Long-term soil accumulation of chromium, copper, and arsenic adjacent to preservative-treated wood.

Abstract

Chromated copper arsenate (CCA) treated wood has been used extensively in outdoor applications, ranging from fence posts to decking and wood foundations. CCA treatment provides long-term protection for wood because chemical reactions that take place during treatment fix the preservative elements in the wood. However, varying amounts of CCA have been reported to leach from the wood (Lebow 1996). The Environmental Protection Agency (EPA) and CCA producers recently reached an agreement to limit future use of CCA for some types of applications. One area of concern is the long-term accumulation of leached CCA in soil adjacent to treated wood structures. A study of CCA concentrations in soil beneath residential decks reported substantially elevated levels of copper, chromium, and arsenic (Stilwell and Gorny 1997). However, this finding conflicted with other studies that reported much lower concentrations of leached CCA components in soil adjacent to forest boardwalks (Comfort 1993, Lebow et al. 2000). The conflicting nature of these findings demonstrates the difficulty of interpreting soil measurements adjacent to in-service structures. The original treatment may be unknown, and there may be little historical data to indicate whether the site was previously exposed to contamination from construction debris or other non-leaching sources. In complicated structures such as decks, it may also be difficult to determine the surface area of the structure that is contributing to soil accumulations in any specific area. One way to overcome these obstacles is to sample soil adjacent to test specimens in exposure sites. The USDA Forest Service, Forest Products Laboratory uses such specimens to evaluate the long-term efficacy of wood preservatives. Treated wooden stakes are buried to one-half their length at exposure sites in Wisconsin and Mississippi. Treatment conditions and original preservative concentration are known for each stake, as are site history and conditions. The spacing of the stakes, and their simple geometry, make it possible to assume that the soil levels are attributable to a specific volume of wood. In this paper, we report soil levels of arsenic, copper, and chromium adjacent to stakes treated with CCA and exposed at test sites in Wisconsin and Mississippi for 22 years. The effects of site location and preservative concentration are discussed.