Lower Hutt water to be permanently treated with E coli source still unknown
A months-long investigation has failed to identify the cause of contamination in Lower Hutt's water supply, which will continue to be permanently treated.
E-coli was found in water drawn from the Waiwhetu Aquifer on three occasions since late last year, and an increase in coliform bacteria was also detected.
The discovery led to an investigation early this year and immediate chlorination of the water supply, which goes to about 74,000 Lower Hutt residents each day.
However, on Thursday, it was revealed the investigation by utilities company Wellington Water failed to identify the cause of the contamination.
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As a result, chlorination and ultra-violet (UV) water treatment would continue to manage the "significant" risks.
The investment is expected to cost Greater Wellington Regional Council about $5 million in the 2017-18 financial year, with operational costs of about $1.5m in 2017-18 and $875,000 a year from 2018-19 onwards.
E-coli was found in two bores in the Waterloo Wellfield and in the Naenae Reservoir inlet main from December last year, the first time any contamination had been recorded in the area.
Environmental regulation manager Al Cross said there were a number of ways the aquifer could have been contaminated.
They included breakages in the wastewater network; water from beneath the ground finding a way into the aquifer through bores; and damage to the aquifer following November's 7.8-magnitude Kaikoura earthquake.
The contamination cause will continue to be investigated despite the decision to permanently treat the water.
"We are fully aware we're dealing with something really complex, and we may not ever really get a full grip of the pinpoint source," Cross said.
"But the most important thing is we do the work to do our best to understand it, and we safeguard the system for the future."
There were up to 400 bores in the area, with about 20 of them used for extracting water, Cross said.
As people drilled into the surface, they could get close to or penetrate the aquifer, which brought a contamination risk.
The council planned to make changes to its consents process for taking water from the aquifer, and to upgrade its monitoring programme to make sure bores are sealed properly.
Regional council chairman Chris Laidlaw said decisive action was required to protect the health of Lower Hutt residents.
"This is a complex area of study in an aquifer that is yet to be fully explored, and it's shown us there are no quick fixes to the issue of contamination.
"We can't take chances when it comes to protecting public health, and that means continuing to chlorinate drinking water from the Waterloo Treatment Plant."