By monitoring the changes in chemistry and temperature of groundwater, surface water, and steam at a volcano over time, scientists can obtain useful information about changes in volcanic activity. These volcanic hydrothermal systems contain elemental components that signal the presence of magma, such as magmatic carbon dioxide or high helium-isotope ratios (3He/4He).
Hydrothermal monitoring occurs at 25 sites in the U.S. portion of the Cascade Range including the area and drainages of 10 volcanoes. Instruments measure water pressure (to calculate flow rate), temperature (to calculate quantities such as heat flow), and conductivity (to calculate concentrations of elements or chemicals). Scientists visit these sites regularly to take samples of liquid and gas and measure the rate of water flow. The results from these measurements indicate baseline conditions, help researchers determine whether hydrothermal activity is changing, and can indicate the presence or absence of fresh magma.
In 2009, the USGS made an effort to develop hourly hydrothermal records in the Cascade Range in order to gain adequate baseline data to better detect and understand anomalies. The 25 selected monitoring sites show evidence of magmatic influence in the form of high 3He/4He ratios and (or) large fluxes of magmatic CO2 or heat. The monitoring sites can be grouped into three broad categories: (1) sites with continuous pressure-temperature-conductivity monitoring and intermittent liquid sampling and discharge measurements; (2) sites with continuous temperature monitoring and intermittent gas sampling; and (3) sites that lack hourly data, but where the USGS has carried out intermittent flux measurements over a period of several decades.
For most of the hydrothermal monitoring sites, correlations have been developed to convert pressure-temperature-conductivity data into a flux of heat or (more often) to the flux of a solute species of interest. We relate (1) specific electrical conductance to lab-measured concentrations of dissolved constituents and (2) pressure (depth of water) to field-measured discharge. The metadata includes descriptions of the sites and methods and plots of the calculated fluxes. The workbook files contain all of the data and correlations upon which those fluxes are based.