Geothermal potential of Idaho

Abstract

The first comprehensive inventory and evaluation of thermal ground water in the State of Idaho since 1925 shows that such water can be expected in wells and springs almost anywhere along the margins of the Snake River Plain or in valleys south of the Plain. In addition, about one-half of the approximately 200 thermal springs in the state emerge from granitic rocks or silicic volcanic rocks along the borders of the Idaho batholith. Although no steam has been observed, water in a few wells is boiling, and temperatures of many springs are only slightly lower. Thermal springs and wells are non-existent in region previously considered to have prime geothermal potential, that area of Idaho bordering Yellowstone National Park. However, general ground-water temperatures are as much as 11°C above mean annual air temperature, and lack of distinct anomalies is caused by high rock porosities and large volumes of non-thermal water. Sodium and bicarbonate are the dominate ions in most waters, although a few highly mineralized springs are the sodium-chloride type. Specific electrical conductance of thermal waters from granitic rocks averages between 300 and 400 micromhos/cm at 25°C; average conductivity of thermal water from volcanic rocks is somewhat higher. Much of the water is unsually high in silica and fluoride, even though concentrations of other minerals are low. Although the only thermal waters used to generate electricity in Idaho flow through Pelton wheels, use of such waters for space heating dates to before 1900. Water commonly is used to heat individual homes, and within the last 10 years water has become an important source of heat for greenhouse operations. Although space-heating use will expand rapidly, power generation from geothermal waters will occur only in the distant future.