Groundwater phytotechnology systems are currently being used to control the migration of contaminant plumes and remove groundwater that flows into containment areas. Trees in such systems function by reducing recharge and using groundwater via transpiration. A stand of deep-rooted trees can act as a biological “pump”, removing a substantial amount of water from the saturated zone. Depth to groundwater compared to rooting depth is an important consideration. Many useful tree species are not naturally deep-rooted, but various cultural practices can produce trees with roots that are deep enough to tap into the water table. Hydraulic control can be calculated or modeled using equations that compare transpired water to ground-water flow and take into account site-specific geologic and climatic variables. However, field measurements should be carried out whenever possible to obtain more precise estimates of groundwater capture and control.