Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilize, and/or destroy contaminants in the soil and groundwater. There are several different types of phytoremediation mechanisms: Rhizosphere biodegradation. In this process, the plant releases natural substances through its roots that supply nutrients to microorganisms in the soil. The microorganisms enhance biological degradation. Phyto-stabilization. In this process, chemical compounds produced by the plant immobilize contaminants, rather than degrade them. Phyto-accumulation. In this process, plant roots sorb the contaminants along with other nutrients and water. The contaminant mass is not destroyed but ends up in the plant shoots and leaves. This method is used primarily for wastes containing metals. Rhizofiltration. Rhizofiltration is similar to phyto-accumulation, but the plants used for cleanup are raised in greenhouses with their roots in water. As the roots become saturated with contaminants, they are harvested and disposed of. Phyto-volatilization. In this process, plants take up water containing organic contaminants and release the contaminants into the air through their leaves. Phyto-degradation. In this process, plants actually metabolize and destroy contaminants within plant tissues. Hydraulic Control. In this process, trees indirectly remediate by controlling groundwater movement. Trees act as natural pumps when their roots reach down towards the water table and establish a dense root mass that takes up large quantities of water.