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KLEINFELDER �- have used the Hantzsche mass balance equation (source: Predicting Groundwater Nitrate- Nitrogen Impacts, Hantzsche and Finnemore (1992)), which is presented below with an explanation for each factor and the value used in our study. The selection of parameters was based on previous studies and investigations performed by our firm and others, San Joaquin County Sewage Standards, communication with Mr. Mike Huggins with SJCEHD, and information provided in a report dated March 19, 1997 prepared by the Questa Engineering Corporation and authored by Mr. Hantzsche for a development in the Stockton area. N,=AW&(1-D) +(_Nh) 6. AW+R Where: Ne = Average calculated nitrate-nitrogen in groundwater resulting from domestic sources and precipitation—mg/l, as N. r. -)4-- A = Number of dwelling units per acre on septic tanks (unit/acre) = 0.04 (estimated based on a maximum of 2 dwelling units per proposed parcel (8 dwelling units maximum) divided by 165.3 acres.) W = Annual wastewater loading per dwelling unit per acre (inches) =2.0 inches (per year per dwelling unit assuming a flow of 150 gal./day/dwelling unit.) Nw = Nitrate concentration of nitrified leachfield effluent(mg/1) = 35 mg/1 (Hantzsche and Finnemore (1992)). Percent denitrification selected based on past discussions with Mr. Mike Huggins of SJCEHD. D = Percent dentrification of leachfield effluent = 0.30 percent (0.1 to 0.35 percent typical per Hantzsche and Finnemore (1992). 0.30 selected given the deep depth to groundwater. R = Annual deep percolation of precipitation (inches) = 11.9 inches (representing 70 percent of average annual rainfall of approximately 17 inches in project area. Remaining 30 percent of rainfall is assumed to drain off-site into natural or manmade drainages.) Nb = Background nitrate nitrogen concentration of rainfall recharge at the water table = 0.5 mg/l (per Technical Memorandum, Hydrologic and Soil Investigation for the Chico Urban Area,page 6, Dames and Moore, April 1996) ` Our calculation is as follows: Ne = 0.04(2)(35)(1 —0.30)+0.5(11.9) = 1.96 +5.95 = .0.7 mg/1 as N 0.04(2) + 11.9 0.08 +.11.9 L. 37050.G01/ST03R1750 November 20,2003 O 2003 xleinfelder,Inc. Page 5 of 6 6.