The USGS Water Science School
Many sewer lines are constructed next to streams to take advantage of the continuous, gradual slopes of stream valleys. Blockages, inadequate carrying capacity, leaking pipes, and power outages at pumping stations often lead to sewage overflows into nearby streams. There are three types of sewer systems:
This picture of a sanitary sewage overflow illustrates a common problem concerning sewage overflows that occur in urban areas. This overflow happened in Sept. 2009 during historic flooding in Atlanta, Georgia, USA. Sanitary sewer overflows occur when sewer pipes clog or pumping stations break down. As shown here, mixed sewage and rainfall runoff overflows from manholes and leaking pipes into nearby streams rather than backing up into homes and businesses.
Combined sewer overflows occur during storms when there is more stormwater flowing than the pipes leading to a treatment plant can handle. The excess runoff flushes human and industrial wastes, oil, toxic metals, pesticides, and litter into streams.
Effects of urbanization Groundwater quality Wastewater-treatment water use