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6 <br /> ` rFNb <br /> = Fraction(%)of nitrate-nitrogen loss due to denitrification in the soil <br /> = Average recharge rate of rainfall - inches per yr(11) <br /> = Background nitrate-nitrogen concentration of rainfall recharge, exclusive 7wa7stewater <br /> influences (ppm NO3-N) <br /> Values and Assum=tions <br /> 68 gals/person/day x 4 people per household x 8 new houses/17 4 Ac x 365 d/yr= 1.7 in. <br /> I= 43,560 cu ft/Ac ft x 7.48 gat/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. <br /> Nb= Assumed to be 0.1 ppm NO3-N <br /> 1.7 in/yr(53 mg N/L) LL--0.-25)–+-(11 in/yr x 0.1 my—/L) <br /> Nr = (1.7 in/yr+ 11 in/yr) <br /> Nr = 5.4 ppm as NO3-N or 24.1 ppm as NO3 <br /> LARK METHOD <br /> Total wastewater nitrogen loading from project: <br /> 66 mg N/L/person x 68 gals/day(257 L/d) x 4 persons/house x 8 houses x 365 d/yr= <br /> 198 kg N/yr=437 lbs N/yr <br /> The following reductions in the nitrate-nitrogen loading from above are assumed, according to <br /> this method: <br /> Septic tanks remove 20%N x 437 lbs=350 lbs <br /> Biomat formation removes 10% x 350 lbs= 315 lbs <br /> Denitrification in soil removes 15%x 315 lbs=268 lbs <br /> 268 lbs N/yr(122 kg N/yr)total nitrogen load. <br /> Determination of total theoretical nitrogen loading to the underlying groundwater: <br /> Assume 122 kg N/yr x 1,000 g/kg 365 d/yr= <br /> 334 grams NO3-N/day - assuming complete oxidation of N to nitrate-nitrogen. <br /> Assumptions: <br /> 1. No rainfall contribution <br /> 2. Average, total (includes irrigation)daily water use per residence= 1,000 gal/day <br /> 3. Groundwater gradient approximately 0.00063 ft/ft(0.63 ft fall/1000 ft) <br /> 4. Majority of property is a rectangle with groundwater flowing north: <br /> Parcels 1-7= 1329 ft x 506 ft= 672,474 ft' Parcel 8 =290 ft x 300 ft= 87,000 ft' <br /> 9 <br /> _ Va!!ey Ag Research <br />