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• Sources of Nitrate <br /> Septic systems and dairies have been found to be the most significant sources of nitrate <br /> in ground water. Orchards that require fertilization in sandy soils are also a significant <br /> source of nitrate contamination in shallow or sandy aquifers in San Joaquin County. <br /> Contribution of Rainfall <br /> An estimate of the quantity and quality of rainfall was required for the nitrate loading <br /> analysis. An estimate of the deep percolation of rain was based on a calculation using <br /> average monthly precipitation of rainfall and evapotranspiration rates for the area. The <br /> calculation first determines whether the average monthly precipitation exceeds average <br /> monthly potential evapotranspiration. If so, then the difference is potential deep <br /> percolation. This method does not consider runoff. Based on the calculation shown in <br /> the following table, the deep percolation of rain (recharge) for the Stockton, California <br /> area is found to be 4.37 inches per year. <br /> Table 3. Estimate of Deep Percolation of Rainfall for Stockton, CA. <br /> Month Average Rain- Average Potential Annual Potential Deep <br /> Fall (inches) Evapotranspiration (inches) Percolation of Rain (inches) <br /> January 3.06 1.24 1.82 <br /> • February 2.73 1.96 0.77 <br /> March 2.19 3.41 None <br /> April 1.30 5.10 None <br /> May 0.46 6.82 None <br /> June 0.11 7.80 None <br /> July 0.02 8.06 None <br /> August 0.03 7.13 None <br /> September 0.22 5.40 None <br /> October 0.79 3.72 None <br /> November 1.76 1.80 None <br /> December 2.71 0.93 1.78 <br /> Totals 15.37 53.4 4.37 <br /> Notes: CIMIS evapotranspiration information at www.cimis.water.Ga.gov for Zone 12. Rainfall average from station <br /> 048560(Stockton Fire Station 4, CA)at www.wrcc.dri.edu. Closest available zone/station to Site were selected. <br /> • LOGE 1628 Page 9 <br />