Abstract
Investigating water within lava tubes offers valuable opportunities for research in cave ecosystems and their importance for Earth and other planetary bodies. This study examines the hydrogeological and geochemical characteristics of an subterranean lake within a lava tube on the Big Island of Hawaii. The lake is located 1.27 km from the coastline, and the opening of the lava tube or "puka" at an elevation of approximately 46 m a.m.s.l. The lake edge occurs inside the lava tube at approximately 20 m a.m.s.l. For a year, fluctuations in lake depth, temperature, and salinity were monitored with in-situ sensors and found to be closely linked with the proximal tidal patterns. Additionally, comparisons with lake depth fluctuations and groundwater elevation data revealed a slight response to rainfall. However, due to the distance between the observation well and the lake, a direct correlation between groundwater changes and lake fluctuations was not evident. The lake exhibits a distinct halocline (5 to 15 ppt), measured with a YSI multiparameter probe at the beginning and end of the research period. The lake reaches a depth of over 20 m towards the back of the cave. An ionic analysis of the major constituents characterizes the water as sodium chloride type with high concentrations of CO (sub 3) (super 2-) + HCO (sub 3) (super -) , relationship to tides, as well as a slight increase in pH with depth (6.62 to 7.75), suggests that the lake can be characterized as meromictic, with an upper brackish layer receiving water from meteoric sources and gradually becoming more saline as it advances into the transition zone of the exposed, basal freshwater lens, responsive to tides. The unique properties of this lake offer valuable opportunities for ecological and paleo-environmental research, as well as highlight its potential as an analog for planetary research.