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Table 4. Result of Nitrate Loading Calculation <br /> Variable Value Units Description <br /> Nc 13.5 mg/L-N Long-term average concentration of percolating effluent <br /> The US EPA recommended MCL for nitrate in drinking water is 10 mg/L-N. In a worst- <br /> case situation, the maximum buildup of nitrates in the aquifer from the use of septic <br /> systems on the Site is predicted to be 13.5 mg/L-N. The result of the nitrate loading <br /> estimate exceeds the maximum drinking water standard. <br /> Discussion of Nitrate Loading Results <br /> Total Nitrate Impact <br /> Based on the method and assumptions described, the result of the nitrate loading <br /> estimate indicates that the existing and proposed development on the Site may cause a <br /> buildup of nitrates in the aquifer above 10 mg/L-N. <br /> There are three mitigating factors that should be considered, however, when evaluating <br /> whether or not this Site is likely to impact ground water with nitrate: <br /> • The Hantzsche and Finnemore equation is a conservative method for <br /> determining potential impacts, and actual nitrate loading experienced on the Site <br /> over time may be less than predicted. The requirement by San Joaquin County <br /> Environmental Health Department that the calculation use rainfall and <br /> evapotranspiration data to determine ground-water recharge is particularly <br /> conservative, as it does not include consideration of recharge from irrigation <br /> water or surface water sources. <br /> • Depth to ground water beneath the Site is estimated to be 93 to 108 feet. San <br /> Joaquin County experienced its highest recent ground-water levels in 1983 and <br /> 1999, but no data is available for the Tracy area during these years. Even <br /> assuming a conservative depth to water of 90 feet, the vadose zone of the new <br /> septic system will be relatively deep. Significant nitrate attenuation would be <br /> anticipated over the course of this span. To maximize the vadose zone <br /> attenuation, a shallow septic system is recommended. <br /> • Based on the clayey soil type present on the Site, it is likely that adsorption of <br /> nitrate ions to clay will occur as percolating effluent travels downward through the <br /> vadose zone. In their original paper, Hantzsche and Finnemore considered <br /> developments with relatively shallow groundwater; as a result, the soil factors <br /> LOGE 1930 Page 12 <br />