What is an aquitard?

An aquitard is defined as a geological formation that restricts the flow of water, effectively limiting the movement of groundwater. This can occur due to the presence of fine-grained materials, such as clay or silt, which have low permeability and do not allow water to pass through easily. As a result, aquitards serve to confine and protect aquifers, the layers of rock or sediment that can transmit significant quantities of water.

When considering the other options, a layer of soil that allows unrestricted water flow refers to an aquifer rather than an aquitard, which contradicts the very definition of an aquitard. The mention of a well that accesses groundwater pertains to the mechanisms for drawing water from aquifers but does not relate to the concept of an aquitard. Lastly, methods to purify groundwater, while important in water management, do not describe any geological features and thus do not pertain to the function of an aquitard. Therefore, the description provided in the correct choice accurately encapsulates the fundamental role of an aquitard in hydrology.