Engineering answer to dirty lake, committee told

Experts say engineering solutions could fix water quality problems at Lake Rotoiti in the next 10 to 15 years.

Diversion of the nutrient-rich water flow coming from Lake Rotorua and the Ohau Channel, and artificially boosting oxygen levels in deeper water, are the likely saviours, says Paul Dell, co-ordinator of the scheme to clean up the highly polluted Rotorua lakes.

A report on scientific studies into the diversion scheme is expected in September.

Mr Dell told Environment Bay of Plenty's regulations and monitoring committee a wall would be built in Lake Rotoiti to stop the nutrients from the channel moving into Okawa Bay and other eastern parts of the lake.

The flow would be straight up the western side of the lake, nearer the intake into the Kaituna River. Another possible diversion being investigated was to get Hamurana Springs water away from the lakes.

About 65 tonnes of nitrogen flowed into Lake Rotorua every year from the springs and this would double over the next 80 years or so, Mr Dell said.

"We have to focus on what will make a difference, and we have to look outside the square."

Lake Rotorua remained, by a long way, the largest source of the nutrients in Lake Rotoiti.

Mr Dell confirmed earlier findings that the use of groynes at the Lake Rotorua entrance to Ohau Channel was an unlikely part of the project because they would not keep many organic nutrients out of the channel.

National Institute of Water and Atmospheric Research scientist Max Gibbs said modelling indicated only about 28 tonnes of the 250 tonnes of nitrogen flowing through the channel would be stopped.

The groynes would stop a large body of inorganic sediment going through the channel - about 14,000 tonnes of the 20,000 tonnes going through now every year.

These sediments, largely sand, helped to form the delta which contributed to making Lake Rotoiti such a good fishing area, he said.

Keeping out the sediment might also cause erosion along the channel banks.

- NZPA