Conditions right for river study

JANINE RANKIN
Last updated 12:00 11/02/2013

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Hot, dry weather and low flows in the Manawatu River are speeding up research to find out what it is about Palmerston North City Council's wastewater discharge that is harming the good bugs downstream of the Totara Rd treatment plant.

Horizons Regional Council plans to review the city's discharge consent conditions, but first needs to understand why algae grows more quickly downstream even when phosphorus is being removed from the discharge.

Horizons manager of science Jon Roygard said a key part of the study was being done using paving tiles placed in the river upstream and downstream of the discharge point.

"Periphyton attaches to the tiles in the same way it would to a stone on the river bed," he said.

"By placing the tiles upstream and downstream we can measure differences in the growth rate in a controlled way. This lets us see what impact the discharge is having on the speed at which the algae grows."

The research, which needs a spell of hot weather and low river levels to run its course, is being supported by a River Lake Ltd ecologist hired by the city council. Investigations into water quality in the Manawatu River in 2012 raised questions for scientists because the amount of phosphorus measured in the river could not be blamed on the wastewater discharge alone.

The theory that phosphorus was building up in the river bed sediment and then squeezing out into the water when flows were low is being tested, with collected sediment samples currently being analysed.

The city council started alum dosing to reduce phosphorus levels in its discharge at the beginning of November 2012.

Weekly water quality sampling and invertebrate (bug) monitoring is also ongoing and, if the weather continues to be dry, equipment to measure dissolved oxygen up and down stream will be installed this month. Horizons plans to draw up new consent conditions to be released for public consultation at the end of April.

Meanwhile, city council water and waste services manager Rob Green will update the council's planning and policy committee on progress on a $1.7 million programme of improvements at the plant today.

The performance of the aeration ponds is being assessed, with a view to breaking down or removing sludge build-up, and checks are also under way to find out if they are leaking into groundwater.

Wet weather could set the algae-tracking part of the programme back to the beginning.

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- Manawatu Standard

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