Water is produced by nature. In clouds, the accumulated moisture is soft and clean.
But once it starts to fall as rain or snow, it washes the air and can pick up impurities in the atmosphere. This is the cause of the acid rain problem we have today.
When it reaches the ground, it can come in contact with additional impurities. Then it percolates into the earth; it may dissolve various elements which add calcium, magnesium, iron, etc. into the water.
The water continues its journey until it enters your home.
Then, after you use it, the water evaporates and begins the cycle all over again.