Chemical tonic for sick lake

By JO-MARIE BROWN

Scientists are preparing to treat one of the central North Island region's sickest lakes with chemicals in a bid to stop toxic blue-green algal blooms appearing over summer.

Lake Okaro, 28km southeast of Rotorua, is one of five lakes in the area with poor water quality because of nitrogen and phosphorus contamination from surrounding land.

Environment Bay of Plenty plans to spray 10 tonnes of alum over the small lake next month to absorb phosphorus.

If it works, blue-green algae should not thrive during summer, posing a health risk.

Environment BoP's manager of environmental investigations, John McIntosh, said alum could be used to treat water in streams which flow into other lakes around Rotorua.

But it was unlikely to be applied directly to large areas such as Lake Rotoiti.

"You'd be looking at a massive treatment."

"The main things we're putting together for Rotoiti is diverting the Ohau channel and oxygenating the main body of the lake," he said.

Alum is a compound of aluminium and sulphate, and is used in ponds and water treatment plants to remove fine suspended particles.

Mr McIntosh did not believe it had been added to a New Zealand lake before, although it has been used in similar situations in the United States and Canada.

He said studies there had looked at the effect on sediment after about 10 years of applying alum, "and they can't detect any adverse effects".

Mr McIntosh said alum was a liquid compound that would lock in phosphorus, fall to the bottom of the lake and work its way into the sediments.

"Whether we will still get the blue-green algal blooms, I'm not sure," he said.

"At Okaro, we get massive nutrient releases from the sediment.

"I can't see that we're going to stop that so we'd be looking at an annual application for quite some time."

The alum trial would cost about $10,000. Mr McIntosh said other measures, such as creating wetlands and retiring land around the edge of Lake Okaro, would also be used.

"There are also other products and we are looking at trialling them, but alum is the most widely used," he said.

* Environment Waikato is cracking down on commercial boat owners who are discharging sewage into Lake Taupo, reports NZPA. It will serve offending boaties with an abatement notice forcing them to permanently disconnect their waste discharge system, under a rule in the Resource Management Act.