Geochemistry of Thermal Water from Selected Wells, Boise, Idaho

Abstract

Samples of thermal water from selected wells in the Boise area were analyzed for chemical composition; stable isotopes of hydrogen, oxygen, and dissolved carbon; radioactive carbon; and dissolved gas concentrations. Chemically, the waters are virtually identical to those of the adjacent Idaho batholiths. Isotopically, the thermal waters are more depleted in deuterium and oxygen 18 than cold water springs in the presumed recharge area. Chemical and isotopic data indicated the presence of two separate geothermal system's Radioactive carbon and dissolved helium concentrations are interpreted to indicate recharge during the Pleistocene. Hot water in or southeast of Boise probably recharges 20,000 to 30,000 years ago, and warm water 2.5 miles northwest of Boise probably recharged at least 15,000 years ago. Recharge to the system during the Pleistocene would have been more depleted in deuterium and oxygen 18 than modern precipitation because of the colder climate during eth Pleistocene.