Identification of fractions of mercury in water, soil and sediment from a typical Hg mining area in Wanshan, Guizhou province, China

Water, sediment and soil in a region with severe Hg contamination resulting from Hg mining and retorting continue to receive anthropogenic Hg loading from discarded tailings and mine waste calcine piles, even after mining has ended. Crucial for potential long-term mobilization and uptake in biota, however, are chemical forms of Hg in the environment. In order to assess potential mobility and bioavailability of Hg in contaminated areas, a refined sequential extraction technique was used for Hg in soil and sediment, allowing identification of the following conceptual Hg fractions: elemental Hg, exchangeable Hg, strongly-bound Hg and organically-bound Hg.This fractionation scheme was used on stream sediment and rice paddy soil collected from the Wanshan Hg mining area in China. Concentrations of total Hg (THg) were as high as 480 mg kg−1 in stream sediment samples and reached 130 mg kg−1 in soil samples. Exchangeable- and strongly-bound Hg constitute 10–30% and 20–40% of the THg, respectively, for most soil and sediment samples. The residual fraction varies considerably (4–82%), with higher Hg concentrations in the sediment samples than in soil samples. Rice paddy soil, which is richer in soil organic matter, also contains a small, but significant amount of organically-bound Hg. Exchangeable-, strongly-bound- and organically-bound Hg fractions are dominantly sorbed to fine particles, which enhances their mobility during high flow in the streams. Elemental Hg is only found in significant proportions proximal to a few artisanal retorts that are still active.