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September 20,2002 Job Number: LG02-261 Page 8 their approach is a good planning tool and provides useful information to planners and local officials but with some real limitations. In Hantzsche and Fennemore's article, published in Ground Water, they mention that "...the equation ignores dispersion, lateral flow, and mixing with ground water flow from up-gradient areas. These processes would generally contribute to additional reduction of nitrate-nitrogen concentrations in ground water to the extent that the �- nitrate-nitrogen concentration of ground-water flow from up-gradient areas is lower." 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, 9 and 10. The units of nitrate used in the mass balance equation are nitrate-N whose maximum drinking water contaminate level is 10 mg/l. Nitrates from the Septic System and Rain Fall The total average nitrate concentration of the typical septic waste is normally considered - to be 45 mg/L-N s (This concentration is not related to the maximum contaminate level for drinking water of 45 mg/L-NO3). The average daily flow of influent into the septic systems on the site was determined by assigning a flow rate of seventy-five (75) gallons per day per person. The primary residence is estimated to be a three(3) bedroom home occupied by 4 people. In addition to the primary residence, a legal two (2)bedroom second dwelling unit (SDU) up to 1,500 SF may be placed on the site at some time in the future. Three (3)people were estimated to occupy the SDU should one ever be built. The following table presents the anticipated daily usage for the property. S Otis,R.J.,Boyle, W.C., 1980,Design manual, onsite wastewater treatment and disposal systems: U.S. Environmental Protection Agency,EPA 625/1-80-012. (See page 100,table 6-1). ?pf RSO > o. Z � O � <'tSOG♦