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LOGE 1832b: De Casas NLS Addendum Page 4 April 17, 2020 For this Site, the long-term average concentration of percolating effluent is found to be 12.0 mg/L-N when calculated as shown. This value exceeds the US EPA recommended maximum contaminant level (MCL) for nitrate in drinking water of 10 mg/L-N. There are several mitigating factors that should be considered, however, when evaluating whether or not this Site is likely to impact ground water with nitrate: • The Hantzsche and Finnemore equation is a conservative method for determining potential impacts, and actual nitrate loading experienced on the Site over time may be less than predicted. The requirement by San Joaquin County Environmental Health Department that the calculation use rainfall and evapotranspiration data to determine ground-water recharge is particularly conservative, as it does not include consideration of recharge from irrigation water or surface water sources. • The map of Nitrate — Land Use Data (2019) prepared by San Joaquin County Environmental Health Department indicates that, within a 1.5-mile radius surrounding the Site, no wells have been found to exceed the MCL for nitrate. This map was included in the Soil Suitability Study prepared for the project (Live Oak, 2020). • Depth to ground water beneath the Site is estimated to be 96 to 100 feet, as discussed in the Soil Suitability Study. San Joaquin County experienced its highest recent ground-water levels in 1983 and 1999. Review of ground-water maps for these years (discussed in the Soil Suitability Study) reveals that depth to water would have been approximately 65 to 70 feet during these high water periods. Even assuming the most conservative (shallowest) ground-water depth, the vadose zone of the new septic system will be quite deep, and significant nitrate attenuation would be anticipated over the course of this span. • Because of the clay soil type present at the Site, it is likely that adsorption of nitrate ions to clay will occur as percolating effluent travels downward through the vadose zone. In their original paper, Hantzsche and Finnemore considered developments with relatively shallow groundwater; as a result, the soil factors included in the equation may not reflect the true nitrate reduction experienced as the effluent travels down through a larger vadose zone. Based on the factors above, the actual nitrate impact to ground water beneath the Site may be considerably less than predicted. Recent San Joaquin County Environmental Health Department policy has been to evaluate each project on a case-by-case basis to determine whether additional measures to mitigate nitrate should be employed. Water Sample Live Oak collected a water sample from the domestic well on the Site on April 16, 2020. Analytical results of the sampling will be provided as soon as they become available.