Viscosity and Density
More specifically, it is the viscosity and density of the fluid that matter. More viscous fluids will flow more slowly through the same rock than less viscous ones. This is important for comparing different fluids (say, oil vs. water – whether you are thinking about an oil reservoir or contamination of groundwater by a gasoline spill). It is also important in considering the effects of temperature, because water viscosity decreases with increasing temperature: it’s less than half as viscous at 90° than at 32° F. So even for the same aquifer, the hydraulic conductivity goes up if it is warmer! This makes some sense – if the water is less viscous (i.e. “thinner”), it will flow more easily through the aquifer.
So…that’s how we define permeability and hydraulic conductivity. But what controls their magnitude? The main factors are grain size and shape, sorting, porosity (degree of compaction or fracture aperture), particle orientation or alignment that affects the tortuosity of the flow path, and cementation. Tortuosity is a measure of how far fluid must go to “circumnavigate” its way around particles: higher tortuosity indicates that water must go farther to get to its destination (a more tortuous path). For all of these mechanisms, the key underlying control on groundwater movement is the viscous resistance resulting from the interaction of the fluid with solid surfaces in the aquifer (grain edges or fracture walls).