]> A condition in which the upper surface of the zone of saturation forms a water table under atmospheric pressure. A boundary between the saturated flow field and the atmosphere along which groundwater discharges, either by evaporation or movement "downhill" along the land surface or in a well as a thin film in response to the force of gravity. A condition in which the upper surface of the zone of saturation forms a water table under atmospheric pressure. The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. A sustainable amount of water within a unit of sediment or rock, below the water table, in the phreatic zone is called an aquifer. The ability of the aquifer to store groundwater is dependent on the primary and secondary porosity and permeability The process or processes that cause the time required for a given radionuclide to move between two locations to be greater than the groundwater travel time, because of physical and chemical interactions between the radionuclide and the geohydrologic unit through which the radionuclide travels. The film of water left around each grain or fracture surface of water-bearing material after gravity drainage (after APHA, 1981). Water of adhesion; (after APHA, 1981). Water that can be extracted by root absorption and evaporation but cannot be moved by gravity or by the unbalanced film forces resulting from localized evaporation and transpiration (after APHA, 1981). Groundwater separated from an underlying body of groundwater by an unsaturated zone (ASCE, 1985). Unconfined groundwater separated from an underlying body of ground water by an unsaturated zone. Its water table is a perched water table. Perched groundwater is held up by a perching bed whose permeability is so low that water percolating downward through it is not able to bring water in the underlying unsaturated zone above atmospheric pressure. The density of a unit of water after it is raised by an adiabatic process to the surface, i.e., determined from in-situ salinity and potential temperature (AGI, 1980). Density that would be reached by a compressible fluid if it were adiabatically compressed or expanded to a standard pressure. An imaginary surface representing the static head of groundwater and defined by the level to which water will rise in a tightly cased well. The ratio of the average linear velocity of groundwater to the velocity of the retarded constituent at C/Co=0.5. The rate of discharge of water from the well divided by the drawdown of the water level within the well. The rate of discharge of groundwater per unit area of a porous medium measured at right angle to the direction of flow. The ratio of the Volume of water which the porous medium, after being saturated, will retain against the pull of gravity to the volume of the porous medium. The volume of water released from or taken into storage per unit volume of the porous medium per unit change in head. The ratio of the volume of water which the porous medium after being saturated, will yield by gravity to the volume of the porous medium. The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head (virtually equal to the specific yield in an unconfined aquifer). The sum of the energy-related components of a soil-water system; i.e., the sum of the gravitational, matric, and osmotic components. A devise used to measure groundwater pressure head at a point in the subsurface. A device used to measure the moisture tension in the unsaturated zone.