Introduction:
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 Types A and B have generally been replaced
by Type C since its introduction in 1968.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.
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