Exploring concealed hydrothermal activity in Lāna'i (Hawaiian Islands) and its hydrogeological effects through geophysical surveys

The Hawaiian Islands are characterized by volcanic activity and intricate hydrogeological systems. Given their isolated location and reliance on imported fossil fuels for energy, exploring for geothermal sources is imperative. Simultaneously, understanding freshwater reservoirs is crucial for sustainable water resource management, considering the islands' susceptibility to climatic variations and increasing demands. This study, situated at the intersection between hydrothermal exploration and freshwater reservoir characterization, aims to contribute to sustainable development across the Hawaiian archipelago. Through integrated analysis of geophysical, groundwater, and mineralogical data from Lāna'i Island, we have identified and characterized concealed hydrothermal activity, proposed a hypothesis for its potential magmatic heat source, and examined associated hydrogeological effects, such as the occurrence of brackish groundwater. Correlations between newly acquired self-potential (SP) data and previously published magnetotellurics (MT) data indicate a hydrothermal upflow within the central and southern regions of the Pālāwai caldera, likely originating from deep magma reservoirs, with the latter supported by prior-collected gravity data. Additionally, X-ray diffraction (XRD) analyses reveal varying degrees of hydrothermal alteration in surface rocks located in the Pālāwai caldera and the rift zone, reflecting past interactions with high-temperature fluids. The spatial alignment of these surface alterations with current upflow zones suggests an active hydrothermal system that still utilizes pre-existing caldera faults, potentially dating back to the shield-building volcanic stage and influencing the formation of warm brackish groundwater within the Pālāwai basin. This hydrothermal influence appears to be restricted to the Pālāwai caldera and the Canyon zone, while the Munro Trail ridge contains a cold freshwater source, likely representing an isolated hydrogeological environment with impounded aquifers. In sum, these findings highlight the existence of a potential geothermal resource at depths of >2 km. • New SP data reveal concealed hydrothermal activity in Lāna'i caldera. • Hydrothermal upflow linked to deep magmatic sources (>2 km depth). • XRD shows past alteration by high-temp fluids in surface rocks. • Hydrothermal system follows caldera faults from shield stage. • Findings support geothermal and water resource potential on Lāna'i.