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M <br /> V <br /> k KLEINFIE LDER <br /> which is resented below with an explanation for <br /> impacts, Hantzsche and Finnemore (1992)), p p <br /> p <br /> each factor and the value used in our study. The selection of parameters was based on previous <br /> studies and investigations performed by our firm and others, San Joaquin County Sewage <br /> Standards, and information provided in a report dated March 19, 1997 prepared by the Questa <br /> Engineering Corporation and authored by Mr. Hantzsche for a development in the Stockton area. <br /> Nc =AWNW 1-D f b R <br /> AW+R <br /> Where: <br /> NC = Average calculated nitrate-nitrogen in groundwater resulting from domestic sources <br /> and precipitation mg/l, as N <br /> A — Number of dwelling units per acre on septic tanks (unit/acre) = 0.625 (estimated <br /> based-on an estimated water usage of 90 gal./day, -a typical water usage for 6 <br /> employees at 15 gal/employee per the SJC PHS/EHD Sewage Standards divided by <br /> 150 gal./day to obtain an equivalent number of homes, divided by 0.96 acres)) <br /> W = Annual wastewater loading per dwelling unit per acre (inches) = 2.0 inches (per year <br /> per dwelling unit assuming a flow of 150 gal./day/dwelling unit) <br /> NW = Nitrate concentration of nitrified leachfield effluent (mg/1) = 70 mg/l typical <br /> (Hantzsche and Fir emore (1992)) <br /> D — Percent dentrification of leachfield effluent = 0.25 percent (0.1 to 0.35 percent <br /> typical per Hantzsche and Finnemore ([1992]). <br /> R = Annual deep percolation of precipitation (inches) = 7.6 inches (representing 80 <br /> percent of average annual rainfall of 9.5 inches in project area. Remaining 20 <br /> percent of rainfall is assumed to drain off-site into natural or mamnade drainages) <br /> Nb = Background nitrate nitrogen concentration of rainfall recharge at the water table = <br /> 0.5 mg/l (per Technical Memorandum, Hydrologic and Soil Investigation for the <br /> Chico Urban Area,page 6,Dames and Moore, April 1996) <br /> Our calculation is as follows: <br /> N� = 0.625 2170)(1 —0.25) +0.5(7.6) = 65.6 + 3.8 = 7.84 mg/l as N <br /> 0.625(2)+7.6 1.25 +7.6 <br /> CONCLUSIONS <br /> Based on the above evaluation, we conclude that wastewater produced by the proposed leach <br /> field should not have a nitrate as nitrogen (N) concentration at the groundwater level greater than <br /> 10 mg/l. Furthermore, the results of our percolation tests indicate that a conventional leach trench <br /> 12573/2002R320 April 16,2002 <br /> OO 2002 Kleinfelder,Inc. Page 3 of 4 <br />