ARSENIC AND ANTIMONY GEOCHEMISTRY IN THERMAL WATERS OF YELLOWSTONE NATIONAL PARK

To understand the source and fate of dissolved arsenic and antimony from geothermal waters, approximately 500 water samples of hot springs, geysers, and rivers have been collected and analyzed of which 420 have been analyzed for As(III/V) redox species and 90 for Sb(III/V) redox species. The majority of samples have been collected from Norris Geyser Basin where a zone of high arsenic coincides with a fracture network of high heat flux and new thermal activity. Arsenic concentrations range from detection (0.0005 mg/L) to 9 mg/L. Antimony and arsenic generally correlate together but a few samples at high concentration do not correlate. Repeated monitoring at the same hot spring demonstrates that arsenic concentration varies with chloride concentration, indicating conservative behavior affected only by boiling and dilution. However, a recently activated thermal area in Norris shows occasional sudden increases in arsenic concentration over time while chloride and other solute concentrations are constant. A few samples with high As/Cl ratios are found in the high-arsenic zone of Norris Geyser Basin and a few samples with low As/Cl ratios are found in the same area but in small stagnant pools where amorphous As2S3 has precipitated. Evidence for orpiment and realgar precipitation can be found in the high-arsenic zone just under the surface of thermal sediments with variable water content. Arsenic occurs as As(III) at the discharge point of most hot springs but oxidizes quickly either within the pool or the overflow, if concentrations of the reduced sulfur species, sulfide and thiosulfate, are low. Reduced sulfur species can prevent biotic and abiotic arsenic oxidation. Antimony also oxidizes in hot spring overflows but at a slower rate than arsenic. The mass flux of arsenic and antimony to the Gibbon River from Norris shows that most of the arsenic and antimony remains dissolved. Little attenuation of these elements downstream in the Gibbon River is observed.