Modeling the impact of logging debris on the dissolved oxygen balance of small mountain streams Public Deposited


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  • Previous research has indicated that considerable amounts of finely divided slash accumulate in small mountain streams following timber harvesting. The subsequent biological decomposition of this organic matter can result i.n the reduction of dissolved oxygen (DO) concentration to levels as low as 0.6 ppm. These DO levels are lethal to the salmon and trout fry which inhabit these waters during the summer months. The purpose of this study was to verify the results of previous research on the biochemical oxygen demand (BOD) characteristics of Douglas-fir needles, western hemlock needles, and red alder leaves, and to develop an oxygen balance model to describe the effect of logging debris BOD on the dissolved oxygen of small mountain streams. The concept of the leaching process was examined and refined. As a result K₄, the leaching rate constant, was introduced to describe the addition of leachate to the stream. This necessitated redefining K₁, the first order decay rate constant, as it had been reported in previous studies. Slash may be described by its ultimate oxygen demand, or potential BOD, and two rate constants, K₁ and K₄. Manometric BOD tests were run for 20-day periods to estimate the potential BOD and the rate constants K₁ and K₄. The 20-day ultimate BOD, K₁ and K₄ estimates at 20°C were: 139mg 0₂ /gm (dry weight), 0.266 and 0.189 per day for Douglas-fir needles, 183mg 0₂/gm (dry weight), 0.202 and 0.089 per day for western hemlock needles, and 226 mg 0₂/gm (dry weight), 0.121 and 0.141 per day for red alder leaves. Laboratory experiments designed to examine the possibilities of occurrence and impact of nitrification on the oxygen demand curve concluded that even though it is possible to observe nitrification under special conditions, coastal streams are generally too nitrogen and carbon poor for nitrification to exert a significant oxygen demand. Even with the addition of large amounts of vegetation to the stream water, an available form of nitrogen must be added to produce detectable amounts of nitrogenous oxygen demand. It was found that the presence of nitrification cannot be detected in small mountain streams by nitrate analysis because of rapid fixation of nitrogen. Thus, the nitrification process appears to be of more concern for evaluating nutrient pathways and fixation than oxygen demand. The leachate of the Douglas-firs western hemlock, and red alder vegetation was analyzed for sugar and phenol concentration and rate of leaching. The vegetation was gas sterilized using ethylene oxide. The sterile vegetation was placed in autoclaved aliquots of water and sealed. The leachate of each vegetation type was analyzed at periodic intervals over 90 days, and the leachate expressed in glucose and gallic acid equivalents. Of the 120 samples treated, only one showed signs of contamination. The sugar and phenolic release rate constants for Douglas-fir and western hemlock were very similar; 0.049 and 0.062 for Douglas-fir sugars and phenols respectively and 0.060 and 0.046 for western hemlock sugars and phenols. All of these rates are less than the leaching rates derived by BOD experimentation. These differences may be due to biological interaction in the leaching process. The leaching rate constants for red alder leaves were much higher than the experimental rates, 0.88 and 0.187 for sugars and phenols respectively. The mean maximum sugar concentration was: 117 mg glucose equivalent / gm (dry weight) for Douglas-fir needles, 86 mg glucose equivalents / gm (dry weight) for western hemlock needles, 121 mg glucose equivalents / gm (dry weight) for red alder leaves, and 50 mg glucose equivalent/gm (dry weight) for dissected Douglas-fir twigs. No rate constant was computed for Douglas-fir twigs because of the lack of fit to an exponential function. Experimentation of the effects of temperature showed & significant response of potential BOD, K₁ and K₄ to variations in water temperature from 15 to 27°C. In general, ultimate BOD, K₁ and K₄ values increased with increasing water temperature. The effect of mass concentration on ultimate BOD, K₁ and K₄ were negligible for vegetative loadings of 4 to 16 gm, fresh weight per liter. The second phase of this study involved constructing a finite difference model of the dissolved oxygen balance in small mountain streams. A fixed cell approach was used to demonstrate the negligible effects of dispersion on pollutant and oxygen concentration. This then allowed the use of the much more efficient Lagrange variation of the finite difference modeling technique, utilizing a moving cell. By using available data and estimates of the distribution of slash in a simulated stream following clearcutting along 991 ft of the stream, DO levels as low as 5.72 ppm were computed for the simulated stream 5890 ft below the top of the clearcut. At these DO concentrations, salmon and trout fry experience "critical" stress conditions. Further model runs indicated that by reducing the length of the clearcut or by removing a portion of slash this condition could be avoided.
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