B-39 An Appraisal of Colorado’s Geothermal Resources


Bulletin B-39 was re-released in 2006. It was first published in 1978 and contains a description of each of the 56 geothermal areas known in 1978 and an explanation of the various geothermometric methods used to estimate the geothermal reservoir temperatures of each area. Because of the increase in price of oil and natural gas during the 2000s, the Colorado public is once again searching for cheap, reliable, and renewable energy sources. Geothermal resources in Colorado may offer opportunities to produce electricity using technologies not available in the 1970s. The objective of this publication is to provide readily accessible information on geothermal resources in Colorado to resource developers, government planners, along with interested businesses and citizens. Digital PDF download. B-39D


The Colorado Geological Survey (CGS) in conjunction with the US Geological Survey (USGS) in 1975 initiated a two-year evaluation of the geothermal resource potential of Colorado as determined by the usage of hydrogeological and geochemical data and geothermometer models. The geothermal resource potential of Colorado is expressed in numerous thermal springs and wells found throughout the western one-half of the state. In most instances the thermal waters of Colorado are unused, with minor amounts of thermal waters being used for recreation, space heating, domestic, and miscellaneous agricultural purposes. Although many energy companies have expressed interest in the geothermal resources of Colorado and have acquired leases to federal, state, and private lands, no large scale development has yet occurred.

During the investigation, 127 thermal springs and wells (temperatures in excess of 20°C (68°F)) were located, and field measurements of such physical parameters as discharge, pH, conductivity, and temperature were made. Water samples were collected for wet chemical and atomic absorption analysis and sent to the USGS, Water Resources Division Central Laboratory in Salt Lake City, Utah, or to Atlanta, Georgia. Spectrographic analyses were performed at the Denver Analytical Laboratory of the USGS. Samples were also collected and sent to the US Environmental Protection Agency (EPA) Radiological Laboratory in Las Vegas, Nevada, for determination of radioactive elements.

Evaluation of the field data shows that there are 49 distinct thermal areas within the state consisting of one or more groups of springs or wells. The temperature of the springs varied from a low of 20°C (68°F) at a number of springs to a high of 83°C (181°F) at Hortense Hot Spring, southwest of Buena Vista. The discharge of the waters varied from a low of less than 0.06 l/s (>1 gallon per minute (gpm)) to a high of 143 l/s (2,263 gpm) at the Big Spring in Glenwood Springs. The total dissolved solids of the waters varied from a low of 91 mg/l (91 ppm) at Spring B at Eldorado Springs, southwest of Boulder to a high of 21,500 mg/l (21,500 ppm) at Graves Spring in Glenwood Springs.

To determine what, if any, chemical and discharge changes might occur at a spring throughout a year’s time, one spring in each thermal area was selected for sampling on a quarterly basis. This investigation showed that no consistent changes occurred throughout the year’s time. The number of springs showing any change in the amount of total dissolved solids, temperature, or discharge were very small, and the changes that did occur were not consistent from one spring to another. For example, one spring might show a change in the total amount of dissolved solids, while another spring might show a change in temperature or discharge. Temperature changes were usually only of a few degrees.

A major effort of this investigation was an appraisal of the reservoir temperatures through the use of four geothermometer models: silica, mixing model, sodium-potassium, and sodium-potassium-calcium. Research has shown that a relationship exists between the concentration levels of certain ions in thermal waters and reservoir temperatures. This relationship has led to the development of the above-named geothermometer models, which are used to estimate the reservoir temperature of the thermal areas.

The range of subsurface temperatures estimates as calculated by the silica geothermometer ranged from a low of less than 20°C (68°F) at a number of springs to a high of 157°C (315°F) at Waunita Hot Spring. The estimated subsurface temperatures as determined through the use of the Mixing Model geothermometer ranged from a low of 15°C (59°F) at Mcintyre Warm Spring to a high of 291°C (556°F) at Waunita Hot Spring. The Na-K-Ca geothermometer estimated temperatures ranged from a low of 4°C (39°F) at Conundrum Hot Spring to a high of 220°C (428°F) at Cebolla Hot Springs.

Other ions, such as chloride, found in the thermal waters, or deposits such as travertine around the springs may be used to make a preliminary appraisal of the reservoir conditions. A generalized appraisal of the thermal systems based on the chloride ionic concentrations may be used to evaluate whether the thermal system is a hot water system or a vapor-dominated system. Such an appraisal was made for the systems in Colorado.