aquitards

An aquitard is a geological formation with low permeability that slows groundwater flow. It typically lies between aquifers and can act as a confining layer that limits vertical exchange, helping to preserve pressure in confined aquifers. Common aquitard materials include clay, shale, siltstone, and other fine-grained rocks; some mudstones and tight sands can also function as aquitards. Although named to retard flow, aquitards are not completely impermeable; water can pass slowly, especially through fractures or at high hydraulic gradients.

The hydraulic conductivity of aquitards is usually orders of magnitude lower than that of adjacent aquifers, commonly in the range of 10^-9 to 10^-7 m/s, but values vary with clay content, porosity, and fracturing. Thickness can range from a few meters to hundreds of meters, and their presence strongly influences groundwater systems by creating confined or semi-confined conditions and by limiting recharge from the surface.

Aquitards are contrasted with aquifers, which have high permeability and readily transmit water, and with aquicludes, which are effectively impermeable barriers. In hydrogeology, aquitards contribute to concepts such as confined aquifers, perched aquifers, and groundwater residence times, and they play a key role in groundwater resource management and contaminant retardation.