MONITORING THE HYDROTHERMAL SYSTEM OF LASSEN VOLCANIC CENTER, SOUTHERN CASCADES

New work on the hydrothermal system at the Lassen Volcanic Center (LVC) includes three areas: water temperature monitoring, mapping of hydrothermal features, and water chemistry. Water temperature and composition has been measured since 2008 to help constrain baseline conditions of hydrothermal activity associated with inputs of heated groundwater and meteoric water from streams, snowmelt and rainfall. Continuous water temperature measurements at the Sulphur Works (SW) site correlate with minor diurnal variations but remain between 80-90°C year round. Spring 2017 temperatures of surrounding streams range from 5.7-31.1°C. Mapped locations of fumaroles and mud pots have migrated as much as 2m since 2006. Boiling mud pots have lower pH (2.1-2.7) than surrounding streams at SW (2.6- 6.4), which inversely correlates with total dissolved solids (TDS) and temperature. Oxygen and deuterium isotope ratios of stream water collected at SW in spring 2017 were measured at CSU, East Bay using a liquid water isotope analyzer (LWIA; Los Gatos Research, Inc.), which range from δ¹⁸O = -11.8‰ to -12.3‰ and δD = -96.0‰ to -99.9‰. SW stream data are closer to MWL values than water collected in 2010, which is consistent with the high precipitation and snow pack of 2017. Warmer streams (~23.9-31.1°C) have the lowest δ¹⁸O, low pH and high TDS while cooler streams (5.7-16.5°C) have the highest δ¹⁸O, high pH and low TDS. These data indicate meteoric water is an important component for the SW hydrothermal system, and that especially in 2017’s high precipitation year, spring run-off overprints hydrothermal fluid temperature and water composition. Ongoing work includes continuing to monitor variations in water temperature and water chemistry at SW and other hydrothermal locations at LVC to characterize trends from pre-drought years (2008-2012), through drought years (2012-winter 2016) and effects of more recent high precipitation and snowpack of 2016- 2017 and beyond.