Leaching and methylation of Hg was studied in mine-waste calcines (retorted ore) collected from abandoned Hg mines in the Terlingua district, Texas, and Mariscal mine in Big Bend National Park.  These mines were of moderate-size on an international scale and produced about 5,000 t of Hg.  At both mines, there are significant mine-waste piles containing elevated Hg concentrations, a portion of which may be available for leaching and transformation to methyl-Hg.  We carried out Hg leaching experiments in mine-waste samples and also measured total Hg and methyl-Hg concentrations.  Methyl-Hg is a bioavailable compound that is toxic to living organisms and is produced from inorganic Hg by microbial activity.  Leaching of mine wastes was conducted to simulate mine-water runoff and interaction with simulated human body fluids.  Methylation of Hg and leaching of Hg from mine wastes into human body fluids have important human health implications.

Calcines contain total Hg concentrations as high as 19,000 µg/g and methyl-Hg concentrations as high as 1,500 ng/g.  Calcine mineralogy consists of variable amounts of cinnabar, metacinnabar, calomel (HgCl₂), elemental Hg, and elemental Hg sorbed onto clay minerals, Fe-oxides, and other particulates.  However, calcines with the highest methyl-Hg concentrations typically contain calomel, elemental Hg, or particulate-sorbed elemental Hg.  Results suggest that Hg(0) oxidizes to Hg(II), which is subsequently bioavailable for microbial Hg methylation.  Total Hg concentrations in stream sediments collected below the mines range from 0.02-0.87 µg/g, showing significant dilution, and methyl-Hg concentrations range from 0.03-0.62 ng/g.  Lower Hg methylation downstream from these mines is probably due to the arid climate, lack of mine-water runoff, and lower methylating microbial activity.  During leaching studies, total Hg concentrations were highest in simulated gastric fluids, ranging from 1.0- 9,500 µg/L, whereas water leachates generally contained the lowest total Hg concentrations, 0.05-220 µg/L.  Total Hg concentrations in water from uncontaminated baseline streams range from 0.003-0.20 µg/L.  Results indicate that ingestion or inhalation of even small amounts of Hg mine waste may lead to increased Hg concentrations in the human body, especially gastric fluids.