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Soil Suitability / Nitrate Loading Study Thor Property m Stockton, CA Irerracon July 19, 2018 m Terracon Project No. NA187029 Based on a review of Table 5, the estimated potential sewage flow rate for the proposed four bedroom residences is 560-gallons per day. 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 35 mg/L-N over the long term. 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 percent to 15 percent. Based on the results of the shallow and deep soil borings, the proposed new leach field is expected to be located within lean clay below two feet bgs. Based on the clay soils encountered during our borings and anticipated percolation rate, a denitrification estimate of 35 percent is anticipated. 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 6. Summary of Nitrate Mass Balance Equation. Result Average Effluent Leachfield Deep Land Use (Nc)* Acres (A) Daily Flow Concentration Denitrification Percolation (mg/L-N) (Q) (gpd) (Nw) (d) of Rainfall (mg/L-N) (R) (in) Proposed 20.4 0.27 560 35 35% 3.30 residence "Maximum contaminant level(MCL)for nitrate in drinking water is 10 mg/L-N. Responsive ■ Resourceful ■ Reliable 15