Submarine groundwater and vent discharge in a volcanic area associated with coastal acidification

Abstract 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 which 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 composition of water and chloride concentration revealed that the springs discharge recirculated seawater mixed variably with terrestrial groundwater. Shallower springs and porewater 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. Plain language abstract Groundwater flow from land to sea could have important coastal impacts but usually is unrecognized. Delicate reefs may be particularly sensitive to groundwater inputs. Yet, few studies have made connections between groundwater and reefs. We investigated submarine groundwater in a volcanic coastal area that hosts the world's most biodiverse reefs. By measuring 222Rn activity in seawater as a tracer for groundwater, we found that groundwater discharge is prevalent throughout the coast. In areas where seawater 222Rn activity was higher, we discovered hydrothermal springs emitting acidic waters (pH~5.4-6.0) and venting volcanic CO2 which 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 composition of water samples indicate that the springs discharge recirculated seawater mixed variably with terrestrial groundwater. Shallower springs and porewater from the seabed have a higher proportion of terrestrial groundwater than deeper springs, which emit mostly recirculated seawater. These waters also interact more with the aquifer matrix. Because different mechanisms of groundwater flow from land to sea are taking place, the groundwater flow could be contributing to the evolution and functioning of the ecosystem. However, groundwater also represents potential pathways for contamination released from land.