Pollution can come from point sources, or single identifiable sources, such as factories or wastewater treatment facilities. Factories, oil refineries and electronic manufacturers typically release one or more pollutants into water they discharge (called effluents). Some factories discharge their effluents directly into a water body. Others treat it themselves before it is released, and still others send waste to sewage treatment plants for handling. Sewage treatment plants treat human wastes and send the treated effluent to a stream or river and ultimately to the ocean.

Polluted runoff (also known as nonpoint source pollution) comprises the majority of pollution in the ocean off California. When it rains or when you water your lawn or you wash your car, water picks up pollutants and carries them into our storm drains. Unlike the sewer system, most cities do not have treatment plants, or even filters, for this runoff. As a result, the pollutants found on streets, parking lots, buildings, yards and golf courses ultimately enter creeks and rivers, and eventually flow into the ocean. The kinds of pollutants found in runoff include motor oil, trash, pet waste, fertilizers, pesticides and dirt. These pollutants harm marine life, endanger human health and lead to costly beach closures.

The four categories of polluted runoff or nonpoint source pollution that enter the coastal ocean are domestic, industrial, agricultural and marine.

Oil spills can cause enormous damage to the marine environment and species, human health, coastal tourism, the fishing industry and recreational opportunities. For example, the April 2010 Deepwater Horizon oil spill was the largest accidental marine oil spill in the history of the petroleum industry, releasing over 4.9 million barrels of oil. The spill area affected 8,332 species, including more than 1,200 fish, 200 birds, 1,400 mollusks, 1,500 crustaceans, four sea turtles and 29 marine mammals. Despite the impacts of oil spills, they are responsible for only approximately 12 percent of the oil entering the ocean each year. According to a 2003 report by the U.S. National Research Council, seventy percent of oil entering the ocean from humans results from consumer uses, largely runoff related to the transportation sector.