Mercury accumulation in soil fractions of podzols from two contrasted geographical temperate areas: southwest Europe and southernmost America

Abstract

The presence of Hg in soil fractions of different size in eight illuvial horizons of podzols from SW Europe and five from southernmost America was studied. Total Hg concentration (HgT) in the bulk soil was very variable, ranging from 11 to 293 ng g−1, attributed to a different intensity of the pedogenetic and weathering processes. The clay fraction showed the highest HgT average with 509 ng g−1 (range 93–2076 ng g−1), followed by the fine silt with a mean of 222 ng g−1 (range 38–836 ng g−1), while the lowest corresponded to the fine sand with an average of 50 ng g−1 (range 2–150 ng g−1). Mercury enrichment in the finer fractions was supported by the high values of the Hg accumulation factor (HgAF, Hg soil fraction / Hg bulk soil) found in the clay (range 1.4–11.5) and in the fine silt fractions (range 0.9–6.8). If the mass of each fraction with regard to bulk soil was considered, mean Hg content followed this sequence: fine silt (31.3 ng g−1), coarse sand (22.7 ng g−1), clay (18.2 ng g−1), coarse silt (15.2 ng g−1) and fine sand (12.5 ng g−1, n = 13). Silt and clay fractions from illuvial horizons of soils from SW Europe were richer in Hg than those fractions from southernmost America. Mercury levels in finer fractions (fine silt and clay), as well as some geochemical indices of Hg accumulation, were closely correlated to total organic C, organo-metal (Al, Fe) complexes and Al and Fe oxyhydroxides, which also exhibited an accumulation in those fractions, suggesting an active role in the Hg enrichment. The results confirmed that illuvial horizons of podzols, particularly those fractions with the lowest size, play a key environmental function preventing the mobilization of Hg in non-polluted soils towards other compartments of terrestrial ecosystems, such as surface waters and groundwater, in which Hg toxicity could be enhanced.