Abstract:
This study was initiated to make some preliminary
evaluations of the effects of red alder (Alnus rubra,
Bong.) leaf fall on stream water quality, particularly
water color. Laboratory tests were also conducted
to better understand the effects of leaf loading, duration
of leaf leaching, and type of leaching on
changes in water quality. The study focused on the
Seaside Municipal Watershed, which is typical of many
municipal systems in the Oregon Coast Range. In these
systems, streamside alder stands are common and water
treatment is limited to chlorination.
The potential for coloration of stream water by
alder leaves is present but in the fall of 1981 there
was no chronic (more than one or two days) low flow
water color problem observed at the Seaside diversion
in the South Fork of the Necanicum River. The color
observed during the lowest flows, which occurred almost
through September, averaged 20 platinum-cobalt
units of true color. While this is slightly higher
than the 5-15 units measured during the winter months
and slightly higher than the maximum standard of 15
units set by the USPH for domestic supplies, it is a
very faint tint and probably would not cause many complaints
by domestic users.
Field results showed that the water color can become
elevated for short periods at the diversion.
There were two occasions when the color was above 20
units (30 units on October 6 and 25 units on November
11). Elevated water color was also observed in a tap
water sample from the city's water works building on
October 27 (45 units). Each of these color, increases
occurred during the beginning of a storm and there was
a subsequent drop in color after the storms. This increase
in color during storms is probably due to the
flushing of storage sites containing dissolved organic
material by rising flows. The other water quality
characteristics measured (dissolved oxygen, conductivity,
pH, nitrate-nitrogen and temperature) showed evidence
of. some relationships with leaf fall, but there
were no signs of water quality impairment at the city
diversion during the autumn of 1981.
Laboratory leachings in filtered stream water
showed that for a given mass of leaves leached in still
water, there is a fairly constant release of color
(2 units/hr) for leaching periods between 2 hours and
4 weeks. Running water was more efficient than still
water in leaching colored material. There also appears
to be a limit to the amount of material that can be
removed from a given leaf mass in the first 48 hours
of leaching. In still water a linear increase in
color occurred with increases in mass loading, and
weight loss (as a percentage of original dry weight)
was constant. Laboratory tests also showed that ultra
violet absorbance may provide a reasonable estimate
of dissolved organic carbon concentrations in
systems dominated by red alder input. This, however,
has yet to be verified with field samples.
In a small stream with lower flows than the South
Fork of the Necanicum River bounded by an alder stand,
color could possibly be a chronic problem during autumn
low flows. The South Fork generally appears to
have too large a flow for this to normally occur.
However, a hypothetical calculation of maximum coloration,
under conditions of very low flow and peak leaf
fall, resulted in a water color of approximately 110
units in this stream. Short-term increases in color
at the beginning of the first few storms would be expected
to be more common, in a stream of this size,
if there is time for organic matter accumulation in
storage sites between storms. Such increases in true
color would likely be accompanied by increases in
apparent color due to sediment, which may even supercede
the coloration due the dissolved or colloidal
organic material.
