Analysis of Continuous Multi-Year Baseline Hydrothermal Monitoring Data for the Cascade Range Volcanoes

Since 2009 the U.S. Geological Survey has systematically monitored hydrothermal behavior at selected Cascade Range volcanoes in order to define baseline hydrothermal and geochemical conditions. Gas and water data have been collected at 25 monitoring sites on ten of the highest-risk volcanoes in the Cascade Range. These sites include summit-fumarole groups and springs/streams that show clear evidence of magmatic influence in the form of high 3He/4He ratios and (or) large fluxes of magmatic CO2 or heat. The monitoring data are intended to (1) be suitable for comparison with other continuous geophysical monitoring data and (2) provide baseline data that will be useful during future episodes of volcanic unrest in the Cascade Range. Site records consist mainly of hourly temperature and (or) hydrothermal solute flux data spanning multiple years. Many of the hydrothermal time series data show considerable variability during quiescent periods, including diurnal, seasonal, and inter-annual variability. Having established baseline conditions, the USGS has decreased its monitoring frequency and archived the data. The purpose of this report is to analyze the baseline data. The analysis examines the solute flux and fumarole temperature measurements that have been collected. The goal is to provide a portfolio of useful and succinct summaries of baseline behavior during quiescent periods from each of the 25 sites. This report defines normal variability for each site and describes large-scale trends in variability among the sites. These patterns include differences in behavior between large and small streams, groundwater- and snowmelt-fed streams, and sites across a wide range of effective elevations. This analysis shows that solute flux sites at smaller streams, groundwater- sourced streams, and streams at higher effective elevations are the most useful for monitoring. This information can be used for site selection for future volcanic monitoring projects