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Where: Nr= Resultant average concentration of nitrate-nitrogen in recharge water- (ppm NO3-N) I = Volume rate of wastewater entering the soil averaged over the gross developed area in inches per yr Nw = Total nitrogen concentration of wastewater in mg NIL d = Fraction (%) of nitrate-nitrogen loss due to denitrification in the soil R = Average recharge rate of rainfall - inches per yr Nb= Background nitrate-nitrogen concentration of rainfall recharge, exclusive of wastewater influences (ppm NO3-N) Values and Assumptions I= 675 als/d x 260 work d/ —175,500 gals =total treated WW/31= 23,463 cu ft= 0.86 in. 7.53 Ac x 43,560 sq ft/Ac = 328,007 sq ft Nw = 10 mg max NIL d = Assume 40% from shallow leachiines since effluent must pass through --6.5 ft clay soil R = 10 in/yr. But 80% of total area covered w/roofs and asphalt 2.0 in/yr. Nb= Assumed to be 0.1 ppm NO3-N 0.86 in/yr(10 mg NIL 1 - 0.40 + 2.0 in/ x 0.1 mg/L) Nr = - (0.86 in/yr+ 2.0 in/yr) Nr = 1.9 ppm as nitrate-nitrogen (NO3-N) or 8.4 ppm as nitrate (NO3) In addition to this calculated reduction in nitrate-nitrogen loading, further decrease in loading should occur from leachfield landscaping. Since the effluent that will be used for subsurface landscape irrigation will have undergone nitrogen treatment to the extent estimated above (10 mg N/L max), uptake by plants will be based on this concentration. Most plants utilize only two forms of nitrogen: nitrate and ammonium (NH4). Approximately 75% of conventionally treated septic effluent is ammonium. However, the proposed treatment system will discharge no greater than 10 ppm nitrate-nitrogen or 45 ppm nitrate, so the ammonium fraction is unknown without quantitative analysis. Therefore,percentages of the maximum nitrate concentration in the effluent will be correlated with the amount estimated to be synthesized by landscape plants: For six months out of each year(April-September), it is theorized the plants will utilize 50% of available nitrate molecules emanating from the treated septic effluent (the remaining 50% is lost to denitrification, immobilization and leaching). The colder months of the year, the plants may synthesize 20% available nitrate. Consequently, an additional 35% (50% + 20%-2 = 35%) reduction in nitrate concentrations may occur, above the anticipated wastewater treatment: 1.9 ppm nitrate-nitrogen - 35% = 1.2 ppm NO3-N or 5.6 ppm NO, potentially entering groundwater. Chesney Consulting