Correspondence Comment on “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal” The recent article titled “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal”, by Khan et al. (1), includes a prediction on long-term impacts to the environment from the disposal of chromated copper arsenate (CCA)-treated wood in Florida municipal solid waste (MSW) or construction and demolition (C&D) debris landfills. The authors extrapolate the results of lysimeter tests to predict the mass of arsenic leaching over the next 2000 years from CCA-treated wood disposed in these landfills. We believe that these predictions are based on several flawed assumptions, none of which are supported by the authors’ own data. These flawed assumptions include (a) use of a constant rate of arsenic leaching, (b) equating leaching of arsenic with actual release to the environment and impacts to groundwater receptors, and (c) misuse of groundwater monitoring data from C&D landfills that clearly show no impacts to date from past disposal practices. As a consequence, the conclusions of the article are seriously misleading. (a) Constant rate of leaching. The authors assume that the rate of leaching of arsenic species will remain at a constant rate (0.048% per year for C&D landfills), based on data presented in Figures 2 and 3. Yet inspection of these graphs shows that the rate of leaching of inorganic arsenic species approaches zero after about 300 days of operation of the lysimeters (lysimeter 4) in the C&D landfill simulation. The rate of leaching is clearly not linear with time as assumed by the authors. A significant reduction in the rate of leaching from a solid (e.g., CCA-treated wood) to a liquid (e.g., infiltrating water) over time is consistent with slow mass transfer due to the physical-chemical processes occurring during the leaching process. Thus, use of a linear rate of leaching significantly overestimates the rate and ultimate amount of leaching of inorganic arsenic species from the CCA-treated wood disposed in the C&D landfills. (b) Assumption that leaching is equivalent to release to the environment. The authors erroneously imply that once inorganic arsenic species have leached from the disposed wood, this will cause undesirable water quality impacts to the groundwater. In order for this to occur, these species must first be leached from the disposed wood, then migrate through the landfill-soil interface. The arsenic species must then be transported through the vadose zone, the thickness of which can vary between several feet to more than one hundred feet, depending on the location in Florida. Finally, if the arsenic species reach groundwater, they must migrate a sufficient distance to impact compliance monitoring wells before ultimately impacting potential receptors. The authors have not evaluated any of these processes under conditions likely to be found in Florida to estimate if, in fact, any impacts are likely to occur in the future. (c) No evidence to show current impacts caused by past disposal of CCA-treated wood. The authors attempt to show that results of four months of groundwater sampling of groundwater monitoring wells located at 21 C&D landfills support their hypothesis that CCA-treated wood disposed in these landfills poses a threat to groundwater quality. However, 10.1021/es060601b CCC: $33.50 Published on Web 06/22/2006
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they conclude that there was “no significant difference between the total arsenic detected in background wells and those of down-gradient wells”. This is consistent with results of an extensive study that we recently conducted on behalf of the Wood Preservative Science Council (WPSC) (2). We analyzed groundwater monitoring data reported by the Florida Department of Environmental Protection (FDEP) from the 116 registered C&D landfills in the state. This database included over 4500 samples collected from February 1992 through July 2004 from three well types (background, detection, and compliance monitoring wells). When viewed as a single population, the arithmetic average total arsenic concentration (9.53 µg/L) was higher in the background samples than in either the detection well (5.82 µg/L) or the compliance well samples (7.31 µg/L). For 70 of the C&D landfills, data were available for samples from both background and compliance wells, and again the background arithmetic average (10.36 µg/L) exceeded the average in the compliance wells (7.17 µg/L). We concluded that there was no evidence that arsenic was migrating out of the C&D landfills in Florida in agreement with the Khan study. Despite the conclusions from their own monitoring data, the authors proceed to speculate that “it may be too early” to observe impacts, and that future impacts are “likely”, even though the authors state that over 5200 tons of arsenic were present in C&D landfills as of 2000. We are not aware of any detailed modeling of the complex phenomena controlling the fate of inorganic arsenic species present in CCA-treated wood disposed in C&D or MSW landfills. However, there is a very large body of peer-reviewed literature on the fate and transport of inorganic arsenic species in the subsurface. Recent articles in Environmental Science and Technology have promoted the use of natural attenuation as a groundwater remedy for certain sites impacted by arsenic releases (3) given the relative immobility of inorganic arsenic species under certain geochemical conditions. The FDEP also recognizes natural attenuation of inorganic arsenic species having established a natural attenuation default source concentration for total arsenic of 500 µg/L for closure of underground storage tanks (4). In summary, we are unaware of any evidence supporting the authors’ speculations on the long-term fate of arsenic associated with disposed CCA-treated wood. Our analysis indicates that releases of inorganic arsenic from the C&D landfills in Florida have not yet occurred and that releases in the future represent a low-risk probability. Therefore, expensive and potentially impractical regulatory programs to restrict the disposal of CCA-treated wood at C&D landfills are neither scientifically justified nor likely to have any significant benefits to the prescribed regulatory requirements for protection of human health and the environment. More careful analysis of this problem is warranted to replace the unsupported predictions made in this article.
Acknowledgments Submitted by the authors on behalf of the Wood Preservative Science Council.
Literature Cited (1) Kahn, B. I.; Jambeck, J.; Solo-Gabriele, H. M.; Townsend, T. G.; Cai, Y. Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal. Environ. Sci. Technol. 2006, 40 (3), 994-999. VOL. 40, NO. 15, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(2) Kavanaugh, M. C.; Kresic, N.; Schwarz, S. C.; Wright, A. P. S. Analysis of Data & Research Concerning CCATreated Wood Disposal & Economic Impact and Practicability Issues; Redoak Consulting: Ft. Lauderdale, FL, 2005; http://www.woodpreservativescience.org/ finalccareport070105.pdf. (3) Reisinger, H. J.; Burris, D. R.; Hering, J. G. Remediating Subsuface Arsenic Contamination with Monitored Natural Attenuation. Environ. Sci. Technol. 2005, 39 (22), 458A-464A.
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(4) Florida Department of Environmental Protection. Storage Tank System Closure Assessment Requirements; Division of Waste Management, FDEP: Tallahassee, FL, 1998; Table 9.
Michael C. Kavanaugh, Neven Alexandra P. (Sandy) Wright Malcolm Pirnie, Inc. ES060601B
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