COMPARISON OF OXYANION GEOCHEMISTRY (AS, SB, MO, V, W, SE) IN YELLOWSTONE'S THERMAL WATERS AND DOWNSTREAM FLUVIAL TRANSPORT

Concentrations of As, Sb, Mo, V, W, and Se have been determined in hundreds of water samples from thermal features in Yellowstone National Park and from the Firehole and Gibbon Rivers. Selenium is consistently less than 1 μg/L everywhere, V is often below detection up to 130 μg/L, Mo occurs up to 360 μg/L, Sb up to 370 μg/L, and As up to 14,600 μg/L. Data on W are limited but concentrations are generally similar to or a bit higher than those of Sb and Mo. Arsenic concentrations are typically 0.5 to 3 mg/L, being highest in one small area of Norris Geyser Basin. The chemically variable composition of one pair of springs indicates mixing between two end-members: a circumneutral pH, high-As, high-Cl water and a low-As, low-Cl acid-sulfate water. The acid-sulfate end-member does not occur in any springs in the area and is hypothesized to derive from evaporation of hot saturated acid soil and sediment pore waters. Arsenic concentrations are linearly related to Cl concentrations in most waters, suggesting that arsenic is largely conservative in the hydrothermal fluid. However, the As-Cl trend for samples taken inside the caldera is substantially different from the trend for samples taken outside (Norris Geyser Basin).

Arsenic concentrations are not attenuated during 20-30 miles of river drainage. Concentrations in the Gibbon River reach more than 100 μg/L dissolved As and those in the Firehole River reach more than 400 μg/L. Apparently, high silica concentrations in thermal waters precipitate in the river and coat the river bed sediments, decreasing its sorption capacity. Also, Sb and Mo concentration profiles are strikingly similar, showing no attenuation. These trace elements are indicative of a deep geothermal reservoir source.

Concentrations of V in thermal waters behave differently; they are < 10 μg/L for nearly all Cl concentrations > 200 mg/L. Higher V concentrations are only found in acid waters of low Cl concentration, regardless of temperature, indicating that V is dominantly derived from acid weathering of near surface rocks. Vanadium is the only element in this group of oxyanions that forms a cation under reduced conditions which may explain its higher concentration in acidic waters.