Hydrology and geochemistry of thermal ground water in southwestern Idaho and north-central Nevada

In southwestern Idaho and north-central Nevada, thermal groundwater occurs under artesian conditions in igneous or sedimentary rocks of Tertiary age. Temperatures of the groundwater range from 30 degrees to more than 80 degrees Celsius. Thermal waters are a sodium carbonate or bicarbonate type; nonthermal waters are a calcium bicarbonate. Chemical geothermometers indicate maximum reservoir temperatures near 100 degrees Celsius. Stable-isotope data indicate recharge to the system occurred when climate averaged 3 degrees to 5 degrees Celsius colder than at present; such conditions existed during Holocene glacial advances 3,000 and more than 8,000 years ago. Residence time calculated on the basis of reservoir volume and thermal-water discharge is 3,400 to 6,800 years. Considering estimates of heat flux in and heat discharged by conduction and convection, about 25.0 cubic feet per second, or about 18,000 acre-feet per year, of 50 degrees Celsius water is required to transport excess heat from the system advectively in groundwater. The conceptual model is one where water has circulated thousands, even tens of thousands, of years. Within model constraints, reservoir thermal energy for this geothermal system is 130x10 to the 18th power calories. (USGS)