Aquifer remediation operations are often behind schedule. Usually, a rather sharp concentration decrease shortly after the start of an operation is followed by a situation in which hardly any concentration decrease is observed. Furthermore, the concentration increases after stopping the groundwater recovery. These phenomena are caused by so-called tailing. An important cause of tailing is the phenomenon that equilibrium is not reached for some of the transport and sorption processes involved. To predict these effects of tailing, IWACO has developed a program SORWACO, which describes the behaviour of solutes along a path line. Processes for which equilibrium is reached quickly as well as processes for which equilibrium is reached only slowly are taken into account. The program has been verified against break-through curves observed in column experiments reported in the literature. The program parameters were calibrated using the data of several experiments. The resulting set of parameter values accurately described the transport for different flow velocities. The fact that quite good results can be obtained without a lot of data from a specific site makes the program a valuable tool for the design of remediation operations. The program is especially useful when extensive input data are not available so that detailed three-dimensional or stochastic models cannot be applied. The use of the program is illustrated by means of a case study. The progress was monitored and the data show good correspondence with the predictions of the program.