The original estimate of heat loss from the Roosevelt Hot Springs geothermal system (RHS) based on integration of conductive heat flow measurements at 30 – 60 m depth was 60 – 70 MWth (Ward et al., 1978). This heat flow map outlines the convective upflow zone coinciding with a 5 km length of the Opal Mound fault, and an inferred outflow zone to the northwest. Assuming a reservoir enthalpy of 1130 kJ/kg (260°C water), the pre-development reservoir upflow was about 60 kg/s. However, contouring of conservative chemical species such as chloride and boron from groundwater wells in the Milford Valley outlines a much more extensive thermal outflow from a 20 km length of the western flank of the Mineral Mountains (Ross et al., 1982). Re-evaluation of the temperature data between 100 and 200 m depth in the original shallow thermal gradient wells supports a thermal outflow zone at least 14 km long, and extending 10 km westward towards the center of Milford Valley. The area with a temperature of at least 40°C at 200 m depth is about 100 km2. Recent analyses of groundwater from five wells in the northern Milford Valley confirm water compositions very similar to the original groundwater compositions from about 1980. Consideration of chloride-enthalpy trends of the pre-development reservoir water and the original RHS spring composition shows predominantly steam loss between the reservoir and the surface spring. The thermal groundwater beneath Milford Valley is consistent with a simple mixing trend between cold groundwater and the RHS spring water with at least 50% dilution and associated cooling. Inclusion of the effects of mixing with cool groundwater at least doubles the pre-development heat and mass flow from the RHS. A deep geothermal exploration well (Acord-1) near the western limit of the outflow has a temperature of 195°C at 3 km depth, a conductive heat flow of 110 ± 20 mW/m2, and may be representative of the regional heat flow beneath Milford Valley. This heat flow, and the higher estimates of predevelopment heat and mass flow from RHS, may be indicative of a significant deep basin-scale geothermal resource. PacifiCorp Energy operates the 36 MWe (gross) Blundell power plant at RHS.

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