Helium in Stream Water as a Volcanic Monitoring Tool

We show that synoptic sampling of streams can be used to characterize volcanic volatiles in groundwater over large spatial scales. Synoptic sampling of dissolved noble gases, ²²²Rn, major ions, and stream discharge was carried out along a 30 km reach of the Gibbon River, near Norris Geyser Basin in Yellowstone National Park, USA. Groundwater discharge location, volume, and composition were estimated by constrained calibration of a stream flow and solute transport model. Estimated groundwater composition from stream modeling was compared to shallow groundwater concentrations measured in nearby springs. ³He, ²²²Rn, and Cl^- _aq signatures in the Gibbon River are indicative of groundwater discharge with a volcanic signature along the study reach. Stream water noble gas isotopic composition has similar isotopic mixing patterns to springs. The model-estimated composition of groundwater discharging to the Gibbon agrees well with observed groundwater composition from nearby springs for all modeled analytes. We present the first observations of elevated mantle helium in stream water and show that stream water can be used as a convenient collection point to estimate spatially distributed groundwater composition and to monitor changes in volatile flux over large spatial areas. These results offer the possibility that stream surveys in volcanic terrain could be a new method for distributed volcanic monitoring at the catchment scale and beyond.