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Question session on Lake Rotorua alum dosing

Wednesday, 27 May 2015 10:00 a.m.

Rotorua residents, iwi and farmers interested in improving water quality in Lake Rotorua can attend a free session next week to find out more about the results of alum dosing in the lake.

Bay of Plenty Regional Council is holding the open session on Tuesday, 2 June at the Novotel Rotorua from 5pm to 7pm, for anyone who would like to know more about the effects of alum dosing of Puarenga and Utuhina to Lake Rotorua.

Regional Council Lake Operations Manager Andy Bruere said alum dosing was designed to reduce external nutrient loads, and was proving helpful in improving lake water quality. The meeting was a follow up on a meeting held with farmers at Ngongotaha last year presented by the Regional Council’s Lakes Chair Dr David Hamilton.

“We will cover the modelled impact of alum dosing on Lake Rotorua water quality and align that with the effects of reducing nitrogen inputs to Lake Rotorua. This will be a science-based presentation on the impact of a range of scenarios on Lake Rotorua water quality,” Mr Bruere said.

“It will help people understand why the lake is currently meeting its long-term water quality goal and what we expect to happen if land use nutrient inputs are reduced,” he said.

Dr Hamilton said water quality modelling of Lake Rotoua had helped to better identify the underlying reasons for improvements in water quality over the past few years. 

“Anecdotally, and with support from measurements, nutrient concentrations have declined, leading to improvements in water clarity and lower levels of algae. The modelling indicates three processes leading to these improvements, each connected with alum dosing of the Puarenga and Utuhina Streams,” he said.

“They include locking up dissolved phosphorus in the two streams, increasing sedimentation of nutrients in the lake due to formation of large particles (flocs) that fall to the bottom of the lake, and reducing previously large releases of nutrients from the bottom sediments of the lake.”

Anyone wishing to attend the session can register online at, or email

Lake Tarawera