Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: IV Acid–sulfate waters

• Published in: Applied geochemistry 2009-02, Vol.24 (2), p.191-207
• Main Authors: Kirk Nordstrom, D., Blaine McCleskey, R., Ball, James W.
• Format: Article
• Language: English
• Subjects: Freshwater
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2008.11.019

Many waters sampled in Yellowstone National Park, both high-temperature (30–94 °C) and low-temperature (0–30 °C), are acid–sulfate type with pH values of 1–5. Sulfuric acid is the dominant component, especially as pH values decrease below 3, and it forms from the oxidation of elemental S whose origin is H 2S in hot gases derived from boiling of hydrothermal waters at depth. Four determinations of pH were obtained: (1) field pH at field temperature, (2) laboratory pH at laboratory temperature, (3) pH based on acidity titration, and (4) pH based on charge imbalance (at both laboratory and field temperatures). Laboratory pH, charge imbalance pH (at laboratory temperature), and acidity pH were in close agreement for pH < 2.7. Field pH measurements were predominantly used because the charge imbalance was ±10%, a selection process was used to compare acidity, laboratory, and charge balance pH to arrive at the best estimate. Differences between laboratory and field pH can be explained based on Fe oxidation, H 2S or S 2O 3 oxidation, CO 2 degassing, and the temperature-dependence of p K 2 for H 2SO 4. Charge imbalances are shown to be dependent on a speciation model for pH values