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Gifu's Hot Spring Museum! Episode 10: Classification of Hot Springs by Water Quality

The quality of hot springs is classified based on temperature, pH, and osmotic pressure as follows.

(1) Classification of Hot Springs by Temperature

The temperature of a hot spring when it gushes out to the surface or when it is collected is called the spring temperature. Hot springs are classified as follows based on spring temperature.
Hot springs are classified as either broad hot springs or cold mineral springs depending on whether the spring temperature is 25°C or above or below 25°C. Hot springs (broad) above 25°C are further classified into low-temperature springs, hot springs (narrow), and high-temperature springs, with boundaries at 34°C and 42°C.

(2) Classification of Hot Springs by Acidity

Hot springs are classified as acidic, weakly acidic, neutral, weakly alkaline, or alkaline based on the hydrogen ion concentration (pH value) at the time of gushing.

Figure 4: Main Hot Springs pH

(3) Classification of Hot Springs by Osmotic Pressure

Whether the components of a hot spring are absorbed into the body when bathing (the transfer of substances inside and outside of living cells) is greatly influenced by the osmotic pressure of the hot spring.
A solution with the same osmotic pressure as human cell fluid (body fluid) is called an isotonic solution, and hot springs are classified based on whether their osmotic pressure is higher, lower, or about the same as that of an isotonic solution.
Hot springs with lower osmotic pressure than isotonic solutions are called hypotonic springs, those with the same osmotic pressure are called isotonic springs, and those with higher osmotic pressure are called hypertonic springs.
These classifications are based on the total amount of dissolved substances (g/kg) or the value of the freezing point.

In the case of therapeutic springs, it is customary to list the classification in parentheses after the spring quality name, as shown in the following example.