SR0081206 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 56 to 59 feet below ground surface for this Site; consequently, significant nitrate reduction would be anticipated as the septic effluent percolates downward. 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. For the project Site, the local percolation and evapotranspiration rates are presented in Plates 14 and 15. The Site is located between the CIMIS Stations at Holt (station 248) and Staten Island (station 242); taking both stations into account will yield a value more representative of the area. During the past five-year period, data from the Holt station were only available for 2017 and 2018; based on the calculation shown, the estimated rate of recharge from percolating waters for the Holt station is determined to be 3.72 inches (Plate 14). A statistical error of 25 percent is associated with this value. Data for the Staten Island station were available for the past three years; based on the calculation shown, the estimated rate of recharge from percolating waters for the Staten Island station is determined to be 12.60 inches (Plate 15). A statistical error of 49 percent is associated with this value. The two stations are then averaged, for a regional deep percolation value of 8.16 inches. LOGE 1939 Page 10