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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 135 to 145 feet below ground surface for this Site; <br /> consequently, significant nitrate reduction would be anticipated as the septic effluent <br /> percolates downward. <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 Holt (station <br /> 248) and Staten Island (station 242); taking both stations into account will yield a value <br /> more representative of the area. During the past five-year period, data from the Holt <br /> station were only available for 2017 and 2018; based on the calculation shown, the <br /> estimated rate of recharge from percolating waters for the Holt station is determined to <br /> be 3.72 inches (Plate 15). A statistical error of 25 percent is associated with this value. <br /> Data for the Staten Island station were available for the past three years; based on the <br /> calculation shown, the estimated rate of recharge from percolating waters for the Staten <br /> Island station is determined to be 12.60 inches (Plate 16). A statistical error of 49 <br /> percent is associated with this value. The two stations are then averaged, for a regional <br /> deep percolation value of 8.16 inches. <br /> LOGE 1943 Page 11 <br />