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w <br /> D. CALCULATED IMPACT FROM NITRATE-NITROGEN LOADING <br /> The calculated (or estimated) nitrate impact to the soil and ultimately to the groundwater is <br /> extremely difficult to detennine due to the number of variables involved. It is assumed that <br /> traditional septic systems will be installed for each residential unit. Consequently. the following <br /> calculations are based on using a conventional or traditional type of septic system consisting of a <br /> septic tank, leachfield and seepage pits. <br /> HANTZSCHEIFINNEMORE E CATION <br /> IN,,,(1 - d) + R Nh <br /> Nr = (I + R) <br /> Where: <br /> Nr Resultant average concentration of nitrate-nitrogen in recharge water - (ppm NO;-N) <br /> I = Volume rate of wastewater entering the soil averaged over the gross developed area in <br /> inches per yr <br /> Nw— Total nitrogen concentration of wastewater in N mg/L (66 mg N/L/day) <br /> d = Fraction (%) of nitrate-nitrogen loss due to denitrification in the soil <br /> R = Average recharge rate of rainfall - inches per yr (13) <br /> Nh = Background nitrate-nitrogen concentration of rainfall recharge, exclusive of wastewater <br /> influences (ppm NO3-N) <br /> Values and Assumptions <br /> 68 gals/person/day x 4 people per household x 4 new houses/20 Ac x._365 d/yr= 0.73 in/yr. <br /> 1 = 43,560 cu ft/Ac ft x 7.48 gal/cu ft x (1 ft/12 in) <br /> Nw= 66 mg/L - 20% reduction from typ. septic tank = 53 mg/L N <br /> d = 10% from biomat and 15% from saturated soil conditions under leachlines and seepage pits. <br /> Nh = Assumed to be 0.1 ppm NO,-N <br /> 0.73 in/yr 53 mg N/L (1 - 0.25) + (13 inlyr x 0.1 mg/L) <br /> Nr — (0.73 in/yr+ 13 in/yr) <br /> Nr = 2.2 ppm as NO3-N or 9.8 ppm as NO3 <br /> 7 <br />