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More than 2500 homes and businesses were affected when untreated water from the reservoir entered the city’s drinking network through an old, decommissioned pipe on August 14.
Because of the potential for contamination, a boil-water notice, which lasted three days, was issued for large parts of the city and businesses were forced to close temporarily.
At the time, the reservoir was being drained to allow work on its $6.6million refurbishment to continue.
A report on the contamination event will be presented at a meeting of Dunedin City Council’s infrastructure services and networks committee today.
Prepared by council 3 waters asset planner Sarah Stewart and infrastructure and networks business resilience project manager Gerard McCombie, the report identifies four key events which led to the untreated water entering the system.
In June, Opus International, which is overseeing the refurbishment, asked the council to arrange for the level of the reservoir to be lowered by 2.5m so work could continue on the project.
It was not until August the automated draining system at the reservoir was used but it was unable to remove enough water and the manual system was needed.
The usual manual discharge system which went through a valve in the face of the dam could not be used, so an alternative was needed.
Removing water from the reservoir with portable pumps was considered but later discounted because of the time it would take.
An email from Opus to the council’s engineering project manager on August 11 stated a quick decision was needed if they were to take the opportunity to maximise time.
Ultimately it was decided a branch line off the automated dewatering pipeline would be used. It was not known at the time that another branch of the pipeline led directly to the city’s treated water network.
The report found the manual system used was not a documented or authorised method.
Had it been discussed during the planning stages, the potential for untreated water entering the network would likely have been identified.
Water was able to flow through the decommissioned pipe as it had not been plated to stop such a thing.
A plan of the site used by the operator on the day of the contamination event showed the valve, which isolated the pipeline, with the words "Plated up, not in operation" hand-written beside it.
The report found the operator took that to mean water could no longer flow through the pipe, when in fact only the inner workings and handle of the valve had been removed. Discarded parts of the valve nearby, along with the written note, also led the operator to believe the pipe was no longer operational. The note could have been written on the plans any time between 1960 and 1992.
Other causes identified in the report included an emergency action plan not being updated after a pipe at the toe of the dam was plugged during reconstruction work.
The potential for possible contamination was also not identified during the project’s risks and hazards assessment.
While none of the issues would have resulted in the contamination event on their own, they all contributed, the report found.
The report identified a range of corrective and preventive measures, such as the identification and disconnection of raw/treated water cross connections and changes to the way capital and operational contracts were managed.
Mayor Dave Cull said he was comfortable with the findings in the report, as it showed the event was caused by combination of events, one of which was historical, and no one person was responsible.If the historical work on the pipeline had originally been noted clearly, the situation would never happened, he said.
The council had already implemented some of the recommendations in the report and would be investigating and considering the others.