20 May 2010, by Sara Coelho
Mercury is a highly toxic metal that damages the environment and causes chronic diseases in animals, plants and micro-organisms. Now scientists have found that the acceptable critical upper limit for mercury in soils is just 0.13 parts per million.
The critical limit was defined at five per cent, which means that 95 per cent of the organisms will be unaffected by 0.13 microgrammes of mercury per gram of soil.
One third of mercury pollution comes from volcanoes and other natural sources. Past and present human activities, such as burning mercury-enriched fossil fuels, are responsible for the majority of mercury emissions.
'Mercury is a global pollutant that can travel very far in the atmosphere,' says Professor Ed Tipping, who researches environmental chemistry at the Centre for Ecology & Hydrology. 'Knowing how much mercury soils can take up will help us to define acceptable critical load of mercury emissions,' he adds.
Tipping and colleagues surveyed the scientific journals and compiled the results of experiments testing the toxic effects of mercury on 52 organisms, including plants, animals and microbes. They only considered tests where the mercury was added to soils in a soluble form and where the exposed organism was in close contact with the soil.
They joined together the maximum amount of mercury in the soil that each of these 52 species could tolerate and concluded that 95 per cent of the organisms would be safe with a mercury concentration of 0.13 microgrammes of mercury per gram of soil.
This may sound like a very small amount, but mercury is so toxic that only 10 microgrammes (or parts per million) per gram of soil are toxic to the majority of the assessed organisms.
There is another route to determining these limits. Organic matter in soils can take up mercury, reducing the amount available in the soil as a pollutant. Soils rich in organic matter, such as peat, can tolerate higher amounts of mercury. This means that 'another useful way of expressing the critical limit of mercury is per gram of organic matter,' says Tipping.
This method gives a critical limit of 3.3 microgrammes of mercury per gram of organic matter in the soil. The value, reported in Environmental Pollution, is considerably higher than the value of 0.5 microgrammes per gram of organic matter currently in use to calculate critical mercury loads.