EVALUATION OF MERCURY CONTAMINATION IN MINE WASTE, SEDIMENT, WATER, AND AIR AT THE MARISCAL MINE, BIG BEND NATIONAL PARK, TEXAS

The Mariscal Hg mine is one of the smaller mines in the Terlingua Hg district of southwest Texas, but is the only Hg mine located within Big Bend National Park (BBNP). Mariscal produced about 50 t of Hg during mining from 1900 to 1943, whereas total Hg production from the Terlingua district was about 5,000 t. Mariscal became part of BBNP when the park was established in 1944. Access to Mariscal is unrestricted, and tourists of BBNP often visit this site; therefore, this study was undertaken to evaluate Hg contamination at this site.

The volume of mine waste at Mariscal is estimated at about 30,000 m³ and Hg concentrations in some mine wastes are as high as 150 mg/g. Principal Hg compounds in mine waste are cinnabar and metacinnabar, but minor quantities of soluble Hg salts and matrix sorbed Hg⁰ were also identified, some of which are available for leaching and downstream transport. Minor mine waste was observed in a stream channel draining the mine, but the concentration of Hg in stream sediment collected > 0.5 km from Mariscal is similar to that in uncontaminated regional baseline sediment. Concentrations of Hg in stream sediment downstream from the mine are generally < 1.06 mg/g, which is the probable effect concentration for Hg, above which harmful effects are likely in sediment-dwelling organisms. Concentrations of methyl-Hg (a highly toxic Hg compound), total organic carbon, and total sulfur are low in mine waste and stream sediment collected around the mine, which suggests low potential for Hg methylation in this dry desert climate. No mine water was observed draining Mariscal during studies from 2002-2007. However, water leach studies that simulate runoff indicate that mine waste may leach as much as 33 mg Hg/L, but the lack of precipitation and runoff at Mariscal results in little transport of water-borne Hg to the Rio Grande about 10 km downstream. Soil gas emissions of Hg from the Mariscal mine are elevated, in some instances > 20,000 ng Hg/m³, reflecting the presence of Hg⁰ in mine waste. Conversely, concentrations of Hg gas measured in ambient air 2 m above the ground surface are several orders of magnitude lower, ranging from 2.5-59 ng/m³. These emission data suggest that, although concentrations of Hg emitted from Mariscal mine waste are elevated, persistent wind in southwest Texas disperses Hg in air within a few meters of the ground surface.