Septic-tank drainfield performance in five Willamette Valley soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pz50h0516

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  • Permeability, effects of age, and water table fluctuations were studied for nine drainfields in five soil types ranging from well to poorly drained. Three sites were in Willamette, one in Woodburn, two in Amity, two in Concord, and one in Dayton soils. Before permeability of drainfields could be evaluated, methods of determining it were tested. After comparing the Public Health Service (PHS) test and the double tube method, the latter proved to be the more reliable. PHS test results were influenced greatly by sediment that eroded off sidewalls during testing. Double tube tests were variable, but nearly all variation was due to entrapped air and/or the heterogeneous nature of soil. When the double tube method was run in soil adjacent to the nine drainfields, infiltration rates from the inner tube correlated better with performance than hydraulic conductivity. Detailed observations were made to correlate movement of effluent in the drainfields with infiltration data. Infiltration rates for the B2t horizon of Willamette soils averaged 74.5 ml/min. Effluent moved mostly vertically from trenches in three drainfields located in Willamette soils. Infiltration rates averaged 9.6 ml/min. at the top of the Woodburn B2t horizon. In Woodburn and Amity (Al) drainfields effluent moved laterally through a porous B1 or AZ horizon and slowly percolated through the B2t horizon. Infiltration rates determined in the middle of the B2t horizons of Amity (A2) and Concord (C1) indicated that these soils should have conducted effluent faster than Woodburn. Such was not the case. Effluent leaving trenches in A2 and Cl drainfields spread laterally through porous subsurface A2 horizons with very little, if any, moving through the subjacent B2t horizons. These observations suggest that the tops of B2t horizons in Amity (A2) and Concord (C1) were less permeable than the middle where infiltration rates were determined. The very slow infiltration rates obtained for Concord (C2) and Dayton soils were in agreement with the direction effluent moved. Nearly all of the effluent traveled on top of the B2t horizon, which is the way Amity (A2) and Concord (C1) drainfields performed. Length of time drainfields had been in operation had two noticeable effects. One, the area of intensely mottled soil around drainfield trenches increased with time. Two, the area of high moisture around trenches increased except for Concord and Dayton soils, which are strongly influenced by impermeable clayey B2t horizons. From field observations and thin sections, the clogging material causing drainfields to deteriorate with time appears to be colloidal ferrous iron compounds. Concentration of coliform microorganisms was used to evaluate the effect of high water tables in drainfields. Coliform densities showed that the area influenced by Amity (A2) drainfield, which is on a nearly level position, increased from 150 sq. meters to 15,000 sq. meters in the winter when the water table remained near the surface for several months. Also it was found that caution is needed in interpreting coliform data when domestic livestock is near (within 0.6 kilometers) of sampling point. Coliform counts in ground water influenced by domestic livestock were the same order of magnitude as those found 5-6 meters away from drainfields.
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