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exist to the depth of the ground-water aquifer, it is possible that any remaining surface <br /> contaminants may be transported to the first saturated zone of the aquifer. <br /> First ground water is predicted at 205 to 334 feet below ground surface for this Site; <br /> consequently, significant nitrate reduction would be anticipated as the septic effluent <br /> percolates downward. Clay soils, like those found on this Site, also tend to inhibit <br /> downward movement of nitrate due to adsorption of the nitrate ions to the soil. <br /> Sources of Nitrate <br /> Dairies, orchards that require fertilization in sandy soils, and septic systems have been <br /> found to be the most significant sources of nitrate in ground water. <br /> Percolating Recharge Waters <br /> One of the most determinant factors in the Hantzsche and Finnemore equation is the <br /> rate of recharge from percolating waters, which mix with the downward-percolating <br /> effluent generated by on-site septic systems. In the original Hantzsche and Finnemore <br /> paper, "average recharge rate of rainfall" was utilized. <br /> Based on San Joaquin County Environmental Health Department policy (personal <br /> communication, March 18, 2019), the recharge input for the Hantzsche and Finnemore <br /> equation should be derived from average monthly precipitation of rainfall and <br /> evapotranspiration rates for the area; the data should be obtained from the California <br /> Irrigation Management Information System (CIMIS). Recharge is calculated by first <br /> determining whether the average monthly precipitation exceeds average monthly <br /> potential evapotranspiration. If so, then the difference is the percolating recharge water <br /> into the aquifer. Runoff and stream infiltration drainage are not factored into the <br /> calculation. This approach likely under-estimates recharge to the aquifer and, <br /> consequently, over-estimates nitrate loading. <br /> The Site is located approximately equidistant from the CIMIS stations at Staten Island <br /> (station 242), Holt (station 248), and Manteca (70); taking all three stations into account <br /> will yield a value more representative of the Site area. Data for the Staten Island station <br /> were available for the past four years; based on the calculation shown, the estimated <br /> rate of recharge from percolating waters for the Staten Island station is determined to be <br /> 11.78 inches (Plate 20). A statistical error of 45 percent is associated with this value. <br /> Data from the Holt station were available for 2017 through 2019; based on the <br /> calculation shown, the estimated rate of recharge from percolating waters for the Holt <br /> station is determined to be 4.04 inches (Plate 21). A statistical error of 21 percent is <br /> associated with this value. Data for the Manteca station were available for the past six <br /> years; based on the calculation shown, the estimated rate of recharge from percolating <br /> waters for the Manteca station is determined to be 3.69 inches (Plate 22). A statistical <br /> LOGE 20-29 Page 11 <br />