In the San Luis Valley, there are two main types of aquifers—confined and unconfined. The confined aquifer is a deep aquifer which is trapped below layers of non-porous material such as clay, locally known as the Blue Clay Layer. The unconfined aquifer sits above these non-porous layers. In areas where the Blue Clay Layer is not found, such as at the Great Sand Dunes and along the mountain fronts, water seeps directly into the confined aquifer. These locations are known as aquifer recharge zones.