Box 7E The problem of methylmercury in Sweden

The environmental history of mercury in Sweden vividly illustrates the problem of hidden pollution. In the 1960s, the toxic effects on birds and other animals from the use of fungicides were obvious and the use of mercury compounds as fungicides was stopped. In the 1970s, emissions from industrial sources (for instance, chlor-alkali plants) were decreased to almost zero. Thus, the mercury problem was thought to be solved in Sweden. In the 1980s, the problem re-emerged as a delayed response. The present mercury content in pike exceeds the level of 1 mg/kg in fish in at least 10 000 lakes in Sweden. Part of this can be attributed to the fact that, even though the source has been drastically reduced, mercury which has accumulated over time, especially in forest soils (mainly bound to organic matter), is able to be mobilised through chemical and biological processes.

Recent results indicate that a substantial amount of the mercury flux is in the methylmercury form which is highly toxic and bio-available. Currently, however, the present leaching of methylmercury forms only a small part of the actual amount stored in Swedish soils (the average methylmercury transport to the lakes per year is less than 0.2 per cent of the total methylmercury stored in soils ­ see Figure 7.2). In addition, the soil stores are still increasing due to atmospheric deposition inputs and, unless the driving forces for the methylation and leaching processes are stopped (by reduced deposition of acidifying compounds - see Chapters 4 and 31), the soil will act as a supplier of mercury for a long time to come (Hultberg et al, 1994).


Figure 7.2 - Input/output balance of methylmercury (MeHg) in a coniferous forested catchment (1 km2) and the methylmercury mass balance of a 50 ha lake in southwest Sweden

Source: Hultberg et al, 1994