In analyzing groundwater flow and mass transport, most commonly the heterogeneity of a geologic formation is quantified by the field mapping of the hydraulic conductivity distribution. In this paper two alternative approaches are proposed. According to the first nonparametric approach, aquifer heterogeneity is characterized in terms of spatial variability in the concentration field generated in a natural-gradient tracer test with a high-energy gamma-emitting radiotracer. In situ sensing technology is used for mapping the concentration distribution, and collected concentration-versus-depth profiles are correlated to produce the local-scale geometric model of aquifer heterogeneities for further numerical groundwater flow and mass transport modeling. In the second parametric approach, heterogeneity is characterized in terms of spatial variability of the groundwater velocity rather than in terms of spatial variability of the hydraulic conductivity. Geostatistics is used to characterize the local-scale velocity variation in terms of velocity integral scales in three orthogonal directions, in the longitudinal (along the mean flow) and two transverse (across the flow) directions. The velocity correlation volume is estimated as 16.68¿2¿0.22 m. It defines the characteristic volume for the ''field-scale'' groundwater flow and mass transport modeling. It was concluded that the field mapping of aquifer structure by measuring concentrations and travel times of a radiotracer leads to a fundamentally new approach to the characterization of geologic heterogeneity and offers a reliable means for solving this complex problem of contemporary hydrogeology in quantitative terms. ¿ American Geophysical Union 1993