An underground layer of permeable rock or sediment that holds and transmits groundwater.
An aquifer is the water-bearing zone a well draws from. In Utah, aquifers range from shallow alluvial gravels in valley bottoms to deep confined sandstone units on the Colorado Plateau. The aquifer's depth, thickness, and permeability decide how deep a well must be, what casing is required, and how many gallons per minute it can sustainably yield.