Aigües subterrànies de Hawaii: un nou recurs

Malgrat la pluja abundant de Hawaii, la principal illa de l'arxipèlag pateix una manca d'aigua dolça, ja que hi ha una gran demanda i l'aigua es filtra i desapareix de la superfície. Això provocava que s'hagués d'agafar aigua de dalt de les muntanyes, que danyava a molts ecosistemes. Fins que, fa ben poc, s'ha descobert l'existència de rius subterranis d'aigua dolça que flueixen envoltats de roca volcànica fracturada a sota de l'illa, gràcies a l'ús de tecnologia d'ones electromagnètiques, que diferencien l'aigua dolça de la salada perquè la segona té una conductivitat més alta. 

Segons el Dr. Attias, que va dur a terme la investigació, el descobriment d'aquests rius subterranis, de fins a 4 km de llargada, pot ser un gran recurs no només per a Hawaii, sinó que potser també per a altres illes similars, com Cap Verd o les Galápagos, ja que es podria extreure a la superfície, com quan s'extreu petroli, però amb menor impacte mediambiental, perquè si hi ha una fuga, és només aigua dolça el que s'escapa. 

A les profunditats de la muntanya Hualalai, a la Gran Illa de Hawaii, flueixen rius subterranis d’aigua dolça.

The New York Times, 25/11/20

There are few things on the island of Hawaii that are more valuable than fresh water. This is not because the island is dry. There is plenty of rain. The trouble is that there is tremendous demand for this water and much of it that does accumulate on the island’s surface disappears before it can be used.

New research by marine geophysicists reveals that underground rivers running off the large island’s western coast are a key force behind this vanishing act.

Fresh water is often pumped on the island from aquifers formed from rain at higher elevations where it is easy to access. The drawback is that if too much water gets pumped to meet demand, little remains to travel through rocks to farms and fragile ecosystems that depend upon it. To make matters worse, recent studies of this water labeled with isotopes and tracked over time have revealed that these aquifers are also heavily leaking somewhere else.

“Everyone assumed that this missing fresh water was seeping out at the coastline or traveling laterally along the island,” said Eric Attias, a postdoctoral researcher at the University of Hawaii, who led the new study published Wednesday in Science Advances. “But I had a hunch that the leak might be subsurface and offshore.”

The big island of Hawaii is like an iceberg. Only a tiny fraction of the island sticks out of the ocean. The rest is submerged. To study the hydrogeology of these sections, Dr. Attias turned to electromagnetic imaging.

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

Dr. Attias’ work shows that within the rock of the island below the waves, there are underground rivers of fresh water flowing 2-½ miles out into the ocean. These rivers are flowing through fractured volcanic rock and surrounded by porous rocks that are saturated with salt water. Between all of this salt water and the flowing fresh water are thin layers of rock formed from compacted ash and soil that appear to be impermeable and thus keeping the two types of water separated. In total, these rivers appear to contain enough fresh water to fill about 1.4 million Olympic swimming pools.

El doctor Eric Attias i el seu equip desplegant el dispositiu de sonda electromagnètica de font controlada a la costa de Hawaii.

“It looks quite plausible that there is a whole lot of fresh water down there beneath the ocean,” says Graham Fogg, a hydrogeologist at the University of California, Davis, who was not involved in the study.

To access this water, Dr. Attias proposes a system similar to an offshore oil platform. “The water is already under high pressure, so little pumping would be needed and, unlike an oil pump, there would not be any threat of pollution. If you have a spill, it’s just fresh water,” he said.

“I am excited to see wells drilled into these offshore aquifers so we can find out how fresh this water is and whether or not we can produce large volumes without pulling seawater into the system,” said Mark Person, a hydrogeologist at the New Mexico Institute of Mining & Technology.

Yet, to Dr. Attias, 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 cautious.

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

Dr. Attias speculates that the discovery could be relevant to other islands, too.

“Given that Reunion, Cape Verde, Maui, the Galápagos and many other islands have similar geology, our finding could well mean that the water challenges faced by islanders all over the world might soon become a lot less challenging,” he said.