Researchers from Unesp point out that the aquifer recharge is not sufficient to replace the extracted water, threatening the region's water supply
A study conducted by researchers from São Paulo State University measured the contribution of rainwater and groundwater to the maintenance of springs and rivers in the Brotas region, in the central part of the state of São Paulo. In the region, urban water supply, agriculture and intensive tourism depend heavily on water resources. The results indicate that rainfall is not able to replenish all the water in the Guarani Aquifer used in various human activities. This puts the sustainability of the system at risk.
Aquifers are the largest sources of drinking water in the world. And the Guarani is the largest transboundary aquifer -- extending underground through several countries. Its total area, approximately 1 million square kilometers (km2), covers stretches in Brazil, Paraguay, Uruguay and Argentina. Two thirds are within national territory, reaching the states of Goiás, Mato Grosso do Sul, Minas Gerais, São Paulo, Paraná, Santa Catarina and Rio Grande do Sul.
In the central parts of the Paraná Sedimentary Basin, the Guarani Aquifer can reach thicknesses of up to 450 meters and be located at depths of over a thousand meters. Although the quantity of fresh water is large, it is finite. Hence the need for studies that enable a thorough understanding of its hydrological mechanisms, as well as actions to monitor consumption and recharge that allow the adoption of management mechanisms that ensure parsimonious use and preservation. It is known that, in some regions, the drop in groundwater levels can exceed 100 meters.
The state of São Paulo consumes approximately 80% of the water extracted from the Guarani Aquifer in the country. Data from 2010 indicated an even higher consumption, over 95%. Wells for urban supply, firstly, and for agricultural irrigation, secondly, are the main factors in reducing the liquid content.
“We monitored springs, rivers, wells and rainfall over eight years, from 2013 to 2021, using stable isotopes of hydrogen [1H-2H] and oxygen [16O-18º] as markers of the origin of the water. We found that around 80% of the water volume from these springs comes from the discharge of groundwater from the Guarani Aquifer System [SAG],” says Didier Gastmans, a researcher at the Center for Environmental Studies (CEA) at Unesp.
He says that even during periods of heavy rainfall, most of the water that feeds the springs comes from the aquifer, with only 20% of the annual discharge coming from newly infiltrated rainwater. To reach these conclusions, the researchers carried out detailed monitoring of the sub-basin, tracking variations in the depth of the groundwater level, the amount of rainfall and the isotopic ratios of the spring water, rainwater and water from a deep monitoring well installed in the region.
“The stable isotopes of hydrogen and oxygen, which make up the water molecule, act as ‘fingerprints' that allow us to identify the origin of the water. Rainwater samples show a large variation in isotope values, reflecting the influence of different seasonal atmospheric processes. Groundwater samples, on the other hand, show a much more constant isotopic composition throughout the year. Something quite similar was observed in spring water, indicating that they are predominantly fed by deep water,” explains Gastmans.
The homogeneity of the groundwater indicates that the aquifer is not directly affected by seasonal effects, being essentially composed of a majority source of groundwater with much smaller contributions from rainwater. On the other hand, the drawdown of the well during the monitoring period suggests that there has been a reduction in recharge rates, due to the decrease in total precipitation volumes and the increase in evapotranspiration. In other words, the amount of water entering the aquifer through recharge is not sufficient to replace the amount of water leaving the reservoir.
“There has always been a false idea that all outcrop areas of the Guarani Aquifer were also recharge areas for the confined and deeper regions of the aquifer. However, our study showed that the recharge that occurs in the outcrop areas contributes fundamentally to the maintenance of the local hydrological system, that is, to the maintenance of river flows and discharges from springs. The groundwater that is currently being overused for various forms of human consumption is, in fact, quite old. Since carbon-14 dating presents several uncertainties, we carried out a project in partnership with the International Atomic Energy Agency in which another tracer was used, a noble gas, krypton-81, which, associated with another isotope, helium-4, provides very precise age values. And we detected ages ranging from 2.600 years in Pederneiras, to 127 thousand years in Bebedouro, 230 thousand years in Ribeirão Preto and 720 thousand years in Paraná”, explains Gastmans.
The Guarani Aquifer supplies water to around 90 million people. During the dry season, its contribution can supply up to 90% of the discharge from springs. Overexploitation, combined with prolonged droughts in the context of the climate emergency, could compromise its ability to sustain the flow of rivers and springs, exacerbating water crises such as those that occurred between 2014 and 2015 and again between 2017 and 2021 in the State of São Paulo.
More information can be found at doi.org/10.1080/10256016.2024.2397469
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