Correspondence Comment on “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal” Khan et al. (1) investigated the speciation of arsenic leachate generated from the disposal of CCA-treated wood using a series of lysimeters operated for a period of up to 17 months. The study additionally calculated leaching rates and forecast the effects of CCA disposal on groundwater quality. Although Khan et al. (1) reported arsenic concentrations in groundwater downgradient of actual construction and debris (C&D) landfills that were in the range of background, they concluded that the effects of disposal of CCA-treated wood in such facilities will likely be observed in the future. The objective of the present communication is to describe the uncertainties in the forecasts of Khan et al. (1), a topic initially raised by Kavanaugh et al. (2), but subsequently refuted by Khan et al. (3). As discussed below, arsenic concentrations in the experimental lysimeters are higher than would be expected from actual C&D landfills, leading Khan et al. (1) to overestimate the effects of CCA-treated wood disposal on groundwater quality. In addition, the justifications provided by Khan et al. (3) for the discrepancies between their lysimeter experiments and groundwater monitoring data are not technically defensible to a degree of scientific certainty. We conclude that there are too many uncertainties in the research conducted by Khan et al. (1) to reasonably forecast the effects of CCA-treated wood disposal, and that their own data confirm this. First of all, we disagree with Khan et al. (3) that the constant leaching rate used in their forecasts may or may not overestimate leaching behaviorsthe weight of evidence suggests that the predictions are high. As pointed out by Kavanaugh et al. (2), the forecasts of Khan et al. (1) were based on leaching rates calculated from a single simulated year, even though the leaching rate of inorganic arsenic species diminished significantly during the course of the experiments. Also, the high proportion of new versus weathered CCA-treated debris used in the C&D lysimeter (a 50:50 ratio was used in Lysimeter 4) would have contributed to overestimated forecasts. The lumber industry sponsored a phase-out of CCA-treated wood for residential uses in 2003 (4), and we expect the result to be a substantially lower proportion of new versus weathered debris in the future. This is important because leaching rates have been shown in several studies to decline over the service life of CCAtreated wood structures. Finally, Khan et al. (1) excluded soil from the lysimeters, even though, according to Reinhardt et al. (5), 8%-12% of waste (by mass) in Florida’s C&D landfills is soil and excavated earth, and it has been shown that even sandy soils have the ability to reduce arsenic concentrations in leachate due to adsorption. In addition to overestimating its forecasts, Khan et al. (1) also introduced uncertainty by using chemical methods that are incapable of distinguishing between As(V) species. Prior research has established that arsenic on the surface, and in aqueous rinsate, of CCA-treated wood is present as a molecule wherein the arsenic is complexed with chromium in a form that is expected to be stable under environmental conditions (6, 7). Although arsenic in bulk rinsate collected from the 10.1021/es061792l CCC: $37.00 Published on Web 12/05/2006
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lysimeter likely contained arsenic in this complexed structure, Khan et al. (1) reported only a total As(V) concentration. Understanding the form of arsenic is relevant to understanding the likely environmental fate of arsenic from wood leachate, because arsenic in the complexed form would be expected to behave in the environment in a manner different from that of dissolved arsenate ions. Finally, the modeling study that Khan et al. (3) cited as evidence that groundwater plumes might not yet have reached downgradient monitoring wells mostly demonstrated that the potential for arsenic migration to occur is very sensitive to infiltration rate, hydraulic conductivity, and the selected distribution coefficient. These model results are consistent with our contention that downgradient arsenic migration is site specific and cannot be forecast using the lysimeter experiments of Kahn et al. (1). In summary, the lysimeter data are interesting and likely present a worst-case leaching scenario. However, Khan et al. (1) provide no evidence to support their claim that elevated arsenic concentrations from the disposal of CCA-treated wood in C&D landfills will likely be observed in the future. Their own data suggest that the current rate of disposal of CCA-treated wood is already very close to the maximum expected (4), and impacts have not been observed to date. Saxe et al. (9), in a separate analysis, evaluated 62 unlined C&D landfills in Florida and found that only six had estimated average arsenic concentrations in compliance wells that exceeded the drinking-water standard. The study found that arsenic concentrations were statistically similar to background at five of those six landfills, and in the last, arsenic was detected above 0.01 mg/L in a compliance well during a single sampling event. We believe that uncertainties such as the effect of debris type, debris size, soil content, chemical speciation, and sitespecific hydraulic and chemical properties (e.g., redox state, adsorptive capacity) make it impossible to use the lysimeter data of Khan et al. (1) to develop a meaningful prediction of the fate of arsenic placed in C&D landfills. Exponent acknowledges Georgia-Pacific Corporation for their support in generating these comments.
Literature Cited (1) Khan, 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, 994-999. (2) Kavanaugh, M. C.; Kresic, N.; Wright, A. P. 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. (3) Khan, B. I.; Solo-Gabriele, H. M.; Jambeck, J.; Townsend, T. G.; Cai, Y. Response to Comments on “Release of Arsenic to the Environment from CCA-Treated Wood. 2. Leaching and Speciation during Disposal”. Environ. Sci. Technol. 2006, 40, 4811-4812. (4) 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. (5) Reinhardt, D. R.; Townsend, T. G.; Heck, H.; Chakrabarti, S.; Cochran, K.; Medeiros, S. Generation and composition of construction and demolition debris in Florida; Report 03-08; Florida Center for Solid and Hazardous Waste Management: Gainesville, FL, 2003. (6) Nico, P. S.; Fendorf, S. E.; Lowney, Y. W.; Holm, S. E.; Ruby, M. V. Chemical structure of arsenic and chromium in CCAVOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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treated wood: Implications of environmental weathering. Environ. Sci. Technol. 2004, 38, 5253-5260. (7) Nico, P. S.; Ruby, M. V; Lowney, Y. W.; Holm, S. E. Chemical speciation and bioaccessibility of arsenic and chromium in chromated copper arsenate-treated wood. Environ. Sci. Technol. 2006, 40, 402-408. (8) Jambeck, J. The disposal of CCA-treated wood in simulated landfills: Potential impacts. Ph.D. Dissertation, University of Florida, Gainesville, FL, 2004. (9) Saxe, J. K.; Wannamaker, E. J.; Conklin, S. W.; Shupe, T. F.; Beck, B. D. Evaluating landfill disposal of chromated copper arsenate (CCA) treated wood and potential effects on groundwater: Evidence from Florida. Chemosphere 2006 (in press).
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Brad Bessinger* Exponent 5335 SW Meadows Road, Suite 365 Lake Oswego, Oregon 97035
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