GEOLOGY OF BOISE, IDAHO: IMPLICATIONS FOR GEOTHERMAL DEVELOPMENT AND ENGINEERING GEOLOGY

Geologic 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 rhyolite 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 deg F hot water at a rate of 600 to 1200 gpm from 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 for recharge of the geothermal aquifer assumes percolation to a depth of 7000+ ft beneath the granitic highlands northeast of the city. Deep percolation is driven by the topographic head. Heated water convects upward through the NW-trending range-front faults. The basaltic tuff unit is responsible for several landslides and for the moderately expansive clays in the eastern Boise foothills. Planning and construction near areas of outcrop of the basaltic tuff unit should be accompanied by thorough geotechnical investigation.