Groundwater flow from land to sea could have important coastal impacts but it is usually unrecognized. Delicate reefs may be particularly sensitive to groundwater inputs. Yet few studies have made connections between groundwater and reefs. Now, a team of researchers has investigated submarine groundwater in the Verde Island Passage in the Philippines, a volcanic coastal area that hosts the world’s most biodiverse reefs, and discovered hydrothermal springs — which they call ‘Soda Springs’ — emitting acidic waters and venting volcanic carbon dioxide (CO2).
A researcher collects gas samples at Soda Springs in the Philippines. Image credit: Jackson School of Geosciences, University of Texas at Austin.
The Soda Springs are linked to a nearby volcano that vents out the gases through cracks in the ocean floor and has probably been doing so for decades or even millennia.
However, the high CO2 levels could make the springs an ideal spot for studying how coral reefs may cope with climate change.
The site also offers a fascinating setting to study corals and marine life that are making a home among high levels of CO2.
“These high CO2 environments that are actually close to thriving reefs, how does it work? Life is still thriving there, but perhaps not the kind that we are used to. They need to be studied,” said Professor Bayani Cardenas, a researcher in the Jackson School of Geosciences at the University of Texas at Austin.
Professor Cardenas and colleagues measured CO2 concentrations as high as 95,000 parts per million (ppm), more than 200 times the concentration of CO2 found in the atmosphere. The readings range from 60,000 to 95,000 and are potentially the highest ever recorded in nature.
The CO2 levels fall quickly away from the seeps as the gas is diluted in the ocean, but the gas still creates an elevated CO2 environment along the rest of the coastline of the Calumpan Peninsula, with levels in the 400 to 600 ppm range.
The team discovered the Soda Springs while researching whether groundwater from the nearby land could be discharging into the submarine ocean environment, which is a phenomenon that is generally ignored by scientists looking at the water cycle.
“It’s an unseen flux of water from land to the ocean. And it’s hard to quantify. It’s not like a river where you have a delta and you can measure it,” Professor Cardenas said.
The scientists tracked groundwater by testing for radon-222, a naturally occurring radioactive isotope that is found in local groundwater but not in open ocean water.
Along with CO2 bubbles, they also found hotspots in the sea floor where groundwater was being discharged into the ocean.
“This is significant because the connection between the groundwater and ocean means that there is a pathway for pollutants from the island to make it to the reef system,” Professor Cardenas said.
“This is particularly important for a place like the Philippines, where coastal development is booming largely because of ecotourism driven by the nearby reefs, but the communities almost always depend on septic tanks instead of modern sewage systems. This means the development could drive pollution to the same reefs the economy relies on.”
The findings are published in the journal Geophysical Research Letters.
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M. Bayani Cardenas et al. Submarine Groundwater and Vent Discharge in a Volcanic Area Associated with Coastal Acidification. Geophysical Research Letters, published online January 3, 2020; doi: 10.1029/2019GL085730
