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of the ground-water aquifer, it is possible that any remaining surface contaminants may be transported to the first saturated zone of the aquifer. Because ground water is so deep in the area (125 to 153 feet bgs), there is potential for significant nitrate attenuation before the effluent reaches ground water. Clay-containing soils, like those observed on the Site, also tend to inhibit downward movement of nitrate due to adsorption of the nitrate ions to the soil. 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. The Site is located within a small rural residential neighborhood surrounded by orchards. 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, "average recharge rate of rainfall" was utilized. 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. The closest CIMIS stations to the Site are Manteca (station 70) and Holt (station 248). Data for the Manteca station were available for the past 12 years; based on the calculation shown, the estimated rate of recharge from percolating waters for the Manteca station is determined to be 3.83 inches (Plate 14). A statistical error of 53 percent is associated with this value. Data for the Holt station were available for the past nine years; based on the calculation shown, the estimated rate of recharge from percolating waters for the Holt station is determined to be 3.99 inches (Plate 15). A statistical error of 54 percent is associated with this value. The values for the two stations were then averaged, for an average rate of recharge of 3.91 inches for the vicinity of the Site. LOGE 2614 Page 9