SR0082191 SSNL - Laserfiche WebLink

Home Browse Search

of the ground-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 120 to 127 feet below ground surface for this Site; consequently, significant nitrate reduction would be anticipated as the septic effluent percolates downward. Clay soils, like those found on this 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. 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). 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 Site is located closest to the CIMIS stations at Manteca (station 70) and Holt (station 248); taking both stations into account will yield a value more representative of the Site area. Data for the Manteca station were available for the past six years; based on the calculation shown, the estimated rate of recharge from percolating waters for the Manteca station is determined to be 3.69 inches (Plate 14). A statistical error of 43 percent is associated with this value. Data from the Holt station were available for 2017 through 2019; based on the calculation shown, the estimated rate of recharge from percolating waters for the Holt station is determined to be 4.04_ inches (Plate 15). A statistical error of 21 percent is associated with this value. The two stations are then averaged, for a regional deep percolation value of 3.86 inches. LOGE 20-22 Page 9