E.coli readings shot up this month to around four times the acceptable maximum limit in the Kawarau River, downstream of the confluence with the Shotover River into which the QLDC releases its treated sewage.
Infrastructure operations manager Simon Mason said the "isolated spike" in test results, which are being investigated by the Otago Regional Council, should be considered in the context of other results and "prevailing weather conditions".
He pointed to a high E.coli sample taken from the Shotover River upstream of the council’s treated wastewater outlet.
"It’s common to observe elevated levels of E.coli in water bodies following rainfall, as stormwater runoff will mobilise animal droppings on the land [as well as other potential contaminants]," Mr Mason said.
However, Victoria University senior research fellow Dr Mike Joy said references to the weather as an explanation for freshwater pollution was not good enough.
River pollution needed to be tackled to protect biodiversity in rivers and oceans as well as human health, he said.
"The failed authorities want to make it sound natural when it sure as hell isn’t — it is an infrastructure failure."
E.coli is a type of bacteria found in the intestines of warm-blooded animals and humans and its presence in water indicates faecal contamination that can be from human wastewater discharges, runoff from farms carrying dung and faecal particles and other causes.
The bacteria can cause gastrointestinal illness for anyone who ingests them.
E.coli levels above 550 viable bacteria per 100ml of water are generally deemed not safe for humans.
The levels recorded at the Kawarau site on December 8 were 2100 viable bacteria per 100ml of water.
The annual average consented limit that the district council must achieve for its treated wastewater is 260 viable bacteria.
Other readings on December 8 revealed wastewater from the treatment plant was only 40 viable bacteria post-treatment, but another test site located close to, and below, where the wastewater discharges into the Shotover River recorded 370 viable bacteria.
A third Shotover test site, 200m further downstream, recorded 900 viable bacteria.
However, a fourth test site on the Shotover River upstream of the wastewater discharge site — just north of the Frankton Ladies Mile Highway — also recorded 1000 viable bacteria.
Results of QLDC testing on December 10 had found "low" E.coli levels across all sampling sites, Mr Mason said.
In May, a district council-commissioned report by Landpro had also blamed periods of elevated E.coli risk on high flows of rain, with a maximum reading of 517 viable bacteria. The report was included in an application made by the QLDC seeking retrospective consent from the ORC for release of treated sewage into the Shotover River, which restarted on March 31.
ORC team leader of investigations Peter Kelliher said the council was investigating the December results as they showed "inconsistencies" between sites.
Additional samples had been taken yesterday and further updates would be provided once the results had been processed.
Downstream samples in the Kawarau River at the council’s Chard Rd site continued to indicate "good" water quality, Mr Kelliher said.
He referred people to the council-supported water agency Lawa organisation’s advice that "water quality can change quickly — especially after rain. That is why, even at sites with generally good water quality, we recommend avoiding swimming for two to three days after heavy rainfall, since runoff from urban or rural land can affect swimming water quality".
The QLDC has faced criticism this year for its decision to revert to releasing the town’s treated wastewater into the Shotover, which it also did prior to 2019.
Mr Mason declined to comment on Dr Joy’s remarks.
