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Gritter of the winter service on a country road in Rhineland-Palatinate

Photo: Harald Tittel / picture alliance / dpa

In the Bible, the phrase "You are the salt of the earth" appears in connection with the "light of the world". Salt as a symbol of the essence of civilization – the crucial ingredient for preserving food in earlier times. And today, to keep icy paths passable in winter.

But the human use of the mineral has a downside for the environment – and thus ultimately for the people themselves. A new study in the journal Nature Reviews Earth & Environment describes an "anthropogenic salt cycle", i.e. a man-made change in the natural circulation of salt through soil, water and air.

The research team from the US universities of Virginia Tech and the University of Maryland led by geologist Sujay Kaushal says they have for the first time systematically recorded how the accelerated industrial production of the past century has affected the global concentration and circulation of salt – and what consequences this has for ecosystems. For about 50 years, the proportion of salt ions in freshwater rivers, in the wake of mining, agriculture, construction or other industrial activities that release salt, has been growing.

»Mass pile-up in slow motion«

Environmental scientist Megan Rippy, who was involved in the project, spoke of a "mass pile-up in slow motion". It's happening so slowly that it's easy to overlook how our rivers, lakes and drinking water reservoirs are becoming increasingly salty." Many ecosystems depend on a finely balanced salinity. In the course of time, species such as fish were threatened with extinction. The cultivation of crops could also fail due to saline groundwater. In addition, medical damage to humans, such as increasing poisoning during pregnancy, has already been observed.

Even in the air, the salinity increases measurably, for example through evaporation from dry lakes – or via aerosols from road salt, which then precipitates in the environment. In the U.S., almost half of salt consumption ended up on the streets in the recent past.

The study describes "chemical cocktails", in which salt ions in the soil combine with sediment and deposited pollutants, and "cascade effects" of salinisation, for example due to forced snowmelt and thus a lack of drinking water reserves in the western United States.

Beet juice instead of road salt

By nature, there is a salt cycle that brings salt from the soil to the surface via geological and hydrological processes and later allows it to be deposited again. However, humanity has considerably accelerated this process and thus ultimately "disrupted", according to the study's verdict – not for the first time, but to an unprecedented extent.

"History is full of ancient cultures that collapsed because they couldn't balance their salt balance," commented co-author Stanley Grant, who directs the Occoquan Watershed Monitoring Laboratory at Virginia Tech in the Washington suburb of Manassas. He hopes that the warning will help to avoid repeating this mistake. The university is also researching alternatives. Instead of road salt, Washington and other U.S. cities are now using beet juice to clear their streets of ice – with the same effect, but less salty.