An aquitard is a geological layer that significantly impedes the movement of water beneath the Earth’s surface. These layers are fundamental to understanding how water flows and is stored in underground environments.
Defining Characteristics
An aquitard is characterized by its low hydraulic conductivity, which means water moves through it at a very slow rate. These layers are typically composed of fine-grained materials such as clay, shale, or silt. The small size of the pores within these materials, and their poor interconnectedness, create significant resistance to water flow. While an aquitard contains water, often with high porosity, its low permeability prevents it from yielding water freely to wells.
Despite slowing water considerably, an aquitard does not completely block its passage. Water can still seep through an aquitard over time, though the rate is substantially reduced compared to more permeable layers. This partial impedance distinguishes aquitards from other geological formations.
Its Role in Groundwater
Aquitards play a significant role in groundwater systems by acting as confining layers for aquifers. They separate different water-bearing formations, often creating confined aquifers where water is held under pressure. This confinement helps maintain the pressure within the underlying aquifer, influencing how water can be extracted.
These low-permeability layers also protect aquifers from surface contamination. By significantly slowing the downward movement of contaminants, aquitards provide time for natural attenuation processes to occur. This allows pollutants to degrade or dilute before reaching deeper, cleaner water sources.
Aquitards also influence groundwater pressure and flow paths. They can cause a distinct change in hydraulic head across the layer, indicating a zone of lower hydraulic conductivity. This can lead to vertical flow components, even if the primary flow in adjacent aquifers is horizontal.
Aquitards Versus Other Layers
To understand aquitards, it is helpful to distinguish them from other subsurface geological formations like aquifers and aquicludes. An aquifer is a permeable layer of rock or sediment, such as sand or gravel, that can store and transmit significant quantities of water for various uses.
In contrast, an aquiclude is an almost impermeable geological formation that essentially stops water flow. While an aquiclude may contain water due to its porosity, it does not allow water to pass through or yield it readily. Clay is a common example of an aquiclude.
An aquitard sits between these two extremes, restricting groundwater flow but not completely preventing it. It can transmit water, albeit slowly, to or from adjacent aquifers, unlike an aquiclude which acts as a near-absolute barrier.