In 2017, Paulo Tarso Oliveira, a professor of hydrology at the Universidade de São Paulo, came across a news report about a small village along the banks of the São Francisco River, one of the main rivers in northeastern Brazil. The article said villagers were experiencing unusually high rates of high blood pressure, and linked the anomaly with the region’s dry climate and low river flow. As the water table dropped, ocean water began infiltrating the region’s groundwater, raising salt levels in the water supply and making people sick.
“Oftentimes, people don’t realize, but surface and groundwater are connected and must be seen as an entirety.”
Intrigued, Oliveira investigated further. Streamflow was slowing, he later found, because wells were pumping water from the aquifer below. “Oftentimes, people don’t realize, but surface and groundwater are connected and must be seen as an entirety,” Oliveira said.
In places where a water table lies beneath a riverbed, the river can leak water into the aquifer below. This process, known as streamflow leakage, occurs naturally depending on underlying rock formations and groundwater levels, but the construction of wells that overpump water from aquifers may intensify the issue.
The situation in the São Francisco basin is not unique, Oliveira and his colleagues found. In evaluating wells across Brazil, the researchers discovered that water levels in more than half the wells were below the level of nearby streams.
Mapping Wells
In 2023, Oliveira and master’s student José Gescilam Uchôa began mapping Brazilian rivers to identify areas at risk of water loss. They relied on public data on river levels and the locations of wells from the Geological Survey of Brazil. The data, however, were insufficient for most of the registered wells. As a result, they focused on 18,000 wells with comprehensive data spread across thousands of rivers in Brazil.
The researchers compared the water level in each well with the elevation of the nearest stream. In 55% of the wells, water topped out below the elevation of neighboring streams.
José Uchôa takes measurements in a river in São Paulo. Credit: Laboratório de Hidráulica Computacional da Universidade de São Paulo
“Our data suggest that the groundwater use is significantly impacting the rivers’ streamflow,” Uchôa said. “This is and will continue to be a growing worry for water management in the country.”
The study, published in Nature Communications, also identified critical regions, including the São Francisco basin, where more than 60% of rivers may be losing water because of extensive groundwater pumping. Pumping is mainly associated with irrigation activities.
In the Verde Grande basin in eastern Brazil, where irrigation is responsible for 90% of water consumption, 74% of rivers may be losing water to aquifers.
Oliveira thinks the results are conservative and that the situation could actually be worse because the researchers did not account for illegal wells. A 2021 study by geologist Ricardo Hirata at the Universidade de São Paulo estimated that around 88% of Brazil’s 2.5 million wells are illegal, lacking a license or registration for pumping.
Hirata, who was not involved with the new work, warned that the new study was limited to only 5% of existing wells, primarily located in regions where groundwater is more intensely exploited.
“Perhaps this is also happening in other parts of the country with high irrigation demands, and we just don’t know it because we lack data.”
He also stressed that though the researchers identified rivers that are potentially losing water to aquifers, these data alone are insufficient to determine whether the rivers are drying up. To assess that, other factors would need to be considered, such as the amount of water extracted from an aquifer compared to the river’s streamflow, how connected the aquifer is to the river, and how much water is drawn from the aquifer in relation to seasonal variations in streamflow.
“The fact that the water level of a well is lower than that of a nearby river doesn’t necessarily affect the river or the aquifer,” Hirata said.
The areas identified as critical by the study are mostly arid regions where stream leakage was expected to occur naturally, pointed out hydrologist André F. Rodrigues at the Universidade Federal de Minas Gerais in Brazil. Rodrigues was not involved with the research.
The study is important because it highlights a growing issue, he said, but more local analyses are necessary to get a more detailed picture of the problem and consider, for example, the effects of climate and seasonal changes. “Perhaps this is also happening in other parts of the country with high irrigation demands, and we just don’t know it because we lack data,” Rodrigues said.
A Growing Issue
Uncontrolled expansion of wells and excessive pumping not only affect people’s health, water supplies, and agriculture but also can make soil unstable, leading to ground sinking (subsidence). Similar phenomena have been observed in regions of China, the United States, and Iran.
The outlook is not good for Brazil. Wells will likely multiply because irrigated land areas are expected to increase by more than 50% in the coming 20 years, according to the Brazilian water agency.
“We will likely see a vicious cycle of degradation, where a decrease in surface water quality and quantity, coupled with an increase in drought periods, will force farmers to drill more wells for food production, further intensifying groundwater extraction and exacerbating the problem,” Oliveira said.
Overexploitation of groundwater is a global concern. Most aquifers have declined in the 21st century, and modeling studies suggest that stream leakage will become more common in the coming decades. Still, the issue is largely overlooked in tropical places such as Brazil, which holds 12% of the world’s renewable water resources.
This oversight is partly due to a lack of funding and surveillance and partly due to a long-standing belief that rivers in tropical and humid countries mostly gain water from aquifers rather than losing it, Oliveira said. “We must act to avoid having entire regions devastated in the future.”
Researchers are calling for more studies and systematic monitoring of wells to identify critically dry areas and assess the impact of drilling new wells on rivers. Brazil has only 500 observational wells that are constantly monitored by the government, compared with 18,000 in the United States, despite the countries having similar land area. “Surveillance is extremely important and highly undervalued,” Uchôa emphasized.
—Sofia Moutinho (@sofiamoutinho.bsky.social), Science Writer
Citation: Moutinho, S. (2025), Brazil’s rivers are leaking,
Eos, 106, https://doi.org/10.1029/2025EO250116. Published on 28 March 2025.
Text © 2025. The authors.
CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
