Tritium in the thermal waters discharging in Loowit Canyon, Mount St. Helens, Washington, U.S.A.

The tritium contents in the Loowit hot springs were determined for samples collected in 1986–1989 and ³H was modeled using two separate methods to estimate fluid ages and determine if an older component of water enters the geothermal system at Loowit. Estimated fluid ages, assuming either well-mixed or piston-flow behavior, indicate fluid ages could lie in the range of 2–184 yr. Refinements to this first model, with the use of geologic, topographic and hydrologic data, indicate average fluid ages are ⩽ 10 yr. Advection-dispersion modeling indicates that average fluid transit times are ∼ 5 yr. and horizontal fluid velocities of ∼ 200 m yr.⁻ can be expected in the Loowit system. Hence, both modeling efforts indicate that water in the Loowit system was recharged after the eruptions of Mt. St. Helens in 1980 and deeper, pre-1980 fluid, stored in the deposits of the volcano, is not required to explain the behavior of the newly formed geothermal system.