STRUCTURAL CONTROLS OF A GEOTHERMAL SYSTEM AND EFFECTS ON LOCAL GROUNDWATER AQUIFER, NORTHERN CACHE VALLEY, IDAHO

The Northern Cache Valley (NCV) of southeastern Idaho is a north-south trending Basin and Range graben that is receiving renewed interest for its geothermal potential. In the 1970s and 80s, geophysical and hydrological studies were undertaken, motivated by the presence of thermal wells and springs in the area. Geothermal exploration in this era culminated with the drilling of geothermal exploration wells by Sunedco Energy Development in 1979 and 1980. The test borehole temperatures were deemed too low (< 120^o C) for power production using technology available at the time. In January of 2014, a water well drilled to 79 meters, encountered Na-Cl-HCO₃ water with a measured bottom hole temperature of 104^o C. Traditional magnesium corrected Na-K-Ca geothermometry of water from that well estimated the temperature of the thermal reservoir to be 204° C. Due to this revived interest, new studies utilizing updated geochemical and geophysical techniques were undertaken. Present understanding of the NCV geothermal system suggests that fluid flow is associated with a fault(s) adjacent to Clifton Hill (aka Little Mountain) – a secondary horst complex rising from the floor of the Cache Valley graben. The existing data from the area is relatively sparse and is not suitable for pinpointing the location of faults thought to be acting as conduits for thermal water to travel from depth to the shallow subsurface. To investigate the locations of these faults and accurately characterize the plumbing of the geothermal system, high resolution potential field (gravity and magnetic) data was collected along lines made across the suspected location of the Clifton Hill bounding faults. Geophysical models of the subsurface using these data in conjunction with existing hydrogeological and geochemical data have aided in better determining fault locations. Groundwater level and aquifer temperature mapping in the area adjacent to the fault system has also been undertaken to explore the behavior of the shallow ground water aquifer in response to the discharge of thermal water through the system’s faults.