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The US EPA recommended Maximum Contaminant Level (MCL) for nitrate in drinking <br /> water is 10 mg/L-N. In a worst-case situation, the maximum buildup of nitrates in the <br /> aquifer from the use of on-site septic systems on the Site is predicted to be 9.7 mg/L-N <br /> for a four-bedroom house, 10.7 mg/L-N for a five-bedroom house, or 11.6 mg/L-N for a <br /> six-bedroom house. <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 regional recharge from <br /> irrigation water or surface water sources. <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. The soil factors discussed in the Hantzsche and Finnemore paper <br /> may not reflect the true nitrate reduction experienced as the effluent travels down <br /> through the vadose zone. <br /> • According to the San Joaquin County map reviewed, several wells in the area <br /> have been tested for nitrate; none are exhibiting nitrate levels above 10 mg/L-N. <br /> Nitrate was detected in the on-site domestic well at a concentration of 4.1 mg/L- <br /> N. The test results suggest that residential development in the area has not <br /> adversely impacted water quality. <br /> Based on the factors discussed above, the actual nitrate impact to the ground water <br /> may be less than predicted. 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 20-21 Page 10 <br />