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The most promising geothermal resource in the eastern United States is warm water stored in the permeable sediments of Cretaceous and younger age beneath the Atlantic Coastal Plain. Optimum sites for high gradients are locations where the heat flow is high and sediment thermal conductivity is low. Low conductivity is characteristic of most of the Coastal Plain sediments.
Heat flow in the eastern United States varies from about 33 mW/m2 to 100 mW/m2. All variations in heat flow in the eastern United States are caused either by differences in the concentrations of the heat-producing isotopes of uranium, thorium, and potassium in crystalline rocks, or by differences in thickness of heat-producing crystalline rocks. The highest concentrations of heat-producing elements, and the highest heat flows, are in the relatively young (ca. 300 m.y.) unmetamorphosed granite stocks and batholiths. Similar granites also are present in the southeast in the basement beneath the Coastal Plain. Thus, optimum sites for the development of moderate-temperature geothermal resources beneath the Coastal Plain require a knowledge of the (1) distribution and thickness of heat-producing granites in the basement, (2) thermal conductivity and thickness of sediments above basement, and (3) nature and extent of aquifers in the sediments above basement.
A site for the first deep geothermal test on the Atlantic Coastal Plain was chosen at Crisfield, Maryland. A temperature of 57°C was found at a depth of 1.26 km. Economic analyses at this site and elsewhere by the Applied Physics Laboratory of Johns Hopkins University, aquifer pump tests, and numerical modeling of the thermal lifetime of a reservoir suggest that geothermal energy may be an important resource at some locations on the Atlantic Coastal Plain.
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