The cause of an E. Coli scare that forced Kawerau residents to boil drinking water earlier this month has been narrowed down to three possibilities.
The Kawerau District Council is preparing a report for the Bay of Plenty District Health Board on the positive E. Coli test.
A summary of the technical report provided to NZME suggests the single contaminated sample could have been as a result of laboratory contamination, a sampling error or a breach due to back-flow contamination from a residential property.
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Kawerau District Council manager of operations and services Hanno van der Merwe acknowledged the E. Coli breach resulting in a three-day Boil Water Notice was significant.
"The testing that is done is an indicator that lets us know whether the system is working as it should or it isn't," van der Merwe said.
"If it is working there should be a zero E. Coli reading and if the reading is anything above zero - for example, one - it tells us there is something wrong."
On January 4 the E. Coli reading was 200.
"It's fair to say it wasn't just a blip on the screen, the reading told us the contamination level was quite high and required immediate attention."
On January 4 a Boil Water Notice was placed on the Kawerau water supply by the Bay of Plenty District Health Board.
Testing on January 4, 5 and 6 all returned clear of contamination and the Boil Water Notice was lifted just before 5pm on January 7.
"During the period we did not receive any reports of people becoming ill," van der Merwe said.
Council staff, in conjunction with drinking water assessors and the Bay of Plenty District Health Board, began an investigation into the contamination and ruled out the most obvious causes.
The investigation included checking the water intake and surroundings, checking the treatment plant and its performance, reviewing all reservoirs and networks around sample points and reviewing the location of the sample points and sampling procedures.
"We did significant investigations and, at the end of the day, we eliminated all likely causes and were left with only three possible reasons. All three are also unlikely but it must be one of them."
Van der Merwe said one of the three was the one most often worried about.
"This is a back-flow event. It means someone had a hose pipe or something inside a dirty water pond when they were filling up or they were using a water blaster or something and there was dirty water in the system.
"And then we have, for a brief moment, lower pressure in the reticulation than outside and this could be a major break in the reticulation. A water blaster, in this circumstance, can cause a backflow by going from the dirty water back into the drinking water.
"Our reaction to that is we are installing backflow preventers at all residential addresses. This is a big job and it won't be done overnight but it will be done."
He said something could have also gone wrong during the water sample collection process.
"In this specific location there is a tree with birds in it and it is possible, but unlikely, the sample was contaminated with bird droppings.
"The third option is if something went wrong in the laboratory. Again, this is unlikely but possible - human error does happen."
Council communications manager Tania Humberstone said that as part of the investigation, letters were delivered to 50-plus residential properties in the Upper Valley area where the contaminated sample was taken.
"Some feedback has been received, but nothing to indicate it would have been the cause," Humberstone said.
Remedial action taken by the council since the Boil Water Notice includes an agreement with the health board to increase the limit on how much chlorine could be used.
The target of Free Available Chlorine Equivalent (FACE) would be upped from 0.35mg/l to a new limit of 0.5mg/l in order to allow for changes in pH and flow rate variations.
Humberstone said the Boil Water Notice had prompted questions about whether the Kawerau Council had invested enough in the infrastructure.
"Developed in the 1950s, there are some areas of infrastructure that are ageing, however, inspections of the reticulation system showed overall, it is still in good condition," Humberstone said.
Last year the council set in motion several high-priority projects including the upgrade of the water infrastructure and replacement of the water pipes in addition to routine maintenance.
It planned to spend $15 million replacing the old water reticulation system over the next five years, on top other projects.
Kawerau mayor Malcolm Campbell thanked Kawerau residents for their patience and resilience during the Boil Notice period.
"It has not been an easy way to start 2020," Campbell said.
"We have completed reviews and audits of all aspects of the water treatment, sampling and supply systems. This has confirmed that the water is safe and our treatment, sampling and testing systems are robust.
"We are now looking forward to getting stuck into the water infrastructure improvement projects as we had planned."