Stormwater runoff is the result of rainwater carrying pollution into storm drains. Pollution, such as trash, pesticides, bacteria, oil, and many other harmful substances, builds up on impermeable surfaces. When it rains, the pollution travels with stormwater down storm drains and eventually ends up in our waterways, often without being treated.
Combined sewer overflow:
About one-third of DC’s sewage system is a combined sewer system. This means that both stormwater and sewage are collected in the same piping system. During periods of heavy rainfall, the combined sewer system may exceed its capacity. When this occurs, the excess flow, known as combined sewer overflow (CSO), is discharged into DC’s rivers and streams. The system is designed to do this to prevent homes, businesses, and streets from flooding. There are generally not outflows into rivers and streams during periods of dry weather. The collected sewage is diverted to Blue Plains for treatment. CSOs can negatively affect water quality because CSOs carry litter, high levels of bacteria, and organic materials that lower the dissolved oxygen levels.
Washington DC has a Municipal Separate Storm Sewer System (MS4) permit that requires the DC government to implement practices to reduce stormwater pollution in the areas of the District that have separated sewers. In the MS4 area, stormwater carries pollution and trash off the street into storm drains that discharge directly into the Potomac, Rock Creek, and small tributaries that feed into the river. The stormwater is discharged back into the river without being treated. MS4 can be a major source of trash. DC’s MS4 permit has led to limits on the amounts of trash that can be discharged into the Anacostia River (called a “TMDL”) but not the Potomac River.