Water Hardness – The mineral content of hard water is high (as opposed to soft water). When water seeps through limestone formations containing calcium carbonate, magnesium, and other minerals, hard water is generated.
This isn’t the best water for making coffee. Because hard water contains more minerals, it has a lower ability to dissolve coffee than soft water.
The hardness of the water and the process of coffee extraction
Surprisingly, each world location has a completely different view of the relationship between coffee and water quality. Espresso has come to represent the quality of coffee brewed with hard water in Italy, where it is popular.
Soft water is more frequent in Japan, where manual brewing techniques are common. The world’s two major coffee cultures have evolved in response to the quality of their natural water.
Water hardness is determined by various factors, which you may learn more about at Wikipedia.