Advection is the transport of dissolved solute mass present in groundwater due to the bulk flow (movement) of that groundwater. Advection alone (with no dispersion or reactive processes occurring) would cause a non-reactive solute to advect (move) at the mean groundwater pore velocity. All solutes undergo advection, however, reactive solutes are subject to influences by other processes detailed below. Molecular diffusion is the movement of solute ions in the direction of the con- centration gradient from high towards low concentrations. It effects all solutes. Mechanical dispersion causes spreading of solute and hence dilution of concentrations, it arises from: the tortuosity of.
