The waters of the Verde Grande River, which cuts through the semiarid regions of Minas Gerais and Bahia, are disappearing because of a phenomenon that could be linked to the depletion of an aquifer in the region.
The SGB (Geological Survey of Brazil) is investigating the reduction in the volume of the watercourse, which about 10 years ago is no longer perennial, that is, it no longer flows in its entirety in periods of drought.
Verde Grande starts in Bocaiúva, north of Minas, and goes as far as Malhada, south of Bahia, where it flows into the São Francisco River, a path of 557 kilometers that crosses 27 municipalities in Minas Gerais and 8 in Bahia.
The latest survey by the SGB, carried out in 2018, indicates that the river loses 560,000 liters of water per hour. The data only takes into account the missing volume, and not the one used, for example, for agriculture. The 560 thousand liters lost per hour are equivalent, for comparison purposes, to 7 Olympic swimming pools per day.
The result worries producers in the region. “Without water, there is no agriculture. And without agriculture, there is no food”, says cattle raiser Flávio Oliveira, from the municipality of São João da Ponte, who is also president of the river basin committee.
The region depends on water from the Verde Grande and from wells, connected to the region’s aquifer, for agriculture and livestock. Of everything that comes out of the river to move economic activities, 90% go to activities in the countryside, according to the SGB. The remaining 10% is for human supply.
In addition to the withdrawal of water from the river for economic activities and the change in the rainfall regime, which impacts the river’s volume, the SGB also became suspicious of one more factor for the reduction in the flow of the Verde Grande. The suspicion is that the aquifer is “stealing” the river water.
The region where the Verde Grande flows is formed by carbonate rocks, which are susceptible to the action of water. As a result, pipelines are formed between the river bed and the aquifer — a kind of large underground water reservoir.
With these pipelines, an exchange relationship between river and aquifer is established, with one supplying water to the other. When it goes from the river to the aquifer, the exchange takes place via pipelines called sinks. In reverse, they are known as surges.
The yellow signal for what could be happening to the river was given even before the water crisis in 2013. “For about ten years, the Verde Grande did not dry out, it was perennial”, points out the hydrogeologist from the SGB, Maria Antonieta Mourão.
In the investigation into the disappearance of the river’s waters, carried out by the SGB in partnership with the ANA (National Water Agency), the fear is that the aquifer is no longer contributing to the river, but only keeps the water from the Verde Grande.
One explanation for this would be the intense use, also for agriculture, of wells that capture water directly from the aquifer, which, with less volume, is no longer able to contribute to the river.
To try to confirm this, the investigation by the SGB and ANA launched a tracer liquid in Verde Grande, which does not cause environmental impact. At the same time, it placed a substance in wells in the region that reacts when in contact with the liquid released into the river.
“The intention is to know how the river’s relationship with the drought is. At what point does the river lose water to the aquifer and at what time the aquifer is supplying water to the river. There are several underground water abstractions and these abstractions may be affecting the river”, says the hydrogeologist from the SGB, who is linked to the CPRM (Research Company for Mineral Resources), of the Ministry of Mines and Energy.
The launch of the tracer took place on Friday (5). The first collection of material placed in wells in the region will take place next Friday (12). Two more pickups will take place. One 35 days and another 75 days after launch.
The release of the tracer took place in a stretch of the river between the municipalities of Montes Claros, São João da Ponte, Verdelândia and Jaíba, all in Minas Gerais
The president of the Verde Grande basin affirms that the result of the investigation carried out by the SGB will be decisive in making future decisions about policies for the use of water from the watercourse. “We are also working on the preservation of springs in the basin. It is mandatory to keep 20% of the forests in these areas. Many producers leave more than that”, he says.
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