Huge risks hanging over Tauranga's water supply are to be dealt with urgently after it was found that rural roads in the hills behind the city were big contributors to stormwater contamination costing ratepayers nearly $600,000 a year.
Tauranga was vulnerable to running dry because its two treatment works frequently shut down whenever it rained heavily across the streams that supplied the city with its water.
Run-off from Oropi Rd and Oropi Gorge Rd have been revealed as major causes of the high sediment loadings in the two streams during storms.
Reports setting out the scale of the problem caused by the roads have forced the Western Bay's three councils to take urgent action, centering on sealing 2km of Oropi Rd.
"Failure to act may lead to a major failure in the Tauranga City Council's water supply," a joint councils report said.
The city council used micro-filtration technology to clean water taken from the Tautau and Waiorohi streams, but when the streams become heavily polluted in heavy downpours, the filters could not cope and the two treatment plants had to be shut down.
The council's water supply manager Peter Bahrs said the Oropi Rd treatment plant was shut on average 20 times a year while Pyes Pa's Joyce Rd plant was shut up to five times a year.
The city's reservoirs are only capable of keeping the city supplied for two days so the Western Bay's three councils have agreed to pay $200,000 each to seal the 2km stretch of Oropi Rd where it crossed the Tautau Stream which supplied the Joyce Rd works.
A consultant's report obtained by the Bay of Plenty Times concluded that sealing Oropi Rd between Gluepot Rd and Pyes Pa Rd would immediately benefit stream health and "greatly diminish" the risk to the Tauranga council's ability to treat and supply drinking water.
This report and a later submission to the Bay of Plenty Regional Council laid part of the blame on the Western Bay District Council and its consultants.
The joint submission from the Tauranga and Western Bay councils said the higher sediment loads in the Tautau Stream were largely from the unsealed section of Oropi Rd. It said these discharges had increased after roadworks were carried out by the district council in 2009.
The other report by Aqua Terra International said the slips and significant discharges into the Waiorohi Stream were linked to recent widening on Oropi Gorge Rd by the district council.
It said the council consultants who oversaw the project did not consider it necessary to apply for consent to widen the road. However, the Bay of Plenty Regional Council believed consent should have been sought, accompanied by a geotechnical investigation.
Aqua Terra said the Western Bay District Council had significantly increased the sedimentation upstream of the water intake structures, had breached the regional council's Water and Land Plan, and needed to pay a significant role to reduce the problem.
Tauranga City Council believed that contaminated run-off from Oropi Rd was the primary source of the fine sediments that caused most of the problems at its Joyce Rd treatment plant.
Higher water treatment costs caused by contaminated water entering the council's two plants were adding an estimated $580,000 to ratepayers' water bills each year.
The district council will plant at-risk slopes and improve sediment run-off controls on Oropi Gorge Rd.
Oropi Gorge Rd impacted on the Waiorohi Stream which fed the Oropi Rd treatment plant. Mr Bahrs said the Oropi Rd plant drew its water much further down the stream catchment than Joyce Rd, meaning run-off was much higher.
The council spent $3.6 million building a water pre-treatment plant at Oropi which was commissioned in January.