Crude oil and refined petroleum products can enter the marine environment through tanker accidents, fueling, tank cleaning, bilge pumping, improper disposal or on-land spills into storm drains. Other hazardous substances may also be spilled as a result of transportation, industrial accidents or illegal disposal practices.

Spills of oil and other hazardous materials occur every year in the Bay and its watershed, each with the potential for serious affects on the water quality and marine resources. Spills can directly kill marine organisms through smothering, hypothermia or acute toxicity. The animals and populations that survive may display symptoms of stress or chronic poisoning, such as increased mortality rates or reproductive failures.

Two natural seeps, one about 2.3 miles off Redondo Beach, the other about 4.5 miles off Manhattan Beach, also release oil into the Bay. Daily flows to the surface from these seeps is estimated at 2 to 18 barrels (84 to 756 gallons), but may be several times this amount during and after local earthquakes.

In addition, natural oil from the vicinity of the La Brea Tar Pits also seeps into the Ballona Creek drainage system, contributing to contamination of the Creek and the Bay, especially during periods of low flow (dry weather).