Why is water pollution a concern if water is continuously cycled through Earth’s systems?

Water pollution is still a concern, even though its cycled through the Earth's system continuously, for many reasons. Firstly, people still continue to dump toxic chemicals, and plastic/ garbage into the water. Plastic doesn't decompose, it continues to sit and pollute the ocean unless it is cleaned up. Then there are also the chemicals. Some of these chemicals don't dissipate, or clear out for many years. There is also oil that has been spilt into the ocean. Even though it was many years ago, it still lingers in the ocean, and is very difficult to clean up. Secondly, there are bacteria and algae that lives in the water, which makes it very harmful for us if not cleaned out. The Earth's system is unable to keep these organisms out of the water. Thirdly, there is constantly water, even though water goes through the cycle, there is still unfiltered polluted water that stays, and then pollutes the filtered water. 

felt like this needed a better answer, so here you are. I got 100% on edgenuity 2020 with this.

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%