Boise, Idaho Geothermal System

Abstract

Geological mapping and data from geothermal water wells have provided information to delineate late Cenozoic geologic units and structures important to understanding the geothermal system of Boise as it is currently being developed. The main geothermal aquifer is a sequence of rhyolitic layers and minor arkosic and tuffaceous sediments of the Miocene Idavada Group. The aquifer is confined by a unit of impermeable basaltic tuffs. The aquifer has sufficient fracture permeability to yield 150-170°F hot water at a rate of 600 to 1200 gpm form wells drilled in the metropolitan area north of the Boise River. In this area the rhyolite lies at a depth of 900 to 2000 ft. A conceptual model of recharge assumes percolation to a depth of 7000+ ft beneath the granitic highlands northeast of the city driven by the topographic head. Heated water convects upward through the NW trending range front faults.