Which mechanism describes water movement that involves bulk movement with the moving water?

• A. Diffusion
• B. Osmosis
• C. Capillary rise
• D. Mass flow

Answer: (D)

Water can move through soil as a moving body, carrying dissolved substances along with it. That bulk movement of the water and its solutes is called mass flow (advective transport). It’s driven by hydraulic gradients—pressure differences and gravity—so the entire water column flows through pores and pushes solutes ahead of it.

This contrasts with diffusion, which is the slow spread of solutes from high to low concentration due to random molecular motion and without a net bulk movement of water. Osmosis is a specific case of water movement across a selectively permeable membrane driven by differences in solute concentration, not the general transport of water through soils. Capillary rise involves water moving upward in tiny pores because of adhesion and cohesion and surface tension; it can cause some bulk motion in those pores, but it’s not the bulk, gravity-driven flow of water carrying solutes through the soil that mass flow describes.