Graduate Thesis Or Dissertation
 

BOD and toxicity of log leachates

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  • A series of log storage experiments were conducted to determine whether leachates derived from water storage of logs are acutely toxic to fish. Log segments approximately 18 inches long and 16 inches in diameter were stored in tanks and held submerged for a period of 7 days. The holding water containing leached materials was made toxic with mercury to retard biological decomposition of the leached substances. Mercury was selectively removed from leachate samples by chelation prior to biochemical oxygen demand (BOD) and bioassay testing. Trout and salmon fry were subjected to the leachate water in short term acute bioassay tests. Results are reported as a median tolerance limit, (TLm), i. e., the concentration of leachate at which 50 percent of the test fish died for any given exposure time. Leachates were also tested for BOD₅, BOD k-rate, chemical oxygen demand (COD), wood sugar and Pearl Benson Index (PBI). Test results show that leachates from Douglas fir stored in fresh water exert a slight acute toxicity to fish. A TLm₉₆ of 20 percent leachate by volume, for a 50 year old Douglas fir log, was the most toxic leachate observed. Leachates from ponderosa pine, hemlock and an older fir log stored under identical conditions produced no measurable acute toxicity. Leachates contained a significant quantity of BOD and PBI exerting substances. The highest BOD₅, (1.36 g/ft² of submerged surface area) was exerted by leachate from a ponderosa pine log segment stored with bark removed. The highest PBI valve (12.5 g/ft²), was observed for leachate from a young Douglas fir log segment. BOD:COD ratios and BOD k-rate ranged widely for the various leachates, but were relatively low which indicated a significant fraction of non-biodegradable substances. Hoffbuhr (9) also observed a high non-biodegradable fraction in samples taken from log storage ponds. Wood sugars were found to account for a large part of the degradable portion of leachates. Leachate from ponderosa pine log with bark intact exerted a high BOD and also contained the highest concentration of wood sugar observed, 0.84 g/ft².
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