The Earth's Water Cycle involves Evaporation, Condensation, and Precipitation. Evaporation purifies the water resulting to the replenishment of freshwater to the Earth.
Water pollution is a concern despite of the water being continuously cycled through Earth's systems because the source of pollution is has not been removed.
Water pollution is defined as the contamination of bodies of water. Pollutants are directly or indirectly released to the bodies of water causing the water to be polluted.
Imagine: A river is filled with garbage, it is polluted. The water from the river undergoes the water cycle but since the garbage is still present in the river, the freshwater that comes from precipitation becomes polluted.
I guess it's DNA replication
