Graduate Thesis Or Dissertation
 

The effects of red alder leaf fall on the water color and other water quality characteristics of a small watershed in northwest Oregon

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  • 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.
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