Stormwater runoff can pick up debris, chemicals, dirt and other pollutants during a rainfall or snowmelt event. The runoff can flow into a storm sewer or directly to a lake, stream, river or wetland. Anything that enters a storm sewer system is discharged untreated into nearby surface waters.
Polluted stormwater runoff can have many adverse effects on plants, fish, animals and people. Sediment can cloud the water and make it difficult for aquatic plants and habitat to survive. Excess nutrients can cause algae blooms. When algae die they sink to the bottom and decompose in a process that removes oxygen from the water. Fish and other aquatic organisms cannot exist in water with low dissolved oxygen levels.
Household hazardous wastes like insecticides, pesticides, paint, solvents, used motor oil and automotive fluids can poison aquatic life. Surface water pollution in these forms can pose a health hazard to animals and people.
Polluted stormwater often affects drinking water sources such as ground water or other surface water features from which the potable supply is drawn. This, in turn, can affect human health and increase drinking water treatment costs.