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-.Poe <br /> June 11,2003 <br /> Job Number:LG03-137 <br /> Page 9 <br /> 6.3' Quantity and Quality of Rain Water <br /> An estimate of the quantity and quality of rainfall was required for the analysis. The estimate of <br /> deep percolation of'rain is based on the average monthly precipitation of rain fall and <br /> evapotranspiration for Tracy, California. The estimate first determines whether the average <br /> monthly precipitation exceeds average monthly potential ET. If so, then the difference is <br /> potential deep percolation. This method does not.consider runoff. See Plate.No. 4 for the data <br /> and calculation. <br /> Average potential evapotranspiration was obtained from the California Irrigation Management <br /> Information System (CIMIS).- Tracy is located within evapotranspiration zone number 12. The <br /> average rain fall for the Tracy area,was obtained from-the USDA (1992)Soil Survey for San <br /> Joaquin County.. <br /> Based on this estimate;the average deep percolation of rain for the Tracy area is estimated to be <br /> 1.07 inches. Please refer to Plate'4 for details regarding the deep percolation of rain. <br /> The nitrate concentration of rain fall was estimated to be 1 mg/L-N whose maximum <br /> contaminate limit for drinking water is 10 mg/L-N. <br /> 6.4 Denitrification Estimate <br /> Denitrification is an important process which occurs within the leach field whereby certain <br /> bacteria utilize.the oxygen in the nitrate (NO3) and release nitrogen gas(N ) to the atmosphere: <br /> i P <br /> Denitrification is expressed as a percentage, and the normally accepted range is 10.percent to 35 <br /> percent. Soils with higher clay content,moist soil conditions, high pH, and organic material <br /> ' denitrify by'about 35 percent and soils with high sand content and fast percolation rates are . <br /> estimated to denitrify at a lower rate, about 10 percent to 15 percent. Due to.the presence of the <br /> dark colored.clayey,soils which exist on the site;the denitrification factor is.estimated to be about <br /> 35 percent for standard septic system effluent. <br /> 6,5 Nitrate Loading Scenario <br /> One nitrate loading scenario was performed to measure the impact to ground water utilizing a <br /> standard type septic system at the site. The previously described assumptions were unitized in - <br />` ` the Hantzshe and Finnemore equation. The variable Nw(totalnitrogen concentration of waste <br />