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Soil Suitability/ Nitrate Loading Study Irerracon Eagle's Nest Motorcycle School : Stockton, California July 27, 2021 m Terracon Project No. NA217040 Based on this method, the average net rainfall recharge for Stockton area is estimated to be 9.17 inches/year. The nitrate concentration of rainfall is estimated to be 1 mg/L-N (Hantzsche and Finnemore, 1992). 2.3.3 Current On -Site Sources of Nitrate The current on-site sources of nitrate include natural processes. 2.3.4 Future On -Site Sources of Nitrate Future on-site sources of nitrate include the proposed septic system and natural processes. 2.3.4.1 Quantity and Quality of Wastewater from Project The following discussion is for determining the long-term wastewater flow generated from the proposed development for the nitrate loading analysis and should not be used for septic system design purposes. Typically, use estimates for the sizing of septic systems include a peak use factor; however, as the maximum daily flow rate for the facility operations is approximately 55 gpd, it was not necessary to calculate a long-term average. The average total nitrogen concentration of domestic wastewater is normally considered to be 20 to 85 mg/L-N with 35 to 45 mg/L-N being typical (Crites and Tchobanoglous, 1998). It is estimated that commercial use will have higher concentrations of total nitrogen when compared to residential use. For the purposes of the nitrate loading estimate, it is estimated that the average concentration for total nitrogen in wastewater for the project is approximately 85 mg/L-N. 2.3.4.2 Denitrification Estimate Denitrification is an important process which occurs within the leach field whereby certain bacteria utilize the oxygen in the nitrate (NO3) and release nitrogen gas (N2) to the atmosphere. Denitrification is expressed as a percentage, and the normally accepted range is 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 are estimated to denitrify at a lower rate, about 10 to 15 percent. Based on the results of the shallow soil borings, the proposed leach field is expected to be located within silty sand. Based on depth to groundwater (Section 1.4.1) and presence of silty sandy and clayey soils, a denitrification estimate of 25 percent was selected. Responsive ■ Resourceful ■ Reliable 15