EVALUATION OF SURFICIAL MERCURY GEOCHEMISTRY IN MINE WASTE, SEDIMENT, AND WATER AT THE ALMADEN MERCURY MINE, SPAIN

Almaden, Spain is the world’s largest Hg mining district and has produced about 250,000 metric tons (>7,500,000 flasks) of Hg during more than 2,000 years of mining. Primary Hg ore consists of cinnabar (HgS) with minor amounts of elemental mercury (Hg^o). Almaden is one of the few presently operating Hg mines worldwide. As a result of the long period of mining and abundant Hg production, there are significant deposits of mine waste in the region that have not been thoroughly studied for environmental effects. We have measured total Hg and methyl-Hg contents in mine wastes, stream sediments, and surface waters to evaluate Hg contamination, and more importantly, conversion to methyl-Hg, a water and lipid soluble compound that is highly toxic to all organisms.

Mercury in mine wastes collected in the Almaden area are highly elevated, with total Hg concentrations ranging from 10->1,000 µg/g, and methyl-Hg ranging from 0.15-3,100 ng/g. Of greatest concern is potential Hg contamination of the Valdeazogues River, a large ecosystem and fishery in the region, part of which flows proximal to the mine. Mercury contents are highly elevated in stream sediments and stream water collected from tributaries flowing from the mine, which eventually drain into the Valdeazogues River. Stream water samples collected within 7 km of the Almaden mine are contaminated with Hg, containing total Hg as high as 13, 000 ng/L, and methyl-Hg as high as 30 ng/L; stream sediment samples collected from corresponding sites contain methyl-Hg contents as high as 83 ng/g. Several surface water samples collected in and near the mine contain total Hg contents in excess of the 1,000 ng/L drinking water standard recommended by the World Health Organization. Total Hg and methyl-Hg in mine waste, stream sediment, and stream water in the Almaden area are among the highest concentrations observed at Hg mines worldwide. These results indicate substantial downstream transport of Hg from the Almaden mine and significant conversion to methyl-Hg in the surface environment.