The history of metal pollution in the city of São Paulo, the largest metropolis in Brazil and the Southern Hemisphere, can be read in the layers of sediment accumulated over the last century. Using paleolimnology – a method that allows the reconstruction of past environmental changes based on sedimentary evidence – researchers reconstructed a century of metal pollution in the capital of the state of the same name from samples collected in the Garças Lake, in the Ipiranga Fountains State Park (PEFI). The study showed a strong correlation between industrialization, population growth and the increase of this type of pollutant. The results were published in the journal Environmental Science and Pollution Research.
The scientists analyzed the concentrations of eight metals – cobalt, chromium, copper, iron, manganese, nickel, lead and zinc – at the bottom of the reservoir, whose sediments have been recorded for about a hundred years.
"Everything that happens in a drainage basin ends up somehow being recorded in the sediments of aquatic environments. We chose Garças Lake because it had never been dredged, which allowed us to preserve the historical sequence of pollutant deposition," says Tatiane Araujo de Jesus , coordinator of the Ecological Engineering Systems Laboratory at the Federal University of ABC (UFABC) and first author of the article.
The researchers used divers to collect sediment cores. These cores are vertical cylinders of deposited material from the bottom of the lake. Since carbon-14 is not suitable for dating relatively recent samples, the layers were dated using lead-210. The physical principle is the same as for carbon-14: the radioactive decay of the isotope. "Lead-210 has a half-life of approximately 22.3 years, so through the activity of this isotope, we were able to assign an age to each layer of sediment, as if we were numbering the pages of a book," explains Jesus.
The results revealed three major periods in the evolution of metal pollution in the city of São Paulo. Layers corresponding to the pre-industrial period, which lasted until 1950, showed low concentrations of metals, reflecting a time when the site was less affected by human activities. It is worth remembering that the reservoir, created by damming the Campanário stream in 1893, was used for water supply until 1928.
In the period 1950-1975, the metal content began to increase progressively. Factors such as the intensification of air traffic at Congonhas Airport (inaugurated in 1936), the growth of automotive and metallurgical plants in the ABC Paulista (an area that covers seven municipalities in the São Paulo Metropolitan Region), and unregulated urbanization contributed to the accumulation of pollutants.
Pollution peaked in the following period, 1975-2000. From the 1970s onwards, there was a significant increase in the concentration of metals such as lead, nickel, iron, chromium and copper. When evaluating the data collected, the local factor must be taken into account, as the period coincides with the installation of the Imigrantes Highway (1974), which intensified vehicle traffic in the region. "We observed that most of the metals detected came from vehicle and industrial emissions," says the researcher.
One of the most striking findings of the study was the decrease in sediment lead levels after 1986, when Brazil banned the use of leaded gasoline through the Vehicle Emissions Control Program (PROCONVE). "Until then, lead was used as an additive in gasoline, and with its ban, we saw a decrease in its concentration in sediments. This shows how environmental policies can have positive and measurable impacts," says Jesus.
Despite the reduction in lead, concentrations of other metals continued to increase throughout the 1990s, particularly cobalt, nickel and copper, likely related to changes in industrial processes. The data indicate that a steel mill near the reservoir, which previously produced steel, began manufacturing metal artifacts during this period, which may have changed the profile of metal pollution.
The study not only revealed the evolution of metal pollution in São Paulo but also highlighted the importance of sediments as environmental indicators. "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," says the researcher.
And he emphasizes that while some reductions in pollution levels have been observed, many metals remain in the sediments and represent an environmental liability. "What we can do now is use these data to set restoration goals. We know what the natural levels of these metals were before industrialization, and we can work to try to reverse some of the impact," he says.
The study also offers reflections on conservation areas, such as the Ipiranga Fountains State Park. "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," warns Jesus.
The results underscore the need for stricter public policies to reduce industrial and vehicular pollution, as well as for environmental restoration efforts in contaminated areas. "Historical data help us understand how we got here and can serve as a basis for more informed decisions about the future of the city's environmental quality," the researcher concludes.
The study was supported by FAPESP through doctoral fellowships awarded to Tatiane Araujo de Jesus ( 04/08071-5 ) and Sandra Costa-Böddeker ( 04/08675-8 ), the second author of the paper.
About FAPESP
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the state of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration.
