The water that runs along the ground or through pipes after a rainfall (or during snowmelt) is known as stormwater. Stormwater picks up sediments, bacteria, nutrients, chemicals, and debris as it runs across lawns, roofs, driveways, parking lots, and residential, commercial, and industrial sites. Laden with a variety of pollutants, stormwater then flows into water bodies and storm sewers that drain into Casco Bay.

Common sources of stormwater runoff in the Casco Bay watershed such as urban development, residential development, construction activities, air deposition, roadways, industrial sites, and agriculture.

There are two primary sources of contaminated stormwater:

1.) point sources conveys stormwater runoff into rivers and the bay through direct, identifiable conveyances such as pipes.

2.) nonpoint sources includes runoff from land or groundwater see page that enters rivers and the bay from diffuse locations such as malfunctioning septic systems, paved areas, feed lots, or manure storage areas. National studies estimate that nonpoint sources of pollution now contribute up to 60 percent of the pollutant load.

While sewage treatment plants are designed to handle a certain amount of flow, as flow increases dramatically during a storm, it can overload the plant. To avoid damage to the sewage treatment plant, a portion of the combined sewage (sewage and stormwater) that would enter the plant is diverted without treatment through relief points known as combined sewer overflows.