Our blue planet is ruled by cycles. There’s the water cycle; the solar cycle; the menstrual cycle, all tiny epicycles on the great orbit that is the Circle of Life. But one you may not have heard of – or even thought of much before – is the salt cycle: how salt, or rather salts, enter and interact with the natural world.
It’s vitally important to life – which is a bit worrying, actually, because it’s also getting thrown pretty noticeably out of whack lately. So what is this saline phenomenon? What’s going wrong? And is there anything we can do about it?
What is the salt cycle?
While the popular health advice is to avoid salt – and that is, for most of us, a pretty good idea – some amount of it is vital for survival. Not just for us, but every animal: “We’re basically an electrical system, controlled by salts,” Sujay Kaushal, a professor of geology at the University of Maryland, told Fast Company last year.
Plants, too, rely on salts to aid the production of chlorophyll; you could even argue that the planet itself relies on salt to survive, since its presence helps the soil underneath our feet to clump together. “Salt has a small ionic radius and can wedge itself between soil particles very easily,” Kaushal explained in a statement. “In fact, that’s how road salts prevent ice crystals from forming.”
But where does all this salt come from? Not the seasonings aisle: like everything in the natural world, it exists in an epic cycle of movement, growth, precipitation, and more.
It works like this: originally, salt comes from the Earth’s mantle – tectonic activity exposes the magma to the ocean or air, where it solidifies into rocks and is gradually worn down by wind and rain, or else leached into the seas via chemical reactions.
Salt ions are released into the soil, where they are taken up by plants and other organisms; other ions may be washed away in rivers or blown into the atmosphere by the wind or sea sprays. A small amount of salt can enter the atmosphere via evaporation, but less than you might think – it simply takes too much energy for that to be common.
In an ideal model, all that salt then gets gobbled up by plants and animals, or else ends up in the sea and soil again, until the whole thing starts again. But there’s a big variable that has yet to be accounted for: us.
Is the natural salt cycle changing?
Humans are, in ecological terms, the ultimate keystone species. Without our intervention, the planet would be markedly cooler, the number of species alive would be much higher – and, it turns out, there would be a lot less salt hanging around.
“Increasing salt production and use is shifting the natural balances of salt ions across Earth systems, causing interrelated effects across biophysical systems collectively known as freshwater salinization syndrome,” begins a 2023 paper from Kaushal and colleagues.
“Anthropogenic activities have accelerated the processes, timescales and magnitudes of salt fluxes and altered their directionality, creating an anthropogenic salt cycle,” it explains. “Global salt production has increased rapidly over the past century for different salts, with approximately 300 Mt of NaCl produced per year.”
Thanks to humans, salt no longer enters the salt cycle just from natural processes like geological uplift and erosion from weather phenomena. Now, the natural world has to cope with salt being extracted en masse for food and road deicing (and changing the sex of frogs as a side effect); with acid rain that has sped up the rate salts are released from rock; with salt-filled wastewater from agricultural and industrial processes, and so on.
In fact, the impact from humans has been so dramatic that researchers now refer to the “anthropogenic salt cycle” – that is, the salt cycle as produced by humans.
Why is that a problem?
Too much salt is bad for your health, and the same is true for the Earth. “If you think of the planet as a living organism,” Kaushal explained, “when you accumulate so much salt it could affect the functioning of vital organs or ecosystems.”
And just like when we consume too much of the delicious crystals, the excess builds up in the system. For us, that means kidney complaints and raised blood pressure; for the planet, it means a massive buildup of salts in waterways, lakes, and even the air and soil. In fact, over the last half-century, research has shown that approximately 2.5 billion acres of soil around the world – an area roughly the size of the continental US – has been salinized to a problematic degree.
And remember, we’re not only talking about “salt” as in the stuff in your condiments cupboard. The term technically refers just to a compound made of positively charged ions and negatively charged ions, so while a lot of salt is indeed sodium chloride, there are also a lot of salts based on other elements.
“When people think of salt, they tend to think of sodium chloride,” Kaushal said, “but our work over the years has shown that we’ve disturbed other types of salts, including ones related to limestone, gypsum and calcium sulfate.”
It’s a big problem, and not just for the salt itself. “When humans add salt to waterways, that salt also releases a lot of dangerous collateral chemicals,” Kaushal explained in 2018. This was a cause of the Flint, Michigan water crisis, and it’s also extremely harmful to natural ecosystems – especially since these “chemical cocktails” can be more damaging in total than as a collection of individual constituents.
“It’s clear that regulatory agencies need to find new ways to address these ‘chemical cocktails’ released by saltier water,” Kaushal said, “rather than looking at individual freshwater pollutants one by one.”
What should we do about it?
So, how do we remove all this extra salt from the system, and reset the salt cycle back to a sustainable status?
Well, that’s a tricky question. “This is a very complex issue because salt is not considered a primary drinking water contaminant in the US,” Kaushal said, “so to regulate it would be a big undertaking.”
Removing the salt that’s already there is also difficult. “Removing salt from water is energy intensive and expensive,” he explained, “and the brine byproduct you end up with is saltier than ocean water and can’t be easily disposed of.”
So perhaps a preventative method is better. In the US, the worst offender against the natural salt cycle is road salt – seriously: between 2013 and 2017, road salts represented 44 percent of US salt consumption, as well as 14 percent of the total dissolved solids that entered streams across the country. And of course, roads need to be de-iced – but in places like Washington DC, city authorities have started using beet juice instead, achieving the same effect with a much lower salt content.
Whatever the solution, researchers stress that time is of the essence. With road salt, for example, “there’s the short-term risk of injury, which is serious and something we certainly need to think about,” Kaushal agreed, “but there’s also the long-term risk of health issues associated with too much salt in our water.”
“It’s about finding the right balance,” he said.
“But do I think it’s a substance that is increasing in the environment to harmful levels? Yes.”
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Source Link: What Is The Salt Cycle, And What's Going Wrong With It?