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to the first saturated zone of the aquifer. As evidenced by the deeper test, the shallow <br /> soil layer at this Site is underlain by more restrictive soils. <br /> Clay soils, like those found on this Site, tend to inhibit downward movement of nitrate <br /> 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), preferably from stations located <br /> within San Joaquin County. Recharge is calculated by first determining whether the <br /> average monthly precipitation exceeds average monthly potential evapotranspiration. If <br /> so, then the difference is the percolating recharge water into the aquifer. Runoff and <br /> stream infiltration drainage are not factored into the calculation. This approach likely <br /> under-estimates recharge to the aquifer and, consequently, over-estimates nitrate <br /> loading. <br /> The closest CIMIS stations to the Site are Manteca (station 70) and Holt (station 248). <br /> Data for the Manteca station were available for the past nine years; based on the <br /> calculation shown, the estimated rate of recharge from percolating waters for the <br /> Manteca station is determined to be 3.56 inches (Plate 14). A statistical error of 55 <br /> percent is associated with this value. Data for the Holt station were available for the <br /> past six years; based on the calculation shown, the estimated rate of recharge from <br /> percolating waters for the Holt station is determined to be 3.60 inches (Plate 15). A <br /> statistical error of 58 percent is associated with this value. The estimated recharge <br /> values from the two stations are then averaged, for an estimated recharge value for the <br /> Site of 3.58 inches. <br /> LOGE 2325 Page 10 <br />