In many parts of Brazil, rivers may be losing water to the ground instead of gaining from it. A recent study found that groundwater levels in several regions are lower than those of nearby rivers, pulling river water underground. This shift, potentially driven in part by intensive pumping of groundwater, could reduce streamflow and have serious consequences for Brazil’s water supply, agriculture, energy production, and ecosystems.

While the study authors emphasize that they can’t say for sure that using wells directly causes river levels to drop, they found a strong correlation between high groundwater use and greater potential river losses, especially in drylands where water extraction is most intense.

“California, India and Iran have all seen how heavy groundwater use reduces river flow, and the signs suggest Brazil is no different,” study co-author Paulo Tarso Sanches de Oliveiras, a hydrology professor at the Federal University of Mato Grosso do Sul (UFMS) and the University of São Paulo (USP), told Mongabay. “Where there’s smoke, there’s fire — and everything points to excessive groundwater use in some local regions of Brazil playing a role in declining river levels.”

The study analyzed data from 17,972 wells and found that 55.4% of them have water levels lower than nearby rivers. In the São Francisco River Basin in eastern Brazil, 61% of rivers are potentially vulnerable to losing water to aquifers (underground layers of sediment, rock and sand). The basin of the Verde Grande River, itself a tributary of the São Francisco River, could have up to 74% of its rivers affected by streamflow loss. Unlike runoff, which is rainwater that flows over the ground into rivers, streamflow relates to the water already flowing within the river itself. A 2020 study in the São Francisco River Basin observed reduced river flow despite no significant decrease in rainfall, suggesting that less precipitation isn’t the cause of the decline.

It indicates an important potential trend: regions where aquifers are heavily tapped with wells could risk losing surface water to the underground reserves.

Both the São Francisco and Verde Grande basins lie near Brazil’s agricultural heartland of MATOPIBA (the border region straddling the states of Maranhão, Tocantins, Piauí and Bahia), and are essential to the country’s commodity-driven economy. These rivers are critical for irrigation, food production, energy generation and cultural activities. As one of the world’s largest food exporters, Brazil’s agricultural success relies heavily on the stability of these river systems. The study also underscores the risk to Brazil’s overall water security. While the country holds 15% of the world’s renewable freshwater, poor management threatens to deplete this essential resource.

“Excessive groundwater removal may reduce a river’s base flow, depending on climatic and geological conditions, impacting downstream users who rely on that water over the long term,” study lead author José Gescilam Uchôa, also from USP, told Mongabay. “This may affect human consumption, livestock and food production — ultimately impacting society as a whole.”

Water management expert Ricardo Hirata from USP, who wasn’t involved in the study, told Mongabay that while the methodology is sound, the research covers only about 5% of the estimated number of wells in the country, and more data are needed to draw definitive conclusions.

“The study highlights the potential impact of aquifer exploitation on rivers, but further investigation and local studies are necessary to confirm these findings,” said Hirata, who heads the Integrated Water Solutions for Resilient Cities project, known by its local acronym SACRE. “The river-aquifer relationship is still underexplored globally, especially in Brazil. Effective water resource management hinges on a better understanding of this connection.”

Illegal well use

The study suggests that the widespread use of both legal and illegal wells in Brazil may be contributing to the potential loss of river water. Of the country’s estimated 3 million wells, about 80% are unregistered and operate without permits, according to Hirata. Together, these unregistered wells extract 17.6 billion cubic meters (4.65 trillion gallons) of water per year — enough to supply the country’s entire population, yet used by less than 20%.

Wells tap into groundwater for agricultural irrigation, drinking water and industrial use. Brazil regulates well drilling and water extraction, but bureaucratic hurdles and a lack of public awareness about the importance of groundwater contribute to widespread noncompliance. 

“Bureaucracy often prevents Brazilian laws from working in practice,” Uchôa said. “Simplifying the process, like allowing online registration [of wells] on a centralized platform, would make compliance easier.

“But if hurdles and the fear of fines persist, people will find ways around the system,” he added. “Raising awareness and ensuring a practical, legal path could improve the situation.”

Economic and environmental implications

Groundwater is often overlooked by both the public and water management professionals, despite its critical role in sustaining rivers and wetlands. Aquifers store 97% of the planet’s freshwater; in Brazil, 90% of rivers are linked to aquifers. 

“This lack of knowledge [of groundwater] not only leads to overexploitation, as the study may suggest, but also hides the opportunities groundwater offers to society, the economy and the environment — especially in a world struggling with water scarcity,” Hirata said.

Reduced river flow causes both environmental and social problems. In the São Francisco River Basin, lower water levels have already led to saltwater intrusion in coastal areas, affecting the health of local ecosystems and human populations. In the state of Alagoas, for example, increased salt levels in drinking water have been linked to health issues such as hypertension, while salt-sensitive plants have been damaged by the intrusion. 

“Upon investigation, [researchers] discovered this was caused by reduced river flow to the sea, resulting in saltwater intrusion,” Oliveira said. “Seawater was invading the coast, increasing the salt content in the water, causing a public health issue.”

The potential economic impact is equally significant. Agriculture, especially in the Cerrado savanna biome and the São Francisco basin, is a major contributor to Brazil’s economy. The depletion of river flows could undermine crop irrigation and disrupt the entire agricultural value chain, which relies heavily on consistent water availability. Additionally, Brazil’s hydroelectric plants depend on steady river flows to generate energy, and reduced water levels could have adverse effects on the country’s energy production and climate resilience in the long run.

The situation is expected to worsen as irrigation demand is projected to double over the next 20 years. Climate change will likely reduce rainfall in Brazil’s north and northeast, while other regions could face longer droughts, affecting rivers and surface water reservoirs. With agricultural expansion driving up water demand, Hirata said that aquifers will likely become the primary water source.

“Groundwater is more resilient to droughts compared to surface water, has a low exploitation cost, and is generally less ecologically impactful. As a result, aquifer exploitation is expected to rise,” he said.

However, overexploitation of these vital resources could harm ecosystems, including rivers, mangroves and swamps. To ensure groundwater is used sustainably, detailed studies are essential for responsible management, according to Hirata.

“It’s important that studies like this one, or others with more detail, be conducted to make the most of groundwater in a sustainable and intelligent way,” he said.

Without proper oversight, groundwater and surface water resources will be depleted, Alexandra Suhogusoff, director of USP’s Groundwater Research Center, who wasn’t involved in the new study, told Mongabay. Farmers often turn to groundwater when surface supplies run low, but weak enforcement and poor hydrogeological studies enable overuse, degrading water quality with salts and agrochemicals, she added.

“We do not want to demonize agriculture, because society depends on it for its survival,” Suhogusoff said. “[But] we need to adopt a more rigorous and efficient approach to the management of these resources, combined with more sustainable agricultural practices and better planned economic processes. Only in this way will we be able to guarantee the availability and quality of groundwater and surface water for future generations.”

Creating solutions

The researchers call for an integrated approach to managing both surface water and groundwater. Furthermore, improved hydrogeological monitoring is essential to track changes in groundwater levels and river flow dynamics. Current data on Brazil’s groundwater resources are limited, with only around 500 monitoring wells in the country compared to 18,000 in the United States. 

“We need investment in science. Once we have this investment, we can create preventative measures,”Uchôa said. “Our national study is more of a warning. It highlights regions facing issues. We need more local studies.”

Local studies will help identify risks and develop preventative measures, he said. For example, if research shows a region is particularly vulnerable, authorities could recommend restricting well drilling near certain rivers. Without adequate monitoring and local research, however, such targeted actions remain difficult to implement.

“Our main challenge is securing long-term investment. Water research requires continuous data. It’s not enough to take one measurement today and another a year from now,” Oliveira said.

Citations:

Uchôa, J. G. S. M., Oliveira, P. T. S., Ballarin, A. S., Neto, A. A. M., Gastmans, D., Jasechko, S., Fan. Y., & Wendland, E. C. (2024) Widespread potential for streamflow leakage across Brazil. Nature Communications, 15. doi:10.1038/s41467-024-54370-3

Conicelli, B., Hirata, R., Galvão, P., Aranda, N., Terada, R., & Gutiérrez, O. J. G. (2021). Groundwater governance: The illegality of exploitation and ways to minimize the problem. Annals of the Brazilian Academy of Sciences, 93(1). doi:10.1590/0001-3765202120200623

Banner image: A central pivot irrigation system being used in soybean plantations in Bahia. This system uses a large amount of water. Image © Marizilda Cruppe/Greenpeace.

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