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Positive results from Lake Okaro trial

Positive results from Lake Okaro trial

Wednesday 28 April 2004

Environment Bay of Plenty will treat Lake Okaro with chemicals again this winter as part of a five-year trial to improve water quality.

In December, the regional council injected aluminium sulphate (alum) into Lake Okaro, the Rotorua lake with the lowest water quality. Manager of environmental investigations John McIntosh says the dose was intentionally low – one part per million – so it would not impact on the lake environment. Because of this, water quality is likely to improve slowly over several years. “It is not a one-hit project but a long-term one,” he explains.

Alum ties up phosphorus in the water before settling as a stable complex on the lake floor. Water with a high nitrogen-to-phosphorus ratio generally has better quality. It is the first time alum has been used in the environment in this way in New Zealand.

Mr McIntosh told Environment Bay of Plenty’s regulation and monitoring committee on Tuesday that he felt “quite positive” about the first application of 13 cubic metres of alum. “It knocked phosphorus down to a low level a few months before it would have happened naturally. It created a big change in the nitrogen-phosphorus ratio. Though only temporary, that was our objective. Now we must wait and see.”

In June, when the lower and upper water levels of the lake mix, staff will test for phosphorus again. “Then we will know for sure if the alum is still in the lake or if it has washed out through the outlet.”

Located near Rerewhakaaitu, Lake Okaro is a small basin-like lake with a maximum depth of 15m. For many years, marine life has only been able to survive in the top five metres of the water. Lake Okaro’s catchment is predominantly in pasture. Wetlands will be built to trap runoff from farmland, helping sustain improvements in water quality.


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