Mercury Distribution and Speciation in Soils Affected by Historic Mercury Mining

Abstract

Vertical distribution and mercury speciation in soil profiles were studied to evaluate the extent of contamination and potential Hg mobility in a historical Hg mining area. Three sites were chosen for a case study of Hg contamination. One site was considered a low impact site located 400 m uphill from the mining activities and two sites were located in a presumably more highly impacted area. Mercury speciation was determined by thermo-desorption analysis. High impact soils in a close vicinity of the mine shaft exhibited relatively high Hg concentrations ranging from 8 to 10 μg g−1 in the surface (Ah) horizons and from 3 to 9 μg g−1 in the underlying mineral soils. Approximately 50–80% of total Hg in all soil horizons of high impact soils was present as relatively insoluble HgS. The remaining 20–50% of total Hg was attributed to Hg adsorbed on mineral surfaces, Fe-oxyhydroxides or clay minerals. The highest Hg concentration in the low impact soils was detected in the Ah horizon (7 μg g−1). Mineral soil horizons contained relatively low Hg concentrations (≤0.6 μg g−1). Over 40% of total Hg in the Ah horizon of the low impact soils was HgS, likely deposited as fine particles derived from past mining activities. The Hg in mineral horizons of low impact soils was exclusively Hg(II) weakly bound to mineral particles. Our study indicated that HgS contamination in the vicinity of a former mining area occurs in most Ah horizons of the area, but contamination of mineral soil horizons appears to be limited to the sites downhill from the mine shaft.