Water, Desert, and Salt
The Atacama Desert in Chile is the driest non-polar desert in the world. Rain gauges have stood in the desert for seventy years but never caught a drop of rain. Here, the ambient temperature gets so hot that it melts and cracks the salt flats—an evaporite layer 4000 feet deep—into jagged pinnacles.
Yet within this extreme environment is the backbone of the world’s lithium production: the Salar de Atacama. The element, so valuable in the production of batteries for electronics, is recovered from brine pools that emerge from subsurface waters in the Salar.
UMass Amherst hydrologist David Boutt, an associate professor in Geosciences, was studying the Salar’s “water budget” with a team of graduate and undergraduate student researchers when they made some intriguing findings.
It had been assumed that the water in the Salar drained down from a specific region of the Andean plateau. But the sheer amount of water showed it had to be originating from an area much vaster than the watershed on the map.
“The watershed is importing water from outside its topographic boundaries,” says Boutt.
So where’s the water coming from?
The researchers collected samples of brine to reconstruct how the water has been arriving over the last 7 to 10 million years. They analyzed the isotopic composition of the water, the dissolved gasses within it, and the balance of water and solutes for factors such as condensation temperature, precipitation rate, and whether or not the water has experienced evaporation.
Boutt estimates the brine in the Salar is “very, very old.”
“We think the water that’s flowing out onto the Salar was put into the system tens of thousands of years ago,” says Boutt, a groundwater specialist. He suspects that the water table was “recharged” during the mid-Holocene when the area was much wetter.
So whence does this ancient water originate, if not the watershed? How long has it been moving slowly through the earth, and how long will that outflow last? Companies that trade in lithium want to ensure that the region is properly managed for the long term. Understanding these water resources can also give insight into how to care for other dry regions around the world.
Compared to the drama of an ocean, groundwater can seem unassuming, yet it has surprises up its sleeve: it can flow uphill, as with a spring, and apparently defy topography. The team’s findings disrupt conventional thinking about the relationship between the input and accumulation of water. “One reason I like groundwater,” Boutt says, “is because it breaks all the rules.”