EMISSIONS OF MERCURY FROM MINES IN THE TERLINGUA MERCURY DISTRICT IN AND AROUND BIG BEND NATIONAL PARK, TEXAS

Total mercury (T_Hg) concentrations were measured in air and soil gas at abandoned Hg mines in the Terlingua district near Big Bend National Park (BBNP), Texas.  Some mines of the Terlingua district are located within 5 km from BBNP and the Mariscal mine is in BBNP.  The Terlingua district produced greater than 5,000 t of Hg during mining from about 1888-1973, but the mines are now inactive.  Terlingua ranks as the fourth largest Hg producing district in the USA.  Recovery of Hg during processing of dominantly cinnabar (HgS) ore is an inefficient and incomplete process resulting in waste containing significant amounts of highly volatile elemental Hg, which are discarded at the site of mining.  Exposure of Hg to humans is of concern in the region, especially at the Mariscal mine in BBNP as tourists often visit this site, but mine waste in the Terlingua area has not been evaluated for Hg emissions.  

Concentrations of T_Hg emitted from mine waste in the Terlingua area ranged from 690 to1600 ng/m³.  However, higher Hg emissions were measured near an old retort at the Mariscal mine in BBNP, where T_Hg concentrations varied from 29,000 to 82,000 ng/m³.  These data indicate that the average flux of Hg emitted from the mines of the Terlingua district may exceed 2,000 ng/m²/hr.  Such Hg concentrations and Hg flux rates are high, but are within the range of concentrations measured from other Hg mines worldwide.  In contrast, T_Hg concentrations in ambient air measured 2 m above the ground surface at the mines studied contained 4.9 to 57 ng/m³, suggesting rapid atmospheric dispersion of Hg gas.  Ambient air also measured at 2 m above the surface at 4 background sites within BBNP contained T_Hg concentrations ranging from 1.2 to 2.1 ng/m³, and is similar to Hg found in the atmosphere worldwide ranging from about 1 to 4 ng/m³.  Soil at these background sites emitted T_Hg concentrations ranging from 2.6 to 5.4 ng/m³.