Since industrialization, mercury input into the oceans has increased significantly.

Rivers worldwide are now transporting about two and a half times as much highly toxic mercury into the oceans as they did in 1850. This is the finding of an international research team based on measurements and computer models. The main sources of the heavy metal are industrial processes, metal mining, small-scale gold mining, and coal combustion, as the group led by Yanxu Zhang of Tulane University in New Orleans reports in the journal "Science Advances."

"Mercury compounds are potent neurotoxins that endanger human health, especially through the consumption of fish," the study authors write. Because mercury is found in the soil, the toxic metal is naturally introduced into rivers through soil erosion. However, the global level of mercury input into rivers was previously unknown. "Previous studies have focused on mercury concentrations in the atmosphere, soil, and seawater, but have largely overlooked rivers," Zhang is quoted as saying in a statement from his university.

The scientists based their model of pre-industrial mercury export from rivers to the oceans on mercury erosion in the catchment area and on the transport power of the rivers. This, in turn, depends on the current and flow velocity of the water. For calibration, Zhang and colleagues compared their simulation results for 1850 with the mercury values ​​measured in the sediment deposits of some rivers. The simulated values ​​tended to be somewhat lower than the measured concentrations, making the model a rather conservative estimate.

According to estimates, in 1850, at the beginning of industrialization, approximately 390 tons of mercury entered the oceans via rivers worldwide each year. Currently, the figure is around 1,000 tons per year.

"Establishing a baseline value for mercury in rivers in the pre-industrial era can serve as an important reference point," Zhang emphasized. This could guide policymakers in reducing mercury releases.

According to the study, the amount of mercury increased particularly sharply in rivers in South Asia and Southeast Asia, as well as in North and South America. "The Amazon's mercury balance now exceeds 200 tons per year, three-quarters of which is attributable to human activities, primarily small-scale mining," Zhang explained.

In addition, the researcher says that heavy deforestation in the Amazon region contributes to increased soil erosion, which also leads to higher mercury levels in the Amazon.

Siberian rivers like the Ob and Yenisei, which flow into the Arctic Ocean, also transport relatively large amounts of mercury into the sea. And just recently, scientists led by Isabel Smith of the University of Southern California in Los Angeles reported that mercury concentrations in the North American Yukon and Koyukuk rivers have increased significantly due to thawing permafrost and the resulting increased soil erosion.

But Zhang and colleagues also report positive results: For example, mercury input into the Mediterranean has decreased compared to 1850. They explain this by saying that dams in the North African Nile catchment area—such as the Aswan Dam in Egypt—retain sediments transported by rivers.

Mercury is one of the most dangerous toxins known. In addition to damaging the immune and reproductive systems, the substance primarily damages the central nervous system, with unborn children being particularly sensitive. For this reason, US authorities advise pregnant women to avoid certain types of fish from certain areas.