Chemical characteristics of thermal water beneath the eastern Snake River Plain

The eastern Snake River Plain aquifer is among the largest and most productive aquifers in the United States. Protection of this resource requires an understanding of the dominant mechanisms that control groundwater chemistry in the eastern Snake River Plain aquifer. To assess the chemistry of the deeper waters of the aquifer, two deep thermal wells and numerous thermal springs were evaluated. The results of this study indicate that the eastern Snake River Plain aquifer is composed of two systems, the upper aquifer, from which most water is produced, and a deep thermal aquifer. The chemistry of the upper aquifer is dominated by Ca-Mg-HCO₃, typical of groundwater in the arid west. The deep thermal system consists of Na-K-HCO₃ water. The difference in water chemistry between the upper and lower aquifers is the result of longer residence times and more water-rock interaction within the deep system. Differences in composition of the deep thermal waters may reflect the variety of types of aquifer host rock.