High levels of pollutants and pathogens often exist in cities as an unintended by-product of development. With so many people living so close to the ocean, much of these harmful materials that makes it into the environment eventually gets washed into the sea. Rainwater rinses the land by collecting materials as it flows downhill, picking up loose soil, pebbles and organic material. As water moves it either soaks into the earth or collects at low points such as rivers, lakes, or the ocean. Soil, rocks and plants, both on land and in aquatic environments act as filters. They efficiently trap and remove from stormwater materials that range in size from large leaves and sticks to invisible dissolved metals. San Diego’s coastal wetlands are particularly important both in their trapping ability and their role as the final filter between land and ocean.
Today, however, California has lost over 90% of its coastal wetlands to development. Throughout our cities we have turned soft earth into hard surfaces such as buildings, roads and parking lots. These surfaces block water from soaking into the earth and speed up the rate at which water flows downhill. Development has stripped away many rocks and plants, which can make areas of land unstable. Although the natural environment still has the capacity to filter storm water, there is much less exposed earth available to do it. Humans have also introduced new materials into the environment, including pollutants such as motor oil, pesticides, animal waste, and trash, all of which get swept up in the torrents of water that flow across the land when it rains.
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