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Integrated Geological, Geochemical, and Geophysical Studies of Big Bend National Park

Mercury Contaminant Studies of Abandoned Mercury Mines at Terlingua and Mariscal Mountain, Texas

Task Contact: John Gray | Task Objectives | Task Highlights & Key Findings | Task Products

Task Objectives

Terlingua is a large mercury (Hg) district generally located just outside of BBNP, although one mine, Mariscal, is located inside the park on Mariscal Mountain. About 5,000 t (150,000 flasks) of Hg were produced from the 1890's to 1970's. Terlingua ranks as the 4th largest producer of Hg in the U.S., and only mines in the California Coast Range and McDermitt, Nevada are larger. Terlingua is a moderate-sized Hg district on an international scale. Mercury mines in the Terlingua district and throughout the U.S. are presently inactive because of low demand for Hg, which is primarily a result of environmental concerns about its toxicity. Mercury has no known biological function and is toxic to all organisms, including humans. The primary concern around abandoned Hg mines is the conversion from inorganic Hg (primarily elemental Hg) to organic forms of Hg. Methyl-Hg is the dominant organic Hg compound, and it is highly toxic affecting the central nervous system in organisms. Methyl-Hg is water soluble and rapidly enters the aquatic food chain. For example, methyl-Hg concentrations in fish muscle are typically 90-100% of the total Hg. Methyl-Hg is then passed onto wildlife and humans who consume such fish contaminated with Hg.

Environmental concern around the mines in the Terlingua district, and all Hg mines worldwide, is any soluble Hg compounds in mine wastes and the potential generation of methyl-Hg at these sites. Mercury ore is mostly cinnabar (HgS), which is generally stable during surface weathering; however, when Hg ore is roasted during processing, soluble Hg compounds such as Hg chlorides, oxychlorides, and sulfates are formed. Oxidation of elemental Hg may produce highly toxic methyl-Hg, especially in wastes and soot from condensers, which are typically found around dilapidated retorts and furnaces. In this task, we studied the methylation process and evaluated the metal generating (leaching) capacity of Hg mine wastes found at these abandoned Hg mines.

The objectives of this task were to: (1) evaluate the process of Hg methylation in Hg mine waste and stream sediment downstream from the mines through the measurement of total Hg and methyl-Hg concentrations, and by measuring rates of Hg methylation and methyl-Hg demethylation; (2) determination of the concentrations of Hg compounds such as Hg(0), HgS, and soluble Hg salts in mine wastes collected from various Hg mines; (3) conduct laboratory leaching experiments to determine the metal-generating capacity of Hg mine wastes; (4) evaluate any adverse effects of Hg to streams and biota in ecosystems downstream from abandoned Hg mines in this region; (5) evaluate potential Hg contamination in soil samples at the Mariscal Hg mine in the park; and (6) measure Hg gas emissions from soil and mine waste at the Mariscal mines and background sites throughout the park.

Photo of Study Butte Mercury Mine
Mercury methylation and mercury leaching were studied at mercury mines in and around the park.

Highlights & Key Findings

High concentrations of methyl-Hg were found around some Hg mines in the area, primarily in retort condenser soot and fine-grained ash in and around Hg mines outside of BBNP. Measurement of Hg species in mine-waste and stream-sediment samples were completed and summarized in an article for the journal, "Applied Geochemistry". Mercury methylation and methyl-Hg demethylation rate measurement studies were also reported in this journal article. Total Hg and methyl-Hg were also measured in several springs and drinking water supplies in the park. These results indicate relatively low total Hg concentrations (≤14 ng/L) in all water samples measured, which is significantly below the USEPA drinking water standard for total Hg of 2,000 ng/L. Results for Hg in water collected in and around BBNP are summarized in USGS Circular 1327.

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