MERCURY SPECIATION IN SOIL SAMPLES FROM THE MARISCAL MERCURY MINE, BIG BEND NATIONAL PARK, TEXAS

Speciation of Hg was studied in soil samples collected around the inactive Mariscal Hg mine in Big Bend National Park (BBNP), Texas, to evaluate Hg contamination and conversion to methyl-Hg. Mariscal was a relatively small Hg mine, having produced approximately 50 t of Hg during mining from about 1900-1943. The Mariscal mine became part of BBNP when the park was established in 1944. At this site, there are significant mine waste deposits containing elevated concentrations of Hg, some of which are potentially transported by wind or converted to methyl-Hg. Methyl-Hg is a bioavailable compound that is toxic to living organisms and is converted from inorganic Hg by microbial activity in sediment and soil. Concentrations of total Hg, methyl-Hg, and Hg(II) were measured in mine waste at the site, soil samples collected along 3-transects that ranged up to 8 km from the mine, and baseline soil samples collected more than 10 km from the mine. Potential contamination of Hg in and around this site is important because the site is open to tourism. Results indicate that only soil samples collected within 500 m of the mine site contain significant concentrations of total Hg ranging from 0.57-11 µg/g, whereas other soil samples collected 1-8 km from the mine contain total Hg concentrations ranging from 0.022-0.18 µg/g, similar to total Hg found in uncontaminated baseline soils (0.012-0.068 µg/g). Methyl-Hg and Hg(II) show a similar pattern where soil collected within 500 m of the mine contains methyl-Hg concentrations ranging from 0.20-0.54 ng/g and Hg(II) concentrations range from 0.43-5.1 µg/g, whereas soil samples collected up to 8 km from the mine contain methyl-Hg concentrations ranging from 0.013-0.29 ng/g and Hg(II) ranges from 0.011-0.14 µg/g. Soil collected from uncontaminated baseline sites were found to contain the lowest methyl-Hg concentrations ranging from 0.015-0.064 ng/g and Hg(II) concentrations ranging from 0.017-0.028 µg/g. Mine waste from Mariscal contains total Hg (6.9-150 µg/g), methyl-Hg (0.055-1.5 ng/g), and Hg(II) concentrations (6.2-78 µg/g), which are as much as an order of magnitude higher than those found in soil surrounding the mine. The Hg data indicate local wind blown transport of mine waste primarily in soil within 1 km of the mine site.