The pleasure of soaking in a secluded hot spring enveloped by the sights and sounds of nature is an experience that many in Japan love to indulge in. The country is renowned for the abundance and beauty of its onsen, or hot springs.
Many onsen users report gaining a sense of inner peace and connection with nature from such an experience. Yet engineering hot water to flow constantly from the depths of the Earth can potentially endanger fragile natural resources — first and foremost, the coveted hot-spring water sources themselves.
Hot springs are deeply rooted in Japan’s culture and history, with the country’s oldest establishment, Dogo Onsen in Matsuyama, Ehime Prefecture, thought to be up to 3,000 years old. As such, they are important pillars for local economies and the tourism industry.
However, the public and private use of hot springs in modern times has had a significant impact on the surrounding environment, as they aren’t an infinite resource, says Yutaka Seki, executive managing director of the Japan Onsen Association.
“It’s inevitable that the environment will be adversely affected if proper use isn’t taken into consideration,” Seki says.
The postwar tourism boom, combined with technological advances, has led in some cases to excessive excavation being carried out to release hot-spring water from underground, where it’s naturally stored, causing its depletion as well as changes in water quality, Seki says.
The popularity of hot springs is a natural consequence of Japan’s geology. The country’s abundance of volcanoes, whose hot magma fluid heats groundwater, as well as “deep water” and “fossil seawater” hot-spring formations, where water is heated by other types of geothermal energy, contribute to the country’s 27,969 onsen sources.
This impressive number is determined by the Hot Spring Act of 1948, which defines an onsen as water, water vapor or gas (with the exception of natural gases whose main component is hydrocarbon) that contain a prescribed quantity of chemical components or whose temperature is 25 degrees Celsius or more. The legislation also sets guidelines to protect hot-spring sources.
“Architecture is one of the first things we should be thinking about in terms of onsen sustainability,” says Iris Law, author of the book “The Onsen Experience: A Guide to Japan’s Hot Spring Sanctuaries.” However, the sheer variety of hot spring facilities — ranging from naturally occurring pools to large-scale resorts in urban areas — complicates the picture.
To understand whether Japan’s modern-day proliferation of bathing establishments, including thousands of lodgings, is compatible with limited environmental resources, we must begin with water: its journey from the sky to below our feet and then above ground again.
How an onsen springs
The large amounts of rain and snow that fall on the Japanese archipelago every year cyclically replenish its groundwater and, therefore, hot-spring sources.
“For an onsen to be considered sustainable there should be a balance between the input, which is rainwater, and the output, which is the hot spring,” says groundwater hydrologist Makoto Yamada, an associate professor at Ryukoku University in Kyoto.
Pure onsen water, known as gensen kakenagashi in Japanese, is devoid of additives and is used unfiltered, and can flow naturally from the ground or be extracted mechanically.
“Normally, in Japan, it’s dug out from the ground, so it isn’t natural. Wherever you are in the country, you can probably excavate and find hot springs if the conditions are right,” Yamada says.
Therefore, even areas that aren’t traditionally famed as onsen hot spots can boost local economies and tourism by unearthing the precious sources.
“The problem with excavating in the same area is that this could lead to the depletion of groundwater resources,” Yamada says.
Environmental standards are in place to avert this, together with the risk of landslides, soil erosion, toxic gas generation, overflowing water, contamination of public water areas, and those posed by noise and vibrations.
“Plans to excavate must be approved by prefectural governments,” which also monitor the consumption of onsen water to prevent its excessive use, says Takahiro Okano, director of the Environment Ministry’s Office for Conservation and Promotion of Hot Springs.
Before digging, especially if to a depth of over a kilometer, the Japan Onsen Association recommends that a local council be held to evaluate the project.
Overall, hot-spring resource management is a local affair, as the Hot Spring Act doesn’t set specific provisions as to how deep excavations can be carried out or how far onsen water can be transported from its original source — though the longer the journey, the more expensive and therefore unattractive to developers it’s likely to be. The law also stipulates that it’s up to prefectures, municipalities and individual establishments to regulate the amount of water and energy that each onsen consumes.
Oita Prefecture is one of Japan’s main hot-spring areas, with its second-largest city of Beppu claiming to have the highest output of hot-spring water — 87,000 liters per minute — and concentration of hot-spring sources — around 2,300 — in the country.
In order to preserve onsen water, a vital resource for the area, the prefecture has set a limit as to how far it can be transported from its original source and prohibits new excavation in certain areas of Beppu.
“Furthermore, when hot water is pumped using a compressor, it’s capped at 50 liters per minute or less to avoid too much of it from being extracted,” says Kenichiro Nakamura from Beppu City’s Hot Spring Division.
Where water and energy meet
Mechanically extracting and allocating groundwater, as well as operating onsen facilities, not only impacts the landscape and hydrological resources, but requires a stable supply of energy.
“Energy and water are strongly connected. If you use a lot of water, you use a lot of energy,” says hydrologist Makoto Taniguchi, deputy director general of the Research Institute for Humanity and Nature in Kyoto.
Efficient resource use is, therefore, of paramount concern.
A promising development in this sense is the “cascade” model, Taniguchi says, whereby onsen water is used for different purposes at different temperatures.
One application is in agriculture. For example, hot-spring water is used to heat greenhouses and cultivate crops such as shiitake mushrooms and chrysanthemums in Beppu, and bananas in Okuhida Onsen in Takayama, Gifu Prefecture.
Another use is for heating, and even cooling, buildings such as houses as well as onsens themselves. In Matsunoyama Onsen in Tokamachi, Niigata Prefecture, steam and hot water are used to heat buildings and floors and to melt snow — as well as to cook a local specialty, tojibuta pork.
The most significant example of multistage use of hot-spring resources is for geothermal energy, which currently accounts for 0.2% of Japan’s electricity demand, even though it’s thought it could cover up to 10%.
Power generation facilities known as onsen hatsuden, usually small-scale binary power plants that use heat from hot springs, are in operation throughout the country.
Such projects, however, have been met with resistance from onsen owners who fear that sharing hot springs will leave them without the resource sustaining their businesses’ very existence.
“Excessive excavation for power generation may lead to the depletion of hot-spring resources, deterioration of spring quality, frequent earthquakes and worsening impact on forest environments,” Seki says.
Onsen owners have, in some cases, blocked geothermal energy development, says Masaho Adachi, adviser to the Japan Geothermal Association, citing the Oguni geothermal plant in Kumamoto Prefecture, a project marred by opposition from hot-spring operators in Kurokawa Onsen.
In other areas, such as Ebino Kogen in Miyazaki Prefecture and near the Yanaizu-Nishiyama geothermal power plant in Yanaizu, Fukushima Prefecture, geothermal energy projects have been blamed for the depletion of hot-spring resources.
The Japan Geothermal Association denies these allegations.
“There are 19 large-scale geothermal power plants and there’s no evidence of them affecting onsens,” Adachi says.
In some cases, onsen establishments and geothermal plants have been able to co-exist. According to Adachi, energy developers are even helping hot-spring operators solve certain problems by applying geothermal knowledge and techniques.
The Takenaka Okuhida geothermal power plant, which began operating in Takayama in March 2021 with an expected annual power generation capacity of 500 megawatt hours — equivalent to the energy needs of around 100 households — was launched on the back of a collaboration with the Okuhida Onsen Cooperative. And Oita, which has seen a rapid expansion of geothermal development in recent years, is home to Japan’s biggest plant, Hatchobaru, which has a generating capacity of 110 megawatts.
Whether you believe geothermal energy or onsens provide the most benefit to society and the environment depends on your position, Yamada says.
From society’s point of view, geothermal power is a viable renewable energy source (which, however, comes with environmental complications), but if it does indeed limit onsen resources, this may negatively impact local communities.
“The best option is to use geothermal energy and maintain enough resources for onsens, but this can be difficult because in some cases we don’t know how to do that,” Yamada adds.
Where the water goes
After having been extracted and having fulfilled its purpose of soothing onsen-goers’ minds and bodies, hot-spring water is eventually returned to nature. In Japan, it’s either treated through sewers or, when it’s too hot, released directly into the environment.
However, “the amount and impact of such effluents on rivers are currently unknown,” Okano says.
This practice is common in Beppu, where hot-spring water over 45 degrees Celsius is drained into the environment by means of underdrains. Together with other researchers, Yamada and Taniguchi conducted a study, published in 2017, evaluating the impact of hot-spring drainage on the water quality and temperature of six rivers in the area, as well as on the fish populations of two of these rivers.
The study found that in the Hirata River, the most affected by hot-spring drainage in the Beppu area, the Nile tilapia, a foreign invasive fish species, made up more than 80% of total fish biomass. Its proliferation was connected to the influx of onsen water, which increased the river’s temperature and enriched it with minerals and nutrients, therefore increasing the availability of phytoplankton, which Nile tilapia feed on, and creating a better habitat for the species.
The study concludes that the effects of onsen drainage on river and coastal ecosystems remain unclear, as the wider implications of its findings are yet unknown.
“There’s still very little research on the effects of onsen water drainage on living organisms in rivers,” Yamada says.
Another issue concerning hot-spring drainage is that of pollutants. Okano explains that onsen effluents are regulated by the Water Pollution Prevention Act, which establishes legal limits for toxic substances such as arsenic, long-term exposure to which can cause cancer and skin lesions.
According to Taniguchi, hot-spring water in Beppu has been found to contain arsenic levels over the legal limit, as the technology to treat the water and reduce contamination is currently unavailable. In response, the Environment Ministry has granted a moratorium period, which has been extended multiple times.
“Eventually, the water reaches the ocean and is diluted by a huge volume of seawater. Plus, no health issues have been detected in people in the area,” Taniguchi says. “But it still isn’t known what the effects of this arsenic contamination are.”
Beneath the surface
What is clear is that the relationship between onsens and ecosystems is still poorly understood.
While on the surface, regulations are in place to protect hot-spring sources and public safety, “these rules are concerned with setting thresholds for risk, not with what is needed to actively promote biodiversity and environmental well-being,” Taniguchi says.
Ultimately, a better grasp of the complex interaction between hot springs and the environment is needed to evaluate whether or not onsens are sustainable. In fact, hot-spring operators themselves have a vested interest in safeguarding natural landscapes, Law says.
“If the surrounding environment isn’t preserved, water quality is negatively impacted. This is something that many onsen owners understand deeply,” Law says.
Law offers the example of Hoshi Onsen Chojukan in Minakami, Gunma Prefecture, located inside Joshin’etsukogen National Park, whose owners have contributed to studying local flora and fauna for generations. The town itself is part of the Minakami UNESCO Biosphere Reserve, and is home to 150 onsen inns and hotels, says Hirokazu Ono from Minakami’s town office.
Many hot-spring establishments have already embraced some of the core principles of sustainability, including efficient use of water, energy and land resources, embracing low-impact architecture, contributing to nature conservation and serving locally grown food.
“If they learn to promote themselves for these activities, not only does this help their core business but, more importantly, it contributes to educating visitors and the community about the importance of environmental preservation,” Law says.
Ultimately, hot-spring users themselves can play a key role in promoting more responsible forms of hot-spring tourism by asking the right questions in their quest for a more transparent onsen experience.
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