Many shales and clays are aquicludes. Aquicludes contain water, but the water cannot flow fast enough through the aquicludes to be extracted for human use. If a well is placed in an aquiclude, it will rapidly run dry because groundwater cannot move quickly enough to keep the well casing filled. Confined and Unconfined Aquifers Unconfined aquifers have permeable sediments or soils between them and the surface. That is, water may freely flow from the surface to the unconfined aquifer.
Artesian springs are natural water flows to the surface.
The waters from artesian springs have greater flow rates than ordinary springs. Because of pressure conditions, water in artesian springs or wells flow directly to the surface without pumping. As shown in Figure 4.14b, the presence of confining layers, sloping aquifers and topographic valleys produce the pressure conditions that cause water to freely flow to the surface. Typically, a saturated lens of sand or other permeable material in the otherwise unsaturated zone is located on top of a shale or other impermeable layer. The cone results from more water being removed from the well than flows into it from the surrounding aquifer.