aquiclude

An aquiclude is a geological formation that is sufficiently impermeable to groundwater to act as a barrier to vertical water movement. It is part of a layered hydrogeologic system and can separate aquifers or confine an aquifer beneath it, influencing groundwater flow and storage.

In hydrogeology, aquifers are permeable layers that readily transmit and store groundwater, while aquicludes transmit negligible amounts of water under typical hydraulic conditions. A layer that restricts flow but is not completely impermeable is called an aquitard. A layer that is effectively impermeable and forms a true barrier is sometimes referred to as an aquifuge or absolute caprock.

Aquicludes typically consist of clay-rich materials such as claystone or shale, or dense, unfractured rocks like certain limestones, granites, or metamorphic rocks. Their low permeability results from fine grain size, low porosity, or limited interconnected pore spaces. Nonetheless, fractures or faults can create leakage pathways, turning an otherwise effective aquiclude into a partial barrier in some locations.

The presence of an aquiclude has important implications for groundwater systems. It can confine aquifers, influence the direction and rate of groundwater flow, protect aquifers from surface contamination, and affect well placement, yield, and hydrogeologic modeling. Understanding the properties and continuity of aquicludes is essential in water-resource engineering and environmental assessments.