Hot springs that look brown or reddish-brown often get their color because iron dissolved in the water reacts with air and oxidizes into reddish-brown iron hydroxide. Even if the water is nearly colorless right after it emerges, it may darken over time.

What matters most here is that deeper color does not mean stronger benefits. Brown color is simply a visible sign of iron. The spring type and how it affects the body should be judged by the spring name and analysis sheet posted at the facility. In this article, we focus on why the color develops, the range of appearances it can take, why the source water and bath may look different, and what to watch for when bathing.

What Is a Brown Hot Spring?

A brown hot spring is a descriptive term based on appearance, referring to hot springs that range from light brown like tea to dark, reddish-brown tones. It is not an official spring type name, but rather a way to describe the color of the water.

This color is often seen in iron-rich hot springs and leaves a stronger impression than clear water. For travelers, it is an easy-to-recognize feature that gives a hot spring its character, but even if two waters look similar, their mineral combinations can differ from facility to facility. Even among "brown" waters, some are dominated by iron, while others get their color or cloudiness from different ingredients.

Why Does It Turn Brown? The Science of Iron Oxidation

The main reason for the brown color is that iron in the hot spring water reacts with air and oxidizes. In the ground or immediately after emerging, iron often exists as ferrous iron (Fe²⁺), a form that dissolves easily, so the water may remain relatively clear.

However, when exposed to air, ferrous iron reacts with oxygen and becomes ferric iron (Fe³⁺), which then turns into iron hydroxide. Because this iron hydroxide has a reddish-brown to brown color, the water gradually changes color over time. In other words, brown is not the color of "old, dirty water"; it is the visible result of iron oxidation.

Among the medicinal spring categories defined by the Ministry of the Environment's mineral spring analysis guidelines, a spring containing at least a certain amount of total iron (the sum of iron(II) and iron(III)) is classified as an iron spring. Brown water is often seen in such iron springs. For the definition of iron springs and how to use them, see What Is an Iron Spring?. Waters that lean more golden or yellowish-brown rather than red may involve iodine or humic substances in addition to iron; these are covered in Golden and Yellow-Brown Hot Springs.

Brown Water Can Look Different in Each Bath

Even when people say "brown hot spring," the actual color can vary. The amount of iron, how far oxidation has progressed, other dissolved minerals, water volume, and how often the bath is refreshed can all change the appearance, even within the same hot spring area.

The hot spring areas listed above are only examples. Even at the same facility, the color can change depending on the source, bath, or season. A single color does not definitively identify the hot spring.

Why the Source Water and the Bath May Look Different

In the same hot spring, the water may look almost clear at the source but brown in the bath. This can also be explained by the timeline of iron oxidation.

In places with little exposure to air, such as the source outlet or inside pipes, the iron may still remain dissolved as ferrous iron and look nearly clear. Once the water flows into the bath and spends more time in contact with air, oxidation progresses and the water turns reddish-brown. It is easier to understand if you think of the color changing as time passes after exposure to air.

Because of this, a continuously refreshed bath with flowing spring water and a bath that stores water may look different in color and clarity. The idea of continuous flow is explained in What Is Kakenagashi?.

Color and Benefits Are Not the Same Thing

When people see brown water, they often assume that deeper color means stronger minerals and greater benefits. That is not accurate. Color is only the visible result of iron oxidation. What the water does to the body depends on the combination of minerals, temperature, bathing time, and more.

In fact, there are many colorless hot springs that still contain rich minerals. Conversely, some brown waters feel gentle, while others may feel more stimulating depending on the person. Color is a helpful clue, but not the answer itself. If you want to know the spring type or benefits, check the posted information or the analysis sheet instead of relying on appearance. For a basic classification guide, see Hot Spring Types for Beginners.

Points to Keep in Mind When Bathing

With brown hot springs, there are several practical points to watch in addition to their unusual appearance. Iron and oxidized minerals can affect clothing, footing, and whether drinking the water is allowed.

• Staining on white towels and clothing: Water containing iron may leave brown stains on white towels, underwear, swimsuits, and similar items. If color transfer is a concern, do not bring such items and use the facility's towels or darker-colored items.
• Slipping on bath edges and floors: Iron hydroxide and other substances may accumulate on the edges and floor, making them slippery. It is easy to focus on the color and forget to watch your step, so move slowly.
• Only drink spring water when permitted: Even if the water looks rich in minerals, never drink it on your own. Drinking is allowed only at certain sources that have official permission and display signage for drinking. Use it only when there is a sign and follow the facility's instructions.

If you have cuts or scratches on your skin, or if the water is strongly acidic, an iron spring may feel irritating. If you are concerned, start with a short bath and stop if it does not suit you.

FAQ

Why is the source water clear but the bath water brown?

Before iron oxidizes, it exists as dissolved ferrous iron and looks nearly clear. When exposed to air, it oxidizes into ferric iron and iron hydroxide, which gives the water a reddish-brown color. The source outlet has less time in contact with air, so it often stays clearer, while the bath water darkens over time.

Are brown hot springs good for your health?

Color depth and benefit strength are not proportional. Brown color is simply the visible result of iron oxidation, and the water's effects depend on mineral type and amount, temperature, bathing time, and other factors. To understand the benefits, check the spring type name and analysis sheet posted at the facility.

Will brown hot springs stain towels or swimsuits?

Water containing iron may leave brown stains on white towels and clothing. If you are worried about color transfer, it is best not to bring such items and to use the facility's towels or darker-colored items.

Is it okay to drink brown hot spring water?

Regardless of appearance, only spring water with drinking permission and clear drinking signage may be consumed. Avoid drinking water without signage, and follow the facility's instructions only when it is officially allowed.

If it looks brown, does that always mean it is an iron spring?

Brown color is often caused by iron oxidation, but iron is not always the only reason. Other minerals or cloudiness may also be involved, so the exact spring type should be confirmed through the posted information or analysis sheet.

Summary

Brown and reddish-brown hot springs often get their color because dissolved iron reacts with air and oxidizes into reddish-brown iron hydroxide. Water at the source may look nearly clear, while the bath becomes darker over time because of this oxidation timeline.

However, deeper color does not mean stronger benefits. Brown color is only a starting point for understanding the hot spring. To know the spring type and its effects, check the facility's posted information or analysis sheet. If you avoid staining white items, watch your footing, and drink only from permitted sources, you can enjoy brown hot springs more fully while understanding why they look the way they do.

Sources

• Ministry of the Environment, "Mineral Spring Analysis Guidelines"
• Ministry of the Environment, "Classification of Medicinal Spring Types"
• Ministry of the Environment, "Notices and Guidelines Concerning the Hot Springs Act" (including contraindications and bathing precautions)