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k" KLEINFELDER NITRATE LOADING CALCULATIONS For new developments, the San Joaquin County Public Health Services Department (SJCPHS) of the Environmental Health Division requires a study to evaluate if discharge from an on-site sewage disposal system at the property could load or increase the nitrate levels in groundwater above the state drinking water MCL standard. As you may be aware, the drinking water MCL standard for nitrate as NO3 is 45 mg/1 and for nitrate-nitrogen as N is 10 mg/1. For the purpose of our evaluation, we have relied upon the application of an equation intended to predict the level of nitrate in groundwater often referred to as the Hantzsche mass balance equation' that is presented below with an explanation for each factor and the values used in our study. N� =AWNW(1-D) + (Nb) AW+r Where: Nc = Average calculated nitrate-nitrogen (N) in groundwater resulting from domestic sources and precipitation—mg/l, as N A = Number of dwelling units per acre on septic tanks (unit/acre) = 0.30 W = Annual wastewater loading per dwelling unit per acre (inches) = 1.75 inches (based on 80 gallons/day) N,,, = Nitrate concentration of nitrified leachfield effluent (mg/1) = 35 to 45 mg/l typical (Hantzsche and Finnemore (1992)) D = Percent dentrification of leachfield effluent = 0.1 to 0.25 percent typical per Hantzsche and Finnemore (1992). 0.1 selected given the sand soil conditions. r = Annual deep percolation of precipitation (inches) = 3 inches (used by Mr. Hantzsche in the Stockton area to reflect the sandy soil conditions anticipated at percolation depths for the leach trenches) Nb = Background nitrate-nitrogen concentration (N) of rainfall recharge at the water table — mg/1 = 1.0 mg/l (per Hantzsche and Finnemore (1992) the nitrate-nitrogen loading, Nb, typically falls in the range of 0.5 to 1.0 mg/0 For the purpose of our analysis, we have assumed that each of the four employees will generate approximately 20 gallons per person per day of wastewater (80 gallons total or approximately equal to one-half a residence). The property encompasses approximately 1.68 acres; therefore the `A' factor would equal 0.5 dwelling units divided by 1.68 acres or 0.30. Our calculation is as follows: Low Estimate N, = 0.3(1.75)(35)(1 —0.1)+1.0(3) = 16.5 +3.0 = 5.52 mg/l as 0.3(1.75)+ 3 0.53+3 N ' Hantzsche N., Finnemore J. (1992), Predicting Ground-Water Nitrate Nitrogen Impacts, Ground Water, Vol. 30, No.4 20-4630-01.G01/2011R986 Page 3 of 5 Copyright 2001 Kleinfelder,Inc. August 16,2001