AMHERST, Mass. — One of the biggest obstacles on the road to the low-carbon energy future is caused by the rare-earth element lithium, a critical component for the batteries that can store the abundant and sustainable energy from renewable sources. The element occurs naturally as a salt in briny oases, called salares, in some of the world's harshest environments, including the "Lithium Triangle" high in South America's arid Altiplano. Mining lithium has the potential to destabilize already sensitive environments that are host to rare flora and fauna, as well as the Indigenous communities that have long made such places their homes.
While earlier research from the University of Massachusetts Amherst has shown that commonly accepted figures quantifying how much water can be withdrawn from salares overestimate the amount of water available by more than an order of magnitude , a recent study led by UMass Amherst graduate student Daniel Corkran uncovers the previously unknown physical mechanisms that govern sustainable water usage. And it overturns some of the commonly held assumptions about what counts as sustainable lithium mining.
It's all about location and water density.
"The question that really drove this study centers around a debate between two different parties in these arid basins," says Corkran. "Some view the types of water available in these basins— fresh, brackish and lithium-containing salt water (brine)—as one continuous water resource, meaning that when lithium companies pump enormous amounts of brine out of the salares, they're using an enormous amount of water. They claim that this usage will greatly affect all the other water demands, both environmental and human, to which that water could be put. On the other hand, lithium companies have pointed out that the brine is 200 times saltier than seawater and so can't support life. Therefore, it's only the fresh water in these salares that matters, and since lithium doesn't occur in the freshwater portions of the salares, there's nothing to worry about. Given the wide gulf between the two conceptualizations and the implications for sustainable water use, we decided to test both hypotheses."
To do so, Corkran and his co-authors, which include members of David Boutt's Hydrogeology Group at UMass Amherst as well as collaborators from the University of Alaska Fairbanks and the University of Dayton, first designed a series of immensely complex model simulations to project the effects of lithium and freshwater pumping over the next 200 years, across a wide range of climactic scenarios and geologic settings. They then checked their modeled results against satellite data from salares in two different regions of the Lithium Triangle, the source of more than half of the world's lithium resources. Each of these salares relies on a different mode of lithium mining: the traditional evaporative technique, which involves evaporating brine, and direct lithium extraction, or DLE, which preserves the brine but can use up to 200% more freshwater.
"Corkran's systematic evaluations revealed two new processes," says Boutt, professor of Earth, geographic, and climate sciences at UMass Amherst and the paper's senior author.
The first involves where in the salar mining companies pump their water. The traditional understanding of the relationship between water and sustainability holds that you can responsibly use only as much water as flows in, and that it's better to use this new water, not the older stuff stored in the aquifer.
But salares are composed of both freshwater portions, located at the edge of the basin near the points where fresh groundwater recharges the wetlands, and briny portions at the center of the basin, with a transitional zone between the two, and Corkran found that the closer a company pumps to the fresh water, the greater the impact is on the salar's wetlands and the faster its shores recede.
"The fresh water is what we shouldn't be touching," says Boutt, who points out that both local agriculture and companies that mine for other precious metals, such as copper, as well as the newer DLE methods of gathering lithium may be having an outsized impact on the salares.
"Instead," says Corkran, "companies should be pumping water from the briniest patches they can find."
Which brings us to the surprise player in this story—density.
Think about what happens to water when you freeze it. Fill a glass jar with fresh tap water, cap it tightly, and then put it in the freezer overnight. When you check on it in the morning, you'll have a broken jar because freshwater is denser than ice. As the water froze, it expanded, gaining volume, losing density and cracking the jar, even though the number of water molecules in the jar remained the same.
Something similar happens in the salares.
Salt water is denser than fresh water. Pound for pound, it takes up less space in the salar. This means that when mining companies pump out dense, lower-volume salt water the effect on the water levels is blunted. But pump out less dense, high-volume fresh water, and the effect on water levels is magnified.
Put another way, you can pump out more salt water with less effect; but pump out fresh water, and the salar's groundwater-dependent wetlands seem to melt away.
Corkran and his colleagues confirmed this conclusion with measurements of two wetlands located in different salares: the Diffuse South Tumisa Discharge Zone in Salar de Atacama and the Rio Trapiche Vega in Salar del Hombre Muerto.
"What this all adds up to," says Boutt, "is that we don't have to be all that concerned about pumping brine. But we need to be very careful with any freshwater usage—whether for mining, agriculture or any other use. And if we do decide to go full steam ahead with DLE technology for lithium, we need to address its needs for fresh water."
