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Soil Suitability / Nitrate Loading Study Irerracon JPM u Lathrop, Califomia May 24, 2022 n Terracon Project No. NA227026 As the facility is proposed to be open daily year-round, a long-term average daily flow rate was not calculated. The total maximum daily flow rate for Phase I and proposed Septic System 1 is 1,275 gpd (Section 1.3.7). 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, Table 4-14). 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 and deep soil borings, the proposed leach field is expected to be located within silty sandy; therefore, a denitrification estimate of 15 percent was selected. 2.3.5 Nitrate Loading Scenario A nitrate loading scenario was performed to determine the impact from the proposed project. 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 nitrate loading calculation for the site. A copy of the nitrate loading calculation is included in Appendix F. Table 10. Summary of Nitrate Mass Balance Equation. Maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L-N Based on the nitrate loading balance, nitrate levels for the proposed Phase I development is anticipated to increase nitrate levels to 3.3 mg/L-N for the project area which is below the MCL of 10 mg/L-N. Responsive ■ Resourceful ■ Reliable 17 Average Effluent Leachfield Net Rainfall Proposed Result (Nc)' Acres Daily Flow Concentration Denitrification Recharge Development (mglL-N) (A) (Q) (9Pd) (Nw) (d) (infyear) (R) (mg/L-N) Phase I — Septic 3.3 92.39 1,275 85 15% 5.58 System 1 Maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L-N Based on the nitrate loading balance, nitrate levels for the proposed Phase I development is anticipated to increase nitrate levels to 3.3 mg/L-N for the project area which is below the MCL of 10 mg/L-N. Responsive ■ Resourceful ■ Reliable 17