Introduction:
Ammoniacal Copper Zinc Arsenate (ACZA) is an improved formulation
of Ammoniacal Copper Arsenate (ACA). ACZA is licensed and being
produced in the United States. This product has essentially replaced
ACA for the protection of poles, piling and timber. In ACZA, 50%
of the arsenic found in ACA is replaced with zinc in the form of
ZnO. The proportions of copper, zinc and arsenic in ACZA are 50%,
25% and 25% respectively. Wood is treated to a retention of 15.6
kg/m3 (1.0 lb/ft3) ACZA for aquatic applications.
Lebow and Morrell (1993) examined ACZA fixation in Douglas fir
and concluded that zinc plays a dominant role in the retention
of arsenic. Although the distribution of copper, zinc and arsenic
within ACZA treated wood was variable; zinc and arsenic levels
were highly correlated, suggesting formation of a zinc-arsenate
precipitate. As in ACA, the controlled evaporation of ammonia
plays an important role in properly fixing the metals to the treated
wood fibers.
Leaching of arsenic, copper and zinc from ACZA treated wood.
Best and Coleman (1981) examined the leaching of ACZA from small
blocks of wood. Using the leach test protocols specified in AWPA
Standard M11(1), and a retention of 19.4 kg ACZA/m3, they observed
that 1.7% of the copper, (0.6% of the zinc and 0.4% of the arsenic
were lost. This study also observed an exponential decrease in
arsenic leaching with time. Little additional arsenic was leached
from the treated wood after five days in deionized water at pH
6.5.
Cooper (1989) reported the results of similar leaching studies
in CCA-C treated Red pine. Cooper found leaching rates of 6% Cu,
1% Cr and 6% As at pH 5.5. Comparing the results of Cooper's (1989)
CCA study with those of Best and Coleman (1981) for ACZA suggests
that ACZA leaches less copper and arsenic than does CCA. Zinc
has not been considered in detail, in this study because the amount
of zinc leached in ACZA is insignificant compared with either
EPA or Washington State Surface Water Standards?
Ruddick and Ruddick (1992) reviewed an AWPA study that examined
ACZA leaching. A sample containing 729 mg of copper, 183 mg of
zinc and 545 mg of arsenic leached 3.3 mg of copper, (0.4 mg of
zinc and 48 mg of arsenic. This formulation deviates from the
2:1:1 ratio of Cu:Zn:As currently used in ACZA. No information
was provided on the sample size or leaching protocols. However
the losses of 0.45% copper, 0.22% zinc compare favorably with
other studies. The 8.8% arsenic leached in this study apparently
results from the 4:1:3 ratio of Cu:Zn:As used in the treatment.
The current 2:1:1 formulation has been found to optimally reduce
the amount of arsenic lost.
No studies were available describing metal leaching from commodity
size ACZA wood products. Based on the paucity of data, no attempt
will be made to predict losses to aquatic environments. However,
the available data suggests that fixation is more permanent in
ACZA than in either CCA or ACA products. Therefore, it seems reasonable
to suggest that until definitive studies describing environmental
levels of ACZA leaching are conducted, the CCA levels reported
in the previous section should be used.
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 will 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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