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f <br /> File No. 20-2921-01 � �` j k'q <br /> KLEINFELDER <br /> March 25, 1992 <br /> and incorporated into septic bio mass. To be conservative <br /> and somewhat in line with the lower limits of published <br /> information, we have used in our study a Nw value of 30 mg/1. <br /> Percent Denitrification of Leach Field Effluent <br /> According to Reference No. 4, denitrifying bacteria requires <br /> a source of carbon for energy. Therefore, denitrification <br /> usually occurs where organic matter is abundant, such as <br /> directly beneath leach lines or sump pits. In the study by <br /> the CRWQCB, deep, sandy and well-drained soils with low <br /> organic matter content were described as having negligible <br /> denitrification potential. Sandy loam soils are described as <br /> having a medium denitrification potential (10 to 20 percent <br /> N loss) while finer textured soil such as silts and clays <br /> have a high potential for denitrification (20 to 40 percent <br /> N loss) . Based on numerous test borings drilled for our <br /> geotechnical investigations performed for the Morada Meadows <br /> subdivision and for many other projects in the northeast <br /> Stockton area, fine-grained soils are predominate beneath the <br /> site area. For this reason, we have used for D a value of 40 <br /> percent. <br /> Annual Deep Percolation of Precipitation <br /> According to the National Weather Service, the 30-year <br /> rainfall normal for this area is on the order of 13-1/2 <br /> inches. We note that in the "Hantzche" equation, the greater <br /> the r the lower the Nc. For this reason we have <br /> conservatively neglected any landscape watering or farm <br /> irrigation that would increase the annual amount of deep <br /> Percolation. We have also assumed in our analysis an <br /> evapotransporation rate of approximately 4 to 5 inches. With <br /> these assumptions, we have used an annual deep percolation <br /> rate of 8 inches in our analysis. <br /> Background Nitrate Concentration of the Deep Percolating <br /> Precipitation <br /> Very little information is available .,on this subject in the <br /> referenced reports. In the Chico area study, the CRWQCB used <br /> a value of 2 . 2 mg/l reflecting primarily the fact that <br /> rainwater is relatively clean. Since surface irrigation was <br /> neglected in the estimate for r, it was felt that the effect <br /> of the higher nitrate concentration in surface waters on the <br /> determination of Nb should also be neglected. Therefore, we <br /> have used a value of 2. 2 mg/1 for Nb. <br /> Equation Calculation <br /> With the above factors, the calculated nitrate concentration <br /> in groundwater resulting from domestic sources and <br /> Copyright 1992 Kleinfelder, Inc. Page 6 of 9 <br />