Why is Ganges water regarded so sacred? ...

What are the scientific or religious reasons for its water being eternally free from any sort of contamination? Is it a fact that all sins are washed away by a dip in the Ganges?

From time immemorial, Mother Ganges has been associated with sages and saints who
had found immense spiritual benefit by bathing in her waters as well as by drinking it.
Fire can burn all other things, but cannot act upon its own self. Flowing down the
Himalayan ranges which contain many a rare herb that possess disinfectant power to a very high
degree, Mother Ganges contains the various chemicals (found in those herbs) in a nascent
condition. As such other pollutants lose their powers to defile the Ganges when thrown into her
waters. All our religious dogmas are based on sound scientific reason.
Yes, all sins are washed away by a dip in the Ganges when there is a real Bhav in your
mind that you are atoning for your wrong doings.

From "May I answer that" by Swami Sivananda (1987-1963). More Informations on Swami Sivananda: on the Website of Divine Life Society, Photographs of Swami Sivananda, German Pages on Swami Sivananda