The lowest specific yields come from clays, which have small particles and voidspaces with large surface areas. Permeability Permeability describes of how easily water is able to move through rock. Permeability is related to the connectedness of the void spaces and to the grain size of the rock. Obviously, a rock could be extremely porous, but if each pore was isolated from the others, the rock would be impermeable and thus make a lousy aquifer. Often volcanic rocks will have many vessicles, but the vessicles will be isolated, rendering the rock impermeable.
Grain size affects permeability in a mannor similar to the way that it effects specific yield.
The thin film of water that clings to the surface of particles is tenaceous. If voidspaces are large, then additional water can easily move past the water coated particle surfaces . However, if voidspaces are small, then the surface film of water can actually choke the movement of additional water through the small spaces. The voidspaces in clay are small and clogged with pendular water. Hydraulic Conductivity Hydraulic conductivity is a measure of how easily a particular fluid will pass through a particular earth material.