What is the ruling on performing ablution with tap water that has a strange odor, resembling the smell of sewage, when sometimes no alternative is available?

It is permissible to purify oneself with tap water that has changed in a way that is difficult to avoid, as long as its change is not known to be due to an impurity. If there is doubt about whether the altering agent is pure or impure, the default is that it is not impure. If one is certain that the water is unfit for purification, one should resort to Tayammum (definition) with pure earth.