Transport of solutes in groundwater is decisively influenced by the heterogeneity of the aquifer. The goal of the present work is the numerical generation of synthetic heterogeneous aquifer models based on a statistical description of heterogeneities in gravel aquifers. Large unweathered outcrops in several gravel pits in northeastern Switzerland were investigated in this context. On the basis of sedimentological observations it was possible to specify distinct sedimentary structures appearing as lenses or layers. All structures have been identified in each of the investigated outcrops. By inspecting and analyzing photographs of the outcrops, it was possible to estimate the probability density functions (pdf) of the geometrical attributes of the sedimentary structures. Disturbed and undisturbed samples were taken from these structures to estimate the pdf of their hydraulic properties, that is, the hydraulic conductivity and the porosity. The information obtained is used to generate distinct numerical realizations by unconditional stochastic simulation of synthetic aquifers having the same statistical properties with respect to hydraulic conductivity and porosity as the investigated deposits. ¿ American Geophysical Union 1994