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ATTACHMENT 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 />nY = <br />Wnw(1-d)+Ri?6 <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 />nu, = 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 Wand 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 />