"Sediment is the major pollutant in most urban runoff. It usually comes from eroding soil, but vehicle exhaust, soot from chimneys, and industrial emissions also contribute to sediment pollution.

Nutrients, such as phosphorus and nitrogen, are another concern. They promote weed and algae growth in lakes and streams, which in turn depletes a water body of its life-sustaining oxygen. Common phosphorus and nitrogen sources include fertilizer, leaves, grass clippings, and vehicle exhaust.

Oxygen is also depleted by pet waste, leaves, grass clippings, and litter that end up in urban runoff. As organic matter decays in lakes and streams, it uses oxygen that fish and other aquatic life need to survive.

Bacteria is another pollution source in urban runoff. Waste from pets and urban wildlife, as well as overflows from older wastewater systems, are common sources of bacterial contamination. In many cities, bacteria levels in urban stormwater often exceed public health standards for swimming.

Finally, toxic pollutants come from a wide range of sources. These are substances that may cause death, disease, or birth defects. They also may interfere with reproduction, child development, or disease resistance.

Toxic pollutants most often found in water runoff can be traced to automobiles, insecticides, weed killers, wood or oil burning stoves, incinerators, and insulators that once were used in electrical transformers and capacitors."
Source