Waitaki District residents are no closer to discovering the source of an E coli spike that contaminated their groundwater.
About 119 bore owners were contacted last month to stop drinking their water following a spike in bacteria levels that could cause abdominal cramps, diarrhoea, vomiting, and serious kidney issues in severe cases.
The Otago Regional Council conducted tests to determine the source of the contamination in the aquifer. However, the tests were inconclusive due to low bacteria concentration during sampling.
Council staff notified residents on 15 June after scientists discovered E.coli spiked at 150 colony forming units (CFU) per 100 millilitres.
That number should be less than one, according to the Ministry of Health drinking water standards.
Council chief executive officer Sarah Gardner said people in the region needed to remain vigilant and should not drink the water.
"The serious health issue that occurred in Havelock North is a stark reminder of the dangers to human health associated with E.coli.
"Lower Waitaki Plains residents who source their drinking water from private bores should still assume their source water is contaminated unless they have effective disinfection in place.
"Even E.coli levels of 2CFU/100ml in drinking water is a risk to human health."
It's unknown how long residents in the Lower Waitaki Plains will be unable to drink from their bores.
The council met in late June to discuss potential solutions to the raised E coli and nitrogen levels by changing the aquifer affected.
At the time, councillors chose to put their decision on hold to wait for more information.
A decision on the matter is expected to happen next month.
The spike was discovered during routine water sampling in the area.
Determining the contamination source required a different type of test, Mrs Gardner said.
"Having high concentrations of E coli present during a spike may allow our scientists to determine what's causing the issue at that exact point in time. We can then advise the community of the appropriate next steps."
Representatives from the council, other district and city councils from across the region have formed The Southern Drinking Water Reference Group to improve the drinking water standards for Otago and Southland.