First images of freshwater plumes in the sea

The first imaging of large freshwater plumes off the West Island of Hawaii may help water planners optimize sustainable yields and aquifer storage calculations. Researchers at the University of Hawaii at Mānoa demonstrated a new method to detect freshwater plumes between the seabed and the ocean surface in a study recently published in Geophysical research letters.

The research, supported by the Ike Wai project of Hawai’i EPSCoR, is the first to demonstrate that marine controlled source electromagnetic (CSEM) imagery towed to the surface can be used to map oceanic freshwater plumes in high resolution . It is an extension of the revolutionary discovery of fresh water under the seabed in 2020. Both of these discoveries are important in a world facing climate change, where fresh water is vital for preserving public health, agricultural yields. , economic strategies and ecosystem functions.

Deep implications

While the CSEM method has been used to detect the presence of resistive targets such as oil, gas and fresh water under the seabed, this study is the first time that CSEM has been applied to the image of the ‘fresh water in the ocean water column, according to’ Ike Wai research faculty affiliate Eric Attias, who led the study.

“This study has profound implications for oceanography, hydrogeology, and oceanic processes that affect biogeochemical cycles in coastal waters around the world,” Attias said. “Using the CSEM, we can now estimate the volumes of fresh water emanating from the water column. This is indicative of the renewability of Hawaii’s underwater freshwater system.”

Submarine groundwater discharge (SGD), the leakage of groundwater from a coastal aquifer into the ocean, is a key process, providing a source of water for people and supporting marine life like fish and the seaweeds. According to Henrietta Dulai, associate professor in the Department of Earth Sciences in the Department of Earth Sciences at UH Mānoa and co-author of the study, the location of offshore sources is extremely difficult to predict due to the geology. unknown underlying and underground water pipes.

“The flow of such high volumes of nutrient-rich, low-salinity groundwater to the ocean is of great importance for chemical balances and the supply of nutrients to offshore food webs,” said Dulai. “It is great to have a method for locating discharge sites and plumes, as it opens up new possibilities to sample and identify the age of water, its origin, its chemical composition and its importance to marine ecosystems in this otherwise oligotrophic region (relatively low in plant nutrients and containing abundant oxygen in the deeper parts of the ocean. “

Four Olympic swimming pools

This study included the 2D CSEM inversion based on electromagnetic data, the calculation of the resistivity to salinity and the volumetric estimation of the freshwater plume. Through the use of CSEM, the research team were able to imagine surface freshwater bodies and multiple large-scale freshwater plumes containing up to 87% freshwater off the island of Hawaii. The results imply that at the study site, substantial volumes of fresh water are present in the area between the seabed and the ocean surface. A conservative estimate for one of the plumes suggests 10,720 cubic meters or approximately the volume of four Olympic swimming pools.

The methodology used in this study can be applied to coastal areas around the world, thereby improving future hydrogeological models by incorporating offshore SGD and optimizing sustainable yields and storage calculations. Attias plans to expand the innovative use of CSEM to further prove its application in freshwater imaging in other volcanic islands around the world.

Attias will present his work at the International Tropical Island Water Conference to be held April 12-15, 2021. Organized by the UH Water Resources Research Center and Hawai’i EPSCoR, this conference brings together water specialists, water managers and community members from around the world. world to share cutting-edge research and learn from each other’s experiences in managing and understanding water resources in a wide range of tropical island contexts.