A large-scale natural gradient dispersion experiment was carried out in a sandy aquifer in the western part of Denmark using tritium and chloride as tracers. For both plumes a marked spreading was observed in the longitudinal direction while the spreading in the transverse horizontal and transverse vertical directions was very small. The horizontal transport parameters of the advection-dispersion equation were investigated by applying an optimization model to observed breakthrough curves of tritium representing depth averaged concentrations. No clear trend in dispersion parameters with travel distance for distances between 50 and 200 m could be found, suggesting that the asymptotic stage was reached within a short distance from the point of injection. A three-dimensional numerical model for flow and transport was applied to the aquifer in order to quantify the dispersivity parameters more closely. The following ''best fit'' dispersivity parameters were identified: longitudinal horizontal, 0.45 m; transverse horizontal, 0.001 m; and transverse vertical, 0.0005 m. ¿ American Geophysical Union 1993