The Bruneau-Grand View area occupies about 1,100 square miles in southwest Idaho and is on the southern flank of the large depression (possibly a graben) in which lies the western Snake River Plain. The igneous and sedimentary rocks in the area range in age from Late Cretaceous to Holocene. They are transected by a prominent system of northwest-trending faults. For discussion purposes, 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-metres 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/l) in the volcanic-rock aquifers, they were generally less than 500 mg/l. Measured chloride concentrations of water in the volcanic-rock aquifers were less than 20 mg/l.
Temperatures of water from wells and springs ranged from 9.5 to 83.0 degrees C. Temperatures of water from the volcanic-rock aquifers ranged from 40.0 to 83.0 degrees C, whereas temperatures of water from the sedimentary-rock aquifers seldom exceeded 35 degrees C. Aquifer temperatures at depth, as estimated by silica and sodium-potassium-calcium geochemical thermometers, probably do not exceed 150 degrees C. However, a mixed-water geochemical thermometer indicates that temperatures at depth may exceed 180 degrees 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.
The thermal waters in the area are believed to be heated by deep circulation in a zone of high geothermal gradient resulting from thinning of the earth?s crust.
