In Brazil, researchers from the Federal University of ABC investigated how metal contamination has evolved over the past century by analysing sediment layers from Garças Lake in Sao Paolo.
They dated sediment cores from the undisturbed lake bed using the radioactive isotope lead-210, which allowed them to determine when different pollutants were deposited over the last hundred years. They analysed eight metals: cobalt, chromium, copper, iron, manganese, nickel, lead, and zinc.
Lead author Tatiane Araujo de Jesus said: ‘Everything that happens in a drainage basin ends up somehow being recorded in the sediments of aquatic environments.
‘Sediments are like an archive: they store evidence of changes in the environment over time. This type of analysis can be useful for guiding strategies to protect and restore the environment.’
The study identified three key pollution phases. In the pre-industrial era (until 1950), metal levels were low, indicating minimal human impact. This period includes the lake’s early use as a water reservoir after its creation in 1893.
From 1950 to 1975, metal concentrations rose due to growing air traffic, urban expansion, and the boom in metallurgical and automotive industries in the region.
The highest levels appeared during 1975 to 2000, coinciding with major infrastructure like the Imigrantes Highway, which amplified vehicle emissions.
The 1986 ban on leaded petrol proved a turning point, after which lead levels in the sediment declined noticeably. However, levels of other metals, such as cobalt, nickel, and copper, continued to rise into the 1990s, a rise which the scientists presume to be down to changes in industrial activity, such as a local steel mill switching production from steel to metal goods.
The fact that the lake is in a conservation area – the Ipiranga Fountains State Park – prompted Jesus to warn: ‘It’s not enough just to surround a place and call it a conservation area. If air pollution and the deposition of pollutants aren’t controlled in the surrounding area, the impact will continue.’
Jesus hope his team’s finding will allow the lake to be restored to better health: ‘What we can do now is use these data to set restoration goals. We know what the natural levels of these metals were before industrialisation, and we can work to try to reverse some of the impact.’
Image: Tatiane Araujo de Jesus
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