THE COLUMBIA PLATEAU GEOTHERMAL ANOMALY: THE ROLE OF GROUNDWATER

Heat flow mapping of the western United States highlights a region of relatively low heat flow coincident with the Columbia Plateau Regional Aquifer System (CPRAS), which is comprised of thick volcanic aquifers of the Columbia River Basalt Group (CRBG). By simulating two-dimensional coupled groundwater and heat flow using best a priori estimates of hydrogeologic parameters and conditions compiled from existing measurements and previous modeling studies, we demonstrate that patterns of groundwater flow expected from hydrogeologic studies can cause variations in near-surface heat flow of the magnitude observed. Heat flow measurements from depths less than 500 m from the CRBG may be affected by groundwater flow and thus insufficient to estimate regional conductive heat flow beneath the CPRAS. A 600-900 m deep transition zone where permeability drops off rapidly has been identified in deep boreholes. This transition zone corresponds to in situ temperatures of ~40-50 ºC, which is postulated to be the approximate temperature range above which secondary mineralization intensifies. Below this transition zone and away from major groundwater discharge zones, the thermal regime is likely conduction-dominated and borehole data may yield reliable estimates of regional heat flow. Given the scarcity of measurements from below the transition zone, significant portions of the CPRAS could be characterized by higher heat flow, and consequently, temperatures of geothermal interest at depth.