Quantifying diffuse contamination: Comparing silver and mercury in organogenic and minerogenic soil.

For both, silver (Ag) and mercury (Hg), the median concentrations in an aqua regia extraction of minerogenic top- and subsoil from continental scale geochemical surveys (Australia, China, Europe) are around 0.02 mg/kg. When the soil O horizon is collected as topsoil sample, the concentration of again both elements is higher by about a factor of 10 (range 7-30), with median concentrations around 0.2 mg/kg Ag and Hg. Geochemical maps of top- and subsoil at different scales for both elements display regional patterns which reflect mainly geology, climate and topography. Anthropogenic sources like mines, power plants, or major cities visually occur only as local anomalies. For Ag in organogenic topsoil the maximum possible input due to diffuse contamination is estimated to be in the 0.02 mg/kg range, about 10% of the median concentration in the soil O horizon. For Hg this value is slightly higher at 0.03 mg/kg. In the soil O horizon Hg concentrations show less variability than in the C horizon. Substantial Hg soil contamination should lead to noticeably increased Hg/Ag ratios.