The Occurrence of Groundwater: Origin and Age
Almost all groundwater can be thought of as a part of hydrological cycle, including surface and atmospheric waters. Relatively minor amounts of groundwater may enter this cycle from other origins.
Water that has been out of contact with the atmosphere for at least an appreciable part of geological period is termed as Ground Water.
This water may have been derived from oceanic or freshwater sources and, typically, is highly mineralized.
The residence time of water underground has always been a topic of considerable speculation. But with the advent of radioisotopes, determinations of the age of groundwater have become possible.
Rock Properties Affecting Groundwater
Groundwater occurs in many types of geological formations; those known as aquifers are of most importance. An aquifer may be defined as a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs. This implies an ability to store and to transmit water; unconsolidated sands and gravels are a typical example.
Clearly, there are various types of confining beds; the following types are well established in the literature:
Aquiclude– A saturated but relatively impermeable material that does not yield appreciable quantities of water to wells, clay is an example.
Aquifuge– A relatively impermeable formation neither containing nor transmitting water; solid granite belongs in this category.
Aquitard– A saturated but poorly permeable stratum that impedes groundwater movement and doesn’t yield water freely to wells, that may transmit appreciable water to or from adjacent aquifers and, where sufficiently thick, may constitute an important groundwater storage zone; sandy clay is an example.
Those portions of a rock or soil not occupied by solid mineral matter can be occupied by groundwater. These spaces are known as voids, interstices, pores, or pore spaces. Because interstices serve as water conduits, they are of fundamental importance to the study of groundwater. Typically, they are characterized by their size, shape, irregularity, and distribution. Capillary interstices are sufficiently small that surface tension forces will hold water within them.
Unconsolidated geological materials are normally classified according to their size and distribution. Evaluation of the distribution of sizes is accomplished by mechanical analysis. This involves sieving particles coarser than 0.05 mm and measuring rates of settlement for smaller particles in suspension.
The water retentive property of a soil or rock is markedly influenced by its surface area. This area depends on particle size, shape and on the type of clay minerals percent. The term specific surface refers to the area per unit weight of the material, usually expressed as m2/g. Relative methods for measuring specific surface are based on retention of a polar organic molecule such as ethylene glycol; these have been related to absolute values derived from statistical calculations of surface area.
Clay particles contribute the greatest amount of surface area in unconsolidated formations. Non-swelling clays such as kaolinite have only an external surface and exhibit specific surfaces in the range of 10-30 m2/g; however, swelling clays such as montmorillonite and vermiculite have internal and external surfaces that yield specific surface values near 800m2/g.
Vertical Distribution of Groundwater
The subsurface occurrence of groundwater may be divided into zones of aeration and saturation. The zone of aeration consists of interstices occupied partially by air. In the zone of saturation, all interstices are filled with water under hydrostatic pressure.
In the zone of aeration, vadose water occurs. This general zone may be further subdivided into the soil water zone, the intermediate vadose zone, and the capillary zone.
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