On the interpretation of tritium ‘age’ measurements of groundwater

An equation for the transit time of groundwater, based on natural tritium measurements and with consideration of the hydrodynamic dispersion coefficient of the aquifer, is derived. This equation is compared with equations based on the ‘piston flow’ and complete mixing models. Several approximations are given, and their validity is indicated for various values of the nondimensional parameters τ and K. It is shown that the piston flow solution is a satisfactory approximation to this equation under many commonly encountered conditions. The conditions of validity of this solution and its geohydrological significance are discussed.