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( <br />i <br />2, Heavy clay soils nay be quite imperneable, so that after the <br />biomat forms, alnost no percolation occurs* If you keep flushing <br />the toilet it will either seep to the surface or back up in <br />your basement. <br />To help eliminate these problsms, during the last 3C years most local <br />governments have adopted a specific range of acceptable percolation <br />rates, measured using a standard test procediire. Briefly, the perc test <br />is done by digging a 6” diameter hole, fill it with water, and measure <br />how fast the water drops. Acceptable rates for a standard trench system <br />aes'^between 0.5 minutes per incli of drop and 60 minutes per inch* <br />IN OaONO, CLAY SOILS PiiEDO]J:NATE, AJO) KANT SITES HAVE PERC RATES SLOrJER <br />THAN 60 <br />To complicate matters, in many clay soils and often i:i layered soils, <br />during some seasons a "perched" water table ov saturated zone occux'S* <br />If a drainfield trench is located in this area, it ray not accept any <br />effluent; again, backup or surface seepage can occur* Also, when the <br />trench is saturated, some of the treatment processes stop working* <br />To eliminate this "system floodir.g" problem, most Codes specify that the <br />bottom of the drainfield trench must be at least 3 feet above the highest <br />known water table or saturation zone* <br />IN ORONO, PANY SOILS HAVE A SEASONAL PERCHED SATURATION ZONE BET-’/EEIT <br />f AND U FEET BELCK THE SURFACE, 1-lAKING THE 3-FOOT SEPARATION IKPCSSIBLE* <br />C* ALTERNATIVE SOLUTIONS <br />1. No Development. <br />2. * City Sewers, i.o. collection system with off-site treatmcnt/disposal* <br />3* Install Trenches An;/way, Cross Fingers. <br />4. Exotic IriT.ovative Systems - such as sewage osmosis; evapotranspiration <br />or vapor beds; in-home recirculating plants; etc. <br />5* klound Systems <br />au Origins <br />- Sand mound with gravity beds used in N. Dakota since early 1960's <br />— U. of V/isconsin Small Scale Waste ^^anage^er t Project studied <br />mounds extensively in l97C's; concluded that pressure-distribution <br />mounds were m.ost effective, minimized failures* . <br />- Found systems vdth pressure distribution first used in Orono in - (9^ <br />1978. 13 installed to date; no failures reported to date/^^-^ <br />- V/PC-40 (Ki’CA recommendations) specifies design and siting'" <br />criteria for mound systems, also appearing in Orono Design Manual <br />b* How and Why l-Iounds Worlc <br />- Manv clay soils wd.th unacceptable percolation rates at the <br />depth of trenches have a layer of loamy topsoil which dees <br />have an acceptable perc rate, and will accept a limited aiount <br />of effluent. <br />- Pressure distribution mounds arc specifically designed to make <br />use of this topsoil layer.