Free lake treatment trial turned down in Rotorua

By JO-MARIE BROWN


One of Rotorua's sickest lakes has been chemically treated in a bid to stop algal blooms from flourishing, but an Auckland company says their product should have been tried first.

Almost 10 tonnes of liquid alum was sprayed over Lake Okaro, 28km southeast of Rotorua, by Environment Bay of Plenty last week.

Potentially toxic algal blooms often form in the small lake because too much nitrogen and phosphorus has seeped into the water from surrounding farmland and septic tanks.

Alum absorbs phosphorus and the regional council hopes the $10,000 trial will restrict algal growth this summer.

But an Auckland company, who have developed their own treatment product, say they are disappointed Environment BoP did not first take up their offer to carry out a free trial on Lake Okaro.

Lakes Restoration Ltd spokesman Neville Johnson said the company had wanted to try using its mineral-based mixture, called ammab, after laboratory tests using water samples from Lake Okaro showed positive results.

The product mainly targeted nitrogen but Mr Johnson was confident it would have also eliminated enough phosphorus to prevent algal blooms from forming this summer.

"It would not have had any detrimental effect on the lake and [if it didn't work] it would have been our financial loss," he said.

Environment BoP's manager of environmental investigations, John McIntosh, said numerous companies had contacted the council with offers to help fix Rotorua's lakes over the past few months and all ideas were considered.

"We haven't ignored any of them.

"We are collating all of them and we will assess them with respect to different lakes if they are appropriate."

Mr McIntosh said Environment BoP had already invested a lot of time and effort to obtain resource consent for alum, and testing a new product would have forced them to repeat that process.

"It also just wasn't the right product for that type of job on Okaro," he said.

Results from the alum trial will take several months to analyse and confirm.

"There's been a lot of work done and nothing untoward happened [when it was applied] so we're very hopeful," said Mr McIntosh.

If the use of alum proved successful, it could be used to treat stream water and other sick lakes around the Rotorua district.

"It's a possibility wherever phosphorus in the water is an issue," Mr McIntosh said.

It is believed that the chemical compound has never been used to treat a New Zealand lake before, although it has been used in similar situations overseas.