William C. Evans Kenneth A. McGee Kurt R. Spicer David R. Sherrod William E. Scott Peter H. Stauffer D. Bergfeld 2008 Samples of gas and water from thermal springs in Loowit and Step canyons and creeks that drain the crater at Mount St. Helens have been collected since October 2004 to monitor the flux of dissolved magmatic volatiles in the hydrologic system. The changing composition of the waters highlights a trend that began as early as 1994 and includes decreasing SO₄ and Cl concentrations and large increases in HCO₃ . Geochemical models indicate that mineral sources and sinks are not the main controls on the changing water chemistry, and carbon and helium isotopes indicate that their sources in the gases and waters have remained unchanged during this time. The present-day molar ratios of C, S, and Cl in the springs approximate ratios measured in plume emissions in August 2005 and provide supporting evidence that changes in water chemistry most likely reflect changes in the release rates of sulfur gases, HCl, and CO₂ from the magma and a varying degree of efficiency of gas scrubbing by the overlying water. Results from coupled chemical analyses and discharge measurements on the creeks yield an estimate of the dissolved flux of magmatic HCl, SO₂ , and CO₂ of around 5.2, 4.7, and 22 metric tons per day, respectively. application/pdf 10.3133/pp175025 en U.S. Geological Survey Pre- and post-eruptive investigations of gas and water samples from Mount St. Helens, Washington, 2002 to 2005 reports