Tapi...saja gatal nak check alQuran, and here what I found :
Characteristics of the Rain water
Now lets reckon some of the characteristics associated with the water descending from the sky i.e.: rainwater.The rainwater is considered exactly as distilled water, resulting from the vaporization of the water from the seas and its condensation in the form of clouds then it descending as rain . Therefore it is thoroughly pure water. The rainwater can remove the dirt on the human skin more than the regular water, thus this water is considered a disinfectant used in medicine .It is free from the viruses and bacteria, also possessing the feature of the absorption of metals, gases and dust particles and any substance that comes into contact with it on a large scale, therefore it is an antiseptic to the atmosphere as well.
After knowing these attributes we find that they meet accurately the single worded expression in the Qur'an about the rainwater, where Allah says: (And We send down from the sky pure water) The criterion, 48. The word "Tahura" (purified) in the language means the removal of dirt and impurities and the abstinence from it as stated in the dictionary.
The Fresh (Sweet) Water
It is noticeable that the Quran differentiates between the word "Tahur"/pure and "Furat"/sweet in its verses for Allah says: (and have given you to drink sweet water.) Al-Mursalat, 27. The water we drink from the rivers, springs and wells is acceptable and taste sweet because it contains quantity of the minerals like iron that causing the water to taste sweet. This definition suits the expression "Furatan"/sweet. The term "Furat"/sweet water in the language is the water with acceptable taste as described in the linguistic dictionaries .While the descending water from the sky is distilled water which possesses the characteristics of sterilization and cleansing and carries not taste! Therefore the Divine described it as "Tahura". So is there repetition in the Quran or is it a miracle?
The water as it descends from the sky is pure but when it mixes with the minerals and salts present in the land and become "Furatan"/sweet. Even though, the Qur'an speaks about the river waters, we find it using the term "Furatan"/sweet and not "Tahuran"/pure.
- extracted from 'Kinds of Holy Water in Holy Quran (http://kaheel7.com/eng/index.php/earth-science/77-kinds-of-water-in-holy-quran)Sorry friend. Yes, there's a risk of our water contaminated with unsafe pollutants. For now, I'll take my chances with the municipal water and their adherence to WHO drinking water standards. In future? Who knows. Wallahu'alam.