Whanganui's Water Part 1- Where does it come from?

Dave Rudolph is the man who oversees Whanganui's water supply. And while it may have its issues he says the supply is the envy of other major centres around the country. Photo / Bevan Conley

Whanganui's water supply has had its issues but some say it's the envy of other major centres around the country. John Maslin reports.

It's transparent, generally tasteless and odourless but it's the main constituent of our planet and is vital to all known forms of life.

It's water and in Whanganui we use at least 25 million litres of it every day, sometimes up to 40 million litres. It's the sort of thing we take for granted but not so the Whanganui District Council's water engineers.

They're the people responsible for getting the water from its sources to the city's three reservoirs in Westmere, making sure its quality is fit for human consumption and ensuring it gets to every home, business and major industry day and night.

Ninety per cent of Whanganui's water comes from three bores at Kai Iwi, pumped under high pressure from there to the reservoirs through 24km of mains pipes.

Each of those bores – about 100m deep - takes between 350 and 400 litres of potable (drinkable) water every second from a vast artesian supply known as the Nukumaru aquifer.

Whanganui consumers are using a tiny amount of that aquifer (about 2 per cent). It's estimated 80 per cent of the water drains into the Tasman Sea. And we've been using it since the mid-1960s.

The Kai Iwi bores are in a low-lying area so it means shunting water uphill using big electric pumps.

Dave Rudolph, council's senior engineering officer and the man who oversees the city's water business, says flows vary depending on seasonal use but in summer it's all about quantity.

"We'll have those bores running from November through to February or March, 24 hours a day, seven days a week."

Peak draw-off can create "scary moments" if there's a breakdown. That's why the council carries an inventory of spares such as extra pumps.

While there's a cost for all this, Whanganui is one of the few large suburban centres in New Zealand that doesn't have the bogey of water restrictions.

It's clean and free of "nasties" but Whanganui's water has its issues. Photo / File

It once did, when every summer meant bans or at least restrictions on watering. The council did implement odds and evens days when home owners could water their gardens depending on their street number. Despite those rules Rudolph says they used to see massive spikes in draw-off between 7pm and 11pm during summer.

"But when we took those restrictions out of the loop, water usage flat-lined. It actually decreased because we were giving users the opportunity to set their own standards. It definitely showed in our figures."

A typical winter day draw-off in Whanganui is about 25 million litres but in summer that ramps up to between 35 million and 40 million litres daily. Each of those Westmere reservoirs holds about 22 million litres so in summer the city will drain off one-and-a-half reservoirs each day.

The water coming from Kai Iwi requires hardly any treatment but the council has been chlorinating it for some years. It's the only chemical additive. From a health perspective it means the water from the bores and that held in the reservoirs have been given an "A" rating by the Ministry of Health which means it's very low risk. The water quality within the city's pipe network has an "A" which indicates an "extremely low level" of risk.

The water is checked annually to align with the national Drinking Water Standards 2005 (revised 2008).

Rudolph says those bores are "secure", meaning no unwanted microbes get into the water and it's not influenced by climate or other factors. While chlorination isn't absolutely necessary, the council has done it because it stands as a second barrier to any potential problem.

He says the incident at Havelock North, where E. coli bugs got into the system and created a wave of sickness in the town, heightened awareness of the need for proper management and treatment regimes.

Whanganui's water pH is pretty stable and hardly changes and it's clear so it doesn't require filtration.

But sampling remains an ongoing process and samples are regularly sent away for scientific analysis. It's a regime that must be adhered to; any missed sample and council will be hit with a "transgression" notice which means more stringent sampling will be demanded.

Getting water to the consumers doesn't end at the reservoirs. The council is responsible for a broad network of main trunks and smaller pipes delivering water to homes and industries across the urban area. It's a big job keeping it running.

The council also manages rural water systems at Fordell, Maxwell and Mowhanau. The first two have their own bores while Mowhanau residents get water from the city supply.

Consumers there can either use the bore water or what they harvest in their own rainwater tanks. The rural supply is mainly used during summer as a top-up to homeowners' water tanks.

But getting the water from bore to household taps comes at a cost and involves a significant infrastructure.