What caused Timaru’s drinking water to change colour?

Our science investigation into what caused the discolouration of Timaru’s drinking water in late 2021 has found that discharge from silage storage was a likely contributor to the problem, but we couldn't rule out other possible contributors.

Timaru residents experienced several weeks of yellow/brown water during the summer of 2021/22 and – although no drinking water health standards were breached – the water colour was unsightly.

What caused the discolouration

Tests at the time showed the colour change was due to elevated concentrations of a natural element called manganese.

Manganese is not harmful to humans at the concentrations observed, but an alternative water supply was provided by Timaru District Council.

Monitoring water quality in the area

Locals were keen to find out what was causing the manganese levels to increase.

Our groundwater science team installed several monitoring bores at locations around the drinking water intake and took other water samples over a period of months.

"An analysis of these samples has shown that silage leachate was a likely contributor to the elevated manganese concentrations which caused the discolouration,” says our Director of Science Dr Tim Davie.

"It’s important to note that there are other possible natural sources of such organic matter, so we are unable to confirm that the silage was the only source in this area.

"None of the drinking water samples at source had concentrations above the maximum acceptable values (MAVs) for drinking water health standards."

Dr Davie says the Timaru discolouration event – while not harmful to health – has already prompted action.

"We are continuing to monitor water quality in the area near the intake and our staff are conducting a review of all silage storage near or within community drinking water protection zones across South Canterbury and the region, and will take appropriate action."

Silage storage

Silage storage located about 500 metres upgradient from the intake, on private property, were removed during the investigation, after which subsequent water sampling showed reduced manganese concentrations. The two silage storage areas at this site were a combined size of approximately 2500 m3. They have both since been removed at the landowner’s expense and we are continuing to investigate this issue and whether any enforcement action is appropriate.

Silage storage is used to store pasture for animal feed for use in winter months.

Silage storage is regulated by rules in the Land and Water Regional Plan and are subject to industry-agreed Good Management Practices. If they are installed as a permitted activity, the rules must still be followed. Farm Environment Plans also audit silage storage management practices. 

We offer free on-farm advice from land management advisors, as well as free consultations with our consents team for landowners to find out more about the rules for silage storage. We also have information on our Farmers’ Hub website. You can find that information by clicking the 'Animals' information box and under 'Feed'.

More information