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water aquifer, it is possible that any remaining surface contaminants may be transported to the first saturated zone of the aquifer. First ground water is predicted at 110 to 132 feet below ground surface for this Site; consequently, significant nitrate reduction would be anticipated as the septic effluent percolates downward. Clayey soils, like those found on the Site, are known to restrict downward movement of water, and may provide nitrogen reduction through adhesion to the soil particles. The soil log included with the well drilling permit (Appendix 4) indicates that alternating layers of clay and gravel or sand extend to 266 feet below ground surface at the Site. Sources of Nitrate Dairies, orchards that require fertilization in sandy soils, and septic systems have been found to be the most significant sources of nitrate in ground water. Percolating Recharge Waters One of the most determinant factors in the Hantzsche and Finnemore equation is the rate of recharge from percolating waters, which mix with the downward-percolating effluent generated by on-site septic systems. In the original Hantzsche and Finnemore paper, only "average recharge rate of rainfall" was considered. Based on San Joaquin County Environmental Health Department policy (personal communication, March 18, 2019), the recharge input for the Hantzsche and Finnemore equation should be derived from average monthly precipitation of rainfall and evapotranspiration rates for the area; the data should be obtained from the California Irrigation Management Information System (CIMIS), preferably from stations located within San Joaquin County. Recharge is calculated by first determining whether the average monthly precipitation exceeds average monthly potential evapotranspiration. If so, then the difference is the percolating recharge water into the aquifer. Runoff and stream infiltration drainage are not factored into the calculation. This approach likely under-estimates recharge to the aquifer and, consequently, over-estimates nitrate loading. For the project Site, the local percolation and evapotranspiration rates are presented in Plate 12. The Site is located closest to the CIMIS Ripon station (249). Data for the Ripon station were available for 2017 and 2018; based on the calculation shown, the estimated rate of recharge from percolating waters for the Ripon station is determined to be 3.12 inches. A statistical error of 46 percent is associated with this value. LOGE 1922 Page 9