In many cases of practical interest the groundwater flow occurs in a layered soil, consisting of permeable water-bearing layers, bounded at their bottom and/or their top surface by layers of very low permeability (clay layers), see figure 3.1. All these layers are more or less horizontal, so that it is reasonable to assume that in the permeable layers (denoted as aquifers) the flow is predominantly horizontal. The less permeable clay layers (sometimes called aquicludes) act as separation layers between the aquifers. When the bounding clay layers are completely impermeable the aquifer is said to be (completely) confined. An aquifer is considered to be semi-confined when the percolation through one or both of the bounding clay layers cannot be completely disregarded but is still small enough to justify the assumption that the flow in the aquifer is mainly horizontal. A third type of aquifer is the so-called unconfined aquifer, in which the upper boundary of the ground water is a free surface. The uppermost aquifer in figure 3.1 might be considered as unconfined. For the three types of aquifers mentioned above some elementary problems are considered in this chapter.
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