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Table 4. Result of Nitrate Loading Calculation <br /> Variable Value Units Description <br /> Nc 11.1 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 11.1 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 proposed development on the Site has the potential to cause <br /> a buildup of nitrates in the aquifer above 10 mg/L-N. <br /> In fact, the actual nitrate loading will most likely be less than calculated. Four mitigating <br /> factors should be considered when evaluating nitrate impacts from this Site: <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 103 to 104 feet. San <br /> Joaquin County experienced its highest recent ground-water levels in 1983 and <br /> 1999; during spring of those years, depth to water was approximately 70 feet <br /> below ground surface. Even assuming this conservative depth to water, the <br /> vadose zone of the new septic system will be relatively deep, and significant <br /> nitrate attenuation would be anticipated over the course of this span. <br /> • Based on the clay 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 /> included in the equation may not reflect the true nitrate reduction experienced as <br /> the effluent travels down through a larger vadose zone. <br /> LOGE 1942 Page 13 <br />