Runoff from paved or impervious surfaces, such as roads, parking lots, driveways and rooftops, can contribute large amounts of polluted stormwater. To prevent flooding, parking lots and streets are often lined with storm drains to quickly move stormwater off the pavement. Because storm drains have underground pipes that channel the stormwater directly to a nearby water body, whatever flows down a storm drain comes out in the closest wetland, stream, or pond, with no treatment. The sewer system is completely different. The sewer and drain pipes do not connect. The sewage gets treated at the Lowell Treatment Plant.