The period of time since groundwater fell as rain can now be estimated by a technique based on the amount of tritium found in groundwater. This technique was developed by Dr. Willard Libby, who was one of the members of the Atomic Energy Commission, and some of his former associates at the Institute for Nuclear Research at the University of Chicago.
Tritium is a radioactive isotope of hydrogen, believed to be formed in the atmosphere from the action of cosmic rays on ordinary hydrogen. Thus, tritium is found in all atmospheric water, such as rain and snow. As a radioactive material, tritium gradually decays or decomposes into simpler substances, and has a known "half-life" of 12'/2 years. That is, one half of the radioactive form is dissipated in 12 1/2 years. An additional half is lost in the succeeding 121/2 years, and so on until the amount remaining is too small to be measured.
Isotopes: forms of atoms of an element that differ in their masses, due to variations in the numbers of mass particles in their nuclei. Hydrogen has three known isotopes: the most common form has only a proton (a relative mass of one and a single positive electrical charge) in its nucleus; a second isotope known as "deuterium" has one proton and a neutron (neutral in charge and also with a relative mass of one) in its nucleus, and thus a relative mass of two; a third isotope known as "tritium" has two neutrons and a single proton in its nucleus, and thus has a relative mass of three.
As the approximate amount of tritium originally present in water as it fell as rain is known and the amount remaining can be measured, the length of time underground can be calculated unless the amount remaining is too small to be detected by the instruments currently available.
Tests of this type on deep well water from several locations in Nebraska indicated underground water ages of about 14 to 61 years.
GROUND WATER ZONES AND BELTS
Tests on Illinois water gave ages of 50 to more than 100 years. (Beyond 100 years, the tritium concentrations could not be measured accurately.) These ages are generally in keeping with the anticipated values when all the hydrologic factors in each area are considered.
As the diagram shows, water must travel through various strata before becoming groundwater. Below the surface it moves first through the subsoil (the belt of soil water), the intermediate layer, the capillary fringe, and finally into the groundwater bed.
These layers vary in depth and are not too sharply defined. In fact, there is a gradual transition from one to another until the groundwater level or zone of saturation is reached. Even after water moves into the topsoil and subsoil, much of it may still return to the atmosphere either through evaporation or transpiration. Water is held in the subsoil by molecular attraction. It is only after sufficient water has accumulated here that it begins to seep downward under the pull of gravity. The subsoil may extend down 50 feet. It supplies the water needed for the growth of vegetation. Consequently, it is extremely important to farmers.
Water in the intermediate belt is generally considered in "dead storage." To all intents and purposes, it is suspended and does not flow into wells. This belt varies from a hairsbreadth to several hundred feet.
Below lies the capillary fringe. Water in this fringe is continuous with the water in the zone of saturation but is held back by capillary action. The thickness of this capillary fringe depends on its composition. In silty materials, it may extend down for several feet. In coarse, gravelly materials it may go down less than an inch. Even in this capillary fringe, water will still not enter well systems. It is only when it reaches the zone of saturation that it may be drawn back up to the surface by wells.
Capillary action, where water touches a solid, capillary action causes the water at that point to rise higher than that portion of its surface not in contact with the solid.
Capillary action is due to adhesion, cohesion, and surface tension. Capillarity is one of the causes of water's rising in the sail as in the capillary fringe. Kerosene rising in the wick of an old-fashioned lamp is another example of this seeming contradiction of the law of gravity.
This zone of saturation forms a huge natural reservoir that feeds springs and streams in addition to our wells. Its thickness varies from two to hundreds of feet, depending on local geologic conditions. The upper surface of the zone of saturation is neither stationary nor level. It possesses many surface irregularities and may range up or down many feet over a period of years at any given location. The fluctuations in its content depending on the amount of recharge and pumpage.
In general, the contours of the water table parallel the surface contours. However, the water table goes deeper under high elevations and rises nearer to the surface under lower elevations. At springs and flowing streams, the surface and water table elevations coincide. Below the economically important zone of saturation lies dense, solid rock. While this rock is known to hold substantial amounts of internal water, there is no practical way of bringing it to the surface.
GROUNDWATER
Under most conditions, groundwater supplies are higher in mineral content than surface waters in the same area. This is due to their longer exposure to rock formations. Exceptions do occur, as when surface waters originate in a region of relatively soluble rock and later flow into an area of less soluble rock. In such cases, groundwaters in the latter area may be lower in mineral content than that of surface waters.
Meanwhile, as water seeps through the ground and adds to its mineral content, much of its suspended matter, color and bacterial content are filtered out. Thus, a deep well is likely to provide water that is clear, colorless and low in bacterial count. Of course, there are exceptions. it might be expected that the deeper wells go, the more highly mineralized are their waters. In some shallow wells, however, the mineral absorption is greater than for deep wells in the same general area.