Which geological unit is characterized by low or no intrinsic permeability?

The geological unit characterized by low or no intrinsic permeability is the aquitard. Aquitards are layers of material, such as clay or silt, that can store groundwater but do not allow it to flow easily through them due to their low permeability. This means that while they can act as barriers to the movement of water, they do not facilitate significant water flow.

In contrast, an aquifer is a geological formation that has high permeability and can transmit significant amounts of water, making it a vital source for groundwater. An aquiclude is even more restrictive than an aquitard, generally referring to a layer that effectively stops water from passing through, although it may still store some water. An aquifuge, on the other hand, is a geological formation that lacks any capacity to transmit water, possessing virtually no permeability.

Thus, the aquitard's key characteristic is its ability to slow down or restrict water flow, highlighting its importance in groundwater hydrology where it separates aquifers from aquicludes or aquifuges.