Impact of CCA Treated Wood in Aquatic Environments
Anticipated Environmental Impacts From the use of Chromated Copper Arsenate (CCA) Treated Wood in Aquatic Environments.
A reprint from: Western Wood Preservers Institute - July 8, 1993
Western Wood Preservers Institute
601 Main Street, Suite 401
Vancouver, WA 98660
Kenneth M. Brooks, Ph.D.
Aquatic Environmental Sciences
644 Old Eaglemount Road
Port Townsend, WA 98368
Literature Review and Assessment of the Environmental Risks Associated With the Use of CCA and ACZA Treated Wood Products In Aquatic Environments
Depletion of preservatives from treated wood can occur by leaching of water-soluble components, physical loss (abrasion) or chemical and biological degradation. In studies of preservative depletion from treated wood, it may be impossible to identify the mechanisms of depletion. When biological or chemical degradation is present, the results from this type of investigation will over-estimate the environmental loading. Examination of the surrounding medium (i.e. water or sediments) may fail to account for preservative depletion by biological degradation. Further, it is very difficult to discriminate leached inorganic metals from the background in field studies.
Leaching rates from treated wood are strongly influenced by the size and surface area of the test samples. The high ratio of surface area to volume found in thin samples will exaggerate leaching, as will increasing the exposed end grain. Other factors influencing the amount of preservative leached from treated wood include: wood species; the presence of heartwood or sapwood and the preservative retention. The waterborne preservatives considered here are fixed in the treated wood. Fixation is temperature and humidity dependent and inappropriate attention to treating protocols can result in significant increases in leaching rates.
Numerous studies have examined treated wood in the form of sawdust, shavings or Cornell coupons. This is done to speed the leaching process. These studies are valuable for assessing the relative permanence of different wood preservatives and the relative propensity of each metal for leaching. However, their results cannot reasonably be extrapolated to predict leaching from full sized commodities used in the environment. When such extrapolations are made, they will grossly overestimate the potential for environmental contamination. These laboratory studies cannot be substituted for good field studies using full size commodities in natural environments.
Leaching of arsenic, chromium and copper from CCA treated wood:
Kamesam first patented CCA in the United States in 1938. There are currently three CCA formulations registered for use in the United States. Types A, B and C vary in their proportions of chromium, copper and arsenic. Types A and B have generally been replaced by Type C since its introduction in 1968. Type C contains 47.5% hexavalent chromium as CrO3, 18.5% copper as CuO and 34.0% arsenic as As205. This report will focus on CCA, Type C.
The fixation of CCA in wood is a chemically complex process. Pizzi (1982) has provided a comprehensive review of the chemistry and kinetic behavior of arsenic, copper and chromium during fixation of CCA in treated wood. During fixation, following impregnation of the treating solution, chromium undergoes conversion from the hexavalent state to the trivalent state. Most of the preservative (>90%) is chemically bound to the wood fibers by reaction with the wood sugars to form insoluble arsenate precipitates. The length of the fixation period is temperature sensitive and can last from several hours at 45øC to two months at 5øC. Studies by Jain and Lagus (cited in Baldwin, 1985) measuring the efficiency of the fixation mechanism, have shown that drying at 21øC will fix 95% of the metals within four days and 99% within five days. Further studies by Alexander (1991) have shown that the rates of fixation in all wood species are significantly inhibited if the wood is allowed by dry extensively during the fixation process. Improper fixation can result in significantly increased leaching of all CCA components. This report will assume that the treater follows proper AWPA fixation protocols.
Factors affecting CCA leaching rates:
Dahlgren ( 1975) suggests that from Me wood treaters point of view, the most important factors determining the leachability of CCA treated products are the concentration and type of preservative, the drying and storage conditions, and the choice of wood species. Important wood properties are the ion-exchange fixation capacity of copper, the natural pH, and the chemical composition and anatomy of the wood. There are several other factors that cannot be controlled by the wood treater.
Summary and Conclusion:
Copper, chromium, arsenic and zinc are ubiquitous in all aquatic environments. Copper and chromium are essential biological micronutrients. However, in localized areas, anthropogenic inputs can increase these background levels above toxic thresholds. The copper, chromium and arsenic metals present in arsenically treated wood products are toxic to aquatic organisms at varying concentrations. Based on this review, it appears that copper is the metal of most concern to aquatic organisms in both fresh and salt-water environments. Water Quality Standards for Surface Waters of the State of Washington published in WAC 173-201A provide adequate safety margins for the protection of aquatic organisms.
The environmental risks associated with the use of CCA and ACZA treated wood products have been evaluated by quantifying the additional metal loading associated with the use of these commodities in aquatic environments and comparing the resulting concentrations with known chronic and acute thresholds. Throughout this analysis, very conservative assumptions have been used. Leaching rates from CCA and ACZA treated products has been shown to decrease exponentially with time. We have used leaching rates observed in freshly treated wood. We have assumed minimal mixing in aquatic environments. In all instances we have assumed that the metals leached into the water are in their most toxic form and that there is no detoxification by natural processes. Neither of these assumptions are valid - we know that there wilt be substantial additional mixing, and numerous naturally occurring detoxification processes have been reviewed. The risk analysis used in this report is extremely conservative. During initial leaching periods, the actual levels of the toxic forms of the contaminants in question are probable one to two orders of magnitude lower than predicted in this study. Within a matter of weeks or perhaps months, the environmental levels are most likely two or three orders of magnitude less than shown by this analysis.
Even with this very conservative approach to assessing the risks involved, this analysis indicates that the levels of contaminants associated with the use of properly treated CCA and ACZA wood products are well below regulatory standards, and will produce concentrations far below those causing acute or chronic stress in even the most sensitive taxa.
More realistic, estimates of the toxicant loading associated with pressure treated wood products should be made. Those estimates require studies on commodity size products designed to specifically address environmental leaching. While this report suggests that the use of CCA and ACZA products will not impact aquatic organisms, the wood treating industry is encouraged to pursue studies that examine the diversity and abundance of benthic organisms living in proximity to treated wood structures, and to better quantify environmental concentrations of contaminants associated with arsenically treated wood.
The predictions and recommendations made in this study presume that wood products are properly treated and fixed. That assumption will only be valid if the industry continues an aggressive environmental quality control program, and if regulators and the consuming public demand high quality, environmentally sensitive products for the projects they permit and build.