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. i <br /> E. SURFACE WATER INFORMATION <br /> Storm.water management is proposed to be an on-site retention basin'adjacent to Corral Hollow <br /> _ <br /> Road that will be constructed with minimal excavation and built=up embankments. This is to <br /> maintain a five-foot separation distance between the floor of the basin.and the depth to the water <br /> table. There should be no impact to percolating septic effluent from the retention basin. In fact, <br /> the retention basin is theoretically dowrigradient to the future.wastewater disposal area, if the <br /> groundwater directional flow is truly,in a westerly direction. Percolating.rainwater from this::basin <br /> into the top aquifer mixing layer may create a dilution effect to the'percolating septic system <br /> effluent. <br /> Groundwater dilution effects can only be truly discerned with groundwater,monitoring and <br /> modeling. According to a scientific paper authored by Shaw and Turyk (1994), reduction of nitrate <br /> concentrations in groundwater can occur_prirnarily through dispersion, or by percolating rainwater <br /> recharge. <br /> Clima Data: Rainfall versus Evapotranspiration <br /> NLS§3.2. It is generally and scientifically accepted that when precipitation exceeds evapotrans- <br /> iration (. to), deep percolation of surface rainwater may,occur. Evapotranspiration refers to water <br /> e poration that takes place both at the soil surface and transpiration by plants, and is defined as <br /> water loss from a large field of 4-to-7 inch tall, cool-season grass that is not water stressed. <br /> _ i <br /> The following Table illustrates rainfall and evapotranspiration for the Tracy area.- Precipitation J <br /> data was obtained from the Soil Survey of San Joaquin County. The-conservative "2 years in 10" ! <br /> data was used and not the "average rainfall" data. Evapotranspiration data-was obtained from the <br /> California Irrigation.Mariagement Information System (CIMIS): <br /> TABLE 5 <br /> ffMonth <br /> PRECIPITATION-AND Eto.DATA FOR;TRACY;CA <br /> 2 Years in 1:0 will have more Eto Average Potential Deep <br /> precipitation than: (in/mo) Percolation(in/mo) <br /> (in/mo) (in/day) <br /> January 2.93 1.55 0.05 1.38 <br /> February 2.21 2.24 0.08 <br /> March 2.38 3.72 0.12 -- <br /> April _1..51. .5.14 0.17 ..May 0.71 6.82 -0.22 -- <br /> June 2.09 7.80 0.26 -- <br /> July -0.28 8.68 0.28 <br /> August 1.05 . 7.75 0.25 -- <br /> 10 <br /> Chesney Consulting <br />