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Blue Lake or Rotomairewhenua of New Zealand contains the cleanest natural water in the entire world. It is protected not only by Nelson Lakes National Park but also by a local tribe for its sacred waters.

If you’re anything like me, you probably wonder why and how Rotomairewhenua came to be this way. Although only seven meters deep, you can see straight through it. Students at Lappeenranta University of Technology (LUT) in Finland conducted a study called “New technology purifies waste water by freezing it first: Possible applications in mineral extraction industry” in January of 2015 that helped me connect the dots on this lake.

In previous posts I have mentioned a water treatment method called solar disinfection, which cooks bacteria to a point at which they can no longer survive. If bacteria can die when the weather is too hot, you can image that they would also die off when the weather is too cold. The cleanest lake in the world, Rotomairewhenua, is colder than the average lake. With a temperature of 5-8 degrees Celsius and an altitude of 1200 meters above sea level, this lake lies at the base of the Southern Alps of New Zealand. Since Rotomairewhenua is so cold, it is likely that bacteria may have died off due to the conditions.While I don’t believe that this lake was formed by glaciers, it is a very cold lake with limited pollution from humans.

People aren’t typically permitted to go near the crystal clear water and swimming is not allowed. The study at LUT proved that when a contaminated Lake freezes over, the frozen portion of the surface will provide water ten times cleaner than the water beneath it. In addition, similar to when you release the pressure in a balloon and particles disperse, bacteria that are elevated above sea level are under less pressure. Since Rotomairewhenua is a shallow 7 meters deep, UV radiation from its high altitude and proximity to the sun could be what is causing harmful bacteria to die off quickly. Thus, the lake bottom would act like the parabolic surface in a solar cooker, killing off bacterial growth and leading to a cleaner lake.

For now, the message is that nature has found a way to survive despite pollution and environmental degradation. Nelson Lakes National Park and local New Zealand tribes are truly protecting something amazing. That thing is Rotomairewhenua, which is said to mean “the lake of peaceful lands” according to the Maori people. Here is the link to the LUT study.