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Nft." ..i August 3, 2004 NOA Project Number: E04081A 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. A'17.4 Agricultural Contribution Mr. Brad Peterson was interviewed at on July 27, 2004 regarding the agricultural application of fertilizer on the Site. Mr. Peterson stated that he and his father run their own fertilizer program and that nutrients are applied to the vines through the drip irrigation system. The young grapevines on the Site are watered for six hours twice a week through the drip irrigation system. This works out to an application of 12 gallons of water per vine per week; there are 11,000 vines on the Site. Nutrients are applied less frequently. Mr. Peterson indicated that he believes frequent shallow waterings with low concentrations of nutrients should result in the most absorption of nutrients with the least waste. 13