|Abstract or Summary
- 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.