AGE-DATING OF SPRING DISCHARGE NEAR MOUNT ST. HELENS AND IMPLICATIONS FOR DISSOLVED GAS TRACER STUDIES IN ACTIVE VOLCANIC AREAS

Despite the notable role of groundwater in volcanic processes, there is little direct knowledge of the hydrogeology of most active volcanoes. Dissolved gas-based tracers research has been minimal in these settings because all such techniques are susceptible to complications where magmatic gas fluxes, high fluid temperatures, and strongly reducing conditions are prevalent.

This study was undertaken to test the utility of dissolved gas tracers for dating of groundwater near Mount St. Helens (Cascades Range, southern Washington). Springs in the Mount St. Helens area (all within 10 km of the crater) were sampled for major and trace gases, ion chemistry, water isotopes and tritium. Gases were measured from both passive diffusion samplers and serum bottles, and good agreement between the methods was found for 6 of 8 samples. The exceptions were from thermal springs inside the volcano crater (upper Loowit Creek springs). Based on low percent saturation of noble gases (all <60% ASW at spring elevation and temperature) and high CO₂ content (84-98% of TDG), trace gases were likely to have been stripped by CO₂ exsolution prior to sampling at the Loowit Creek sites.

All samples other than the Loowit springs had noble gas ratios with good statistical fit to a closed-system equilibration model (CE model). Modeled recharge temperatures and excess air contributions were used to constrain initial CFC and SF₆ concentrations in equilibrium with recharging groundwater. After correcting for excess air, 5 of 6 non-Loowit samples fall within a tracer plot space defined by the exponential mixing model and the 50% exponential/piston flow mixing model curves on CFC-12, SF₆ and ³H cross-plots. Resulting residence time estimates were constrained to within ±5 years in most cases. Terrigenic ³He contributions were evaluated using helium mixing models in order to derive ³H/³He age estimates, but uncertainty ranges were higher than for CFC-12 or SF₆ because of magmatic helium. CFC-11 concentrations were significantly higher than atmospheric equilibria, presumably also due to a magmatic source. It is concluded that tracer investigations involving multiple approaches and careful data screening can provide insights into the hydrology of challenging volcanic systems.