In recent centuries, cities and towns worldwide have transformed the soil they sit upon. Natural surfaces have been covered or replaced with impermeable paving like asphalt, concrete or tiles, for example. And when the rain comes, the surfaces make it impossible for water to filter through to the soil, producing so-called urban runoff.
On the one hand, it increases the risk of flooding; the soil can no longer absorb the rainwater. On the other, it transports contaminants like microplastics, fertilizers and hydrocarbons to large bodies of water such as rivers, without any prior purification treatment.
Microplastics—particles less than five millimeters small—end up in our oceans, contaminating the water. As the material travels to the ocean, it traps and absorbs various pollutants, dragging them along for the journey. And then it gets to the ocean. Pulled along by the tides, the microplastics can even give invasive species a ride from one place to another.