National Geographic and NPR did a study few years ago :
"Hollick speaks with DS Bhargava, a retired professor of hydrology, who has spent a lifetime performing experiments up and down Ganges in the plains of India. In most rivers, Bhargava says, organic material usually exhausts a river's available oxygen and starts putrefying. But in the Ganges, an unknown substance, or "X factor" that Indians refer to as a "disinfectant," acts on organic materials and bacteria and kills them. Bhargava says that the Ganges' self-purifying quality leads to oxygen levels 25 times higher than any other river in the world."Hollick's search for a scientific explanation for the X factor leads him to a spiritual leader at an ashram and a biologist in Kanpur. But his best answer for the Ganges' mysterious substance comes from Jay Ramachandran, a molecular biologist and entrepreneur in Bangalore.