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Spatial moment analysis is used in this paper to study the asymptotic, long‐time behavior of the depth‐averaged solute plume for transport in a perfectly stratified aquifer. The solute is assumed to adsorb onto the aquifer solids according to a first‐order reversible kinetic rate law; steady, unidirectional, horizontal flow is assumed with arbitrary vertical variation in pore water velocity, dispersion coefficients, and adsorption reaction parameters. We derive general formulas to calculate the effective dispersion coefficient governing the transport of the depth‐averaged plume. The results demonstrate that overall longitudinal spreading of the plume results from three distinct factors: local Darcy scale longitudinal dispersion, vertical variations in the pore water velocity and retardation factor, and adsorption kinetics. For the example of a two‐layer aquifer, a simple nonequilibrium index is derived which shows that deviations from …