Arsenic and antimony in geothermal waters of Yellowstone National Park, Wyoming, USA

A total of 268 thermal spring samples were analyzed for total soluble As using reduced molybdenum-blue; 27 of these samples were also analyzed for total Sb using flame atomic absorption spectrometry. At Yellowstone the ClAs">ClAs atomic ratio is nearly constant among neutral-alkaline springs with Cl > 100 mg L⁻¹, and within restricted geographic areas, indicating no differential effects of adiabatic vs. conductive cooling on arsenic. The ClAs">ClAs ratio increases with silica and decreases with decreasing ClΣCO3">ClΣCO3; the latter relationship is best exemplified for springs along the extensively sampled SE-NW trend within the Lone Star-Upper-Midway Basin region. The relationship between ClAs">ClAs and ClΣCO3">ClΣCO3 at Yellowstone suggests a possible rock leaching rather than magmatic origin for much of the Park's total As flux. Condensed vapor springs are low in both As and Cl. Very high ClAs">ClAs ratios ( > 1000) are associated exclusively with highly diluted (Cl < 100 mg L⁻¹) mixed springs in the Norris and Shoshone Basins and in the Upper White Creek and Firehole Lake areas of Lower Basin. The high ratios are associated with acidity and/or oxygen and iron; they indicate precipitation of As following massive dilution of the Asbearing high-Cl parent water.

Yellowstone Sb ranged from 0.009 at Mammoth to 0.166 mg L⁻¹ at Joseph's Coat Spring. Within basins, the ClSb">ClSb ratio increases as the ClΣCO3">ClΣCO3 ratio decreases, in marked contrast to As. Mixed springs also have elevated ClSb">ClSb ratios. White (1967) and Weissberg (1969) previously reported stibnite (Sb₂S₃), but not orpiment (As₂S₃), precipitating in the near surface zone of alkaline geothermal systems.