Picton residents may have thought krill were back in the harbour, but the red tinge seen yesterday was actually dye that was being used to test the sewerage system.
The Marlborough District Council hired scientists from Cawthron to check the discharge processes and dilution rate of the new system by pumping six litres of non-toxic red tracer dye into the treated wastewater outlet pipe at 10am yesterday.
Marlborough District Council assets and services manager Mark Wheeler said a very small plume of dye was visible in the harbour about 2pm, the red patch lurked about 300 metres out from the former Carey's Boatyard on the west side of the harbour.
"It seems likely this was stirred up by early afternoon ferry movement as, until then, the dye had been sitting about six to eight metres below the water surface."
The scientists will analyse the data to check the sewage outfall diffuser is "functioning as per its design" and report back to the council in March.
It said the testing was one of the conditions of the consent granted for the new sewerage system.
In a statement to the Marlborough Express sent earlier this week, the council said its staff did preliminary work in preparation for the official tests on Thursday last week to give Cawthron staff background information, though that advance work did not show any trace of dye reaching the harbour that was visible to the human eye.
Cawthron staff were briefed on the results before they began their work yesterday.
Failure to see the dye might mean it remained below the surface, trapped under a temperature layer.
- The Marlborough Express