What characteristic defines an unconfined aquifer?

An unconfined aquifer is characterized by being in direct contact with the surface and is directly recharged by surface water. This means that rainfall, surface runoff, and other precipitation can seep into the aquifer without any impermeable layer obstructing the infiltration process. Because there are no confining layers above it, the water level in an unconfined aquifer can fluctuate based on surface conditions, such as seasonal changes in rainfall and evaporation rates.

In contrast, a confined aquifer is overlain by an impermeable layer, which restricts the direct flow of water into it and can result in pressure within the aquifer. Additionally, the presence of saline water is not a defining characteristic of unconfined aquifers, as they can contain fresh or saline water depending on the location and environmental conditions.