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[SunkenCiv]

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http://www.infinitepower.org/resgeothermal.htm

Figure 11. Geothermal Gradient Contour Map of the United States

One question that commonly arises regarding geothermal energy is whether or not it is a renewable resource. The answer hinges on how the resource is developed. Certainly the heat within the earth, like the sun, is limitless compared to human activity. However, the waters that are tapped in geothermal development are finite. Hydrothermal (hot water) aquifers will be diminished whenever water is withdrawn faster than it is recharged. Overexploitation at some facilities in California, for example, resulted in a lower than expected output. If water is reinjected into the field after extracting heat (as is done in some locations), then the resource may be said to be truly renewable. Otherwise, it is simply mined, much as a petroleum reserve.