Figure 9-1.–Hydrologic cycle and zone.two layers of clay maybe a layer of sand which is bothsufficient, the Well may be free flowing. An aquifer(fig.porous (holds water) and is permeable (allows the waterto flow). (See fig. 9-2).A porous and permeable formation that can yieldwater in usable quantities is called an aquifer (fig. 9-3).If an aquifer is not confined by an impermeable layerabove it, the aquifer is said to be under water tableconditions and is subjectcd to atmospheric pressure. Ifan aquifer is confined both above and below and therecharge area or source of water is higher than the pointwhere a well is to be located, the water will be undergreater than atmospheric pressure and will rise to somepoint above the water table. When the pressure is9-3) under this condition is called an artesian.Sedimentary, igneous, and metamorphic rockformations are potential sources of water. Sedimentalrocks are the most common aquifers. They are formedby the accumulation of sediments that have beendeposited by water, wind, or ice. Limestone is anexample of a sedimentary rock and is formed from theaccumulation of chemical compounds and minerals thatsettle together out of water. Water percolating throughcracks and joints in a dense limestone deposit results inunderground streams and lakes. Throughout thick,nearly horizontal layers of limestone, as well as shale9-2
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