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