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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. The Hantzsche and Finnemore method utilizes the "average recharge rate of <br /> rainfall' in calculating potential nitrate impacts to the aquifer. Not all rainfall will <br /> percolate into the aquifer; evapotranspiration, the sum of evaporation and plant <br /> transpiration, will significantly reduce percolating effluent, especially in summer months. <br /> Rainfall recharge (deep percolation of rain) is a significant factor in the nitrate loading <br /> estimate; however, data on rainfall recharge rates are not widely available. Hantzsche <br /> and Finnemore present average rainfall versus estimated rainfall recharge data for three <br /> locations in California: <br /> Table 4: Rainfall Data Cited by Hantzsche and Finnemore, 1992 <br /> Bolinas Mesa Area Chico Area Baywood/Los Osos <br /> • Average rainfall 30.9 22.5 20.0 <br /> (in/yr) <br /> Estimated rainfall 14.4 16.8 12.0 <br /> recharge (in/yr) <br /> Rainfall recharge as <br /> percentage of 46.6% 74.7% 60.0% <br /> average rainfall* <br /> 'Calculated by Live Oak <br /> In these locations, the rainfall recharge is calculated to be 46.6, 60.0, and 74.7 percent <br /> of average rainfall. For the purpose of this report, Live Oak has averaged those values <br /> and selected 60 percent of rainfall to represent the estimated rainfall recharge for San <br /> Joaquin County. The San Joaquin County Environmental Health Department has <br /> confirmed that this approach should accurately characterize rainfall recharge within the <br /> County (personal communication, January 6, 2017). <br /> Rainfall data for the north Manteca area was obtained from the Western Regional <br /> Climate Center. The Stockton Metro Airport station (048558) reports the average total <br /> annual precipitation from October of 1948 through June of 2016 to be 13.76 inches. By <br /> • LOGE 1714 Page 14 <br />