Modeling leachability of metals from preservative-treated wood during rainfall events

Abstract

Alkaline copper quaternary (ACQ) has been used recently as a wood preservative. This study evaluated leachability of metals from ACQ-treated wood during rainfall events, and developed statistical models to predict copper leaching. Four red pine lumber leaching systems (all with a lumber volume of 0.14 m3) were constructed outdoors: a pile of large (15 cm x 15 cm x 51 cm) new ACQ-treated lumber, a pile of small (7.6 cm x 7.6 cm x 51 cm) new ACQ-treated lumber, a pile of large new untreated lumber, and a pile of small new untreated lumber. It was found that the occurrence of leaching process was significantly affected by rainfall depth, rainfall duration, and relative air humidity. Leachate volume could be predicted with rainfall depth, daily average temperature, and interval time after the previous rainfall event. Leaching percentage (leachate volume divided by rain volume) was significantly affected by lumber size and preservative-treatment. Arsenic, copper, chromium, and zinc in the leachate were mostly in dissolved forms. Average copper concentration in the ACQ-treated wood leachate (4025 µg/L) was much higher than the New York State permit limit for stormwater discharge associated with timber products (68 µg/L). Lumber size did not make a statistically significant difference in leachate copper concentration. Copper leaching was found to be a dynamic process that was affected by weather conditions. In the leachate from the ACQ-treated systems, copper concentration was well predicted by a regression model based on rainfall depth, rainfall duration, leachate pH, and interval days after the previous rainfall event. Cumulative copper mass in the leachate from the ACQ-treated systems was highly correlated with cumulative rain depth.