Amazônia has an "underground ocean"
Elton Alisson/Agência FAPESP/DICYT Amazônia has an underground water reservoir whose volume is projected to be more than 160 trillion cubic meters, according to estimates by Francisco de Assis Matos de Abreu, professor at the Federal University of Pará (UFPA), presented during the 66th Annual Meeting of the Brazilian Society for the Advancement of Science (SBPC) held through July 27, 2014, on the campus of the Federal University of Acre (UFAC) in Rio Branco.
The volume is 3.5 times larger than that found in the Guarani Aquifer, the underground fresh water reservoir beneath the surface of Uruguay, Argentina, Paraguay and mainly Brazil that stretches 1.2 million square kilometers (km2).
“The underground reservoir represents more than 80% of all the water in Amazônia. Water from the Amazon rivers, for example, represents only 8% of the biome’s hydrological system, and the surface waters have more or less the same percentage,” Abreu said during the event.
Knowledge about this “underground ocean” is still very limited, however, and further studies are required to assess its potential use for human consumption as well as preserve it, given its importance in maintaining balance in the regional hydrographical cycle.
According to Abreu, research into the Amazon Aquifer began only 10 years ago when he and other researchers from UFPA and the Federal University of Ceará (UFC) conducted a study about the Alter do Chão Aquifer in the Santarém district of the state of Pará (PA).
The study indicated that the aquifer, situated against the backdrop of one of the most beautiful river beaches in Brazil, had an underground freshwater reservoir with an estimated volume of 86.4 trillion cubic meters.
“We were stunned by the results of the study and decided to investigate further. To our surprise, we discovered that the Alter do Chão Aquifer is part of a hydrogeological system that includes the sedimentary basins of the Acre, Solimões, Amazon and Marajó rivers. Altogether, the surface of these four basins is approximately 1.3 million square kilometers,” Abreu said.
Dubbed the Greater Amazon Aquifer System (GAAS) by the researcher and his colleagues, the hydrogeological system began to be formed during the Cretaceous Period, nearly 135 million years ago.
As a result of the geological process that occurred during this period, an extensive sedimentary cover, thousands of meters thick, was deposited among the four sedimentary basins, Abreu explained.
“The GAAS is a transboundary hydrogeological system because it covers other South American countries. However, 67% of the system lies in Brazil,” he said.
One of the limitations to utilizing the water available in the reservoir, however, is the lack of knowledge about its quality, the researcher pointed out. “We would like to get more information about the quality of the water in the reservoir to determine whether it is suitable for consumption.”
“We estimate that in the medium term, the volume of water in the GAAS used by industry or for agricultural irrigation will be very small because of the size of the reservoir and the depth of the wells built in the region today, which do not exceed 500 meters and have a high flow capacity of 100 to 500 cubic meters per hour,” he said.
Because this underground reservoir represents 80% of the water of the Amazônia hydrological cycle, it needs to be viewed as a strategic reserve for Brazil, says Abreu.
“In interactions between the forest and the hydric resources associated with the Earth’s rotation, the Amazon region transfers nearly 8 trillion cubic meters of water annually to other regions of Brazil. This water, which is not used by the population that lives in the region, represents a colossal environmental service provided by the biome to Brazil because it sustains Brazilian agribusiness and the rainfall patterns responsible for filling the reservoirs that produce hydroelectricity in Brazil’s southern and southeastern regions,” he said.
According to Ingo Daniel Wahnfried, professor at the Federal University of Amazonas (UFAM), one of the main obstacles for studying the Amazonia Aquifer is the system’s complexity.
Because the reservoir is composed of large rivers with different depths of sedimentary layers, it is difficult to determine flow data regarding the underground water for the entire Amazon hydrogeological system.
“There are some studies underway, but many more are needed. For example, we need to determine how vulnerable the Amazon Aquifer is to pollution,” said Wahnfried, who obtained a fast-track doctorate on a FAPESP scholarship.
Unlike the Guarani Aquifer, which is only accessible around its edge because a two-kilometer-long layer of basalt covers the water reservoir, the areas around the Amazon Aquifer are completely open.
In forest areas, there is no risk posed by the aquifer exposure. However, in urban areas, such as the capitals of various Amazon states, it could represent a serious problem. “We still do not know the level of vulnerability of the Amazon Aquifer system in cities such as Manaus,” Wahnfried said.
According to the researcher, similar to the surface water (of the rivers), the underground water is widespread and available in Amazônia. In the state of Amazonas, 71% of the 62 municipalities use underground water (but not from the aquifer) as a main source of public supply, even though the state is covered by the Negro, Solimões and Amazon rivers.
Among the 22 municipalities of the state of Acre, however, four are fully supplied by underground water. “Although these municipalities are in the middle of the Amazon region, their public supply systems do not use the region’s rivers,” Wahnfried said.
Some reasons for the considerable use of underground water in the Amazon region include its easy access and good quality, which represent a lower risk of contamination than surface water.
Furthermore, the water level of the rivers in the Amazon region varies significantly over the course of the year. For example, Wahnfried said, some cities in the region are only a few meters away from a river during rainy periods. In dry periods, however, the level of the river can decrease 15 meters, and the city finds itself 200 meters away.