When should you draw the line?

We've recently been discussing Eastern religions in my world geography class, and right now we're talking about Hinduism. The city of Varanasi is the holiest city in India, and the Ganges River, or Ganga ma[which means Mother Ganges], flows through it. Now the Ganges is a very holy river to them, it actually falls down from heaven. So pilgrims to the river bathe in it, drink from it, and bury dead in it. Of course, sewage is also dumped in it.

Now the prompt: When should the line between religion and science be drawn?

Obviously, this water is severly polluted. I think that the fecal matter count is 4,000 times above the accepted count. The people of Varanasi believe that Mother Ganges will clean herself, but some priests are also lobbying for government aid.

One of the arguments in class was, once the water is clean[if we can clean it at all], how do we keep it clean? Someone suggested changing the religion, to stop the people from burying dead and bathing in the water. Now, I don't necessarily agree with that. It's their belief! But that's what the problem is. It is very perpelexing indeed