Miyagi Prefecture

Katanuma

What is Katanuma? | A highly acidic lake in Naruko Onsen where even fish cannot live [Miyagi Prefecture]

Located in the Naruko Onsen area of ​​Osaki City, Miyagi Prefecture Katanuma a highly acidic crater lake , which is extremely rare even in Japan . The emerald green lake surface captivates visitors. Behind this beauty lies an amazing natural phenomenon that springs from within the earth.

The formation of lagoons born from volcanic activity

Katanuma is a small lake with a circumference of about 1.3 km, located about 1 km southeast of Naruko Onsen Station. It was formed about 1,200 years ago. It is thought to be caldera lake or maar lake

The lake is about 16 to 21 meters deep and is a closed lake with no rivers flowing in from outside. This means that the lake is heavily influenced by volcanic gases and groundwater that well up from underground

A highly acidic lake, rare in the world

Katanuma

The most distinctive feature of lagoons is their extremely high acidity .

In the past, a survey conducted in the 1930s recorded an extreme pH value of 1.4, earning the lake the nickname "Japan's most acidic lake." However, due to environmental changes and changes in water quality over time, multiple recent surveys have reported that pH value is generally between 2.2 and 2.6

Although the pH is in the 2 range, this is a much stronger acidity than typical acid rain (pH 4.5 to 5). It is also highly irritating to human skin, so it is not recommended to come into contact with the lake water

Why is it so acidic?

Volcanic gases (mainly hydrogen sulfide) still constantly bubble up from the bottom of the lake, which oxidize in the water to form sulfate ions (SO₄²⁻), keeping the water highly acidic. Recent surveys have put the sulfate ion concentration in Katanuma at 240-250 mg/L, which is equal to or higher than the acidic springs found in hot spring areas.

In addition, the lake water has high concentrations of iron and chloride ions, making it heavier than it looks and highly corrosive, so caution is required

A lake with a unique ecosystem where fish cannot live?

Naturally, fish cannot survive in such an extreme environment, as it is too acidic and deadly to most living organisms

the midge in the lagoons . These organisms form a unique ecosystem and are the subject of research by scientists.

Color changes and seasonal expressions

Katanuma

Katanuma is known for changing the color of its water depending on the season and weather. On sunny days, it shines a vibrant emerald green, but on cloudy days or during the autumn foliage season, it appears a deep blue-green or grayish color

Katanuma

The lake freezes completely in winter, and in spring the inflow of meltwater causes slight changes in the acidity and turbidity of the water. The lake has a two-layer " thermocline ," and it is interesting to note that the lake exhibits a complex water cycle throughout the year.

Regarding the accuracy of the expression pH2

On the internet and in tourist brochures, the lake is sometimes described as "an extremely acidic lake with a pH of 2," but this is merely an approximation . The latest observations show that the pH range is mainly between 2.2 and 2.6, so strictly speaking, it is somewhat inaccurate to fix the pH at 2.

Even so, the acidity of the lagoons is by far the highest in Japan, and remains extremely rare even on a global scale

Caution and Manners

Currently, you can drive to the lakeside and a walking path has been built, but you should avoid directly touching or entering the lake water. Also, the area is affected by volcanic gases, so you need to be careful when staying there for long periods of time or in bad weather

Katanuma is a precious place where you can experience the mysteries of nature, combining beauty and harshness. Its unique environment teaches us about the geological dynamism of Japan, a country with many volcanoes.

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