A natural phenomenon going back more than 5000 years is being blamed for parts of the stormwater and wastewater systems on the Marsden City development failing, but ratepayers will have to pick up the repair bill that could run into the millions.
Last year the Northern Advocate revealed that sections of the stormwater and wastewater systems on the multi-million-dollar Marsden City development had failed, causing pipes and manhole covers to slump.
Whangarei District Council said the situation could cost several million, but when then Labour List MP Shane Jones later raised the issue in Parliament, he said the infrastructure "collapse" had put a potential $20 million liability on ratepayers. However, the council said that figure was way off the mark, but it had yet to determine the full costs.
The problems cover stormwater pipes and manholes, and wastewater manholes on parts of Roosevelt, Theodore, Casey and Abraham roads in the Marsden City development at Ruakaka, covering 91ha and 190 lots.
The infrastructure of the private development was vested to the council after completion between September 2009 and September 2010 after a council consultant had approved it.
The problems came to light in early 2014 when manhole covers started to slump.
Marsden City developer Oliver Scott did not want to comment for this story. However, he said previously the infrastructure now belonged to the council after it had ticked it off as complying with its consent and he wanted the matter sorted out as soon as possible.
After a 10-month investigation into the problem, WDC group manager infrastructure and services, Simon Weston said the work had determined that a key factor in the pipe failure was the type of soil that the development had been built on.
"The phenomenon is a naturally occurring process known as acid sulphate soils that are routinely encountered in more tropical parts of the world, though seldom cause issues in New Zealand," Mr Weston said.
"The soils occur in land that was covered by water when the sea level was higher than it is currently during a period 5000 to 10,000 years ago. While under sea, the sea bed accumulates minerals that get bound up in the soil structure. The minerals may leach out over time, or may be locked in, particularly when a swamp forms over the old sea bed," he said.
"If the minerals leach out or remain locked up in the soil they cause no problems. However, if the soil is rapidly exposed to air, for example through drainage, the minerals can change composition to form strongly acidic groundwater. The binding up of the minerals in the sea bed requires warm conditions, hence the prevalence of this type of soil throughout Australia and the tropics."
Mr Weston said this is what is believed to have happened at Marsden City.
"It is likely that draining of the site during construction exposed underlying acid sulphate soils to air which, in turn, generated groundwater with a low pH. The low pH has caused damage to the concrete stormwater pipes, which in turn damage the road when they fail," he said.
Mr Weston said once the cost of repairs had been determined - likely in the next few weeks - ratepayers would initially pick up the tab to fix the problem via WDC pan charges.
He said questions of liability have not yet been answered, but if any action was taken in the future over the failures, the council would claim the costs of the repairs back.
Mr Weston said in normal conditions concrete pipes would last 80 to 100 years, but some in the Marsden City development may only last up to 40 years.