Submarine Groundwater and Vent Discharge in a Volcanic Area Associated With Coastal Acidification

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We investigated submarine groundwater discharge (SGD) in a volcanic coastal area that hosts the world's most biodiverse reefs. Measurements of 222Rn activity in coastal seawater, a tracer for groundwater, indicated prevalent SGD. In areas where seawater 222Rn activity was generally higher, we discovered hydrothermal springs emitting acidic waters (pH ~5.4–6.0) and venting magmatic CO2 that brought local pCO2 levels up to 95,000 ppm. The collection of vents raised CO2 and lowered pH over 1–2 km of coastline. The hydrogen and oxygen isotope compositions of water and chloride concentration revealed that the springs discharge recirculated seawater mixed variably with terrestrial groundwater. Shallower springs and pore water have a higher proportion of terrestrial groundwater than deeper springs, which emit mostly recirculated seawater. This suggests that different SGD mechanisms are present. The SGD could be contributing to the evolution and function of the biodiverse ecosystem, but it also represents myriad pathways for contamination.