The concentration of solute undergoing advection and local dispersion in a random hydraulic conductivity field is analyzed to quantify its variability and dilution. Detailed numerical evaluations of the concentration variance &sgr;c2 are compared to an approximate analytical description, which is based on a characteristic variance residence time (VRT), over which local dispersion destroys concentration fluctuations, and effective dispersion coefficients that quantify solute spreading rates. Key features of the analytical description for a finite size impulse input of solute are (1) initially, the concentration fields become more irregular with time, i.e., coefficient of variation, CV=&sgr;c/⟨c⟩, increases with time (⟨c⟩ being the mean concentration); (2) owing to the action of local dispersion, at large times (t>VRT), &sgr;c2 is a linear combination of ⟨c⟩2 and (∂⟨c⟩/∂xi)2, and the CV decreases with time (at the center, CV≅(N)1/2 VRT/t, N being the macroscopic dimensionality of the plume); (3) at early time, dilution and spreading can be severely disconnected; however, at large time the volume occupied by solute approaches that apparent from its spatial second moments; and (4) in contrast to the advection--local dispersion case, under advection alone, the CV grows unboundedly with time (at the center, CV∝tN/4), and spatial second moment is increasingly disconnected from dilution, as time progresses. The predicted large time evolution of dilution and concentration fluctuation measures is observed in the numerical simulations. ¿ 1998 American Geophysical Union