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Response to Comments on “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal” The primary summary statement made by Kavanaugh et al. (1) is that CCA-treated wood should continue to be disposed within unlined construction and demolition (C&D) landfills, that regulatory action should not be taken to restrict the disposal of treated wood to C&D landfills, and that releases of arsenic from C&D landfills represent a future low risk probability. First, we would like to emphasize that statements by Kavanaugh et al. (1) are unfounded as there is overwhelming evidence that supports continued arsenic leaching from CCAtreated wood after disposal within C&D waste and that the quantities of arsenic associated with CCA-treated wood are very large. The fate of arsenic released from C&D landfills should be made on a case-by-case basis, with consideration to the hydrochemical characteristics of a particular landfill site. Below we emphasize these points, which have been overlooked by Kavanaugh et al., as we address their specific comments in detail. Constant Rate of Leaching. Our manuscript (2) clearly states that the scenario evaluated was one of constant leaching rates obtained experimentally over a one-year period in which the simulated C&D landfill underwent typical degradation stages. Arsenic concentrations during this period were relatively constant indicating that anything other than a constant leaching rate would require speculation about the chemistry and infiltration rates over time. A constant leaching rate provides a starting point and draws attention to one possible scenario, which may or may not overestimate leaching behavior. Assumption that Leaching is Equivalent to Release to the Environment. We have not assumed in the original manuscript that leaching is equivalent to release to groundwater. The manuscript clearly states that the relatively low arsenic levels observed in the groundwater can be attributed to the slow release of arsenic from disposed CCA-treated wood, the adsorptive capacity of soil for arsenic, and dilution effects. The paper emphasizes that the amount of arsenic associated with CCA-treated wood is large (33 000 metric tons for Florida, (3)) and this quantity can leach into the soils below landfills where CCA-treated wood has been disposed. Resulting groundwater impacts will depend on the specific location, soil characteristics, geology, and hydrogeology of each location where CCA-treated wood is disposed. No Evidence Showing Current Impacts From Past Disposal of CCA-Treated Wood. One point not emphasized in the manuscript was that the 21 C&D landfills evaluated in this study represented a subset of landfills that met State standards. There were several additional landfills that were not sampled that were out-of-compliance. Even within the group of compliance landfills, there was evidence of elevated arsenic concentrations in a subset of them. Kavanaugh et al. analysis viewed all of the landfill data “as a single population.” * Corresponding author phone: 305-284-3489; fax: 305-284-3492; e-mail:
[email protected]. † Present address: U.S. Environmental Protection Agency, Office of Water/Office of Science and Technology Health and Ecological Criteria Division, Washington, DC 20460. ‡ Present address: University of New Hampshire, Environmental Research Group, Durham, NH 03824. 10.1021/es068006x CCC: $33.50 Published on Web 06/22/2006
2006 American Chemical Society
However, we argue that data for all landfills cannot be lumped together because hydrogeologic and geochemical characteristics vary among landfills. Efforts at modeling arsenic beneath C&D landfill systems show impacts to downgradient wells over a period ranging 60-300 years (4). This range in time is attributed to the ranges in leachate concentrations and sorption coefficients typically observed in Florida soils. Each landfill should therefore be evaluated individually due to differences in chemical and hydrogeologic characteristics. Kavanaugh et al. speculate that arsenic binds indefinitely to soils below landfills and fail to recognize possible arsenic desorption mechanisms attributed to soil type, and microbial and chemical conditions occurring within the subsurface. Arsenic releases can be enhanced through the presence of phosphates (5), sulfates (6), and dissolved organic matter (7); all common components of landfill leachate, with sulfur compounds playing a significant role in C&D landfill leachate due to the presence of gypsum drywall in the waste. Additionally, liners composed of clay have on occasion failed at retaining arsenic, and the low redox state of landfill leachate can induce low redox conditions in groundwater below landfill sites thereby mobilizing arsenic (8). Given the complex chemistry of landfill leachates, it is difficult to predict the degree of sorption or if arsenic would remain bound over time. Summary. Florida groundwater resources represent more than 80% of the state’s drinking water supplies. This resource is also of high quality with background concentrations (2.2 µg/L) below but near the current maximum contaminant level for arsenic (10 µg/L). Kavanaugh et al. fail to acknowledge important characteristics of CCA-treated wood which could impact arsenic concentrations in groundwater and ultimately the majority of the state’s drinking water supplies. These characteristics include the large quantities of arsenic used in the production of CCA-treated wood, evidence showing arsenic releases during in-service use and during disposal, and the fact that arsenic releases are cumulative over time. Given these characteristics, there is a potential for contamination of groundwater supplies to levels that would exceed drinking water standards. Hence, caution is highly recommended when disposing of this material in landfills, in particular C&D landfills which in Florida do not have bottom liners, as the long-term containment of arsenic by soils underlying the landfill systems cannot be guaranteed.
Literature Cited (1) Kavanaugh, M. C.; Kresic, N.; Wright, A. P. S. Comment on “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal”. Environ. Sci. Technol. 2006, 40, 4809-4810. (2) Khan, B. I.; 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, 994-999. (3) Khan, B. I.; Solo-Gabriele, H. M.; Townsend, T. G.; Cai, Y. Release of Arsenic to the Environment from CCA-Treated Wood. 1. Leaching and Speciation during Service. Environ. Sci. Technol. 2006, 40, 988-993. (4) Jambeck, J. R. The Disposal of CCA-Treated Wood in Simulated Landfills: Potential Impacts. PhD Dissertation, University of Florida, Gainesville, FL, 2004. (5) Jackson, B. P.; Miller, W. P. Effectiveness of phosphate and hydroxide for desorption of arsenic and selenium species from iron oxides. Soil Sci. Soc. Am. J. 2000, 64, 1616-1622. (6) Ladeira, A.; Ciminelli, V. Adsorption and desorption of arsenic on an oxisol and its constituents. Water Res. 2004, 38, 20872094. VOL. 40, NO. 15, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(7) Redman, A. D.; Macalady, D. L.; Ahmann, D. Natural organic matter affects arsenic speciation and sorption onto hematite. Environ. Sci. Technol. 2002, 36, 2889-2896. (8) Keimowitz, A. R.; Simpson, H. J.; Stute, M.; Data, S.; Chillrud, S. N.; Ross, J.; Tsang, M. Naturally occurring arsenic: Mobilization at a landfill in Maine and implications for remediation. Appl. Geochem. 2005, 20, 1985-2002.
Jenna Jambeck‡ and Timothy G. Townsend Department of Environmental Engineering Sciences University of Florida Gainesville, Florida 32611-6450
Yong Cai Bernine I.
Khan†
and Helena M. Solo-Gabriele*
Department of Civil, Architectural, and Environmental Engineering University of Miami P.O. Box, 248294 Coral Gables, Florida 33124-0630
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Department of Chemistry & Biochemistry and Southeast Environmental Research Center (SERC) Florida International University Miami, Florida 33199 ES068006X
