Environ. Sci. Technol. 1999, 33, 399-405
Interpreting, Correlating, and Predicting the Multimedia Concentrations of PCDD/Fs in the United Kingdom JOHN G. COLE AND DONALD MACKAY* Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada K9J 7B8 KEVIN C. JONES AND RUTH E. ALCOCK Institute of Environmental & Biological Sciences, Lancaster University, Lancaster, LA1 4YQ U.K.
Reported concentrations of 17 polychlorinated dibenzop-dioxin and furan congeners or congener groups in ten environmental media of the U.K. have been compiled. The relative equilibrium status of each congener is deduced with respect to its concentration in urban air which is regarded as the primary source of multimedia contamination. This is accomplished by calculating the equilibrium lipid partitioning (ELP) concentration for each congener and medium using partitioning information derived from measured physical chemical properties. It is shown that the extents of departure from equilibrium conditions between media are consistent for all congeners except the octa-chlorinated compounds. This method of analyzing monitoring data and the consistency of intermedia ELP concentration ratios facilitates the interpretation of the fate and levels of these persistent chemicals in a multimedia environment and yields an approximate predictive capability by which levels in one medium can be used to estimate levels in other media. Benefits of this ELP concentration approach are discussed as a means of enhancing the more complete and detailed interpretation of monitoring data for persistent compounds.
Introduction Cumulative atmospheric emissions are believed to be the primary source of polychlorinated dioxin and furan (PCDD/ F) residues in the environment. These substances are produced by a range of processes including municipal and solid waste incineration, metal smelting, domestic coal combustion, and vehicular emissions (1, 2). The major sources are thus likely to be industrialized and urbanized centers. Subsequent transport in, and deposition from, urban air causes dioxins to enter other and more remote media of the environment such as soil, herbage (vegetation used as animal feed), sewage sludge, inland waters, sediment, and ultimately human adipose tissue as a result of food consumption. Considerable efforts have been devoted to compiling source inventories and concentration databases and reconciling these data within the context of mass balances (3). Although the atmosphere is the primary medium of discharge, it is essential to quantify the presence of PCDD/ * Corresponding author phone: (705)748-1489; fax: (705)748-1569; e-mail:
[email protected]. 10.1021/es980729w CCC: $18.00 Published on Web 12/19/1998
1999 American Chemical Society
Fs in other media since they can represent major sources of exposure. For example, the air-vegetation-livestock-human pathway is probably the most important route of human exposure (4, 5). Air-soil exchange is critical because soils are probably the major environmental repository of these substances (3). Successful efforts have been made in the U.K. and elsewhere to reduce primary emissions, thus it is likely that the multimedia concentrations are in a dynamic state of adjustment, being controlled by a variety of kinetic and thermodynamic factors. Our objective in this study is to gather multimedia concentration data for a set of PCDD/F congeners in the UK and examine the levels in terms of their relative equilibrium status. We thus examine how far media concentrations are displaced from equilibrium, e.g. how do air concentrations compare with concentrations in equilibrium with the soil? Further, we explore if these displacements are consistent from congener to congener. If consistent patterns of multimedia partitioning are revealed they may assist monitoring and assessment programs, and could even form the basis of an approach for correlating and predicting multimedia concentrations. The alternative to this partitioning-based approach is an empirical multiple regression which lacks mechanistic insights. Although the data are only from the UK, it is believed that the findings may apply to other industrialized temperate regions. We emphasize that this is an exploratory or screening-level assessment in which the aim is to determine the feasibility of this approach in the expectation that, if successful, it may be refined in more detailed future studies. Equilibrium Concentrations and Fugacity. The concentrations of chemicals in the environment are generally expressed in units such as pg m-3 or ng g-1 which do not directly convey any information about the equilibrium status, chemical potential, or fugacity of the compounds between the various environmental phases. We suggest that when interpreting multimedia monitoring data it is useful to report both concentrations and fugacities or another equilibrium criterion. When the fugacities of a chemical are equal in two media, such as air and water, the two media are in equilibrium with respect to mass diffusion. Chemicals generally diffuse from media of higher fugacities to those of lower fugacities. The magnitude and sign of the fugacity difference or the value of the fugacity ratio can provide useful insights into contaminant fate. One approach is to convert all concentrations to fugacities and compare these fugacities as quotients. Fugacity quotients are now routinely calculated for air-water exchange (6) and fish-water bioaccumulation (7). An equivalent approach is to calculate, for each medium, the equilibrium partitioning concentrations in a single medium such as air, water, or lipids by multiplying the reported concentration by the appropriate partition coefficient. For hydrophobic organic chemicals the primary media of accumulation and exposure are organic matter and lipids, thus the use of lipids as the base phase for comparing concentrations is attractive. Concentrations in biota are thus lipid normalized, and those in other media such as air or water are multiplied by the chemicals’ lipid-air or lipid-water partition coefficients. We refer to these concentrations as “Equilibrium Lipid Partitioning” or ELP concentrations. Observed environmental concentrations are, in general, not at equilibrium, thus for a specific chemical ELP concentrations or fugacities are generally unequal. Local contaminant sources, degradation processes, and nondiffusive processes such as deposition in association with particles VOL. 33, NO. 3, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. PCDD/F Concentrations in Environmental Compartments air, pg/m3 urban rural
river water, pg/m3
soil, ng/kg urban rural 1.7 0.1 2.2 0.2 5.4 0.9 11.9 2.9 5.4 0.6 246 23 2690 161
2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD
0.01 0.04 0.05 0.10 0.08 1.06 3.80
0.002 0.01 0.01 0.02 0.02 0.22 0.73
