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NOW ..Of _ August 31, 2004 NOA Project Number: E04060A The average total nitrogen concentration of the domestic waste water is normally considered to be 20-85 mg/L-N (Crites and Tchobanoglous, 1998). For the purpose of the nitrate loading estimate, the long-term average concentration for total nitrogen in waste water is estimated to be 45 mg/L-N. 7.2 Quantity and Quality of Deep Percolating Rain Water An estimate of the quantity and quality of rainfall was required for the analysis. The estimate of deep percolation of rain is based on the average monthly precipitation of rain fall and evapotranspiration for Lodi, California. The estimate first determines whether the average monthly precipitation exceeds average monthly potential evapotranspiration. If so, then the difference is potential deep percolation. This method does not consider runoff. See Plate 9 for the data and calculation. Average potential evapotranspiration was obtained from the California Irrigation Management Information System (CIMIS). Lodi is located within evapotranspiration zone _ number 12. The average rain fall for the Lodi area was obtained from the USDA (1992) Soil Survey for San Joaquin County. Based on this estimate, the average deep percolation of rain for the Lodi area is estimated to be 6.84 inches. The nitrate concentration of rain fall was estimated to be 1 mg/L-N (Hantzsche and Finnemore, 1992). 7.3 Denitrification Estimate Denitrification is an important process which occurs within the leach field whereby certain bacteria utilize the oxygen in the nitrate (NO) and release nitrogen gas (N) 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. A denitrification factor of 10 percent was used for the Site. 14