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h� KLEINFELDER gal/week (assuming 4,200 gal/week/lot) x 52 weeks/year x 0.1337 cubic fugal)/(30 acres x 43,560 sq ft/acre) x 12 inches/ft) N, = 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 that includes both the proposed subdivision is as follows: N, = 4.02(35)(1-0.25) + (9.8)(0.5) = 7.99 mg/I as N (4.02 + 9.8) Given the above factors, we estimate that the subdivision would require an average waste/sewage flow of almost 90,000 gal./week in order to exceed the state drinking water standard for nitrogen (N) of 10 mg/I in groundwater. Conclusion Based on the above evaluation, we conclude that at the groundwater level, the nitrate concentration of the wastewater produced by the maximum possible number of leach fields allowed should not exceed the state standards. Closing We have prepared this letter in substantial accordance with the generally accepted geotechnical engineering practice as it exists in the site area at the time of our study. No warranty is expressed or implied. 23197.G02/STOL057 February 14,2006 Copyright 2006 Kleinfelder,Inc. Page 3 of 4