NELSON MAIL: Blue Lake a New Zealand treasure

Water in the Blue Lake is the clearest freshwater ever reported.

"This light that does not touch us, does not travel the whole distance, the light that gets lost, gives us the beauty of the world, so much of which is in the colour blue." - Rebecca Solnit, A Field Guide to Getting Lost

There is a Japanese couple who have stayed here two nights in their orange tent. They want to stay longer, but food is running low.

"This place has a very strong spiritual power," the man says. "I feel it."

There are three women eating crackers with cheese, avocado and olives. They drink from plastic tea cups: "It's wine, not tea." They giggle, huddle and gossip.

There is a man and two women who look like they belong to the earth - bronze skin, knotted hair.

All have walked here carrying backpacks, following the West Sabine River, hiking through forests and valleys, up steps formed of dirt and tree roots, over boulders and scree.

There are many trails that lead here, but all take at least a day to traverse.

It is appropriate that one of New Zealand's greatest natural treasures is hard to find.

The Blue Lake - the clearest known freshwater in the world, a diamond in Nelson Lakes National Park's rough - glows blue-violet and nuclear green.

The evening breeze ripples the surface of the water and as the sun sinks below the Mahanga Range - craggy hills that soar above and preside over the lake like sentries - the blue is swallowed by shadow.

It is ethereal. That's one way to describe it.

Most people use the words that people use when words fail them.

Words like, "amazing", "awesome", "stunning", "mind-blowing" and other, ultimately, meaningless superlatives.

Doug Forster, of Christchurch, might have been the most honest man to write in the intentions book at Blue Lake Hut, about 100 metres from the water, on January 7.

In blue pen, he devotes seven letters to the Blue Lake.

"No words."

Important discovery 

Rob Merrilees, a former Niwa hydrologist who now works at the Tasman District Council, is credited as being the first scientist to notice the Blue Lake's clarity and unique coloration during a tramping trip with his wife in December, 2001.

It looked similar to Waikoropupu Springs, near Takaka which, at that time, was the world's clearest known freshwater with an average horizontal visibility of 63m.

"He noticed this little lake looked blue-violet coloured," Niwa principal scientist Dr Rob Davies-Colley says.

"He suddenly thought the colour alone suggests the water is extremely optically pure, almost lacking any organic matter, or algae, or sediment, so it could also be extremely clear."

However, Merrilees waited six years before telling Davies-Colley that he suspected the Blue Lake might be a rival for Waikoropupu Springs.

"I hung off for a long, long time. I was afraid of what would happen if the place became popular," Merrilees says.

Davies-Colley was initially dismissive of Merrilees' observations because, typically, lakes are not as clear as spring water.

"It would have to be a very unusual situation for a lake to be comparably clear, but when he said the blue-violet colour, I thought that might be something unusual."

It wasn't until March, 2009, that the men trekked to the Blue Lake, carrying a large black disc, a tape measure, and an underwater periscope.

Merrilees braved the 6-degrees Celsius water and attached the black disc, 1m in diameter, to a stake driven into the lake bed at wading depth. They extended a 50m tape measure from the disc along the lakeshore, but it wasn't long enough.

At that distance, the disc was easily visible through the periscope. So they borrowed the hut's washing line cord and attached scraps of string and a few boot laces to extend the tape measure to about 65m.

The next morning, when the fine sediment in the lake had re-settled, the disc was visible at about 70m.

"We knew that the water really was a rival for Waikoropupu Springs in terms of exceptional visual clarity and optical purity," Davies-Colley says.

The men returned from the trip and, believing they had discovered something special, applied for funding to conduct more rigorous testing.

Lake of peaceful lands

For centuries, Maori have known that the Blue Lake is special.

Known as Rotomairewhenua, or the lake of peaceful lands, it was traditionally used by Ngati Apa ki te Ra To iwi to cleanse the bones of dead men.

Women's bones were cleansed in Lake Constance, which feeds the Blue Lake via an underground river.

The ceremony released the spirits of the dead on a journey from the Blue Lake along the West Coast and Te Tai Tapu, known as the sacred pathway, to Farewell Spit and, ultimately, Hawaiki, or the afterlife.

The bones were then buried in the Sabine Valley.

The water is taonga, a treasure that links past, present and future generations and provides a path to the hereafter, according to the iwi.

It symbolises the life force that is contained in the natural world and binds the physical and spiritual worlds.

Ngati Apa ki te Ra To cultural manager Kiley Nepia says he felt a spiritual connection with the lake when he visited for the first time last year.

"I really got a sense of why our ancestors chose that particular area to do those types of rituals.

"When you arrive there you just get this real sense of serenity.

"It's an isolated location but you could almost feel the love that they would have carried those types of rituals out in."

Today the Blue Lake is tapu, or sacred.

"Very much like baptismal water, or the blessed water used in church practices, these are holy waters to the people of Ngati Apa ki te Ra To," Nepia says.

Visitors are asked not to wash themselves, their clothing or dishes in the water.

Swimming is allowed, but the temperature keeps most people out.

World's clearest freshwater

A year after their initial testing, Davies-Colley and Merrilees returned to Blue Lake with Niwa oceanographer Mark Gall on March 9, 2010, having secured funding to conduct a series of scientific investigations.

The Conservation Department allowed the use of a helicopter to fly in specialist equipment and an inflatable boat, and Ngati Apa ki te Ra To approved of the study. Following a similar process to their initial tests, the men used a black disc submerged in the water, a survey line between the disc and the boat, and an underwater periscope. 

They visited the lake six times in 2010 and 2011 and the results confirmed their earlier findings. The horizontal visibility was consistently above 70m and the highest reading was 81.4m. The theoretical visibility for pure water is about 80m. It was the breakthrough they had been hoping for.

They had just discovered the clearest known freshwater in the world.

Further testing revealed that the Blue Lake is also clearer than almost all sea water on earth, except for some areas of the South Pacific Gyre around Tahiti and Easter Island. 

"There may well be other systems somewhere in the world that are comparable but there's not very much wriggle room between the clarity of Blue Lake and pure water," Davies-Colley says.

"If there's anything else in the world clearer, it could only be very slightly clearer."

Exactly how the Blue Lake is so clear remains a mystery. The water comes from Lake Constance via an underground river and emerges from springs about 35m above the Blue Lake.

However, Lake Constance, located above the tree line at 1335m, is not nearly as clear. Glacial flour from the surrounding mountains makes Lake Constance appear cloudy, although it contains little organic matter. So the water must undergo a natural filtration process underground, between the two lakes. Scientists believe the water passes through glacial gravel from the last ice age about 12,000 years ago, and landslide debris, which act as a sieve for particles and organic matter.

"That cloudiness, those particles, are pretty much all taken out as the water moves underground," Davies-Colley says.

"There must be something in that material, perhaps glacial silt, that acts as a filter of further particles. We're not sure exactly."

The lake also has a protection mechanism. It flushes every three days, meaning any nutrients or contamination are washed away.

Further funding is needed to study the filtration as well as the biology and ecology of the lake.

No mad rush

The number of visitors to the Blue Lake has been gradually increasing since 2011, the year Niwa's research was made public.

There were 906 people who wrote in the intentions book at the hut last year, compared with 784 in 2011, and this summer has been the busiest yet.

John Wotherspoon, Nelson Lakes conservation services manager for DOC, says the popularity of Te Araroa - a tramping trail from Cape Reinga to Bluff that opened in 2011 and passes by the Blue Lake - was also having an impact on visitor numbers. But there hasn't been a mad rush of people as was feared.

"I think the distance from anywhere is probably its saving grace to a large degree. The fact that it's a solid two days to get up there and a fair altitudinal climb with it, that's enough to push a lot of people, who casually think it's a nice idea to see the clearest water, to not do it."

Restrictions on flying in Nelson Lakes National Park also prevent people from landing helicopters at the lake for things like wedding photos and scuba diving expeditions, Wotherspoon says. Aside from regular visits by hut wardens, controlling weeds and Canadian geese is the only extra work DOC has had to do at the lake. It tends to take care of itself like it has done for centuries.

Wotherspoon says the Blue Lake is the Everest of freshwater.

"It is at the world scale. I think that is really, really special. I just hope it retains that status. I'd be happy to have somewhere fresher, somewhere clearer discovered, but I certainly don't want to see that lake degraded in any form."

It commands respect

The first time I visited the Blue Lake, in March last year, I was tramping with a friend who brought a pair of swimming goggles with him. The idea was that he'd dunk his head under the water to see how clear it was.

I remember the trek from West Sabine Hut, following the violent river. I remember the last painful ascent to Blue Lake Hut at 1190m above sea level. We dried our wet clothes and aired our sleeping bags in the sun.

I remember walking down the path that leads to the lake and seeing the sign that says: "Please respect this place of exceptionally clear water that is sacred to Ngati Apa. Do not wash yourself, your clothes, your dishes or use any soaps or detergents."

I remember seeing the Blue Lake and being moved by its beauty. It commands respect, I wrote in my journal that day.

We stood on the lakeshore and exchanged meaningless superlatives.

The goggles never came out. We didn't even touch the water. Something told us not to. We were quiet for longer than is usually comfortable.

It was a clear, sunny day and the lake glowed blue-violet and nuclear green.

- Story from The Nelson Mail