How pure is your water?

How pure is your water?

Water when it falls from the sky is as pure as it gets, so why is pollution such a problem, and why do our water authorities need to work so hard to give us safe drinking water?
If the natural order of life is for us to drink naturally pure water to maintain our health and flush toxins from our bodies whilst at the same time carrying nutrients into our cells, why is it that certain “safe” permitted levels of agricultural and industrial pollution are allowed in our drinking water?