A question mark hangs over Wellington's water supply after serious concerns were flagged about whether eight wells could provide water after a major earthquake.
A new GNS Science report has cast doubt on the assumption that the Waiwhetu aquifer in Lower Hutt would be able to pump water within 24 hours of a major shake on the Wellington Fault.
In response, Greater Wellington Regional Council is now urgently investigating the resilience of the wells used to pump water from the aquifer to treatment plants.
The aquifer can supply more than 100 million litres of water a day, and currently supplies about 40 per cent of the region's water.
Water supply general manager Chris Laidlow said in a worst-case scenario, it could costs tens of millions of dollars to improve the wells or create an alternative supply.
In 2012, a report to the council said the aquifer would cope well in a major quake, and had "provided some confidence that aquifer water would be available fairly quickly after a major seismic event".
But the latest findings from GNS found the aquifer could be more susceptible to liquefaction than originally believed.
"They're now saying that the liquefaction zones are much further north than originally thought, which increases the likelihood of damage in the wells," Mr Laidlow said.
The council was still digesting the information and working out the impact on its emergency recovery plans.
If the aquifer was able to be used quickly after a quake, that would allow water to be supplied to Lower Hutt and Wainuiomata - about 25 per cent of the region's population, he said. The focus of planning and investment had been in Wellington and Porirua, which would likely be cut off from water supplies due to pipe damage.
But if the Waiwhetu aquifer was also out of action, they may require a rethink, he said.
It was too early to estimate, but if more investment was required it would probably be tens of millions of dollars, he said.
However, he also stressed that there were many unknowns, and much more work had to be done to assess how the eight wells would be individually affected by a quake.
If only a couple were damaged, the region could manage. But if six or all of the wells were out of action, that "changes the picture quite substantially" and alternatives such as building a new well field further up the valley may have to be considered.
Council staff would be gathering more information with the aim of reporting back to councillors in April. Any new plans and funding would be included in the 2015 Long-Term Plan.
Wellington Civil Defence Emergency Management regional manager Bruce Pepperell said he needed more talks with scientists to understand the report.
Pepperell said the best plan B was for a household to be self-sufficient, and investing in the 200-litre rainwater tanks being sold through councils was a good start.
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