A meta-analysis revealed that while there is often a greater degree of contamination of soils and sediments with metals, contamination by mercury (Hg) is a large concern owing to its toxicity at low concentrations. The case study in Guyana characterized Hg and concentrations of other metals (As, Cd, Co, Cu, Mn, Ni, Pb, and Zn) in soil and sediment within gold mined areas. Metal concentrations were low in soil and sediment of sampled gold mines, while the concentrations of Hg were much higher in soil and sediment and contamination was not localized to the mine site. Moss bags and Hg passive air samplers (MerPAS) were deployed to measure atmospheric Hg around a gold mine in Mahdia, Guyana over a 90-day period as well as a 2-day period that encompassed a periodic burn (typically 2-hr). Mercury in moss and MerPAS were positively correlated over both deployment periods, but Hg concentrations measured during the 2-day event were several-fold higher in both moss and MerPAS compared with the 90-day exposure demonstrating that most of the Hg sorbed to both moss and passive samplers is lost during periods of inactivity. Using the 2-day deployment as a conservative estimate of atmospheric Hg exposure, Hg air concentrations around the burning station exceeded 100,000 ng m-3 averaged over a 48-hr period, and moss Hg concentrations were greater than 250,000 ng g-1 around the burning station, although Hg concentrations in both media decreased rapidly with distance. Mercury concentration in soil and sediment was strongly related to organic matter content, which tended to be higher away from the mine site. These controls of organic matter (carbon; C) cycling on Hg distribution and movement are clear at sites exposed to high atmospheric Hg and exist at the global scale, although Hg:C ratios in environmental media are greatly elevated at the gold mine site compared with the global average. Locally sourced biochar did not sufficiently improve physical properties (porosity) in overburden soil, which showed the worse plant response, possibly due to the high clay content that contributes to the "baked" condition of these soils and restrict root growth.
Author Keywords: Biochar, Environmental contamination, Gold mining, Mercury, Metals, Organic matter