Stormwater runoff from the Philadelphia region, whether served by separate stormwater sewers or combined sewers, impairs the streams and rivers of the city.
What is stormwater?
When it rains, some water is absorbed through pervious surfaces—such as vegetated areas with uncompacted soil, sand, or gravel that allow the passage of water. Other water, called stormwater, flows over impervious surfaces—such as rooftops, sidewalks, and streets that obstruct natural infiltration.
Impervious cover exacerbates the problem of stormwater when runoff flows directly into the nearest storm drain without being mitigated. If untreated before entering our waterways (including the Schuylkill and Delaware rivers, which we use as sources of drinking water), this contaminated water can have a detrimental effect on water quality.
The more impervious surfaces there are in the city, the more polluted stormwater enters the sewer system, increasing the total volume of water the city's infrastructure network must handle.
Explanation of different stormwater management systems
Below ground lies a vast network of underground pipes. In Philadelphia, we have two types of sewer systems that - in total - are 3,000 miles long.
In areas with combined sewers, a single pipe carries both stormwater from streets, houses, and businesses as well as waste water from houses and businesses to a water treatment plant.
In areas with separate sewers, one pipe carries stormwater to the city's streams while another carries wastewater to a water treatment plant.
When it rains and the volume of combined stormwater and wastewater is larger than the combined sewer system's capacity, the mixed stormwater and wastewater is discharged into the city's streams at a combined sewer outfall (CSO) before it is treated.
In the separate sewer system, stormwater is not routed to a treatment plant and is discharged directly to a stream. Pollutants picked up as the stormwater ran off the city's impervious surfaces are discharged into the streams.
Problems caused by stormwater
Contaminated water bodies are only one of many interrelated problems affected by stormwater. Stormwater volumes that exceed the sewer system's capacity can cause backups and result in street and basement flooding. Waterways and wetlands are degraded by pollutants in stormwater as natural habitats are destroyed, and biodiversity suffers. Impaired streams do not support healthy aquatic communities, do not meet uses designated by the State, do not serve as amenities to the community, and occasionally cause property damage due to flooding. When our waterways are not as healthy as they can be, we lose out on water-related recreation opportunities.