28 July 2007

The Water Of Life

Brad Ewing recently posted an article in his blog (environmental economics...) on how people in some areas deplete non-renewable water supplies at their peril, and how dams and irrigation can increase evaporation so much that once large rivers like the Nile or the Ganges are reduced to a mere trickle by the time they reach the sea.

This reminded me of a piece of news I picked up a couple of months ago (details here). In Tanzania, old seismic data from oil searches in the Dar es Salaam region have been used to discover a huge underground water reservoir, an aquifer (definition here), 600 m below the surface. Dar es Salaam is a harbour city, and thus the aquifer is also deeply below sea level. The reason why the water is not salty is pressure. So much, in fact, that water gush out of test-wells like a large fountain. It seems like a truly extensive and convenient water supply for the growing capitol of Tanzania, now hosting a population of roughly 2.5 million people and growing fast.

But where does this water come from, and why is it under pressure?

According to the company who collects the cash from this project (www.agwa.no), it is a huge geological formation that collects rain water from almost all of Tanzania. They have estimated that the aquifers in the area contain about 680 cubic km of water, and claim that it is regularly replenished by seepage through the ground in the rainy season. The pressure comes naturally because the Tanzanian inland is higher ground.

But what will happen to inner parts of Tanzania when the coastal region of Dar es Salaam taps into this resource? To my knowledge, no attempt has been made to consider this yet.

Instead, it is maintained that the alternative to tapping this resource is to erect a dam in the nearby river Ruvu. This river is already in trouble because of deforestation of the drainage basin, and does not hold enough water in the dry season to supply the city. A dam would come with its own set of environmental complications (eg. increased evaporation and destruction of yet more wildlife habitat), and the deep aquifer is therefore heralded as a more "friendly" and environmentally sustainable solution.

The logic involved seems uncannily familiar. Point to something that's proven bad, expose all the benefits of the new solution, and dismiss skepticism with face value arguments because there are no firm investigations on consequence. If that doesn't work, point to the short-term economic or social consequences. In this case; "the people need clean water NOW". Agwa is, in typical Norwegian company tradition, boasting its environmental responsibility right now. Unfortunately, part of the same tradition is to forget their responsibilities as soon as the project don't generate cash anymore. Being a small company, they wouldn't have the reserves to spend on any cover-up operation either, should they screw up badly.

So what can go wrong, then? Are there any real risks involved?

Well, I'm not an expert in neither hydrology or geology, so I don't really know. Curiosity peaks, however, and some questions seem very interesting. Such as:

How much water enters the aquifer each year? -This would give a good measure on how much water you can sustainably take out. An answer would have to include data on both the amount of precipitation, evaporation and surface runoff. Not terribly difficult to obtain, but surely costly and time-consuming to a small company like Agwa.

The well extends to 600 m below sea level and is tapped close to the ocean. How much can the pressure in the aquifer be reduced before seawater seeps in and make the water salty?

I think both inquisitive and inquiring minds would like to know. Wonder if any of them works in the Tanzanian government...

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