Mangrove mayhem

For much of the weekend the Brazilian town of Vila velha will be without water. The culprit is a burst pipe which has to be replaced, necessitating the closure of much of the town's water supply.


Now, the pipe runs through an old mangrove swamp. Mangrove mud is stuffed full of bacteria. Mangrove trees produce about a kilo of litter per metre per year, which has to be broken down, not to mention debris from fish, crabs and shrimp which make a rich organic soup. In fact, there are so many bacteria that the available oxygen is used up and anaerobic bacteria (which can survive without oxygen) flourish, including those called “sulphate reducing bacteria” - these are the ones that produce hydrogen sulphide, the “rotten egg” smell. Unfortunately, the smell is not the only problem.

The action of anaerobic bacteria causes corrosion on the surface of metal objects. Additionally, the waterlogged soil of a mangrove swamp is high in iron pyrites, and when exposed to oxygen this forms sulphuric acid. As the water level in coastal mangrove swamps goes up and down with the tide and through the seasons, sometimes aerobic, sometimes anaerobic, metal there gets attacked from both sides.


The pipe in this case was made of 1 cm thick cast iron, and laid about 30 years ago. Gradual corrosion and water pressure inside the pipe eventually caused it to burst so it had to be replaced, but as it was now 6m below ground (due to a landfill project) that was easier said than done. Hence the chaos!