Amazon’s deep secret revealed

PETER SPINKS
September 05, 2011

It is, they say, the mightiest river on Earth. And now, it seems, there is one even mightier flowing far beneath the surface.

South America's legendary Amazon - the world's widest and second-longest river - contains more water than the Nile, America's Mississippi and China's Yangtze put together. Now a team of Brazilian scientists has found preliminary but compelling evidence that the Amazon has an even more gigantic underground partner that is as long as the river but many times wider.

The underground behemoth (groundwater flow or aquifer is how geoscientists describe it) rises beneath the Andes and travels through several subterranean basins before coming to the surface as it nears the southern Atlantic Ocean.

The Rio Hamza, named after the head of the research team that found it, is believed to lie about four kilometres beneath the 6000-kilometre-long Amazon River.

The Hamza is also thought to be of similar length to its surface sister and flows from west to east, say the team leaders, Elizabeth Pimentel and Valiya Hamza of Brazil's National Observatory.

But that is where the similarities end. Although the Amazon varies in width from one to 100 kilometres, the Hamza is suspected of being between 200 and 400 kilometres across throughout its sinuous course.

Another key difference is the rate of water flow. While the swift-flowing Amazon reaches speeds of up to five metres a second, the much slower Hamza meanders along at less than a millimetre every hour. This means that, in the space of a second, the Amazon drains something like 133,000 cubic metres of water, while the Hamza probably dispatches less than 4000 cubic metres.

‘‘Without fully understanding what's happening in the Amazon, it would be true to say that many rivers have aquifers below them,'' says CSIRO hydrogeologist Glen Walker in Adelaide.

‘‘These are caused by earlier versions of the rivers sometimes operating at times when rainfall is higher and rivers are steeper. They accumulate gravels and sands, which later get covered as rivers deposit more material from eroding mountains.''

Water can move faster through the sands and gravels than through other materials, Dr Walker explains. ‘‘These make it easier to pump the groundwater so it can be used.''

An interesting question is where this aquifer discharges, says Willem Vervoort, an associate professor in hydrology and catchment management at Sydney University. ‘‘In other words, what depends on the existence of this water? It would have existed already for a long time, so there will be specific ecosystems that rely on this water - possibly even somewhere off the coast where the water pushes out into the ocean and creates an area of low salinity.''

How it was found

Hints that the Amazon had a deeper counterpart first emerged after data was gathered from more than 200 inactive oil wells that had been drilled about three decades ago by Brazilian petrochemical giant Petrobras.

‘‘Oil occurs in sediments that also contain water,'' says Ryan Vogwill, who supervises research students and runs a masters course in hydrogeology at the University of Western Australia.

‘‘Once the oil has been removed and the groundwater system has recovered from the effect of removing the oil, these wells can be used to study groundwater. By combining an understanding of the geology, groundwater levels and groundwater temperatures, the scientists were able to reconstruct the deep-flow system.''

The abandoned wells yielded some unusual temperature fluctuations. ‘‘The scientists would have used spatial patterns of temperature to define water flow,'' Dr Walker says. ‘‘Water flowing through the rocks causes them to cool.''

Using the temperature measurements, the research team produced a mathematical model that predicted that the Hamza existed. The model also suggested that the groundwater travels almost vertically through rocks to depths of about 2000 metres before flowing almost horizontally.

The research findings might account for the relatively low salinity levels near the Amazon's mouth, the team believes. They also suggest that the Amazon rainforest might have two drainage systems, with the Amazon draining the surface and the Hamza draining deeper sedimentary layers.

Verification

The Brazilian team's results were presented last month in Rio de Janeiro at the International Congress of the Society Brasileira Geophysical.

‘‘The techniques and conclusions, at first glance, are credible, but they need to be verified,'' Professor Vogwill says. ‘‘The scientists are working to confirm the work by 2013, so I'd say that would include independent review.''

Groundwater explained

It may not often make the headlines, but groundwater is essential to Australia. In the country's arid interior, it is the main - sometimes the only - source of water.

Even nationally, almost a quarter of Australia's water supplies originate from groundwater of one kind or another. The proportion is higher during years of poor rainfall when surface supplies are stretched.

Does the Amazon discovery suggest that Australia's artesian basins are deeper and more complex than is widely believed? ‘‘Most groundwater systems are complex because the water travels through rocks that have been formed by complex processes,'' Dr Walker says. ‘‘The nature of rocks influences the way in which water flows both horizontally and vertically and also affects water quality.''

It is unlikely that something the scale of the Hamza would exist in Australia, Professor Vervoort says. ‘‘We have alluvial and other aquifers under or associated with most rivers. A range of ecosystems depend on these sources and many are also used for providing good quality water for humans.''

Groundwater resources, scientists say, will be increasingly attractive due to the likelihood that surface runoff in southern Australia will become less reliable as rainfall patterns shift.

Earth science future

Understanding groundwater requires a solid grasp of earth science. Geology strongly influences the shape of the Earth and this controls where rivers flow, Professor Vogwill explains. ‘‘The deeper that groundwater occurs, the more expensive it is to study.''

Drilling of deep new wells costs millions of dollars each, he says. ‘‘That's why the Brazilians used their oil wells. They were already there and saved hundreds of millions of dollars in drilling,'' he explains.

‘‘As we increasingly impact on deep groundwater systems - through groundwater abstraction, coal seam gasification, geothermal power projects, carbon dioxide disposal and tight shale gas extraction - we will need to invest more in understanding them.''

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