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There are three mitigating factors that should be considered, however, when evaluating <br /> whether or not the 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 50 to 65 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 more conservative depth to water, the vadose zone of the new septic <br /> system will be relatively deep, and 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 /> • Because of the clay soil type present at 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 /> included in the equation may not reflect the true nitrate reduction experienced as <br /> the effluent travels down through a larger vadose zone. <br /> Based on the factors above, the actual nitrate impact to ground water beneath the Site <br /> may be considerably less than predicted. Nevertheless, for protection of the ground <br /> water it is recommended that only a shallow septic system utilizing leach lines or a filter <br /> bed be installed on the Site. Recent San Joaquin County Environmental Health <br /> Department policy has been to evaluate each project on a case-by-case basis to <br /> determine whether additional measures to mitigate nitrate should be employed. <br /> LOGE 20-01 Page 13 <br />