Dioxin Toxic Equivalent Concentrations in Wood Ash

Abstract

The purpose of this study was to quantify toxic equivalent (TEQ) concentrations of polychlorinated dibenzo- p -dioxins (PCDD), polychlorinated dibenzofurans (PCDF), and polychlorinated biphenyls (PCB) in ash residues generated by wood-fired boilers in Washington state (USA). With non-detects (ND) set to one-half the detection limit (DL) and employing mammalian toxic equivalency factors (TEF) recommended by the World Health Organization (WHO), TEQ ranged from 0.36 to 11000 ng/kg ( n = 13). When the three highest samples were removed, mean TEQ declined dramatically from 840 ng/kg ( n = 13) to 2.2 ng/kg ( n = 10) with a corresponding fall in standard deviation from 3000 to 2.0 ng/kg. Two TEF methods (WHO vs . U.S. Environmental Protection Agency [USEPA]) were compared and three ND methods were evaluated (denoted by N1, N2, and N3 corresponding to ND = 0, ND = 0.5 DL, and ND = DL, respectively). Although TEQ was significantly correlated (Bonferroni P <0.05) across the three ND methods, median TEQ differed significantly ( P <0.05) among the ND methods with N1<N2<N3. For N2 and N3 methods, median PCDD/PCDF TEQ was significantly higher (Bonferroni P <0.05) when calculated with TEFs employed by WHO vs . USEPA. When wood ash is used as a liming agent or soil amendment, modeled steady-state soil TEQ may be in the range of regulatory benchmarks, depending on wood ash TEQ content and application rate. Overall, these data illustrate the high variability in wood ash TEQ, the notable impact of TEF and ND methods on reported TEQ, and the potential human and ecological concern associated with land application of wood ash and increased soil TEQ.