The main cause for abundant coliform bacteria in the river Ganga is

They are a genus of bacteria that live in the human intestine and, if present in water, can contaminate it.
These germs are released into the water as a result of the disposal of unburned bodies, which also pollutes the water. At most interstate boundaries, the  coliform bacteria  level in the Ganga River is found to be 3 to 12 times higher than the acceptable amount.