Initial tests show no lead contamination in the Waikouaiti River, but there are still more results to come and the Otago Regional Council has ramped up surveillance.
The river feeds the reservoir, which has tested positive for high levels of lead.
Otago Regional Council (ORC) is working with Dunedin City Council (DCC) on the response. ORC chief executive Sarah Gardner told Checkpoint it is responsible for drinking water at source.
"We took water samples on the morning of 4 February after the warning was issued by DCC not to drink the water. The results have come back today and those results down no lead at detectable levels in the river water."
ORC had also tested river sediments but was waiting for those results, she said.
"It could be that the source of the contamination is sitting in the sediment and polluting the water from there, rather than being mobilised in the water itself.
"These results that we have today, they give us a snapshot, a point in time, they also give us some direction for further investigation. But really what we need to do is an in-depth investigation that will involve extensive surface water and sediment monitoring of the river.
"It's only if you do that, you have a number of samples and you get a number of results, that you can generate some confidence in what is actually happening."
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Testing for lead was not part of the standard state of the environment suite of testing the council did routinely, she said.
"We have no record of any lead in the water at that site. But that's not to say that we have a complete record, or even extensive testing of lead at that site.
"I don't think we've discounted any possibilities... We've had a look at some particular sites, we've had a look at consent monitoring in the area – that hasn't given us any cause for concern. That's why we're continuing to investigate.
"We'll keep doing those investigations until we get a satisfactory result, whether that's being able to confirm with confidence that the river water isn't the issue, or whether that's finding something that is as the source of contamination."
The water pipes were DCC's responsibility, Gardner said.
"Dunedin City Council is the water supplier, and they are responsible for the network. We are responsible for protecting drinking water quality at source."
A catchment investigation was underway, she said, which would include extensive surface water and sediment monitoring of the river.
"It's certainly a significant deal more of monitoring and testing than we would have done routinely."
A flyover by DCC identified two sites of interest that Otago Regional Council has tested, including a site with containers and old tyres near the river and Waikouaiti water treatment plant, and a site with clean fill and waste at the headwaters of the Waikouaiti River.
Both sites have been ruled out as sources of lead contamination.