Chemical trial bid to improve Lake Okaro water

For immediate release: Friday 17 October 2003 Rotorua’s Lake Okaro will be dosed with chemicals next month in a bid to improve its ailing health.

Environment Bay of Plenty will sprinkle small amounts of a compound called alum (or aluminium sulphate) over the lake in a trial due to begin in November. Scientists hope the alum will absorb phosphorus from the water and improve its deteriorating condition.

Located 28km from Rotorua, Okaro is a small lake of 32ha surrounded by dairy, sheep and beef farms. It is a popular destination in summer for boating, fishing and water skiing.

John McIntosh, the regional council’s manager environmental investigations, says the catchment has not been intensively farmed and hosts only a few houses. However, nutrients like nitrogen and phosphorus have built up in the lake over many years, encouraging the algal growth that affects water quality.

He says the lake’s sediment now holds large amounts of “historic” nutrients that release into the water and promote annual blue--green algal blooms over summer, making the lake unsafe for bathing.

Environment Bay of Plenty’s regulation and monitoring committee received a draft working paper on Lake Okaro catchment’s management at its meeting on Tuesday. It set out several options for improving lake quality, including developing a wetland around one of the two streams feeding into it. A wetland will help filter nutrients from the streams, Mr McIntosh says.

He hopes the alum trial will also have a positive outcome. Alum works by removing phosphorus from the surface waters, increasing the ratio of nitrogen to phosphorus. That encourages the growth of less harmful algae and discourages blue-green algae.

“We hope the alum will improve quality enough for people to safely use the water this summer.” If the trial works, Environment Bay of Plenty is likely to use alum, or a similar substance called Phoslock, every year for at least five years. It will cost about $10,000 annually.

Mr McIntosh says the chemical is not toxic. Its use has been approved by local iwi, landowners, Fish and Game and other key parties.

The committee’s report was developed as part of the Rotorua Lakes Protection and Restoration Action Programme being carried out by Environment Bay of Plenty, Rotorua District Council and the Te Arawa Trust Board. Action Plans are a key component in the programme.

The goal for Lake Okaro is to reduce the nutrient load to the water by up to 0.9 tonnes of nitrogen and 0.02 tonnes of phosphorus annually.

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