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k" KLEINFELDER N, =AWN, - + N0 r AW +r - Where: N, = Average calculated nitrate in groundwater resulting from domestic sources and precipitation —mg/l, as N A = Number of dwelling units per acres on septic tanks (unit/acre) = 0.44 (single home per lot) and 0.89 (two homes per lot) W — Annual wastewater loading per dwelling unit per acre (inches) — 2.0 inches (based on 150 gpd/residence) N,,, — Nitrate concentration of nitrified leachfield effluent (nig/1) = 45 mg/1 typical (Hantzsche and Finnemore (1992)) D = Percent dentrification of leachfield effluent = 0.35 percent (Selected given the large depth to groundwater(120.feet+) and a majority of the soils encountered consist of fine clays and silts that tend to increase denitrification) R = Annual deep percolation of precipitation (inches) = 4.0 inches (increase over the 3 inches used by Mr. Hantzsche in the Stockton area to reflect the sandy soil conditions anticipated at the percolation depths for the leach trenches) Nb = Background nitrate concentration of the deep percolating precipitation mg/1 =2.2 mg/l as N Our calculations are as follows: Single Home per Lot NC _ .44 (2) (45) (1 —0.35) +2.2 (4) = 25.7 + 8.8 — 7.07 mg/1 as N .44(2) +4 0.88 +4 Two Homes per Lot 1 N, .89 (2) (45) (1 —0.35) +2.2 (4) = 52.1 + 8-8 10.5 mg/1 as N .89(2) +4 1.78 -A 1 3 One of the factors that may be required by the SJCPHS is to detennine the existing value of nitrate in the groundwater below the site. Based on our previous discussions with Mr. Huggins 3 of SJCPHS, we have been advised to use a background nitrate (NO3) concentration of 10 mg/1 in the groundwater. While Mr. Huggins has not provided a background level of nitrate as N in the groundwater, based on previous testing by our firm the value of nitrate as N compared to NO3 is usually about 25 percent or less. In other words, if 10 mg/1 is expected for NO3, the initial or background level of nitrate as N might be expected to be on the order of 2.5 mg/l or less. Accordingly, the total nitrate concentration determined in groundwater from the leach fields from one and two homes per lot are 9.6 and 13 mg/1, respectively. The Hantzsche equation has several limitations when applied to this project. The equation does not take into account the large depth to groundwater or the 120 feet of interlayered silts and clays underlying that site. Both factors can contribute significantly to the denitrification of the waste water. Using Designated Level Methodology (DLM) in accordance with the California Regional 20-4476-01.G02/201 1 Ri 242 Page 5 of 6 ©2001 Kleinfelder,Inc. October,11,2001 j