Much of the water that returns to Earth as precipitation runs off the surface of the land, flowing downhill into streams, rivers, ponds and lakes. This water eventually reaches the ocean or large lakes.
Surface runoff is important to aquatic ecosystems, since the water running off the land eventually supplies these systems. Water, though, is not the only thing that runs off the land. Water also carries water pollutants and soil, and both are deposited into aquatic ecosystems.
In some cases, water moving through the ground during infiltration can even pick up pollutants present in the soil. In such an instance, these pollutants are added to the streams and rivers when the water reaches the surface, or remains in the groundwater.
There are many sources of water pollution. Some types of pollution can be traced directly to a particular spot, such as a factory, or industrial plant. These sources of water pollution are easier to control, because the actual point where the pollution is being added to the water can be identified.
Other types of pollution come from more widespread sources and are more difficult to control. For example, fertilizers spread on fields, runoff of livestock wastes from farming operations, soil resulting from erosion, pesticides sprayed on lawns and crops, and materials washed from streets into storm drains come from a variety of sources and are thus difficult to monitor.