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• The estimated flow values discussed here should not be used for the design and sizing of the septic system. The design and sizing of the septic system should utilize the San Joaquin County Environmental Health Department's On-Site Wastewater Treatment Systems Standards (2017) which incorporate peak and surge flow factors. The estimates used in the On-Site Wastewater Treatment Systems Standards may be greater than those discussed here. Based on four bedrooms total at 140 gallons per bedroom per day, the maximum average daily flow for the Site was estimated to be 560 gallons per day (gpd). A typical residential concentration of 35 mgiL of nitrate as N was selected (Crites and Tchobanoglous, 1998). Denitrification Reductions Denitrification is a process that occurs in the septic system drain field whereby certain bacteria oxidize the nitrate (NO3) in the waste water and release nitrogen gas (N2) to the atmosphere. Due to the biologic processes in the shallow soils, nitrogen in the drain field may undergo a reduction between 10 percent to 35 percent. Soils with higher clay content, moist soil conditions, high pH, and organic material denitrify by about 35 percent, and soils with high sand content and fast percolation rates tend to denitrify at a lower rate of about 10 percent. The soil on the Site was found to be silty clay with sand. A denitrification factor of 35 • percent was selected for the Site because of the fine-grained shallow soil near the proposed new residence. Nitrate Loading Estimate A nitrate loading scenario was employed to determine the impact from the project to ground water. The scenario utilizes the variables discussed previously, which represent the estimated conditions and uses associated with this project. The following table presents a summary of the assumptions for the nitrate loading calculation for the Site, as presented in Plate 8. • LOGE 1902 Page 7