The global re-cycling of persistent organic pollutants is strongly retarded by soils

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Abstract

‘Persistent organic pollutants’ (POPs) are semi-volatile, mobile in the environment and bioaccumulate. Their toxicity and propensity for long-range atmospheric transport (LRAT) has led to international bans/restrictions on their use/release. LRAT of POPs may occur by a ‘single hop’ or repeated temperature-driven air–surface exchange. It has been hypothesised that this will result in global fractionation and distillation—with condensation and accumulation in polar regions. Polychlorinated biphenyls (PCBs)—industrial chemicals banned/restricted in the 1970s—provide a classic illustration of POP behaviour. A latitudinally-segmented global PCB inventory has been produced, which shows that ∼86% of the 1.3×106 tonnes produced was used in the temperate industrial zone of the northern hemisphere. A global survey of background surface soils gives evidence for ‘fractionation’ of PCBs. More significantly, however, very little of the total inventory has ‘made the journey’ via primary emission and/or air–surface exchange and LRAT out of the heavily populated source regions, in the 70 years since PCBs were first produced. Soils generally occlude PCBs, especially soils with dynamic turnover of C/bioturbation/burial mechanisms. This limits the fraction of PCBs available for repeated air–soil exchange. The forested soils of the northern hemisphere, and other C-rich soils, appear to be playing an important role in ‘protecting’ the Arctic from the advective supply of POPs. Whilst investigations on POPs in remote environments are important, it is imperative that researchers also seek to better understand their release from sources, persistence in source regions, and the significant loss mechanisms/global sinks of these compounds, if they wish to predict future trends.

Section snippets

Introductory remarks

The global transport of persistent organic pollutants (POPs) is of current interest and concern (Klecka et al., 2000). Industrial (e.g. PCBs), agricultural (e.g. DDT, Lindane) and by-product (e.g. polychlorinated dibenzo-p-dioxins and -furans) chemicals have been detected in remote Arctic and Antarctic regions, far from the areas of their use and emission. Concern over the toxicology of these compounds has led to international efforts to control their use and disposal and understand their

PCBs as model POPs

PCBs are model POPs and ideal marker compounds for global re-distribution studies. They were first manufactured in 1929 and their many uses included transformer, capacitor and lubricating oils and adhesives and sealing materials (de Voogt and Brinkman, 1989). Concerns over their bioaccumulation and toxicity led to them being banned throughout much of North America and western Europe in the late 1970s. We compiled information from the literature that accounts for a total global PCB production of

A global PCB usage inventory

To characterise the pulse of PCBs entering the global environment, detailed estimates of their global use were produced. Historical production was estimated by coupling information on production rates in various countries with information on the chemical composition of the technical mixtures that were manufactured. The spatial and temporal patterns of consumption in 114 countries were estimated, based on reported import and export figures for PCBs and assumptions using Gross National Product as

The role of soils in the global mass balance

Fugacity-based mass balance models can be used to estimate the proportion of compound emitted to the atmosphere, that partitions to soil (Mackay, 2001). Once POPs are released to the environment, the soil compartment can dominate the mass balance, because of the strong affinity of POPs for soil organic matter (SOM) (Mackay, 2001). Soil receives a substantial proportion of the emission inventory through cumulative atmospheric deposition and direct inputs at the surface from spillages or to the

Soils and their role in retarding re-cycling

Soils are clearly very influential in retaining POPs and preventing (or retarding) their large-scale global re-distribution. Fractionation of the congener mixture in soils provides further evidence for this. Fig. 2 shows the percentage of tri- through to octa-chlorinated homologues which contribute to the total PCB concentration in a sub-set of 43 soils taken on a transect in 1998 from the southern UK (50°N; average temperature 10 °C) to northern Norway (70°N; 0 °C) and Bear Island (76°N;

The re-cyclable ‘pool’ of PCBs

Ambient air total PCB concentrations in rural areas are typically of the order of 10–100 pg m−3 (Cousins and Jones, 1998, Lee et al., 1998) implying that the global atmospheric burden of PCBs is only ca. 10–100 kg at any one time. This is a tiny fraction of the global inventory, or the loading in surface soil. The atmospheric burden is subject to rapid turnover (Lee et al., 1998) and constitutes part of a small ‘environmentally mobile’ fraction of the total PCB burden. This also resides in

Implications for POPs transport to polar regions

Polar regions have provided a focus for research and concerns about POPs (e.g. Wania and Mackay, 1996, Jensen et al., 1997), and international regulatory efforts (UNECE, 1998), because of the POPs' extreme persistence in cold environments and the link to lipids in foodchain transfer/biota. Indeed, high exposures in native Inuit populations and adverse effects on the endocrine system of polar bears, the Arctic top predators, have been linked to the current ambient levels of POPs (Jensen et al.,

Implications for future perspectives on POPs

By the same token, this study shows that soil residues are (and will remain) higher close to source, while the best estimates of emissions suggest that the bulk of the PCBs manufactured/used (perhaps >70%) have not entered the environmental pool, because they are still associated with diffusive source materials. PCBs clearly ‘escape’ from source very slowly, implying that ambient levels will continue to decline only slowly in source (Sweetman and Jones, 2000) and remote (Hung et al., 2001)

Acknowledgements

This study was conducted as part of the GLOBALSOC project, with financial support from the European Union (ENV4 CT97 0638). We are grateful to our GLOBALSOC collaborators, particularly Dr. J. Axelman, Dr. O. Gustafsson and Professor D. Broman at the University of Stockholm and Dr. J. Grimalt at the CSIC in Barcelona for many helpful discussions. We also thank the numerous individuals who helped with sampling throughout the world.

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