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�.J May 25, 2004 ` NOA Project Number: E04061A Page 15 The Hantzsche and Fennemore approach is "...a conservative (worst case) first approximation of ground-water nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation. This (the equation and approach) is for estimation of long-term effect(i.e., over years)on ground-water quality, and is not intended for prediction of seasonal changes" (Hantzsche and Finnemore, 1992). The method takes into account development area, soil denitrification factor, rainfall volume, rainfall nitrate concentration,the waste effluent quantity, and waste effluent nitrate concentration. r The formula, assumptions and calculations used for the analysis are shown on Plates 9 through 37. The units of nitrate used in the mass balance equation are nitrate-N whose maximum drinking water contaminant level is 10 mg/L. 7.1 Quantity of Quality of Domestic Wastewater From Proposed Project 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 35 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 Stockton, California,the closest location to the site for which data was readily available. The estimate first determines whether the average monthly precipitation ` exceeds average monthly potential ET. If so,then the difference is potential deep percolation. This method does not consider runoff. See Plate 8 for the data and calculation. Average potential evapotranspiration was obtained from the California Irrigation Management Information System(CIMIS). Farmington and Stockton are located within evapotranspiration zone number 12. The average rainfall for the Stockton area was used for the site and was .. obtained from the USDA (1992) Soil Survey for San Joaquin County. Based on this estimate, the average deep percolation of rain for the Farmington area is estimated to be 3.38 inches. The nitrate concentration of rain fall was estimated to be 1 mg/L-N,whose maximum contaminant limit for drinking water is 10 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 (NO3) and release nitrogen gas (N2) to the atmosphere. L