Aquicludes

An aquiclude is a geological formation or layer that is effectively impermeable to groundwater flow, acting as a barrier to vertical movement of water. By having very low hydraulic conductivity, aquicludes confine adjacent aquifers and help create confined groundwater systems or artesian conditions when groundwater is under pressure.

Most aquicludes consist of materials with low permeability, such as clay, shale, dense sandstone, or evaporites. The key characteristic is the near stoppage of groundwater flow, rather than a specific mineral composition. In practice, the term is sometimes used with variable strictness; some authors reserve aquiclude for layers with negligible flow and use aquitard for layers that retard but do not completely prevent flow. A related term is aquifuge, which denotes a layer that neither stores nor transmits groundwater.

Distinguishing terms in hydrogeology: an aquifer is a permeable unit that stores and transmits groundwater; an aquitard (or confining bed) slows down flow; an aquiclude essentially blocks flow. The presence of an aquiclude can influence groundwater level, hydraulic head distributions, and the movement of contaminants. It also has practical implications for water-supply design, well depth planning, and subsurface engineering, since encountering an aquiclude can create pressure changes and affect borehole stability.

Overall, aquicludes play a crucial role in delineating groundwater basins and shaping the hydrologic behavior of aquifer systems, even though they do not themselves store significant quantities of water.