When precipitation falls (Figure 1) it may: (i) be evaporated from the earth's surface or from the leaves of plants (evapotranspiration) whose roots have taken up the moisture from the soil; (ii) flow along or near the surface of the earth in watercourses of ever-increasing size until it reaches the ocean; (iii) infiltrate down through the pores or crevices of the earth's mantle either at the point where it falls or at some distant point to which surface flow has carried it. Water which evaporates from the earth's surface or bodies of water is ready to start the cycle over again as precipitation.
When water is added to dry or unsaturated soil it is held in the voids between particles by capillary forces. Once the voids are saturated, however, the water is free to descend under the effect of gravity. As long as there is sufficient water to maintain saturation, the water will descend until it is stopped by some impervious layer, such as rock or highly impervious clay. The water can then flow laterally through the voids or rock crevices above the barrier. If there are significant differences in surface elevation, the water may flow out along the impervious layer at some lower point called a spring. If a hole is made vertically down into the saturated layer, water will flow into the hole. If the saturated layer has sufficient interconnected voids, water will flow through it relatively rapidly. When the saturated layer yields water in economic quantities, it is called an aquifer and the hole made into it could be developed into a well. The lack of resistance to flow through porous material is called permeability. In general, fine grained material such as clay or silt is low in permeability; sand is of medium permeability, and gravel is most permeable. Fractured rock varies in permeability depending on the degree and pattern of fracture. The quantity of water which can be stored in an aquifer is equal to the total volume of voids between the solid particles. The fraction of the total volume of an aquifer made up of voids is called porosity. If the voids are interconnected, aquifers of high porosity also tend to have high permeability.
Fig. 1 Hydrologic cycle
Sometimes groundwater is trapped under an impervious layer. An aquifer thus located is called a confined aquifer. If the inflow area to a confined aquifer is higher than the confining layer where a well penetrates it, the water will be under pressure and will rise in the well to some level above the confining layer. Such a well is referred to as artesian. If the water rises to the top of the well a "flowing well" results.
Obviously some locations offer better chances for successful wells than others. Clues which can be helpful in selecting well locations are (i) locations and depth to water of existing wells; (ii) existence of springs and/or streams; (iii) relative locations of infiltration areas and rock outcroppings which might constitute an impervious layer; and (iv) existence of known phreatophytes (plants requiring abundant water, whose roots frequently extend to the water table). In some areas of uniform geology, such as certain alluvial deposits in valleys, wells can be constructed anywhere with equal success.
In the absence of any clues or data, a test boring can be carried out by one of the methods described under small diameter wells. Such a boring can be carried out relatively quickly and cheaply and can save considerable time, money and frustration in the long run.
When a well is pumped, the water in it drops to some level below the static level (Figure 2). The water surface in the aquifer then forms a "cone of depression" as it slopes from the static level at some fairly large radius, R, to the well whose radius is r. If the well completely penetrates the aquifer with the static height of water being H and the height of water during pumping, h, then theoretical considerations give:
Q = yield or rate of pumping (e.g. m3/hr, litres/sec, etc.)
K = permeability of the aquifer
Fig. 2 Flow into a well
(H - h) is known as the "drawdown" of the well. If the drawdown is small compared to H, then the term (H + h) is approximately equal to 2H and the yield, Q, is approximately proportional to the product of H times the drawdown. This shows the importance of penetrating the aquifer to an adequate depth. By contrast the yield, Q, is much less responsive to changes in well diameter, since it is inversely proportional to the natural log of the ratio, (R/r).