Ian Connellan
The Hawaiian island chain has been a tropical holiday destination for decades, renowned for its warm waters, fine surfboarding waves and extraordinary landforms.
As climate change bites, it also faces one of the most pressing problems of volcanic islands worldwide: the likelihood of drought due to poor freshwater reserves.
Now, a study by University of Hawaii (UH) researchers has found twice as much freshwater stored offshore from Hawaii Island – at 10,432 square kilometres the archipelago’s largest landmass – than was previously thought.
The findings, published in Science Advances, were obtained through a ground-breaking method called marine controlled-source electromagnetic (CSEM) imaging. They reveal a new way in which large volumes of fresh water are transported from onshore to offshore submarine aquifers along the coast of Hawai’i Island.
This new mechanism may provide alternative renewable resources of fresh water to volcanic islands worldwide.
“Their evidence for separate freshwater lenses, stacked one above the other, near the Kona coast of Hawaii, profoundly improves the prospects for sustainable development on volcanic islands,” says Brian Taylor, from the UH Mānoa School of Ocean and Earth Science and Technology.
The study revealed the onshore-to-offshore movement of freshwater through a multilayer formation of basalts embedded between layers of ash and soil, a departure the previous groundwater models of this area. The freshwater layers may extend as far as four kilometres offshore.
“Our findings provide a paradigm shift from the conventional hydrologic conceptual models that have been vastly used by multiple studies and water organisations in Hawaii and other volcanic islands to calculate sustainable yields and aquifer storage for the past 30 years,” says study author Eric Attias.
“We hope that our discovery will enhance future hydrologic models, and consequently, the availability of clean fresh water in volcanic islands.”
Co-author Steven Constable, who developed the CSEM system used in the project, has devoted his career to developing marine electromagnetic methods such as the one employed.
“It is really gratifying to see the equipment being used for such an impactful and important application,” he says. “Electrical methods have long been used to study groundwater on land, and so it makes sense to extend the application offshore.”
CSEM imaging distinguishes seawater from fresh water through measurements of the electrical resistivity – a material’s resistance to electric current – of rock formations. The surface-towed CSEM system used had the capability to study the electrical structure of the subsurface up to 500m below the seafloor in water depths of up to 100 metres.
Columbia University associate professor Kerry Key, who uses electromagnetic methods in his work but was not involved in this study, lauds the new electromagnetic technique as “a game-changing tool for cost-effective reconnaissance surveys to identify regions containing freshwater aquifers, prior to more expensive drilling efforts to directly sample the pore waters. It can also be used to map the lateral extent of any aquifers already identified in isolated boreholes.”
Geochemist and study author Donald Thomas says the findings confirm the presence of much larger quantities of stored groundwater than was previously thought. “Understanding this new mechanism for groundwater… is important to better manage groundwater resources in Hawaii.”.
It’s thought that similar offshore freshwater systems might also be a feature of the island of Oʻahu, Hawaii’s most populous island and site of its capital, Honolulu. CSEM imaging hasn’t been used at Oʻahu.
Electromagnetic studies of other volcanic islands across the globe suggest the presence of similarly layered offshore hydrogeological formations, the researchers note, indicating potential new sources of fresh water for these islands as well.