Seattle: When it rains, it pollutes

When rain falls in undeveloped areas, the water is absorbed and filtered by soil and plants. But in built-up cities like Seattle, the water washes over roofs, roads and pavements, picking up a cocktail of pollutants on its way: petrol, tyre particles, heavy metals, pesticides and even animal excrement. Most of it ends up passing directly into the Puget Sound, transforming the city's waterway into a toxic home for marine life.