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r7encounters nding Analysis nce is made to the encountered groundwater table at 1 I feet below existing grade. This shallow groundwater table may induce a phenomenon known as the"mounding effect'by which percolating effluent the water table,or restrictive stratum and cannot disperse laterally in a certain time frame. Consequently, a mound forms under the disposal field creating saturated flow conditions and decreasing the distance the effluent must travel under unsaturated flow for effluent treatment to occur. An equation developed by Finnemore and Hantzsche(1983)is used below to predict the long-term maximum rise of the mound: h=H+Zm=2 where: It=distance from boundary to mid-point of the long-term mound, in ft H=height of stable groundwater table above impermeable boundary, in ft Zm=long-term maximum rise of the mound, in ft Substituting known and estimated values for the variables,we find the following: H=The height of stable groundwater above an impermeable boundary is unknown since there are no on-site or adjacent well logs to determine an impermeable boundary. Therefore,it will be assumed that a boundary exists at 100 ft bgs,H= 100- 11(Existing water table)= 89 ft. Long-tern maximum rise of mound is estimated at 1 ft plus the apparent water table elevational rise as determined in the monitoring wells(11 ft-8 ft(highest depth to groundwater) = 3 ft+ I ft rise of the mound=4 ft). Therefore, It= 89+(4=2)=91 Z _ �� 4 I v �1O.Sv 'Sy'I-0.Sv where: Q=average daily flow in ft'/day A=area of disposal field in ft' C=mounding equation constant L=length of disposal field in ft K=horizontal permeability of soil in ft/day n=mounding equation exponent Sy=specific yield of receiving soil in percent t =time since the beginning of wastewater application in days Substituting known constants for the variables,we find the following: Q =2,108 gpd(From Max. flow volume calcs.,Page 12) . 7.48 gals/ft=281.8 ft'/day A =3,400 ft(From filter bed sizing calcs,Page 16) C=Length to width ratio = 3.3,therefore,C= 1.75 L= 102ft K=Using average vertical permeability as most conservative=38 min/in: 1440 min/day_38 =3.2 ft/day It=7(See above) n=Length to width ratio =3.3,therefore,n= 1.700 Sy= 12% t =3,650 days(10 yrs) 4 =0.145 x 246 x 0.008043 x 2.35=0.674 ft It appears that the maximum mound height that may occur under the filter bed, and above the highest anticipated depth to the water table of 8 ft is 0.67 feet. This would leave a marginal distance of approximately five feet between the soil/effluent interface and the top of the theoretical mound: soilleffluent interface =2 ft below existing grade + 5 ft separation distance= 7 ft below grade. 8 Chesney Consulting