Paddling away from a polluted paradise

For Rotorua, a city perched on the edge of a sprawling, shallow lake, the issue of how to dispose of sewage has always posed a problem.

In the early 1990s - amid growing concern about algal blooms and Lake Rotorua's declining water clarity - local authorities thought they had the answer.

Treated sewage, which had been pumped into the lake for decades, would instead be fed through a pipeline and sprayed on to 400ha of the Whakarewarewa Forest. All would be well.

But now, 11 years on, scientists have discovered that the problem is still far from solved.

"One of the hopes was that with the sewage inputs being taken away, the quality of the lake would improve," said Professor David Hamilton, an expert on the water quality of lakes.

"That's certainly not happened as fast as anyone would have liked, partly because large volumes of nutrients are tied up in the lake's sediments and partly because lakes are big systems.

"You can't just shut something off one day and all's well very shortly afterwards."

Professor Hamilton has been appointed by Environment Bay of Plenty to investigate how the health of the region's lakes can be restored.

He described Lake Rotorua as being "fairly stable" but still quite sick.

Its condition has improved considerably from the 1970s and 1980s, when the people living around it were unwittingly helping to destroy what attracted people to the area.

Maori settled around Lake Rotorua from about 1350, but the city began to take off only in the 1890s.

Native forest around the lake's edge was milled for timber and cleared for farming, and septic tanks were installed as houses sprang up.

But the combination of farmers using fertilisers on their land, stock living around the lake's edge and a burgeoning population meant more and more nutrients starting flowing into Lake Rotorua.

Excess nitrogen and phosphorous stimulated the growth of phytoplankton and, later, blue-green algae, which formed blooms that are harmful to human health.

Work to reduce nutrient in-flows began in earnest in 1975 when a Government-funded scheme saw most streams and lake frontages fenced to prevent stock access and soil erosion.

It was also planned to pipe human waste directly from Rotorua's sewerage plant into the Kaituna River, bypassing Lake Rotorua and Lake Rotoiti. However, a subsequent Treaty of Waitangi claim scuttled the idea in 1984.

Instead, local authorities turned their attention to a revolutionary proposal which would see treated sewage sprayed on to Whakarewarewa Forest.

Rotorua District Council district engineer Paul Sampson said leading-edge technology was used when the wastewater treatment plant was upgraded in 1992.

"It was the first time anywhere in the country, to my knowledge, that nutrients were stripped out of a major urban area's sewage."

The plan was to remove as much nitrogen and phosphorous from the sewage as possible at the plant before spraying the end product over the forest and wetlands.

Rotorua's soil absorbed the phosphorous and it was thought the trees and plants would take up the nitrogen.

The idea worked - to a certain extent. Nutrients entering the lake from Rotorua's sewage peaked at 183 tonnes a year in 1985 and fell to 30 tonnes by 1999.

But the council has since discovered that nitrogen was not being absorbed by the forest as well as had been hoped.

"The reality is that the trees, once they've grown and got their canopy up, aren't that enamoured about whether they've got nitrogen or not, so it's flowing back into streams and into the lake," Mr Sampson said.

After another re-think, a $500,000 methanol plant is being built alongside the existing operation. It will be used to remove even more nitrogen before spraying occurs.

The council also plans to install reticulated sewerage schemes at settlements around other lakes in the area as communities begin to address the damage that their septic tanks have been doing.

Although Mr Sampson acknowledged that septic tanks were part of the problem, he warned that lakeside sewerage schemes were expensive - they were likely to cost $36 million all up - and would take a long time to deliver noticeable results.

"What we want to do is make sure we get the best bang for our buck," he said.

"If you want your lakes fixed as quickly as possible, then you need to make sure that for every dollar you spend, you get the maximum nutrient reduction."

District council figures show that stopping one tonne of nitrogen from entering the lake costs just over $1 million if pastoral land is retired or converted to some other use such as forestry. To achieve the same result from reticulated sewerage schemes would cost almost $3 million.

"If the public want to spend $36 million, good on them," said Mr Sampson. "But if you take this one-to-three ratio you could spend $10 million and get the same effect."

Individual communities, with the help of local authorities, will decide over the next year or so what options to take to improve their lakes' health.

Mr Sampson said that now people realised the scale of the problem it would become politically unacceptable not to install reticulated sewerage schemes in all settlements.

The debate about who should foot the bill was only just beginning, he said. "But at the end of the day, it's going to be the community that pays."

* This is the second in a five-part Herald series looking at the toxic threat to some of our magnificent lakes.