Stormwater runoff becomes polluted as it flows over the surface of the land and picks up contaminants. Since it isn’t filtered through the ground the runoff carries these contaminates to waterways. Some contaminates, like pesticides, fertilizers, oil and soap, are harmful in any quantity. Others like bacteria and nutrients from pet waste, grass clippings, and leaves can harm receiving waters if in large quantities. Any surface can accumulate these contaminates including lawns and agricultural land but the biggest concern for the City are impervious surfaces. Impervious surfaces are manmade areas that do not allow any infiltration of water through them to the ground below like roads and buildings. These areas are most likely to have a build-up of contaminates in comparison to lawns or other open space in the City.