In this paper large-scale dispersion coefficients for tracer transport in a radially diverging flow field with cylindrical geometry in an unbounded domain are investigated. The effect of small-scale dispersion as well as small-scale diffusion on the large-scale dispersion is analyzed. Macrodispersion coefficients are derived analytically from ensemble-averaged second radial cumulants of the tracer concentration distribution. The cumulants are calculated to second order in the fluctuations of permeability of the heterogeneous porous medium. A macrodispersion coefficient is found, which is proportional to the mean velocity field. It is shown that the macrodispersivity is modified because of the impact of the small-scale diffusion and small-scale dispersion. The vertical small-scale mixing leads to a decrease of the macrodispersivity. Small-scale diffusion makes this decrease time-dependent. Horizontal diffusion, however, leads to an increase of the macrodispersivity. ¿ 2001 American Geophysical Union