UNUSUAL PLUMBING OF THE PAGOSA SPRINGS GEOTHERMAL SYSTEM, COLORADO, USA

The Pagosa Springs geothermal system, Archuleta County, Colorado, is one of several thermal springs and wells that are to the west-southwest of the San Juan volcanic field. Pagosa Springs was selected in 1978 for study for a proposed district heating system. New data collected from the 1978 proposal included geophysical surveys, and six ~100 m heat-flow holes were drilled to define the extent of the anomaly. Two “deep” wells were drilled west of the main (Mother) spring), a slim “observation” well (O-2), that was abandoned at a depth of 240 m because of caving, and a production well (P-1), drilled to 452 m, and produced for the district heating system.

Additional geophysical exploration was conducted in the area by the Geophysics field camp of the Colorado School of Mines every year from 2012 through 2018. Pagosa Verde drilled three additional deep temperature gradient wells on the periphery of the anomaly in 2018 and 2019. Combining data from all studies an unusual plumbing system for the Pagosa Springs geothermal system has been deduced.

Using a simple flow-model, the depth of water circulation required for heating of meteoric water to the spring temperature is ~2.1 km; the maximum time for ascent is of the order of years with a discharge rate of ~40 l/s. The water rises from the top of the Precambrian at a depth of 418 m in a pipe-like conduit to the base of the Dakota Sandstone at 133 m. Between the Dakota base and its top at 72 m about half the volume of the ascending hot water leaks laterally into the Dakota to form a circular, surface, thermal-gradient anomaly. From the top of the Dakota the remaining hot water rises again in a pipe-like conduit to the surface spring. The spring has been plumbed to a vertical depth of at least 305 m at which depth the line ran out. The temperature profile in the production well (P-1) inverts below the Dakota, indicating that the lateral flow in the Dakota is less than a couple of hundred years old.