The Gore water supply was deemed not safe for drinking last Friday afternoon because of nitrate levels.
The ban on drinking from the supply was lifted on Monday night after three days of clear tasting with the level now well below the limit.
But University of Otago research fellow Dr Tim Chambers, who has studied nitrate concentration in the water, said it would not be the last supply around the country impacted.
"We've been seeing it in the private suppliers for a while. Usually these public suppliers are better protected, but you know, the flood is coming, in terms of the nitrate contamination," he said.
‘There's only so much that the land can sustain and some of these suppliers, while they're more protected than people in private suppliers, they're not immune. They might be deeper bores or they might be better protected in some ways, but they're generally still vulnerable."
He said nitrate contamination had been discovered when there were changes in dairy intensification.
Councils and other sectors had to be a little bit more deliberate about what land use was permitted in what areas and to what intensity.
"It's hard in some of these areas where you're drawing from groundwater because the pollution can come from such an enormous area. Because it all filters down into the same groundwater and then you're pulling your water from there. Really, you have to have a catchment level approach to the amount of nitrogen that you're putting into the system."
Gore had a nitrate reading level of 11.4mg per litre, 0.1mg above the 11.3mg maximum acceptable level.
Dr Chambers said the 11.3mg level was established in the 1950s and it protected against one condition that affected infants who were less than 6 months old.
"But there's been quite a lot of research done in the last decade examining the relationship between nitrate and other health conditions."
He is leading a study in New Zealand looking at nitrate in drinking water and pre-term birth risk. The results are due later this year.
"All of our environmental regulation related to drinking water is sort of establishing, well, we can do this much land use until it hits that threshold. But what we've been calling for for a long time is that you have to have a cautionary approach to this and trying to figure out that actually maybe it makes more sense to have a different threshold for where you start to intervene."
Federated Farmers Southland president Jason Herrick
defended the use of synthetic nitrogen fertilisers. He said farmers adhered to the cap and it was too early to play the blame game.
Environment Southland has started an investigation into the nitrate outbreak. — Additional reporting Steve Hepburn
