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A 1 1 A1.,1UVM1N 1 1 <br /> Recommended Alternate Nitrate Loading Analysis <br /> For Heavily Developed/Impervious Site Conditions <br /> 1. Formula. Calculate the resultant n,value (weighted average percolate NO3-N concentration) <br /> as in the Hantzsche-Finnemore Equation, adjusted as explained below. Note that the key <br /> difference is expressing the recharge factors (wastewater and rainfall percolation)in acre-feet <br /> per year AFY), rather than inches per year averaged over the development-recharge area <br /> (highlighted). <br /> n, _ Wn,,(1—d)+Rnb <br /> (W+R) <br /> where: n, =resultant average NO3-N concentration in recharge water,mg-N/L <br /> W = average annual volume of wastewater entering the soil, acre-ft/yr(AFY) <br /> n,,, =total nitrogen concentration of wastewater,mg-N/L <br /> d =fraction of NO3-N loss due to denitrification in the soil <br /> R = average annual volume of rainfall recharge contributed by the project site <br /> area,AFY <br /> nb = background NO3-N concentration of rainfall recharge reaching the water <br /> table due to percolation through the native soils(exclusive of development <br /> effects). <br /> 2. Data and Assumptions. The input data and assumptions for the various factors are the same <br /> as per the basic Hantzsche-Finnemore_ Equation, except for converting W and R values to <br /> annual volumes (AFY). This is done simply for W by projecting the total annual wastewater <br /> discharge and converting from gallons/year to AFY. <br /> Developing an estimate for R is a more involved water balance procedure that should be <br /> well-documented in the supporting analysis. For a heavily developed/impervious site with an <br /> adjacent retention basin(as proposed), it would be reasonable to include the retention basin <br /> as part of the site recharge area, using an approach such as the following: <br /> a) Estimate the average annual volume of runoff drained from the building site into the <br /> dedicated retention basin. This would include all rainfall (average year conditions), less <br /> the amounts: (1) retained and evaporated from paved surfaces; and(2) absorbed and <br /> percolated in the pervious areas such as leachfield and landscaping around the buildings. <br /> A-1 <br />