Groundwater, stored beneath the Earth’s surface, is a major resource for drinking, agriculture, and industry. This water saturates spaces within different types of rock and sediment layers, rather than existing in underground rivers or lakes. These geological formations determine how much water is stored and how easily it can be accessed. Understanding the differences between these layers is fundamental to managing water supplies. This article defines and differentiates the two primary subsurface formations that manage groundwater flow and storage: the aquifer and the aquitard.
Defining the Aquifer
An aquifer is a saturated geological layer capable of storing and transmitting significant quantities of water to wells and springs. Aquifers are defined by high porosity and high permeability. Porosity is the total volume of empty space within the rock or sediment, determining how much water the layer can hold.
Permeability, quantified by hydraulic conductivity, is the ability of the aquifer to allow water to move through it. Materials like loose sand, gravel, and highly fractured bedrock have high permeability because their pore spaces are large and well-connected. These properties allow the aquifer to readily yield usable amounts of water when a well is drilled. Aquifers are the primary target for human water extraction, serving as a reliable source for municipal and irrigation needs.
Defining the Aquitard
In contrast, an aquitard is a subsurface layer characterized by low permeability, which significantly restricts water flow. While an aquitard may possess high porosity and store a substantial volume of water, the small size and poor connectivity of its pore spaces prevent the water from moving easily. Fine-grained materials like clay and shale often have high porosity but function as aquitards.
The low hydraulic conductivity means that the aquitard cannot transmit or yield water efficiently to a pumping well, even though water may slowly seep through. Its primary function is not as a water source, but rather to retard the vertical movement of groundwater. This geological resistance influences the pressure and movement of water in neighboring layers.
The Fundamental Distinction
The fundamental difference between an aquifer and an aquitard lies in their ability to transmit water, which directly impacts their function as a water resource. This distinction is centered on permeability, which is the measure of how easily water flows through the material. An aquifer has high permeability and high water yield, acting as a productive source of water.
An aquitard, conversely, has low permeability, resulting in a negligible water yield insufficient for human use. The geological material also differs significantly. Effective aquifers are typically composed of coarse, well-sorted materials like sand and gravel. Aquitards are often composed of fine-grained, poorly-connected materials such as clay, silt, or unfractured igneous rock, functioning as a retarding barrier to groundwater flow.
Layering the Earth’s Water System
Aquifers and aquitards rarely exist in isolation; they are often stacked in layers, creating complex hydrogeological systems. The interaction between these layers defines the two main types of aquifers: unconfined and confined. An unconfined aquifer has an upper boundary that is the water table, which is open to the atmosphere and receives direct recharge from the surface.
A confined aquifer, however, is sandwiched between two aquitard layers, which act as confining units. Because these layers isolate the water, the water within a confined aquifer is typically under pressure greater than atmospheric pressure. This pressure can cause water to rise significantly above the top of the aquifer when a well is drilled, a condition known as an artesian system.