There are few things on the island of Hawaii that are more valuable than fresh water. It’s not because the island is dry. There is a lot of rain. The problem is that there is tremendous demand for this water and much of it that builds up on the island’s surface disappears before it can be used.

New research by marine geophysicists shows that underground rivers flowing off the west coast of the large island are a key force in this act of escape.

Fresh water on the island is often pumped from aquifers that are easily accessible from rain at higher altitudes. The downside is that if too much water is pumped to meet needs, little is left to travel through rocks to farms and fragile ecosystems that depend on it. To make matters worse, recent studies of this isotope-marked water, which has been tracked over time, have shown that these aquifers also leak heavily elsewhere.

“Everyone assumed that this missing freshwater seep down the coast or migrated along the side of the island,” said Eric Attias, a postdoctoral fellow at the University of Hawaii, who led the new study published in Science Advances Wednesday. “But I suspected the leak might be subsurface and offshore.”

The big island of Hawaii is like an iceberg. Only a tiny part of the island protrudes from the sea. The rest of them went into hiding. To study the hydrogeology of these sections, Dr. Attias to electromagnetic imaging too.

Seawater conducts electricity exceptionally well because of the presence of dissolved salt ions. In comparison, fresh water is a rather poor conductor. Dr. Aware of these diverse electrical properties, Attias worked with a team at the Scripps Institute of Oceanography to tow a 3,200-foot system behind a boat that emitted electromagnetic fields through the submerged coastal rocks near the Hualalai Volcano on the west coast.

Dr. Attias’ work shows that in the rock of the island beneath the waves, freshwater underground rivers flow 2½ miles into the ocean. These rivers flow through broken volcanic rocks and are surrounded by porous rocks that are saturated with salt water. Between all this salt water and the flowing fresh water there are thin layers of rock made of compacted ash and earth, which appear impermeable and thus separate the two types of water from one another. Overall, these rivers appear to contain enough fresh water to fill about 1.4 million Olympic swimming pools.

“It’s pretty plausible that there’s a lot of freshwater down there under the ocean,” says Graham Fogg, a hydrogeologist at the University of California at Davis who was not involved in the study.

To gain access to this water, Dr. Attias proposed a system that is similar to an offshore oil platform. “The water is already under high pressure, so only little has to be pumped and, in contrast to an oil pump, there is no risk of contamination. If you spill something, it’s just fresh water, ”he said.

“I am very excited that wells are being drilled in these offshore aquifers so we can find out how fresh this water is and whether we can produce large quantities without drawing seawater into the system,” said Mark Person, a hydrogeologist at New Mexico Institute of Mining & Technology.

For Dr. Attias, however, the real beauty of the find is its location, and he says that collecting the water would not deprive any ecosystems on the island of hydration.

Dr. Fogg was more careful.

“The fresh water they discovered is clearly being actively fed by the island’s aquifer,” he said. “This means that the entire groundwater system is connected and our drainage of this new water could adversely affect the island’s ecosystems and water availability for pumps on the island.”

Dr. Attias speculates that the discovery could be relevant to other islands as well.

“Given that Reunion, Cape Verde, Maui, the Galapagos Islands and many other islands share similar geology, our finding could well mean that the water problems islanders around the world are facing will soon become much less of a challenge” , he said.

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