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sábado, 24 de setembro de 2016


sinkholes
The sinkholes around the Dead Sea can be up to 24 meters deep, though most are much shallower.
alonkorngreen/iStockphoto

Can a controversial canal stop thousands of sinkholes from forming around the Dead Sea?

On the edge of the Dead Sea, the ground is caving in. Trucks and small buildings in Israel and Jordan have fallen into pits, beaches and plantations have closed, and roads been rerouted to avoid the more than 5500 sinkholes that pockmark the region. Now, scientists think they have a better idea of what’s causing these sinkholes to form—and how to stop them.

The Dead Sea is shrinking dramatically, falling 0.9 meters a year. The Jordan River—its main source of freshwater—has been mostly diverted for agriculture, and industrial plants have kept siphoning off Dead Sea water for mineral extraction. Scientists think that the receding shoreline is the driving force behind the sinkholes, but they’ve lacked hard evidence of what’s happening underground.

So, as part of an initiative by the Geological Survey of Israel to study the Dead Sea region, Imri Oz, a hydrogeologist currently at Technion - Israel Institute of Technology in Haifa, and his colleagues built their own version of the Dead Sea. In a sand-filled Plexiglas tank in the lab, they constructed a miniature cross section of the sea’s shore. The Dead Sea’s famously salty water flows in from a hole on one side, and freshwater (representing the flow from nearby underground aquifers) seeps in from the other. The team used information from boreholes drilled around the Dead Sea to estimate the depth of different layers of clay, gravel, salt, and sand and their respective particle sizes.

In the model, sinkholes grew just as they do in the real world: As the Dead Sea’s water level drops, freshwater flows in from nearby aquifers to replace it. The freshwater flows through underground layers of salt and dissolves it, leaving behind unstable caverns. Small pockets then collapsed to form depressions in the sand. Scaled up to the real world, those depressions are sinkholes. The model also gave insight into the timing of sinkhole formation: Large blocks of salt were more likely to collapse all at once, whereas smaller salt deposits showed a slump on the surface before caving in.

Through ongoing studies into the structure of the Dead Sea’s salt deposits and ground-penetrating radar studies to track incipient sinkholes, scientists can now identify the worst spots. Stopping new sinkholes from opening, however, is more difficult. The most popular proposed solution has been a canal or pipeline to refill the Dead Sea using water from the Red Sea or the Mediterranean Sea—the nearest unlimited water sources. Politicians have hailed various proposals as peace projects—opportunities for cooperation among Israelis, Jordanians, and Palestinians, all of which could benefit from the water as it made its way toward the Dead Sea. But no pipelines have been built, in part because of the need for more study.
satellite images of the Dead Sea
Satellite images reveal the Dead Sea’s retreat, as well as the growth of mineral-extraction evaporation ponds in the southern basin.
NASA's Earth Observatory
To find out whether the strategy would work, Oz’s team simulated the effects of a Red Sea–Dead Sea pipeline by adding to their tank slightly salty water—like that of the Red Sea. The results were encouraging: The new “sea water” formed a layer on top of the even saltier Dead Sea water, and flowed back up into the simulated aquifer, effectively plugging it. Though the seawater still dissolved the salt layers, it did so much more slowly than the flow of freshwater. In the real world, salt deposits would dissolve about 10 times more slowly than they currently do if the Dead Sea were refilled, the researchers report this month in the Journal of Geophysical Research: Earth Surface.

“It’s really interesting to see the dynamic evolution that the model predicts, and how it explains what we observe on the surface,” says Simone Atzori, a geophysicist at the National Institute of Geophysics and Volcanology in Rome, who was not involved with the study. “Through this model, they tried to give us visual access to this phenomenon.”

Of course, sand in a Plexiglas tank does not reflect the complexities of actual sinkholes. “This is always the problem with the laboratory, that you’re taking the real world and putting it into a smaller system,” Oz says. The model also can’t account for the potential environmental impacts of a pipeline, such as massive algal blooms. The addition of seawater might cause algae to grow out of control, choking out microorganisms better suited to a saltier environment, or turning the water red. Environmentalists also worry that the seawater could trigger the growth of tiny, floating gypsum crystals that could whiten the upper layers of the Dead Sea, raising its temperature and speeding its evaporation.

As an alternative to the pipeline, a local environmental group called EcoPeace Middle East has proposed restoring the flow of water in the Jordan River, which has been dammed and diverted until only 10% of its former flow reaches the Dead Sea. To do this, they argue that the mineral industry should be charged for the Dead Sea water used to fill evaporation ponds, which yield minerals like potash and magnesium. “We have specific steps for what can be done, and they’re all doable, but that means changing the status quo,” says Mira Edelstein, the Jordan River projects coordinator for EcoPeace Middle East in Amman.
Now, a much-reduced version of the pipeline seems to be the most prominent solution: This year, bidding began to construct a pipe that would bring briny Red Sea wastewater from a new desalination plant in Jordan to the Dead Sea. According to Edelstein, the flow from this pipeline won’t be enough to stem the Dead Sea’s shrinkage, and it may be canceled because of expense (more than $900 million). But in the meantime, researchers will keep studying the collapsing shoreline, fine-tuning their theories to peer into the Dead Sea’s uncertain future. “If you put together all the possible information, you have a better vision of the situation,” Atzori says.

*Correction, 23 September, 10:47 a.m.: The article has been clarified to note the involvement of the Geological Survey of Israel in the study. In addition, it has been corrected to state that the likely addition of seawater, not freshwater, could cause algal blooms in the Dead Sea.
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DOI: 10.1126/science.aah7336

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