Geochemical Evaluation of Shallow Dilution of Geothermal Water in the Little Bitterroot Valley, Montana

Abstract

The Little Bitterroot Valley is a normal fault bounded valley in western Montana in filled with Tertiary and Pleistocene sediments. Hot geothermal water rising along a deep circulation fracture system discharges into a shallow confined Pleistocene gravel aquifer where extensive mixing with cold water occurs. Water from wells in this aquifer show a wide range in temperature and chemistry. The water chemistry shows distinct halos for Li, B, C1- an TDS in the mixing g zone, indication the leakage zone is more extensive than well temperatures suggest. Warm temperatures are obtained only lover a gravity high which probably represents shallow bedrock in close contact with the aquifer. Relatively low Li+ concentrations in the geothermal water and fractionation of B from Li+ in some wells suggest that a fraction of the hot water may be rising as steam. Maximum subsurface temperatures are estimated at 130-220°C by extrapolating Na-Ca-K geothermometer data for mixed waters. The hot water movement is associated with the boundary fault on the east margin of the valley; this fracture aquifer may be highly transmissive.