An unconfined aquifer is a water-bearing layer whose upper limit is the water table, open to the atmosphere through the soil above it.
An unconfined aquifer has no low-permeability layer above it. Its top is the water table, where the water pressure equals atmospheric pressure. Most shallow dug wells and many borewells draw from this kind of aquifer. When the water table falls, the pores above it actually drain. The volume released per unit area per unit fall in level is the specific yield, commonly 0.01 to 0.30. That is hundreds or thousands of times more than a confined aquifer releases for the same fall, so the same volume of pumping lowers the level far less and the effect spreads more slowly. Rain and irrigation water can reach an unconfined aquifer directly, so it recharges relatively quickly. The same path lets nitrate, pesticides and sewage reach it, which makes it more open to pollution. Its transmissivity also changes with the water level, because the saturated thickness changes. In heavily pumped areas, a falling water table can cut well yields well before the aquifer is empty.