Exploration for Blind Geothermal Resources in the State of Hawaiʻi utilizing Dissolved Noble Gasses in Well Waters

Following the Play Fairway Geothermal Analysis of the University of Hawaiʻi we sampled 23 water supply wells across the state of Hawaiʻi for dissolved noble gases. Noble gases are an ideal blind geothermal tracer because they are chemically inert and conservative, and because their isotopic signature among the atmosphere, meteoric water, crust, and mantle is distinct. We supplement our newly-collected noble gas data with those collected by Ormat on Hualālai Volcano (Hawaiʻi Island) and Haleakalā (Maui) and by the USGS Hawaii Volcano Observatory for the Sulfur Banks fumaroles at the summit of Kilauea Volcano on Hawaiʻi Island. While dissolved helium concentration ([He]) and R/Ra = (3He/4Hem)/(3He/4Hea), are not geothermometers, values in excess of air saturated water (ASW) are correlated to increased heat flux, and globally all magmatic geothermal systems for which there is data show elevated R/Ra values when compared to ASW. On all Hawaiian Islands [He] and R/Ra correlate to structural features related to volcanic rifts and calderas. On Kilauea at both the Sulfur Banks and Puna Geothermal Venture R/Ra of steam reaches a maximum of 15. This high value is representative of mantle plume 3He, though is significantly lower than an R/Ra value of 35 measured in rock inclusions on the youngest and active Hawaiian volcano at Lōʻihi Seamount to the East of Hawaiʻi. The still active magmatic system at Mauna Loa has a measured mean R/Ra of 11 in rock inclusions. For all the Hawaiian Islands and volcanoes except Kilauea and Lōʻihi, the maximum R/Ra we observed in water is approximately 8, which coincides with the value of the upper mantle and mid-ocean ridge basalts. An R/Ra of 8 on the island of Lānaʻi was found in groundwater with temperatures of 65 C at 1060 m, while an R/Ra of 8 on Kauaʻi was found in water with a temperature of just 28 C at 140 m.