When water is stored on land, less water enters the stream during a storm. The stored water will be used by plants and will refill the ground water supply. Tree and plant stems slow the flow of surface water, allowing sediment to drop out. Bacteria in soil eat hydrocarbons such as oil and grease. Heavy metals bind to organic matter, such as leaves, and do not enter the water.
Tree roots drink the water stored in the soil.
They absorb heavy metals from the water and store them in plant tissues. N and P are the ingredients in fertilizer, and do great damage to the Chesapeake Bay. N and P pollution comes from agricultural areas, golfcourses, lawns and other landscaped areas when more fertilizer is used than is necessary for the plants. Tree roots prevent soil erosion. The mesh of deep roots holds onto the soil so that the stream waters do not wash it away.