Using electrical conductivity to monitor geothermal solute flux in major rivers of Yellowstone National Park

Abstract:

Thermal output from the magma chamber underlying Yellowstone National Park (YNP) can be estimated by monitoring Cl flux in major rivers draining the park. The U.S. Geological Survey (USGS) and the National Park Service have collaborated on Cl flux monitoring towards this end since the 1970s. Researchers collected water samples from the major rivers in YNP, but funding restrictions, winter travel, and the great distances between sites limits the number of samples collected annually. The use of electrical conductivity, which is relatively easy to measure and can be automated, as a proxy for Cl enables a more consistent monitoring of thermal output. To accomplish this, it is crucial to accurately quantify the relationship between electrical conductivity, Cl, and other geothermal solutes (SO₄, F, HCO₃, SiO₂, K, Li, B, and As) along the Madison, Firehole, Gibbon, Snake, Gardner, and Yellowstone Rivers. Conductivity measurements were made every 15 minutes adjacent to USGS stream gages, allowing for the determination of solute fluxes. In addition, continuous conductivity measurements can be used to identify changes in river chemistry as a result of geysers eruptions, rain events, or changes in thermal inputs as a result of earthquakes or other natural events. Depending on the site, we have collected 2 to 5 years of conductivity measurements. Except for some trace elements (Fe and Hg), most solutes behave conservatively, and the ratio of geothermal solute concentrations are constant. Hence, dissolved concentrations of Cl, SO₄, F, HCO₃, SiO₂, K, Li, Ca, B, and As correlate well with conductivity (R² > 0.96). The use of conductivity to estimate solute concentrations and fluxes will provide a greater understanding of the systematics of the Yellowstone thermal output and allow for monitoring of many more solutes at a much higher temporal frequency.