Deadly algae bloom bacteria detected in dam

Small traces of a deadly bacteria have been found in a Picton dam, but it has not made it into the town’s drinking water.

Cawthron Institute found low-level cyanotoxic bacteria, caused by an algae bloom, in the Essons Valley Dam – a water supply that supplies Picton households.

In high concentrations, the bacteria can kill animals and infect fish and shellfish, causing food poisoning.

Marlborough District Council operations and maintenance engineer Stephen Rooney wanted to reassure residents that only a small algae bloom had been detected and was not enough to pose a health risk.

“At the levels it’s been detected, there is no health impact,” Rooney said.

The samples Cawthron Institute took showed a cell count of 34,000. A cell count of more than 50,000 was needed to pose a significant health risk, he said.

However, it was the first time this particular bacteria had been detected at that level.

The Essons Valley Dam had been switched off and all household water was being supplied from the Speeds Rd well.

“The water in the dam had become quite discoloured and was quite warm and had virtually no flow,” Rooney said.

“With these constituents, it’s hard to treat so we stopped it and it never got in to households. We are not actually required to stop using the water with this level of bacteria . . . [but] we have opted not to draw on this supply.”

National Water Standards were still being met, despite the presence of the bacteria, he said.

However, the council had notified the public health unit in Marlborough, as per protocol. Further tests would be done before the water supply from the dam was switched back on, he said.

“We will do further testing but we don’t anticipate we will switch it back on until we’ve had some reasonable rainfall.”

Rain was needed to flush out the algae bloom, Rooney said.

It would flow through the Waitohi Stream reaching the sea at its discharge point.

“However, the level of algae we are talking about is likely to have dispersed by then, through the natural aeration, which will occur as it flows through the stream system.”

Rainfall would bring cooler weather, which would break up the bloom, he said.

A sign was put up yesterday to warn people not to come in contact with the water, despite the dam being behind a locked gate, he said.

Rooney asked Picton residents to continue to conserve water now that the Essons Valley Dam supply had been switched off.

Household hosing restrictions in Picton remained in place, with residents in odd-numbered houses allowed to water their gardens with sprinklers on odd-numbered days only, and those living in even-numbered houses allowed to use sprinklers on even-numbered days.


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