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

Located in the Naruko Onsenkyo hot spring area of ​​Osaki City, Miyagi Prefecture, "Katanuma" is known as one of Japan's rarest highlyacidic crater lakes. The emerald green surface of the lake captivates visitors, but behind its beauty lies an astonishing natural phenomenon emanating from deep within the Earth.

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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 1200 years ago. Itcaldera lake" or "maar lakeis thought to be a

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

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A highly acidic lake, rare in the world

The most distinctive feature of Lake Katanuma is itsextremely high acidity.

In the past, surveys in the 1930s recorded an extremely low pH of 1.4, earning it the title of "Japan's most acidic lake." However, recent studies have reported that due to environmental changes and changes in water quality over time, the currentpHis generally fluctuating 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) continue to seep from the lakebed, which oxidize in the water toform sulfate ions (SO₄²⁻), keeping the water highly acidic. Recent surveys have shown that the sulfate ion concentration in Lake Katanumais 240-250 mg/L, which is equivalent to or higher than the levels found in acidic hot springs 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

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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

Nevertheless, it has been discovered that Lake Katanuma is home to acid-tolerant insects such as the Japanese midge (Chironomus sibiricus) and special bacteria that thrive in acidic environments. These organisms form a unique ecosystem and are the subject of research for scientists.

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Color changes and seasonal expressions

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

Furthermore, the lake freezes completely in winter, and the influx of snowmelt in spring can slightly alter the acidity and turbidity of the water. The lake also exhibits an interesting feature: it has a two-layered "thermoclimate," resulting in a complex water circulation throughout the year.

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Regarding the accuracy of the expression pH2

While it is sometimes described on the internet and in tourist brochures as a "highly acidic lake with a pH of 2," this is merelyan approximate description. Recent observations show that the main pH range is 2.2 to 2.6, so fixing it as "pH 2" is somewhat inaccurate in strictly speaking.

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

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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

Lake Katanumais a precious place where you can experience the mystery of nature, which combines beauty and harshness. Its unique environment teaches us about the geological dynamism of Japan, a country with many volcanoes.

Katanuma <Information>

• Name: Katanuma
• Address: Naruko Onsen Yumoto, Osaki City, Miyagi Prefecture, 989-6823
• Phone number:
• Official URL:Osaki City Official Website – Katanuma

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