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11655 S UNION ROAD Project No. 2020-00095 Page 15 <br /> <br />4.4 Current and Future On-Site Groundwater Uses <br />The use of ground water for drinking water is currently utilized and further planned for the proposed <br />development. There is currently one domestic well onsite which will serve both residential <br />structures. <br /> <br />5.0 SOURCES AND IMPACT OF NITRATE TO GROUNDWATER <br /> <br />5.1 Current Off-Site Sources of Nitrate <br />Septic systems and agriculture in the area may be the most significant sources of nitrate. In an <br />attempt to determine the septic system density for the vicinity of the site, our office reviewed a recent <br />aerial photograph available from GoogleEarthTM. We counted the clusters within approximately one <br />square mile of the site, with the assumption each building cluster has one septic system. Using this <br />method, we determined there are approximately 50 septic systems per square mile in the immediate <br />vicinity of the site. This is a low number of septic systems, with the majority of these systems located <br />on large tracts of agricultural land. <br />As discussed above, the domestic well was sampled for Nitrate as N; the analysis reports the <br />groundwater is 15 mg/L, which is above the EPA maximum contaminant level of 10 mg/L. <br /> <br />5.2 Contribution of Rainfall <br />An estimate of the quantity and quality of rainfall was required for the nitrate loading analysis. The <br />estimate of deep percolation of rain is based on the average monthly precipitation of rain fall for Lodi, <br />California, based on the nearest weather station to the site available. The estimate first determines <br />whether the average monthly precipitation exceeds average monthly potential evapotranspiration. <br />If so, then the difference is potential deep percolation. This method does not consider runoff. See <br />Plate 8 for the data and calculation. <br />Average potential evapotranspiration was obtained from the California Irrigation Management <br />Information System (CIMIS). The site is located within evapotranspiration zone number 12, as well <br />as the Manteca station (Station 70) used in the analysis. The average rainfall for the area, as well as <br />the average evapotranspiration data, was obtained from CIMIS (https://cimis.water.ca.gov/) where <br />the average rain fall for the Manteca area was found to be 12.32 inches per year for years 1988 to <br />2018. <br />Based on this method, the average deep percolation of rain for the Lodi area is estimated to be 2.74 <br />inches. The nitrate concentration of rain fall was estimated to be 1 mg/L-N (Hantzsche and <br />Finnemore, 1992). <br /> <br />5.3 Current On-Site Sources of Nitrate <br />The current onsite 3-bedroom residence, and associated septic system and rainfall are <br />recognized as potential current on-site sources of nitrate. <br />Petralogix Engineering, Inc. <br />26675 Bruella Road, Galt, Ca 95632 <br />(209)-400-5729 <br />www.petralogix.com <br />I I