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September 20,2001 .Job Number: LG01-098 Page 9 The Hantzsche and Fennemore equation'provides "...a conservative (worst case) first approximation of ground-water nitrate-nitrogen concentration resulting from the combined effect of on-site sewage disposal systems and precipitation. This (the equation and approach) is for estimation of long-term effect(i.e., over years) on ground-water quality, and is not intended for prediction of seasonal changes." The Hantzsche and Fennemore' equation takes into account development area, soil denitrification factor, rainfall volume, rainfall nitrate concentration, the waste effluent quantity, and waste effluent nitrate concentration. The formula, assumptions and calculations used for the analysis are shown on Plates 8 and 9. The units of nitrate used in the mass balance equation are nitrate-N whose maximum drinking water contaminate level is 10 mg/l. Denitrification is an important process in septic systems that normally occurs in the leach line whereby certain bacteria utilize the oxygen in the nitrate (NO3) and release nitrogen gas (N2) to the atmosphere. Denitrification is expressed as a percentage, and the normally accepted range i is 10 percent to 35 percent. Soils with higher clay content, moist soil conditions, high pH, and organic material denitrify about 35 percent and soils with very high sand content and fast percolation rates are estimated to denitrify at a lower rate, about 10 percent to15 percent. Due to the occurrence of silty sand surface soils, the denitrification factor is estimated to be about 15 percent in_a standat-d septic system leach line. I An estimate of the quantity and quality of effluent is required for the analysis therefore the following information was used in the calculation. The deep percolation of rain was estimated to be 5.01 inches and the calculation is shown on Plate 8. The nitrate concentration of rain fall was estimated to be 0.17 mg/L-N.' Nitrates from Septic System The quantity of effluent waste water was estimated to be 5 gallons per person per da ; Table 6 resents the highest antici ate_d use for the facility. The facility will most likely not be operating at this high occupancy during the average year. The average nitrate concentration of the typical septic waste is normally considered to be`45 mg/L N.$ , $ Otis,R.J.,Boyle,W.C., 1980,Design manual,onsite wastewater treatment and disposal systems:U.S. ; Environmental Protection Agency,EPA 62511-80-012. (See page 100,table 6-1). rpEtSo O a rSO� i } h k