The results and preliminary interpretation of two tracer experiments conducted under natural flow conditions are presented. The experiments were conducted in a horizontal fracture at a depth of 10 m in a shale and limestone formation. The transport of a conservative tracer was followed over a distance of as much as 40 m in a 27-borehole array. Observation of the pathways of tracer migration and interpretation of the transport process using a one-dimensional model showed that transverse dispersion and matrix diffusion are likely the predominant processes responsible for spreading and slowing the migration of the solute. Groundwater velocity estimates obtained from the tracer experiments are also compared to estimates obtained using point dilution methods. ¿ American Geophysical Union 1995