Drought highlights water supply

17:00, Jul 31 2013
Priyan Perera
MAHURANGI RIVER: Watercare Services Northern Water Supply manager Priyan Perera atop the Mahurangi River water intake pipe.

The vulnerability of Warkworth's water supply was brought into sharp focus during summer's drought as the Mahurangi River dwindled, the town's usual source nearly drying up.

January was the driest month ever for Warkworth with only 6mm of rain falling, the lowest in any month since records began in 1966.

An average of a million litres of fresh water daily goes through the plant, so this meant trucking water from Snells Beach to top up supplies for around three weeks.

Nolwenn Lagadec
WATER TREATMENT: Watercare Northern Operations controller Nolwenn Lagadec at the Warkworth water treatment station.

As the call went out for residents to curb their water use, that fell to 700,000 litres, ending the need for extra water.

Watercare is looking for more reliable and lower risk water sources in the north, with bores to underground aquifers the preferred choice.

Warkworth's population of more than 4000 is expected to easily double in the next 30 years.


Aside from fluctuating river levels, the water is also exposed to fluctuations in the suspended sediment when the river floods, as well as risk from runoff, Watercare Northern Water Supply manager Priyan Perera says.

Sediment levels alone can rise 3000 per cent in a flood.

This sees a complex system needed to clean the water at the Warkworth water treatment plant.

Suspended solids like clay and soil, along with micro-organisms in the river water, are removed by adding aluminium sulphate and filtering the water through coal filters and fine sand filters.

Taste and smell are taken out with powdered activated carbon.

The water is then disinfected by using both UV filters and chlorine, with a dose of caustic soda to correct the filtered waters pH.

Bore water doesn't need the same level of treatment, making it cheaper to supply.

The former Rodney District Council applied for a resource consent in 2007 to take water from a bore on Sanderson Rd, close to the town.

The Auckland Council gained a consent last year. An Environment Court appeal by another bore user into the same aquifer was resolved out of court in March.

But it will still be two years before the bore is up and running. A year's monitoring of the water will be needed as part of the consent before building can take place.

Both water operations will run for the first five to10 years but will move mostly to the bore over time, Mr Perera says.

The bore goes down 200m, through thick layers of overlying siltstone, mudstone and clay, into a deep sandstone aquifer. The layers give a highly impermeable barrier to any contaminants filtering down, Mr Perera says.

Most of the aquifer water comes from the Mahurangi River catchment.

Wellsford is Rodney's only other river drawn town supply, coming from the Hoteo River.

While its population is not expected to rise past around 3,500 in the next 30 years there are also plans for Wellsford to get an alternative aquifer source.

Watercare is investigating a couple of sites.

A new source is also sought for Helensville.

The Mangakura Stream dam and sandhill wetlands currently supply the town.

Rodney Times