It is uncertain when mankind first started extracting groundwater by artificial means such as wells or infiltration galleries. Early humans most likely drank from surface streams. They may also have discovered groundwater through the discharge of natural springs in some parts of the world, and used this source in addition to surface streams. As streams dried up in hot weather, people learned to dig into the alluvium to find water below the surface.
It is known that the world’s climate has changed significantly over geologic time. Evidence from fossil plants, pollen, rodent middens, tree rings, and even clay-mineral formation has shown that precipitation and temperature have varied widely. For example, near the end of the Pleistocene Epoch (the Ice Age), the climate was much wetter in the American southwest than it is now, and the water table in southern Nevada was 100 meters higher than at present (Winograd and Doty, 1980). It is not entirely clear what caused climate change in the past, although volcanic eruptions and tilting of the earth’s rotational axis have been suggested.
Many climatologists believe that anthropogenic factors are now effecting a slow, but perceptible change in the atmosphere that is causing gradually warming temperatures. These factors include changes in land use with overgrazing and burning of forests, and the introduction into the atmosphere of abnormal amounts of certain gases.
The gases, commonly known as greenhouse gases, include carbon dioxide (CO2) and methane (CH4) as the prime suspects in maintaining the greenhouse effect. This effect is the atmospheric entrapment of heat from the sun which, in turn, is expected to cause greater evapotranspiration in some areas and greater precipitation in others. As a result, the climate is expected to change significantly within the next century, causing a reduction of precipitation in some areas and a great increase of precipitation in other areas.
Although much research has been accomplished in estimating the potential effects of climate change on surface water supplies, little attention has been directed toward the possible effects on groundwater except to estimate the increase or decrease in recharge rates and amounts that can be expected. Recent work by Leap and Belmonte (1992), Leap (1993), and Reichard (1995) illustrates the role of increased pore pressure in the opening of fractures in rocks with consequent increase in hydraulic conductivity.
It has been known for some time that a slight increase in fracture width (aperture) can cause the flow rate through a fracture to increase by a power of three; this is known as the Cubic Law (Witherspoon, et al., 1980), and is given as
Most rocks of aquifer quality may contain tens, hundreds, or even thousands of fractures per cubic meter, and in order to quantify flow through such rocks for practical purposes, it is more convenient to assume that the fracture density is great enough that the flow system can be treated as a porous medium. Jones (1975) performed a series of experiments measuring flow through cores of fractured carbonate rocks under different pressures in order to discover the relationship between intrinsic permeability and effective stress. The rock contained enough fractures that an equivalent porous medium could be assumed, and the relationship sought was determined to be
Leap (1993) postulated that these relationships between permeability, pore pressure, and effective stress could be significant in understanding and predicting changes in permeability if the water table rose during a period of increased recharge consequent to increased precipitation. Reichard (1995) used this equation as a basis for a modeling study to determine what the effect of a significant recharge-induced water-table rise would be on the hydraulic conductivity of an aquifer after a climate change which would cause significantly increased recharge. His studies showed that under such conditions, the hydraulic conductivity of fractured aquifers could increase as much as 15 to 30%. More research is needed in this area, but the results thus far are significant because they suggest that not only will flow rates be expected to increase, but transport rates of contaminants could also increase as well.