What was that rust-coloured stuff in the Whanganui River?

Rust-coloured water discharged from a stormwater pipe into the Whanganui River is the result of harmless iron-oxidising bacteria, council engineer says. Photo / supplied

There is rust-coloured brown water discharging into the Whanganui River most of the time - but it gets more visible in summer when the river runs clear.

The latest discharge was photographed by Hayden Signal from Taupo Quay, probably on January 28 during a high tide when water in the river was clear and the discharge was visible. Most of the time it runs into a brown river and is unnoticed, Whanganui District Council senior stormwater engineer Kritzo Venter said.

The water is from Churton Creek and runs through the Springvale catchment. The catchment has iron-sand, and bacteria living in the pipes there oxidise the iron, making for this red-brown discharge.

Questions about it are some of the most frequent the council gets, Venter said.

A similar discharge has been seen at times at the North Mole fishing platforms, alarming fishers. It's also the result of bacteria oxidising iron as water flows through the Springvale and Castlecliff areas, Venter said.

The bacteria are harmless, but they can increase in number and create a brown gelatinous slime that clogs pumps and drains.

The bacteria, also known as iron ochre, are sometimes visible on the surface of groundwater. At those times they give it an oily orange sheen.