The Role of Moisture Content in Above-ground Leaching
S Lebow, P Lebow
This paper reviews previous reports on the moisture content of wood exposed above ground and compares those values to moisture contents obtained using simulated rainfall and immersion methods. Laboratory leaching trials with CCA-treated specimens were also conducted and the results compared to published values for leaching of CCA-treated specimens exposed above ground outdoors. Previous researchers have reported that the moisture content of wood exposed above ground may range from over 80% to minimums of approximately 10%. In most studies, the maximum moisture content fell within 40% to 55% for horizontal specimens and between 30% to 50% for vertical specimens. Minimum moisture contents were generally in the 10% to 15% range. Average moisture contents reported for horizontal exposures ranged from 21% to 26%, while the averages reported for vertical exposure were 19% and 25%. The simulated rainfall method produced a maximum moisture content of 49% in CCA-treated lumber specimens, but did not provide realistic drying between rainfall events. A 14-day continuous immersion with lumber specimens yielded an average final moisture content of 52% in CCA-treated specimens. Both methods produced cumulative copper and chromium leaching values slightly below, and arsenic values slightly above, those reported for wood exposed outdoors. Trials with smaller specimens indicated that immersion and drying schedules could be adapted to create moisture contents similar to those reported in service. Leaching methods could be improved by developing a better understanding of moisture conditions in wood exposed above ground, and of the role of drying cycles in preservative leaching.