For hundreds of years humans have been adding chemicals and other pollutants to the earths water supply. can earths water ever be clean and pure again? what is clean and pure water?

Pure water is a colorless, odorless liquid with no molecules
except H₂O in it.  You can buy it at the supermarket, or make
it at home if you have the equipment and know how to use it.
At the supermarket, it's sold in gallon jugs, marked "Distilled
Water".

It turns out that pure water tastes bleah, and you would not
want to drink it for very long.

The Earth's water cycle is constantly purifying our water.
When the sun warms the ocean's surface and evaporates
water, what evaporates and rises into the atmosphere is
pure water.  The pollutants don't evaporate.  The clouds,
the rain, and the rivers are self-purifying.  Another example
of a process of nature that makes life possible here.

The junk is collecting in the oceans.  We're lucky that the oceans
have such huge volume that we haven't been able to significantly
pollute that entire layer of the Earth and kill off all sea life yet. 
But we're working on it.