What is Stormwater?
The Pavement Effect
Have you ever noticed water flowing down the street when it rains? Have you ever wondered where the water flows to? Have you thought about what’s in the water?
When it rains onto a forest or a field, some of that rain is absorbed by the ground, replenishing groundwater that is used by many for drinking water. Some of the rain is taken up by plants, and some of it simply evaporates. But very little of the rain flows over the ground.
In a more developed setting, such as our cities and towns, rain falls onto pavement, or other surfaces such as roofs, sidewalks, parking lots, and driveways that don’t allow the water to be absorbed by the ground. The water that you see flowing down the street is called stormwater runoff.
It’s Not Just Rain
When stormwater hits the pavement, it picks up and mixes with what’s there.
That might include:
- oil, grease, and automotive fluids;
- fertilizer and pesticides from gardens and homes;
- bacteria from pet waste and improperly maintained septic systems;
- soil from poor construction site management;
- sand from wintertime snow removal;
- soap from car washing;
- debris and litter.
So the water flowing down the street is not just rain; it’s polluted water, and it heads directly to our local water bodies.
Why is it an Issue?
Swimming, And Fishing, And Flooding…Oh My!
How exactly does stormwater pollute? Here are a few ways:
It contributes sediment to water bodies, which can make it difficult or impossible for aquatic plants to grow. This can destroy aquatic habitats for fish and aquatic life.
It contributes excess nutrients (for example, phosphorus and nitrogen) to water bodies,which can cause algae blooms. Algae blooms remove oxygen from the water, which in turn, kills fish and other aquatic life.
It contributes bacteria and other pathogens to water bodies, which can wash into swimming areas and create health hazards. This is usually what has happened which beaches are closed.
It contributes debris to our water bodies. Litter such as plastic bags, six-pack rings, bottles, and cigarette butts wash into lakes, streams, rivers, and the ocean, and can choke or suffocate aquatic life such as ducks, fish, turtles, and birds.
It contributes household hazardous wastes such as insecticides, pesticides, paint, solvents, used motor oil, and other auto fluids to our water bodies. People (and even pets and other animals) can become sick from eating diseased fish and shellfish or ingesting polluted water.
It affects drinking water sources. This, in turn, can affect human health and increase drinking water treatment costs.
Many people assume that stormwater flows down storm drains and then to a treatment facility. Unfortunately, that is almost never the case. Stormwater either flows directly into local waters or down storm drains, which channel it into local water bodies. The polluted runoff closes swimming beaches and fishing grounds, threatens water resources, harms natural areas, and contributes to flooding.
What Is A CSO And Why Isn’t It The Solution?
When the topic of stormwater surfaces, many people think of CSOs, Combined Sewer Overflows. Before we discuss that issue, we need to sort out the difference between storm drains and sanitary sewers:
When rain water flows across pavement and down a storm drain, that water is almost always piped directly to the nearest stream, river, or bay. That water almost never goes to a treatment facility. As the graphic illustrates, most storm drains simply collect rain water and channel it away to prevent flooding, carrying polluted runoff to local water resources.
Sanitary sewers carry wastewater or “sewage” from homes and businesses through an entirely separate piping network below city streets. Manhole covers allow access for maintenance, but there are no open drains or grates. This wastewater flows to a municipal wastewater treatment facility where it is treated, and that treated effluent is discharged to local rivers and the Bay. Today, businesses are required to pre-treat their wastes before discharging into the system. And many wastewater treatment facilities are being upgraded to improve the quality of wastewater discharged to local waters.
The exception to the above are combined sewers, where the storm drain and sanitary sewers have a connection. These are a problem in older urban areas in Pittsburgh, where the stormwater and wastewater lines were originally interconnected. In dry weather, both the stormwater and the sanitary waste go to a treatment facility. With small storms, the system has enough capacity to treat both the wastewater flow and the additional stormwater. But in larger storms, the pipe overflows, resulting in discharge of mixed stormwater and untreated sewage directly into rivers.
However, because most stormwater pollution is not channeled through a combined sewer, we have to tackle this problem through other means!