[en] A tracer breakthrough curve (BTC) usually has a tailing effect that cannot be explained by the conventional dispersion-advection model. In this study, the chemical, biological and physical mechanisms for the BTC tailing are discussed. Models developed in the recent literature to solve the BTC tailing effect were reviewed and compared. A general group transfer model developed herein extends the conventional dispersion-advection model to include concepts of mobile-immobile pores and nonequilibrium adsorption-desorption in nonhomogeneous media. This general group transfer model was combined with a nonlinear regression program to identify effective hydrogeologic parameters from experimental BTCs obtained in both homogeneous and nonhomogeneous media. (Auth.)