Human error is to blame, not a mechanical fault, for the spillage of sulphuric acid into the Mataura River.
Earlier this week, it was reported that 500 litres of concentrated sulphuric acid was released into the river from the Gore wastewater treatment plant. However, that number has since been corrected to 500ml.
Gore District Council 3 Waters operations manager Aaron Green said no remnants were found in the river.
It was shut down as a precaution, he said.
At the time of the spill the river was flowing at 2,976,000 litres per minute.
"500mls of acid going into nearly 3,000,000 litres of water a minute means the environmental impact was negligible."
The dose rate was calculated at 0.00001%, he said.
Gore staff monitored the river’s acidity levels throughout the day and continued to do so since, he said.
On Thursday, staff walked 500m downstream on both sides of the river to check for any sign of an effect on aquatic life. No effects were found, he said.
As a result of the incident, further staff training and installation of an over-dosing alarm would occur.
Environment Southland integrated catchment management acting general manager Donna Ferguson said contamination of any kind to a waterway is of concern and could have impacts on plant and aquatic life.
"Our on-site investigation at the time did not identify any environmental effects," Ms Ferguson said.
A cautious approach was always advised when there was a chance water had been contaminated, she said.
No delayed effects were anticipated and an investigation is ongoing.
