Mercury in United Kingdom topsoils; concentrations, pools, and Critical Limit exceedances.

The median total mercury concentration in 898 UK rural topsoils, sampled between 1998 and 2008, was 0.095 μg g(-1). Approximate adjustment for unreactive metal produced an estimate of 0.052 μg g(-1) for reactive Hg. The highest concentrations were in the north and west, where organic-rich soils with low bulk densities dominate, but the spatial pattern was quite different if soil Hg pools (mg m(-2)) were considered, the highest values being near to the industrial north of England and London. Possible toxic effects of Hg were best evaluated by comparison with soil Critical Limits expressed as ratios of Hg to soil organic matter, or soil solution Hg(2+) concentrations, estimated by chemical speciation modelling. Only a few percent of the rural UK soils showed exceedance, and this also applied to rural soils from the whole of Europe. UK urban and industrial soils had higher Hg concentrations and more cases of exceedance.