Sources and Fates of Polychlorinated Dibenzo-p-dioxins, Dibenzofurans and Biphenyls: The Budget and Source Inventory Approach

STUART J. HARRAD

1 Introduction

Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) have attracted considerable attention in recent decades, owing to concern over their potential adverse effects in humans and wildlife, which are compounded by their ubiquitous environmental presence and resistance to degradation. Amongst the 75 possible PCDDs, 135 PCDFs and 209 PCBs, there exists wide variation in physicochemical properties, bioaccumulative tendencies and toxicity. Figures 1 and 2 illustrate the basic structures and nomenclature of both PCDDs, PCDFs – collectively referred to as PCDD/Fs – and PCBs.

This chapter reviews our knowledge of several key issues pertaining to the environmental presence of these compounds. Constructing source inventories for a group of chemical pollutants permits the targeting of specific sources in order to reduce environmental emissions and hence human exposure, whilst the establishment of environmental budgets facilitates the identification of major reservoirs, and quantification of the extent to which a given pollutant has been released into the environment and been subsequently 'lost' via either biodegradation or environmental transport.

2 Physicochemical Properties and Environmental Levels

PCDD/Fs and PCBs possess low vapour pressures and water solubilities, along with high octan-1-ol/water partition coefficients (KOW values), which are listed for selected congeners in Table 1. When the long biological lifetimes of these chemicals are taken into account (human half-lives of up to 27.5 years have been reported for some PCBs), it is unsurprising that PCDD/Fs and PCBs display significant bioconcentration on ascending food chains, and this is borne out by a summary of their levels in the ambient environment (Table 2).

3 Environmental Budgets

Background and Limitations

In essence, establishing an environmental budget involves quantifying and ranking different environmental compartments as reservoirs of a given pollutant within a defined section of the environment, such as an individual country. The basic principle of an environmental budget is the derivation of a representative concentration for each environmental compartment considered (e.g. 10 µg kg-1 of soil), and its multiplication by an estimate of the volume occupied by that compartment. Whilst obtaining an accurate estimate of compartment volume is not as easy as it would at first appear (requiring answers to such questions as: to what depth are relatively immobile pollutants like PCDD/Fs and PCBs incorporated in soils, and what is the volume occupied by a compartment as loosely defined as 'biota'?), much of the uncertainty involved in environmental budgets is due to problems in deriving representative pollutant concentrations. To illustrate, several attempts have been made to construct environmental budgets for both PCDD/Fs and PCBs. In each case, the accuracy of such efforts is restricted by the extremely limited database relating to concentrations in different environmental media and spatial variations in such concentrations. With regard to spatial variations, information regarding concentrations of these compounds in rural and remote locations is especially scarce. The significant temporal variations in PCB concentrations reported by some authors also hamper efforts to construct a meaningful budget, and budgets conducted using data recorded over a number of years may be subject to significant inaccuracies.

To illustrate the difficulties in deriving representative mean PCB concentrations in an environment as heterogeneous as the open ocean, whilst Tanabe cited a ΣCPCB concentration of 0.6ng dm-3 in North Atlantic seawater, Harrad et al. employed a value for North Atlantic and North Sea seawater (levels in this latter area significantly exceeded the former) of 0.12ng dm-3. Although Harrad et al. noted the higher estimates of others, they suggested that their own concentration estimate may have been too high, taking into account the concentration decline observed with increased sampling depth, and the fact that the bulk of the samples on which their estimate was based were taken only 6 m below the surface.

Clearly, the derivation of representative concentrations for each of the environmental compartments considered is crucial to the accuracy of any environmental budget. To illustrate, whilst Harrad et al. calculated the ΣPCB burden of a seawater volume of 1.14 x 1017 dm3 (including the North Sea) to be 14t, Lohse used a representative concentration of 3.5 ng ΣCPCB dm-3 to derive a seawater ΣCPCB loading of 150t for the North Sea alone, a volume of 5.25 x 1016 dm3. Such significantly different conclusions concerning the burden of a comparatively well-characterized location illustrates the extent of uncertainty associated with the construction of budgets, and particularly their dependence on accurate concentration data.

Despite these limitations, the construction of environmental budgets plays an important rôle in efforts to understand the environmental fate and behaviour of PCDD/Fs and PCBs, and the following section will examine a selection of the most detailed conducted to date.

PCDD/Fs

Harrad and Jones constructed a budget for the terrestrial UK environment. Unfortunately, although freshwater and freshwater sediments were considered, the absence of sufficiently detailed data relating to PCDD/F contamination of terrestrial biota and the marine environment, as well as the difficulty in deriving a representative concentration for an environmental compartment as diverse in composition as biota, meant that the significance of these potentially important reservoirs of PCDD/Fs was not quantified. The findings of this exercise (summarized in Table 3) were that, within the UK, topsoil represents easily the most important reservoir for tetra- to octachlorinated dioxins and furans, with other compartments such as freshwater sediments, ambient air, freshwater and vegetation making comparatively negligible contributions to the overall burden.

PCBs

The comparative ease of PCB measurement has generated a relatively detailed database relating to the presence of these compounds in the environment. As a result, the distribution of...