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Table 6. Inputs and Results for Nitrate Loading Calculation, <br />Recharge Derived from Reduced Rainfall Method <br />Variable Value Units Description <br />420 gpd Effluent flow rate <br />Nw 35 mg/L-N Effluent stream concentration <br />A 1.00 acres Site area <br />Nb 1.00 mg/L-N Concentration of rain <br />0.25 constant Denitrification factor <br />13.3 inches per year Recharge rate of percolating waters <br />5.6 inches per year Uniform waste water loading (calculated) <br />Nc 8.5 mg/L-N Long-term average concentration of percolating <br />effluent <br />Results of Nitrate Loading Calculations <br />For this Site, the results of the two methods are similar, 8.8 and 8.5 mg/L-N. <br />The US EPA recommended MCL for nitrate in drinking water is 10 mg/L-N. In a worst- <br />case situation, the maximum buildup of nitrates in the aquifer from the use of septic <br />systems on the Site is predicted to be 8.5 to 8.8 mg/L-N. The result of the nitrate <br />loading estimate is below the maximum drinking water standard. <br />Uncertainty <br />For either method of calculating ground-water recharge, and thus nitrate loading, the <br />calculation should be understood to include a significant amount of uncertainty. For the <br />evapotranspiration reduction method, the standard deviation of the values for deep <br />percolation of recharge waters is found to be 6.2 in. The recharge rate can be <br />expressed as 12.6 in +/- 6.2 in, which results in a recharge rate of 6.4 to 18.8 in. <br />Uncertainty exists in the other variables as well, including the waste discharge rate, <br />nitrate concentration of septic effluent, and soil denitrification factor. Nevertheless, <br />when applied consistently, the Hantzsche and Finnemore equation provides a useful <br />screening tool to identify situations where mitigation may be advisable to prevent nitrate <br />impacts. <br />LOGE 1906 Page 15