upgradient areas may also <br />increase over time from other land uses such as agriculture. Nitrate in groundwater occurs as a result of the <br />application of fertilizers, livestock waste, and untreated septic tank waste. Nitrate is mobile and often