Water cycle is, simply saying, the circulation of water in earth. In fact, the water in the earth's biosphere is used and reused again and again. This is called water cycle or continuous movement of water between the earth and the atmosphere. It involves the following mechanisms:

• Evaporation: changing of water from liquid to gas

• Transpiration: Release of water vapor from plant leaves

• Condensation: changing of vapor to liquid (cooled down)

• Precipitation: Water that returns to the earth (water droplets in clouds become large enough and there comes the rain).

What's the relation of water cycle and pollution? 

According to the water cycle, naturally, water around us will be absorbed to the land (soil) and rivers will stream from the upstream to the downstream and released to the sea.  In normal situation organic pollutants are biodegraded by microbes and converted to a form that brings benefits to the aquatic life. And for the inorganic pollutants, in the same situation, don't bring to much hazards because they are widely dispersed and have almost no effect to the environment which they are released to.

In a small scale, both inorganic and organic pollutants safely decompose throughout the stream, their concentration decrease in the sea, and they don't harm the sea ecosystem and its distribution. But in an excessive scale, communities in beach and estuary will be affected by the pollutants, and can heavily harm them.