A number of factors may contribute to poor water quality. Some of the main culprits are runoff of fertilizers and pesticides from yards, oil and grease from cars, and sediment from construction sites. Trash, run off of hazardous materials, dumping of waste, and spills are other pollutants that degrade water quality. Natural occurrences such as algae blooms, although often triggered by pollution, may also contribute to a decline in water quality.