What is Polluted Runoff?
Runoff is water that falls in the form of snow or rain and instead of sinking into the ground, runs off the land to a lower spot. If that water comes into contact with dirt or other contaminants along the way, it becomes polluted.
Runoff can be polluted in both rural and urban areas. Sources of polluted runoff in rural areas include farm fields, barnyards, streambanks and other places where the land is not paved. When runoff water carries soil, nutrients, pesticides or other contaminates with it, it has the chance to pollute other waterbodies that are downhill from the source.
Imagine a farm field on a summer day when a summer shower passes. If the field is flat, most of the water will sink into the ground, but if there is some slope to the land or if the ground is already saturated, some water will flow into the ditch. The water will carry small soil particles with it from the upstream field into a ditch. The ditch flows into a small stream which in turn flows into a larger stream. Some of the soil may drop out along the way, but some of it may be flushed farther and farther downstream, eventually leading to one of the Yahara Lakes.
The soil that travels in the water may contain phosphorus and nitrogen—nutrients that are essential to the growth of healthy crops. Excess nutrients not used by the crops may end up in one of our lakes and instead feed lake weeds and algae.
Polluted runoff from rural sources is a serious problem in our lakes, and may be responsible for as much as 90% of the pollution we see in the form of weeds and algae.