Groundwater that flows through different sized pores at different rates along flow paths and perpendicular to flow paths is caused by

• A. Advection
• B. Dispersion
• C. Diffusion
• D. Capillary action

Answer: (B)

Groundwater dispersion is driven by heterogeneity in pore sizes that creates a range of local flow speeds. Water moving through larger pores travels faster while water in smaller pores moves slower. As these different flow paths carry water at different rates, the plume spreads along the mean flow and also mixes across neighboring paths, producing spreading perpendicular to the main flow direction. This combination of spreading along and across flow paths is what we call dispersion. Advection is the bulk transport at the average velocity without the spreading, diffusion is the molecular mixing that is typically slower and more limited to small scales, and capillary action concerns vertical rise in tiny pores rather than directional spreading of a flowing groundwater plume.