An unconfined aquifer is a source of underground water that seeps through a permeable layer of dirt or sand directly to the ground surface above. Since the pressure of such aquifers is equal to or less than atmospheric pressure, drilling may not result in increased water levels in the area.
In contrast, confined aquifers are sources directly below impermeable layers of rock or clay that prevent the water from reaching the ground above. Thus drilling often results in higher water levels due to the release of pressure. Since confined aquifers are at less risk of contamination from leaking or spilling, they are better suited for use as drinking water wells.