More than half of Brazil's rivers are at risk of reduced flow due to water seeping into underground aquifers. This is the result of an analysis of 17,972 wells throughout the country. Of these, 55.4% had water levels below the surface of the nearest rivers. This difference in hydraulic level creates a gradient that favors the seepage of water from the river into the subsoil, which can turn rivers into water flow losers. The study, conducted by researchers from Brazil and abroad, was published in the journal Nature Communications.
"Due to climatic conditions and intense agricultural activity, the São Francisco River basin and the MATOPIBA region [which includes the states of Maranhão, Tocantins, Piauí and Bahia] are particularly critical areas, both of which are highly dependent on groundwater for irrigation and human consumption," says Paulo Tarso Sanches de Oliveira , second author of the study, professor of hydrology and water resources at the Federal University of Mato Grosso do Sul (UFMS) and of the Postgraduate Program in Hydraulic Engineering and Sanitation at the São Carlos School of Engineering of the University of São Paulo (EESC-USP).
In the case of the São Francisco basin, 61% of the rivers analyzed showed a potential loss of water flow to the aquifer, a result attributed to the intensive use of groundwater, mainly for irrigation. The situation is even worse in the basin of the Verde Grande River, a tributary of the São Francisco, which stretches across northern Minas Gerais state and southwestern Bahia. In this case, the potential loss of flow affects 74% of the rivers. "These two basins are crucial for agriculture and hydroelectric power generation in Brazil. What's happening threatens not only local sustainability but also water, food and energy security on a large scale," comments Oliveira.
The indiscriminate drilling of wells, whether for agricultural irrigation or private consumption, is a major factor. A study published in 2021 by Ricardo Hirata and collaborators showed that at that time there were about 2.5 million tube wells in Brazil and that more than 88% of them were illegal, without a license or registration for pumping. The volume of water pumped, in the order of 17.6 billion cubic meters per year, would be enough to supply the entire Brazilian population, but was enjoyed by less than 20% of the population.
In addition to the fact that pumping can seriously compromise river flows, affecting the availability of water for human consumption, aquatic ecosystems and the landscape itself, excessive use of groundwater can cause land subsidence – that is, the sinking and even collapse of the surface. "This scenario has already been observed in India and California, and Brazil could face similar problems if there isn't adequate planning and control. The warning is all the more pertinent in light of projections that show an increase of more than 50% in irrigated areas in the country over the next 20 years, which will put even more pressure on surface and underground water resources," the researcher stresses.
Planning and control are essential because, despite having 15% of the world's renewable freshwater, the country is already facing major water challenges that are likely to be exacerbated by the worsening climate crisis. "The Cerrado biome, which is home to important aquifers and strategic rivers, as well as being the main area of agricultural expansion and responsible for 70% of the country's maize production, is one of the most vulnerable regions. The balance between rivers and aquifers in the region could be compromised by recent changes in land use and land cover, driven by the advance of the agricultural frontier and, above all, the growing demand for irrigation," Oliveira points out.
To address this scenario, the researchers involved in the study emphasize the need to integrate surface and groundwater management. Tools based on remote sensing and field data can help map critical regions and guide public policy. In addition, investments in hydrogeological monitoring are crucial. "Brazil has the potential to expand irrigation in a sustainable way, but it needs to better plan the integrated use of groundwater and surface water to avoid negative impacts," says José Gescilam Uchôa (https://bv.fapesp.br/en/pesquisador/730946/jose-gescilam-sousa-mota-uchoa), lead author of the article.
He was responsible for the exhaustive collection of information on the 17,972 wells studied. For this work, carried out during his master's research, Uchôa used the database of the Geological Survey of Brazil ( SGB ). Now a Ph.D. student at EESC-USP under the supervision of Edson Cezar Wendland , who also signed the article, Uchôa is researching the impact of land use and occupation, as well as climate change, on hydrological flows between groundwater and surface water in an area where the Guarani aquifer is located. The research is supported by a scholarship from FAPESP .
Oliveira and Uchôa argue that it is still possible to minimize the problem, but that effective measures cannot be postponed because, as things stand, the depletion of the water system is already affecting the health of the population. "In 2017, there was a significant increase in cases of high blood pressure among residents of a small village on the coast [of the state] of Alagoas, who consume water from the São Francisco River. It was later found that the problem was related to the ingestion of a higher concentration of salt in the water, caused by the intrusion of seawater into the river as a result of the reduction in its flow," explains Oliveira. The issue was the subject of media reports .
The study is of great importance for Brazil, which could face increasing water stress with serious consequences for water supply, food security and ecosystems. But its reach is even greater, as it serves as a global call to rethink water management strategies in tropical countries, where growing groundwater use is threatening surface water resources.
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.
