Urban runoff is also called surface runoff. It is in reference to rainwater or melting snow that is unable to infiltrate the naturally porous ground due to concrete surfaces. The water rushes down city streets (or over parking lots, etc.) collecting pollutants such as spilt gasoline, motor oil, heavy metals, pet waste, and litter (such as cigarette butts). This can then flow directly into the storm sewer system carrying all the accumulated pollutants with it. Most cities' (including Toronto's) storm sewers discharge directly into the nearest watercourse or lake. There is often no treatment of this water. Everything that gets picked up on the surface ends up in our rivers and lakes.
The pollutants cause considerable damage to the biological health of the stream or lake system, including any wildlife that depends on the water for survival. For example, small amounts of detergent can remove the protective coating on the skin of fish, or even kill fish eggs. A common herbicides can also wreak havoc in the river and stream ecosystems.
Outside of the city or in green spaces runoff has a chance to infiltrate the ground. In these cases, the water gets filtered through the soil, trapping harmful chemicals and nutrients from the water into the soil. That is why city parks are great places for rainwater capture, provided they are clean of pesticides and animal waste.
New solutions are constantly being invented to combat the challenge of urban runoff. Stormwater collection ponds are in place in new residential areas. These ponds are specially created to capture water during heavy flow, allowing pollutants to settle out of the water before the water is released back into the natural environment. The collection ponds also reduce river erosion caused by high velocity water flow beating away the banks in times of storms. Another new mitigation measure for urban runoff is the creation of green lots. These allow water to seep through the seemingly hard surface instead of water running across the pavement into a sewer. The water can percolate through the porous surface and absorb right into the ground under the parking lot. This allows pollutants to be caught in the soil and to reduce the amount of sewer flow in peak times.