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' Performa Inc. • • May 7,2003 Page 13 Based on this estimate, the average deep percolation of rain for the Stockton area is estimated to be 5.01 inches. Please refer to Plate 3 for details regarding the deep percolation of rain. The nitrate concentration of rain fall was estimated to be 1 mg/L-N whose maximum contaminate limit for drinking water is 10 mg/L-N. 6.4 Denitrification Estimate Denitrification is an important process which occurs within e leach field wherebv certain bacteria utilize the oxygen in the nitrate(NO3) and release nitrogen gas(NZ)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. Due to the sandy soils which exist on the site,the denitrification factor is estimated to be about 10 percent for standard septic system effluent. ' We understand from Orenco Systems Incorporated(OSI) that when the effluent has passed ' through a sewage treatment system, leach field denitrification is not as effective due to the loss of available tri in the effluent. Therefore the demtnfication factor of treated wastewater disposed in the leacld_is.estimated to be 5%for sandy soils. ' 6.5 Nitrate Loading Scenarios ' Three nitrate loading scenarios were performed to measure the impact to ground water of both standard septic system effluent and to measure the impact of using a waste water treatment system at the site. For each scenario,the previously described assumptions were unitized in the ' Hantzshe and Finnemore equation. The variable Nw (total nitrogen concentration of waste stream)was adjusted in two scenarios to reflect a possible range of total nitrogen removal using an Orenco Systems Inc. Advantex waste water treatment system. Leach field denitrification(d) is less effective for treated waste water therefore denitrification was assumed to be 5% for the Advantex scenarios. Considering that 10 mg/L-N nitrate from the effluent is very difficult to achieve and a greatly dependent on influent concentrations of waste water,we believe that it is much more realistic to expect 50%to 75%total nitrogen reduction(Nw) in the nitrate loading scenarios. ' The OSI AdvanTex treatment system was designed to reduce BOD, TSS, and nitrate of domestic and commercial waste water. We understand that the AdvanTex system regularly reduces BOD5 and TSS to below 10 mg/L and reduces total nitrogen typically 50%, sometimes ups to 75+%. The actual percent of total nitrogen reduction will be effected by the alkalinity of the water, loading rate, and the circulation settings of the system. Othei comparable waste water treatment systems achieve similar results. a45�W