Pre- and post-eruptive investigations of gas and water samples from Mount St. Helens, Washington, 2002 to 2005

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.