Despite being surrounded by water, island communities like Hawaii rely on collecting freshwater stored in the ground for drinking, irrigation, and commercial industries. But Hawaii's water is at serious risk — climate-driven droughts are making groundwater scarce.
The solution to Hawaii's problem may lie in the islands' unique geology. Using a new technique that relies on tracing electrical resistance, a team of geohydrologists has discovered a never-before-seen way the islands' volcanic soil collects and hides away freshwater beneath the ocean's salty surface.
Accessing this freshwater could give Hawai'i and other volcanic islands a more sustainable and future-proof solution to collecting water during times of need.
The new study was published Wednesday in the journal Science Advances.
A fresh hypothesis — The researchers behind the discovery started their detection work after re-examining existing calculations of the expected versus the measured freshwater on the island — there was a 40 percent or so discrepancy in the amount that should be there, as opposed to the amount that seemed to be there. This begged the question: Where was all the missing freshwater?
Traditional geohydrological studies focus mainly on sources of freshwater found floating on the surface of seawater, and may miss water hidden in porous rock layers, like basalt, the researchers say.
To figure out where the leftover 40 percent of freshwater was hiding, the team decided to expand their search criteria and focus their attention beneath the salty ocean, using a new kind of imaging technique to reveal what lurked in the rock below the surface.
In search of freshwater — Instead of boring into the rocky seafloor in search of this freshwater, the team used a less invasive method called marine controlled-source electromagnetic imaging to measure changes in electrical resistance in the waters near the islands' shore. Electrical resistance changes dramatically between salty water versus freshwater. To take the measurements, the team released torpedo-like imaging systems off the back of their boat into the ocean near the shoreline.
Diving just under the ocean's surface, the imagers pinged electromagnetic signals to detect the salinity of the water hidden in the rock.
The study's first author and postdoctoral geophysics researcher at the University of Hawai'i, Eric Attias, said in a video accompanying the research that this process is much like performing an MRI scan on the brain.
The measurements reveal where all that missing water has been hiding — and then some. The researchers found massive freshwater reservoirs sandwiched between layers of ocean water-laden basalt.
In total, the researchers estimate these reservoirs contain a whopping 920 billion gallons of freshwater.
Close to a trillion gallons of water is a lot to miss, despite years of searching.
It is possible this hidden freshwater was able to sneak away from the Hawai'ian shore via channels through porous ash and basalt, the team speculate in the study. A little bit of this freshwater is released from the seabed in the form of freshened water springs, but most of it stays confined within the ash and soil.
If validated, this would be the first time anyone has ever documented this natural process.
What's next — Scientists will ultimately need to drill into the rock to see if the results hold true, but the researchers seem confident the volume of this reservoir will actually be higher than they suspect.
The researchers point out this much hidden water could play a big role in providing freshwater to the island — and other islands like it — in a more efficient, and ecologically sound, way.
Abstract: Conventional hydrogeologic framework models used to compute ocean island sustainable yields and aquifer storage neglect the complexity of the nearshore and offshore submarine environment. However, the onshore aquifer at the island of Hawai‘i exhibits a notable volumetric discrepancy between high-elevation freshwater recharge and coastal discharge. In this study, we present a novel transport mechanism of freshwater moving from onshore to offshore through a multilayer formation of water-saturated layered basalts with interbedded low-permeability layers of ash/soil. Marine electromagnetic imaging reveals ∼35 km of laterally continuous resistive layers that extend to at least 4 km from west of Hawai‘i’s coastline, containing about 3.5 km^3 of freshened water. We propose that this newly found transport mechanism of fresh groundwater may be the governing mechanism in other volcanic islands. In such a scenario, volcanic islands worldwide can use these renewable offshore reservoirs, considered more resilient to climate change-driven droughts, as new water resources.
A previous version of this story used the unit 'ton' to describe the freshwater reservoir but it should actually be measured in gallons. Inverse regrets the error.