Geothermal investigations in Idaho. Part 2. An evaluation of thermal water in the Bruneau-Grand View area, southwest Idaho

Abstract

The Bruneau-Grand View area occupies about 1,100 square miles in southwest Idaho and is on the southern flank of the large depression in which lies the western Snake River Plain. The igneous and sedimentary rocks in the area range in age from Late Cretaceous to Holocene. The aquifers in the area have been separated into two broad units: (1) the volcanic-rock aquifers, and (2) the overlying sedimentary-rock aquifers. The Idavada Volcanics or underlying rock units probably constitute the reservoir that contains thermal water. An audio-magnetotelluric survey indicates that a large conductive zone having apparent resistivities approaching 2 ohm-meters underlies a part of the area at a relatively shallow depth. Chemical analysis of 94 water samples collected in 1973 show that the thermal waters in the area are of a sodium bicarbonate type. Although dissolved-solids concentrations of water ranged from 181 to 1,100 milligrams per litre (mg/1) in the volcanic-rock aquifers, they were generally less than 500 mg/1. Measured chloride concentrations of water in the volcanic-rock aquifers were less than 20 mg/1. Temperatures of water from wells and springs ranged from 9.5/sup 0/ to 83.0/sup 0/C. Temperatures of water from the volcanic-rock aquifers ranged from 40.0/sup 0/ to 83.0/sup 0/C, whereasmore » temperatures of water from the sedimentary-rock aquifers seldom exceeded 35/sup 0/C. Aquifer temperatures at depth, as estimated by silica and sodium-potassium-calcium geochemical thermometers, probably do not exceed 150/sup 0/C. The gas in water from the volcanic-rock aquifers is composed chiefly of atmospheric oxygen and nitrogen. Methane gas (probably derived from organic material) was also found in some water from the sedimentary-rock aquifers.« less