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n � <br /> 3.2 Mounding Analysis <br /> Reference is made to the encountered slowly permeable silt/clay stratum found at 15 ft b.g.s. in <br /> each 15 ft test borings. If the depth to groundwater is taken to be 22 ft,then it will be the <br /> encountered stratum that may induce a phenomenon known as the"mounding effect." This <br /> theoretically occurs when percolating effluent encounters the water table, or restrictive stratum <br /> and cannot disperse laterally in a certain time frame. Consequently, a mound forms under the <br /> disposal field creating saturated flow conditions and decreasing the distance the effluent must <br /> travel under unsaturated flow for effluent treatment to occur. An equation developed by <br /> Finnemore and Hantzsche (1983) is used below to predict the long-term maximum rise of the <br /> mound: <br /> h=H+Zm . 2 <br /> where: h=distance from boundary to mid-point of the long-term mound, in ft <br /> H=height of stable groundwater table above impermeable boundary, in ft <br /> Z,,,=long-term maximum rise of the mound, in ft <br /> Substituting known and estimated values for the variables,we find the following: <br /> H=The height of the slowly permeable boundary is 15 ft. Therefore, it will be assumed that a boundary <br /> exists at 15 ft b.g.s.,H=22 (Highest measured water table depth)- 15 =7 ft. Long-term maximum rise of <br /> mound is estimalted at 5 ft. Therefore, h= 7+ (1-2)=7.5 <br /> Z 1 A f (4/n 1 Kh los (Sy) -os� <br /> where: Q=average daily flow in ft'/day <br /> A= area of disposal field in ft' <br /> C=mounding equation constant <br /> L=length of each disposal field in ft <br /> K=horizontal permeability of soil in ft/day <br /> n=mounding equation exponent <br /> Sy=specific yield of receiving soil in percent <br /> t —time since the beginning of wastewater application in days <br /> Substituting known constants for the variables,we find the following: <br /> Q =25,080 gpd(From Max. flow volume calcs.,Page 17)_7.48 gals/ft'=3,353 fe/day <br /> 1 A =3.1 Acres= 135,036 W(Max.Available Disposal Area) <br /> C=Length to width ratio = 1.6,therefore, C=2.0748 <br /> L= 100 ft(Unknown Number of Lengths) <br /> K=Using average vertical permeability as most conservative=36 min/in; 1440 min/day-36 min/in=3.3 ft/day <br /> It=75 (See above) <br /> n=Length to width ratio = 1.6,therefore,n= 1.7552 <br /> S,=8% <br /> t =3,650 days(10 yrs) <br /> Zm =0.0515 x 104.4 x 0.0598 x 21151 =0.68 ft <br /> It appears that the maximum mound height that may occur under the disposal area of 3.0 acres is <br /> 1.85 ft. This would leave a distance of approximately 11.2 feet between the soil/effluent interface <br /> and the top of the theoretical mound: soil/effluent interface =2 ft below existing grade+ 5 ft <br /> separation distance= 7 ft below grade. <br /> Page -12- <br /> Chesney Consulting <br />