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�,. KLEINFELDER A = Number of dwelling units per acres on septic tanks (unit/acre) (estimated based on 15 lots, assuming 2 homes per lot (30 homes tots)),- divided by 30 acres) W = Annual vwastewateLAQpding per dwelling unit per acre (inches) = 2.0 inches (based on 150 gpd/residence NW = Nitrate concentration of nitrified leach field effluent (mg/1) = 35 mg/I typical (Hantzsche and Finnemore (1992)) D = Percent dentrification of leach field effluent = 0.25 percent (0.1 to 0.25 percent typical per Hantzsche and Finnemore (1992). 0.25 selected given the large depth to groundwater and the presence of interbedded low permeable clay layers.) r = Annual deep percolation of precipitation (inches) = 9.8 inches (representing 70 percent of the average annual rainfall of 14 inches in project area. Remaining 30 percent of rainfall is assumed to drain off-site into natural or manmade drainages) Nb = Background nitrate concentration of the deep percolating precipitation - mg/I = 0.5 mg/I (per Technical Memorandum, Hydrologic and Soil Investigation for the Chico Urban Area, page 6, Dames and Moore, April 1996) Our calculation is as follows: N, = 1.0 (2) (35) (1 - 0.25) + 0.5 (9.8) = 52.5 + 4.9 = 4.86 mg/I as N 1.0 (2) + 9.8 2 + 9.8 we have also used the Hantzsche mass balance equation (source: Predicting Groundwater Nitrate-Nitrogen Impacts, Hantzsche and Finnemore (1992)), which is presented below. _ I Nw)(1-D) + R(Nb) N0 - ( (I+R) Where: N, = Average calculated nitrate-nitrogen in groundwater resulting from leach field source and precipitation - mg/1, as N. I = Volume rate of waste water entering the soil averaged over the gross developed area = 4.02 inches per year calculated as follows: (63,000 23197.G021STOL057 February 14,2006 Copyright 2006 Kleinfelder, Inc. Page 2 of 4