Biogeochemical Fractions of Mercury in Soil Profiles of Two Different Floodplain Ecosystems in Germany

A special sequential extraction (SE) procedure for mercury (Hg) was conducted to determine biogeochemical fractions of Hg and their controlling factors in four contaminated soil profiles located in two distinct floodplain ecosystems which differ in their industrial histories and thus in their Hg loads. The first study area is located at the Wupper River (Western Germany) and the soil profiles reveal sum of Hg (Hgₛᵤₘ) concentrations up to 48 ppm. The second study area is located at the Saale River (Eastern Germany) and the soil profiles have Hgₛᵤₘ concentrations up to 4.3 ppm. The majority of Hg was found in fraction IV (FIV, Hg⁰) for both study areas, indicating its anthropogenic origin. Moreover, we have detected Hg in fraction V (FV) and in fraction III (FIII). As Hg in FV is mostly associated with Hg sulfides being formed under reducing conditions, it indicates reduction processes which usually occurred during flooding. Mercury in FIII (organo-chelated Hg) exhibits a moderate mobility and a high methylation potential. Between Hg in FIII and hot-water-extractable carbon (CHWE) as a measure of easy degradable, labile soil organic matter, we found a significant correlation. Sum of Hg seem to have a high affinity to organic carbon (Cₒᵣg). The concentrations of Hg in the mobile and exchangeable fractions FI and FII were low. Moreover, the significant positive correlation between iron (Fe) and Hg in FIV indicate an interaction between Hg and Fe. The majority of the Hg in our soils is considered to be relatively immobile. However, since the formation of more mobile Hg species via oxidation or methylation might occur in floodplain soils, the low Hg concentrations in mobile fractions should not be underestimated due to their high mobility and potential plant availability.