Hydrogeology In Yellowstone National Park
Monitoring Stream Discharge for Geothermal Activity
Jan 14, 2009
Yellowstone National Park is world famous for its hots springs and geysers, both evidence of the geothermal activity caused by the massive hotspot, or magma chamber, below the park. Scientists at the Yellowstone Volcano Observatory (YVO) use a combination of hydrogeology and geochemistry to monitor volcanic activity in the park. A technique called chemical geothermometry is used to measure the chemical composition of ground water discharge in the park.
Chemical Geothermometry and Geothermal Monitoring
Chemical geothermometry is a method used by geochemists for estimating the temperature of a fluid at depth before it cools on its way to the surface. Moreover, the concentration of certain chemicals in the water at the surface can be related to what the concentration of those chemicals must have been while the water was underground and hotter. The idea that as water cools, its chemical composition changes is what makes chemical geothermometry so useful in Yellowstone. Groundwater in Yellowstone has many different chemicals in it, but the one that geochemists are most concerned with is chloride. In Yellowstone hot springs, the chloride concentration is about 400 parts per million (ppm) in water that is 335-340 degrees Celsius.
The relationship between geothermal activity and chloride concentration is a bit complicated and involves many chemical and thermodynamic equations. In a USGS report written by Irving Friedman and Daniel Norton and titled, "Data Used for Calculating Chloride Flux Out of Yellowstone National Park for the Water Years 1983-99," USGS scientists examine chloride measurements from park rivers for clues about the underlying magma chamber and related plumbing systems.
The basic process goes like this:
- Ground water is heated by the geothermal activity in the park and is released through geysers and hot springs, eventually flowing into streams around the park
- All the streams in the park flow into four major rivers: the Yellowstone and Madison Rivers east of the continental divide; and the Snake and Fall Rivers west of the continental divide
- The chloride that was once contained in the heated ground water eventually makes its way to one of these four rivers
- Scientists measure the concentrations of chloride in the water from these rivers
- Knowing the chemical geothermometry of chloride, scientists can calculate what the temperature of the water must have been underground based on the chloride concentration in the rivers
Hydrogeology Provides Clues to Underground Activity
As the concentration of chloride measured in the rivers changes, scientists at Yellowstone can relate this to changes in the temperature underground. What scientists are measuring here is referred to as the chloride flux - the change in chloride concentration. The chloride flux is calculated by multiplying the chloride concentration times the river discharge rate. Changes in the chloride flux provides scientists with some clues as to what the changes in the heat flux may be.
Heat flux information is extremely important as it gives rare insight into what changes may be occurring in the magma chamber and geothermal plumbing of the park, especially when seismic activity has occurred. Furthermore, this information is being used to calculate the impacts that commercial development near the park, such as drilling and mining, may have on the thermal features.
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