Rainwater is insufficient to replenish the Guarani Aquifer owing to overuse and drought, study warns

The Guarani Aquifer is the source of drinking water for some 90 million people and is being overused in several areas of São Paulo state (Brazil). The researchers deployed stable isotopes to estimate the relative contributions of rainwater and groundwater to the maintenance of springs in the region.

By José Tadeu Arantes  |  Agência FAPESP – A study conducted by researchers at São Paulo State University (UNESP) in Brazil measured the contributions of rainwater and groundwater to the maintenance of springs and rivers in the vicinity of Brotas, a city in the central portion of São Paulo state located in the Upper Jacaré-Pepira sub-basin, where the urban water supply, agriculture and tourism rely strongly on groundwater.

An article on the study is published in the journal Isotopes in Environmental and Health Studies. 

The results show that the volume of rainwater is insufficient to replenish the Guarani Aquifer and is exceeded by the amount of water withdrawn for use in human activities, endangering the sustainability of the system.

Aquifers are the planet’s largest sources of potable water. The Guarani Aquifer is the largest cross-border groundwater reservoir in the world, with a total area of approximately 1.19 million square kilometers (km²) spanning Brazil, Paraguay, Uruguay and Argentina – two-thirds in Brazil, where it traverses the states of Goiás, Mato Grosso do Sul, Minas Gerais, São Paulo, Paraná, Santa Catarina and Rio Grande do Sul.

In the central part of the Paraná sedimentary basin, the Guarani Aquifer can be as thick as 450 meters and as deep as 1,000 meters underground. Although the amount of water in the subterranean reservoir is huge – totaling 30,000 cubic kilometers (km³) in stored volume and 2,000 km³ in available volume – like any natural resource it is finite and prone to contamination by pollutants. Hence the need for scientific research aimed at a profound understanding of its hydrologic mechanisms, and for action to monitor water consumption and recharge as a basis for management strategies that guarantee parsimonious use and conservation. In some areas, the level of the underground reservoir is known to have dropped more than 100 meters. 

São Paulo state consumes about 80% of the water withdrawn from the Guarani Aquifer in Brazil. Data for 2010 indicates an even larger share, surpassing 95%. Urban wells and crop irrigation are the main uses. The downtrend in the reservoir’s water volume requires urgent attention, particularly in the context of the current climate emergency and recurring periods of severe drought.

“We monitored springs, rivers, wells and rainfall for eight years between 2013 and 2021, using stable isotopes of hydrogen [¹H-²H] and oxygen [¹⁶O-¹⁸O] as tracers of water origin. We concluded that about 80% of the water in springs and wells comes from the Guarani Aquifer,” said Didier Gastmans, last author of the article. He is a researcher at São Paulo State University’s Center for Environmental Studies (CEA-UNESP) in Rio Claro and heads its Water Resources and Environmental Isotopes Laboratory (LARHIA).

Even at times of heavy rain, most spring water comes from the aquifer, according to Gastmans: recently infiltrated rainwater accounts for only 20% of the annual discharge from springs. 

The researchers’ detailed monitoring of the sub-basin covered variations in groundwater depth, rainfall volume, and isotope ratios for spring water, rainwater and water from a deep well they bored for this purpose.

“Stable isotopes of hydrogen and oxygen, which form the water molecule, function as ‘fingerprints’ that point to the origin of water. Our rainwater samples displayed significant variation in isotope values, reflecting the influence of different seasonal atmospheric processes. The isotopes in the groundwater samples were far more constant during the course of a year, as were those in the spring water samples, showing that they came mainly from deep water sources,” Gastmans said.

The homogeneity of the groundwater samples suggested that the aquifer is not directly affected by seasonal variations but is replenished mainly by subterranean sources, with runoff contributing a much smaller proportion. On the other hand, the water level in the well dropped during the monitoring period, as the recharge rate fell owing to a decrease in rainfall volume and a rise in evapotranspiration. In other words, the amount of water entering the aquifer via recharge is not sufficient to offset the amount leaving it, and this is a major cause for concern.

“There’s always been a false idea that all outcrops of the Guarani Aquifer are also recharge areas for confined parts of the aquifer, which are deeper, but our study shows that recharge in outcrop areas is essential to maintain the local hydrologic system, especially river flows and spring discharge. The groundwater now being overused owing to human consumption is actually very old. Because carbon-14 dating can be imprecise, we partnered with the International Atomic Energy Agency to use krypton-81, a noble gas isotope, and helium-4, a stable isotope of helium. This combination produces very precise dating results, and we detected ages ranging from 2,600 years in Pederneiras to 127,000 in Bebedouro, 230,000 in Ribeirão Preto and 720,000 in [the state of] Paraná,” Gastmans said. Pederneiras, Bebedouro and Ribeirão Preto are municipalities in the state of São Paulo.

Sustainability

The Guarani Aquifer supplies water to some 90 million people and accounts for up to 90% of spring discharge in the dry season. Overuse and long periods of drought in the context of the climate emergency could diminish its capacity to sustain river flows and spring discharge, leading to acute water shortages such as those that occurred in São Paulo state in 2014-15 and 2017-21.

“Understanding how the aquifer is recharged and estimating the relative contributions of rainwater and groundwater are the first steps to assure its sustainable use. Large-scale monitoring and good management are the next steps,” Gastmans said.

FAPESP supported the study via two projects (15/15749-2 and 18/06666-4).

The article “How much rainwater contributes to spring discharge in the Guarani Aquifer System: insights from stable isotopes and a mass balance model” is at: www.tandfonline.com/doi/full/10.1080/10256016.2024.2397469.