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• The nitrate concentration of rainfall was estimated to be 1 mg/L-N (Hantzsche and Finnemore, 1992). Quantity and Quality of Domestic Wastewater Mr. Chashmawala proposes to construct two fourplexes on the Site; according to Mr. Gilman, each will fourplex consist of three units with three bedrooms and 2.5 bathrooms and one unit with three bedrooms and three bathrooms. The estimated sewage flow rates for various uses are available from a number of references. The uses presented below were guided by the San Joaquin County On-Site Wastewater Disposal Standards (2003), Crites and Tchobanoglous (1998), and US EPA (1980). We estimate a maximum use of 120 gallons per day per bedroom for the fourplexes. 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 Disposal Standards which incorporate peak and surge flow factors. The estimates used in the On-Site Wastewater Disposal Standards may be greater than those discussed here. The maximum average daily flow for the Site was estimated to be 2,880 gallons per day (gpd). A typical residential concentration of 35 mg/L of nitrate as N is 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. A denitrification factor of 30 percent was selected for the Site because of the fine- grained nature of the shallow soil at the proposed septic system location. 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 11. • LOGE 1623 Page 9