EVALUATION AND SIMULATION OF INCREASED GEOTHERMAL WITHDRAWALS, BOISE FRONT GEOTHERMAL AQUIFER, BOISE, IDAHO

An extensive geothermal aquifer system underlies the Boise area in faulted igneous rocks and interbedded sediments along the Boise, Idaho Foothills. Geothermal water (78°C) is used for space heating in a variety of residential, commercial, and public buildings (including the State Capitol). A proposed 50% increase in geothermal water use by the City of Boise led to concern among geothermal water users regarding the potential long-term impacts to the geothermal aquifer system. The purpose of this study was to provide insight and tools for the long-term management of the Boise geothermal aquifer system. The study included a recent, detailed measurement of water levels and/or pressures, refinement of the hydrogeologic framework, and the development of a computer model (using FEHM and PEST) to simulate potential changes in hydraulic heads and water temperatures.

Ground water levels in the Boise area show local declines beginning in 1984; water levels began to increase in response to injection of spent geothermal water by the City of Boise in 1999. Initial thermal and hydraulic simulations suggest that the aquifer is able to support the proposed level of increased withdrawals and re-injection. The modeling and model calibration effort helped define future data collection needs to help fill in gaps in understanding of the system.