The water that flows along the ground after a rainfall, or during snowmelt, is known as stormwater. As it crosses roofs, roads, and parking lots, stormwater picks up pollutants like oil, pesticides, bacteria, sediments, and heavy metals.  Those pollutants are then washed into the storm sewers and streams that eventually drain into Casco Bay.  Polluted runoff can degrade water quality in the Bay, impair fish and wildlife habitat, and reduce biological diversity.

In the Casco Bay watershed, most contaminants in stormwater come from  nonpoint sources, such as oil leaking from cars, fertilizers and pesticides washing off lawns, failing septic systems, and fuel spills from recreational boats.  Such seemingly small sources add up to significant water quality problems in urban streams throughout the watershed.


In developed parts of the watershed, impervious surfaces such as roads, parking lots, rooftops, and compacted soils increase the volume of stormwater runoff by preventing rain water from seeping into the soil.  Instead, those surfaces direct pollutant-laden runoff into stormwater drainage systems, which empty into Casco Bay.  High stormwater volumes can also increase flooding, erode stream channels, and reduce groundwater recharge.


When a community’s sanitary waste and stormwater runoff flow in the same underground pipes, the system is called a combined sewer.  During heavy rainfall, stormwater can overwhelm the capacity of such sewers or of sewage treatment plants, causing discharge of untreated sewage mixed with stormwater into Casco Bay waters.  Those discharges are called combined sewer overflows (CSOs), and they result in the introduction of millions of gallons of polluted water to rivers and the Bay annually.  Pathogens from CSOs can lead to human health threats and the closings of beaches and shellfish beds.