Effect of Precipitation Pattern on Leaching of Preservative from Treated Wood and Implications for Accelerated Testing

There is a need to develop improved accelerated test methods for evaluating the leaching of wood preservatives from treated wood exposed to precipitation. In this study the effects of rate of rainfall and length of intervals between rainfall events on leaching was evaluated by exposing specimens to varying patterns of simulated rainfall under controlled laboratory conditions. Lumber specimens were pressure-treated with 0,5; 1 or 2,% solutions of chromated copper arsenate (CCA) and exposed to 762 mm of rainfall at rates of 2,5; 8 or 25 mm/h. Intervals between rainfall events were increased for some specimens. Leachate was periodically collected and analyzed to quantify leaching of arsenic, chromium and copper. The quantity of CCA elements leached, per unit rainfall, was consistently greatest at the lowest rainfall rate for all solution concentrations evaluated. Incorporation of additional time between rain events increased leaching at the lowest rainfall rate, but this effect was less noticeable as rainfall rate increased. The results of this study indicate that simulated rainfall leaching tests should be conducted using rainfall intensities at the lower end of those that are typical for the region of interest. The lengths of intervals between rainfall events may also increase leaching, but use of lengthy resting intervals conflicts with the goal of accelerated testing. Further research may be warranted to optimize time between rainfall events and assess the effect of drying periods between rainfall events.