GEOTHERMAL SOLUTE FLUX MONITORING USING ELECTRICAL CONDUCTIVITY IN THE FIREHOLE RIVER, YELLOWSTONE NATIONAL PARK

The Firehole River, Yellowstone National Park (YNP), receives discharge from some of the largest and most active geothermal areas in the Park including Upper, Midway, and Lower Geyser basins. The thermal output from the underlying Yellowstone magma chamber can be estimated by monitoring the Cl flux in the major rivers and since the 1970s the U.S. Geological Survey (USGS) and the National Park Service have collaborated on Cl flux monitoring. For many years 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 goal of this study was to accurately quantify the relationship between electrical conductivity and Cl and other geothermal solutes (SO₄, F, HCO₃, SiO₂, K, Li, B, and As) at two sites along the Firehole River (above Old Faithful and near Madison Junction). Electrical conductivity is relatively easy to measure and can be automated by adding a sensor to the existing instrumentation at USGS stream gages. In addition, conductivity measurements were made every 15 min to assess rapid temporal variations that may relate to changes in river chemistry as a result of geyser eruptions, rain events, or changes in the hydrothermal system caused by earthquakes or other natural events. The conductivity in the Firehole River is primarily dominated by Na, Cl, and HCO₃ where their relative contribution to the conductivity at the lower gage are 39, 28, and 17 %, respectively. Except for some ultra-trace elements (Fe and Hg), most solutes behave conservatively and the ratio of geothermal solute concentrations are constant in the Firehole River. Hence, dissolved concentrations of Cl, SO₄, F, HCO₃, SiO₂, K, Li, Ca, B, and As correlate well with conductivity (R² > 0.96) in the Firehole River. The 2010 annual flux for Cl (16.9 kt/yr), SO₄ (2.9 kt/yr), F (2.1 kt/yr), and HCO₃ (32.4 kt/yr) determined using automated conductivity sensors are in good agreement with previous studies. The use of conductivity to estimate solute concentrations and fluxes will provide a greater understanding of the systematics of the Firehole River and allow for monitoring of many more solutes at a much higher frequency than what is currently being used.