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� o� <br /> . � 0 , o � <br /> � �-� ��� CITY of ORONO <br /> ,���,' � <br /> II � ,�� °l��=�' ti% M,��oe� <br /> �� "� �, �`�':t;.�..�} �,�, Post O�ce Box 66 <br /> '� `i�;,��;�sr��� Crystal Bay.Minncsots 55323�0066 <br /> �,�kES�K�g,�i <br /> October 10, 1991 <br /> Mr. Randy Rosengren <br /> 1709 North Farm Road <br /> Long Lake, riinnesota 55356 <br /> Re: Septic System Repairs <br /> Dear Randy: <br /> This letter is to follow-up our discussions in September regarding <br /> repairs to your septic system. As we discussed, from your monitoring <br /> of the system, there does not appear to be excessive water use nor .is <br /> there any significant visible ground water seepage into the tanks. <br /> However, it was clear during our inspection on Monday, September 16th, <br /> that 80$ of the pump discharge to the mound was draining back as soon <br /> as the pump turned off. Further, the rock bed in the mound was <br /> observed to be in a continuous saturated state while the sand bed <br /> below the perimeter of the rock bed was not saturated. It would <br /> appear that the effluent being pumped into the rock bed is perched <br /> above the sand bed due to some hydraulic barrier. <br /> In theory, a "biomat" should normally form between the rock bed and <br /> the sand layer, acting as a valve to create an unsaturated f low of <br /> effluent downwarcl through the sand bed. The aeration that occurs <br /> during this unsaturated flow helps treat the effluent before it <br /> reaches the original topsoil for absorption into the ground. The area <br /> of the rock bed is sized based on a presumed acceptance rate of 1.2 <br /> gal lons per square foot of rock bed per day, the expected f low rate <br /> through the biomat once it is formed. <br /> It seems likely that abnormal factors have affected the permeability <br /> of the biomat at the interface between your rock bed and sand. These <br /> factors might include sand containing excess fine materials; clay or <br /> silt particles that have washed down through the rock bed and formed a <br /> barrier at the rock-sand interface; or possibly a site location that <br /> encourages runoff from the adjacent driveway through the sand bed of <br /> the mound, which would lead to an intermittent saturated condition in <br /> the sand bed which would not allow the biomat to break down at the <br /> rate it is being formed. This would yield a thicker biomat with <br /> slower permeability and ultimate hydraulic failure of the system. <br /> TELEPHONE-473-7357�FAX-473-0510 <br />