Wells and Pumps for Greenhouses
An aquifer is an underground formation of permeable rock or loose material that can produce useful quantities of water when tapped by a well or as discharge via a spring. From the point of view of geological structure and expansion, aquifers are of different types. From the geological angle, on the basis of existence or non-existence of water table, aquifers have been divided in following two categories: 1) unconfined aquifers and 2) confined aquifers (See Figure 13.1).
Unconfined and Confined Aquifers
Unconfined aquifers are bound by the water table; that is, they have no confining rock layers over the top of them. A well sunk into an unconfined aquifer will encounter water when the well reaches the water table, which is the approximate level at which water will stand in the well. The depth to the water table usually fluctuates to some extent depending on the season of the year and the amount of local precipitation that directly recharges such aquifers. Because the water table generally conforms to the local surface topography, the depth to water increases in areas of higher relief and decreases in areas of lower elevation.
Consolidated and Unconsolidated Aquifers
Aquifers also can be classified as consolidated aquifers and unconsolidated aquifers. A consolidated aquifer holds water in interconnected spaces between rock layers, fractures, small cracks, pore spaces, and/or solution channel openings. Limestone, granite, and sandstone are some of the rock types with consolidated aquifers.
Water wells located in unconfined aquifers are known as water-table wells. The water level in these wells is the same as the water level in the surrounding aquifer. Water wells located in confined aquifers are known as artesian wells. Wells are designed to be open to the aquifer; that is, water is free to move into the well from the aquifer. When completely at rest, the water level in a well and the groundwater level in the aquifer outside the well are equivalent.
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