Aconfined aquifer is found in the unsaturated zone found between two impermeable layers the top layer of the saturated zone separated from the main groundwater

A confined aquifer is found between two impermeable layers.

Explanation:

The roof of this aquifer and its base are made up of permeable rocks. Regardless of its size it will always be full (saturated) with water. In this storage the water pressure at the top of the saturation is much greater than the atmospheric pressure at that same point. By this factor, when opening an artesian well there is a large volume of water being pumped out of the aquifer. In this classification of aquifer, the process of contamination is very slow, but there is a great difficulty to reverse this situation. These confined aquifers have the so-called indirect recharge that are always located in the middle of the sedimentary basins.

oxygen +1 most likely