In general, it is said that tea is approximately 98% water. For that reason, what kind of water you use is very crucial. The hardness of water determines the taste of the teas. Of course, when drinking tea it all comes down to whether you like it or not. So some of the things I mention may not be the best taste for you, but it is something that is considered in Japan when making a nice cup of tea. My suggestion is not be that tense about the water but it is good to know that water does to teas. I would like to go through briefly what hard water is and what is the best for Japanese green tea.
What is hard/soft water?
Hard water is a water that has high mineral content, and soft water is a water that has less mineral content. This will depend which region the water comes from. In general, water from Japan is soft and water from Europe are hard. In the states it will depend on which part of the country the water comes from.The water hardness depends on the land form. As water moves through that land’s soil and rock, it dissolves small amount of minerals and carries them around, and this amount of mineral determines the hardness.As I mentioned Japan is said to be soft water(less mineral). Since it is an island that is filled with mountains and hills, due to gravity, water goes through the soil and rock in a very short time. In addition, Japan’s land forms are said to lack in minerals to begin with because the it is in the volcanic areas (volcanic areas does not carry much minerals).On the other hand, Europe’s land has many limestone terrain, which is rich in minerals. In addition, the land is more flat compared to Japan and the water takes more time going through the soils and rocks dissolving abundant minerals.

Is there a clear criteria for hard and soft water?
Since I am not a scientist, I will not get into how it is calculated, if there’s other ways of calculating etc..
Just to say briefly, it is the amount of the calcium and magnesium dissolved in water. In the states, there is a standard that is set by the United States Geological Survey* that breaks down water into four categories. 
1. Soft (0-60mg/L)
2. Moderately hard (61-120mg/L)
3. Hard (121-180mg/L)
4. Very hard (181mg/L or more)
This is the standard currently that is used in Japan too.

Water Hardness Category

The best part of Japanese tea is to enjoy the balance of the deep umami and the little bit of bitterness. The water hardness that brings out this perfect balance of this is said to be from 30 to 80mg/L. If it is too soft, the bitterness will come out too strong. On the other hand, if the water is too hard there will be no bitterness.

So which water should I use?
If you can find out if your region’s tap water is within the 30-80mg/L range and safe, I believe it will be the best to use tap water by boiling it. If not, we will recommend to use bottled water especially for making iced type green teas.
We have made a chart below for reference. It is a hardness comparison chart of bottled water by brands. We were not sure what brands are popular in the states, so we have randomly picked the popular ones in Japan. As a result, I think it is the best to use Crystal Geyser or Volvic for Japanese green tea. I personally use Crystal Geyser when drinking cold teas. For hot water, I always boil tap water, since Japan’s tap water is relevantly soft. 

*Please keep in mind that you may have the same water brand in your country, but it may not be sourced from the same place as the ones sold in Japan. Major water bottle companies some times have several sources and I am sure that when sources changes the hardness will change too. Please keep in mind that hardness may vary in your country although its the same brand.

Water Hardness Comparison Chart
Depending on what region you live, the water will vary, and also what kind of bottled water you can purchase may vary too. If you have time, we would recommend to check out the hardness and look for the right water to get the best taste.I understand that it may be a little too much to get into the details of what water you use, but in Japan it is a quite important criteria when making a tasty cup of tea.Hope you found a whole new perspective about teas.
Thank you for reading:)

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