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t Soil Suitability&Nitrate Loading Study <br /> Munoz Property, Escalon, CA Page 10 <br /> Project No. 605.1 <br /> P June 18, 2009 <br /> evapo-transpiration rate (ET). The rate at which precipitation exceeds evapo- <br /> transpiration is the rate at which percolating meteoric water is available to <br /> a leach nitrates to groundwater. According to the National Weather Service <br /> (http:l/cdo.ncdc.noaa govt), the mean annual rainfall at the Stockton Airport (the <br /> nearest reporting station) is 13.84 inche5,--The rate of evapo-transpiration is <br /> available from the website of the California Irrigation Management Information <br /> System (CIMIS) ttp://wwwcimis water.ca. ov/cimisn. California has been <br /> divided into zones, and Escalon f 1`s-4.r.,. Zone 12. The average annual evapo- <br /> transpiration rate for Zone 12 is 53.3 inches. Iyrecognition of the fact that <br /> 4 <br /> most of the precipitation falls d _ng. <br /> the winter months, while most of the <br /> evapo-transpiration occurs during the summer, the folio i <br /> wing table was prepared � <br /> to provide a month-by-month breakdown <br /> F <br /> Month Mean Precip. Mean ET Percolation <br /> (in. in.) in. j <br /> Jan. 2.71 1.24 1.47 <br /> + Feb. 2.46 1.96 0.50 <br /> Mar. 2.28 3.41 <br /> F` A r. 0.96 5.10 <br /> May 0.50 6.82 <br /> Jun. 0.09 7.80 <br /> { Jul. 0.05 8.06 <br /> Aug- 0.05 <br /> 7.13 _ <br /> Sep. 0.33 5.40 _ <br /> Oct. 0.82 3.72 <br /> Nov. 1.77 1.80 <br /> Dec. 1.82 0.93 0.89 <br /> 2.86 <br /> Based on these data, all of the percolation occ between December and <br /> February, and the total annual percolq- ioe is 2.8'6 i r <br /> F, <br /> n ra hes( he assumption { <br /> is also made that rain water contains tl mg/L U nitro e as N. <br /> Denitrification <br /> Denitrification of raw sewage occurs from solids settling in the septic tank and <br /> from the action of certain bacteria in the leach field, which convert dissolved <br /> nitrates into nitrogen gas. The denitrification effect is expressed as a <br /> ` ` percentage, and it varies with septic stem design, soil <br /> p Y g , type, and pH conditions. <br /> The denitrification that occurs through solids settling is accounted for in <br /> estimating the nitrate concentration of septic tank effluent. The denitrification <br /> that occurs in the leach field from bacterial activity typically is betw�l0 and <br /> 35 percent. In this case, it is assumed to be 25 percent as suggested b <br /> Hantzsche and Finnemore (1992). y <br /> it <br /> 2937 Veneman Ave.,#B240 Geo-Phase Environmental Inc. Phone: (209)569-0293 <br /> Modesto, CA 95356 Fax: (209)569-0295 <br />