$1m dredging wasted on lake - expert

LYN HUMPHREYS
Last updated 05:00 01/07/2014
dex
ANDY JACKSON/FAIRFAX NZ
Dex Knowles

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Spending $1 million of ratepayers' money to clean up Pukekura Park's main lake won't work, a former council manager says.

"To adopt a million dollar dredging solution to the Pukekura Park Lake water quality issue at this time is unlikely to provide desired long-term results," Dex Knowles, a former Taranaki Regional Council manager, said.

He has a lifetime of experience in water quality management.

A year ago attempts to dredge the Pukekura lake failed.

New Plymouth District declined to approve $1.2m for a new dredging operation, but the proposal will be considered in future long-term plan discussions.

The sediment levels of copper, lead and zinc in the lake have been found to be above the Australian and New Zealand Environment and Conservation Council guidelines.

Knowles said there was no affordable or practical way to eliminate the cause of pollutants entering the lake.

"The lake is in a stream and is a settling pond that traps material carried by stream flow," he said.

Urban streams generally carried a high level and wide range of pollutants which wash in from surrounding lands or are discharged from stormwater systems.

While dredging would provide some relief in the short term it would be a high-cost, high-risk task. "Silt, sand and pollutants settle to the lakebed. Lake water warms as it stays in the lake for a long time adding to the water quality problem by enhancing aquatic weed growth and further deterioration."

The most effective way to deal with the problem, as shown in other parts of New Zealand and further afield, is to put long term control and management plans in place, Knowles said.

New Plymouth District parks manager Mark Bruhn said the council had a number of strategies aimed at improving the lake water quality.

Traps had already been placed within the system - such as had been done with the installing of a holding bay within the Bowl of Brooklands lake.

Recently, council members decided not to proceed with re-dredging work and to consider re-tendering the work in the next long term plan process, Bruhn said.

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