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Casa Diablo Hot Springs and Geothermal Facility, Long Valley Caldera, California

The Casa Diablo geothermal power plant is located on the southwest side of the resurgent dome in Long Valley Caldera along a major fault system.

Casa Diablo Hot Springs and the Mammoth geothermal power facility v...
Casa Diablo Hot Springs and the Mammoth geothermal power facility viewed toward the west.

The fault forms the east side of the down-dropped block that transects the resurgent dome. This down-dropped block is called the lateral graben.

The power plant taps into the caldera's hydrothermal system at a point where the hot water is only a few hundred feet below ground. Production wells supply hot water (170°C) to three generating units with a combined capacity of about 45 megawatts of electricity. Heat exchangers transfer the thermal energy from the water to isobutane, which vaporizes and drives turbine generators.

Water that is pumped out of the ground to the generating units is injected back underground near the power plant after being used to produce energy. Although no net withdrawal of geothermal water occurs, the process affects the hydrologic system. In response to this, the USGS hydrologic monitoring program makes routine measurements of discharge and chemistry at Hot Creek and monitors other areas around the resurgent dome to look for physical and chemical changes.