Rising sea threatens city's tap water

The future of Wellington's drinking water supply could be under threat from rising sea levels, new research warns.

The region could be forced to spend tens of millions of dollars relocating water bores to tap alternative sources as salt water encroaches on a major underground aquifer, Unitec Institute of Technology researcher Gregory De Costa has found.

Up to 40 per cent of the region's drinking water comes from a large aquifer under the Hutt Valley drawn up from pumping stations, many of which are coastal and low-lying.

That freshwater source supplies more than 100 million litres a day and is connected to the ocean with downward pressure from the water flowing downhill keeping the salt-free groundwater of the Waiwhetu aquifer from mixing with the sea beneath it.

But as sea levels continue to rise over the next several decades, this balance will be thrown out. Increasingly, sea water could contaminate drinking water supplies at coastal pumps, De Costa's research showed.

"We need to manage the pumping of the water so exploitation doesn't take place to the extent that we degenerate the aquifer," he said.

It was vital authorities like Greater Wellington Regional Council, responsible for drinking water in the city, Porirua and the Hutt Valley, understood how mounting oceans would affect its aquifers, he said.

Pumping stations closer to the ocean could be moved upstream and the aquifer carefully monitored, with cut-off points introduced, he said.

"If the water drops below a certain point you stop pumping."

Developing alternative water sources could cost between $30 million for additional storage lakes to $150m for a brand new source and treatment plant.

Wellington Harbour is currently experiencing the largest sea level rise in the country - about 2mm a year.

One cause was climate change, but the city was also sinking into the ocean, according to NIWA scientists.

Slow-slip seismic movement - essentially earthquakes happening over a weeks or months - in the last few decades were thought to be behind the city's drop. By the end of the century, the sea could rise 0.8m.

Greater Wellington water supply general manager Chris Laidlow said modelling to capture the impact on the Hutt Valley aquifer would be completed in the next few months.

That would help the regional council understand when they would have to reduce the levels of water pumped from the Waiwhetu source. "It will bring forward the need for a new source."

In the next 15 to 20 years, the council was planning to spend $30m building water storage lakes at Kaitoke as another drinking water resource. But under current estimates, it was not thought rising seas would have a significant impact on water supplies until the 2060s, he said.

But boosted oceans levels will have another unpleasant side-effect, especially for residents of Petone.

With the groundwater table pushed up by sea-water, the low-lying suburb will be more prone to flooding.

"Under climate change, we're not only seeing the sea level rising, but waves and storm surges becoming greater and greater. The ocean will be beating in on certain days of the year."

The city's reliance on the Waiwhetu aquifer could also be put to the test following a major quake.

A report by GNS Science released earlier this year showed the liquefaction zones were much further north than expected, meaning many pumps could be knocked out of action.

The Dominion Post