Advection describes mass transport simply due to the bulk flow of water in which the mass is dissolved, or movement of solute as a consequence of groundwater flow. Advection is the primary process by which solutes move in the groundwater. The direction and rate of transport coincides with that of groundwater. Mass transport takes places at the average linear velocity of the groundwater. The average linear velocity (average pore velocity) is calculated as the Darcy velocity divided by the effective porosity. The effective porosity is the part of the pore space where water is actually flowing. The effective porosity is smaller than the total porosity, as it only includes that void space that forms part of the interconnected flow paths through the medium and excludes void space in isolated or dead-end pores.