how dirty does water have to be to be considered polluted?

Answer 1

Dirt is not considered "pollution". Water can be very dirty and still safe to drink. Pollution is the introduction of chemicals or organics that make the water unsafe for human contact.

Answer 2

In the case of streams, the EPA maintains a list of polluted waters. They are determined by applying the EPA's list of clean water parameters to the known water chemistry of the stream in question. If the parameters are exceeded the stream is considered to be polluted.