'Integrating Knowledge to Inform Mercury Policy'

Scientists at the Centre for Ecology & Hydrology 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.

Professor Ed Tipping, who researches environmental chemistry at the Centre for Ecology & Hydrology, 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 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.

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.

Planet Earth Online 20 May 2010

Tipping, E, et al, Critical Limits for Hg(II) in soils, derived from chronic toxicity data, Environmental Pollution (2010),  doi:10.1016/j.envpol.2010.03.027