composing  any  stratum  to  transmit  water  under pressure is called its “permeability.” The property of the material of any stratum to contain interstices, or openings,  is  called  its  “porosity.”  Both  the permeability  and  porosity  of  the  rock  formation determine  whether  groundwater  can  be  found  in suitable amounts at any particular location. When water falls on the ground, the part of it that becomes groundwater by reason of the soil or surface characteristics is absorbed into the earth. It is then either  held  in  suspension  or  flows  downward  by gravity to a point beyond which it cannot pass. It then flows in any direction provided the permeability of the particular  stratum  holding  it  permits.  When  the permeability of the stratum does not permit flow, the water remains confined at that point. As more water percolates downward through the soil or rock, the top level of the confined water rises until flow becomes possible in one direction or the other through a more permeable formation. More water must come from a higher level to sustain such a flow. Finally, the amount of water percolating from the higher levels balances the amount of water flowing laterally away, and the top level of the main body of groundwater is stabilized. The upper surface of this main body of groundwater, when  stabilized  under  any  condition  of  flow, constitutes  the  water  table  for  any  specific  locality. However, the water table is not fixed because it rises and falls according to the varying amounts of water percolating from above (called the “influent” supply) and those amounts flowing away or withdrawn (called the “effluent” flow). A stratum that bears groundwater is termed an aquifer. Water beneath the surface of the earth occurs in three zones (fig. 7-2) as follows: 1. The zone of soil moisture is where water is temporarily  held  in  pore  spaces  by  capillarity  and  other soil conditions. Water in the zone of soil moisture may evaporate directly or through transpiration by plants or it may percolate downward into the zone of aeration and then to the zone of saturation. 2. The zone of aeration, or zone of percolation, is beneath the soil layer where both water and air are present in the pore spaces. Wells ending in the zone of aeration produce no water. Sometimes in the cooling- off process or because of other external and internal influences, a stratum of material that does not permit the passage of water has been heaved about into a cup- shaped formation at a point in the zone of aeration higher than the established water table. In time, this cup is filled with groundwater and a “perched” or false water table is established. This is a serious problem to those  attempting  to  develop  a  groundwater  supply.  The perched supply, if pumped, is soon exhausted and requires seepage from above for replenishment. Many of these perched supplies result from folded clay formations  in  the  zone  of  aeration  that  stop  the percolation of water downward. Perched water never forms a dependable water supply. 3. The zone of saturation is where all pore spaces are filled with water. The top of the saturated zone is called the “water table.” It is not flat, but has a variable depth beneath the surface, depending upon surface topography, rainfall, and direction of water movement, rock structure, and porosity. Permeable rocks in the zone of saturation yield water to wells. Q1.    What are the two categories of water? Q2.   What   are   the   three   zones   of   subsurface groundwater? THE QUALITY OF WATER LEARNING  OBJECTIVE:  Identify  and understand  types  of  waterborne  diseases, impurities in water, and types of treatment processes. Preliminary to discussion of water production, the “quality” of available water supplies must be briefly considered. Whether the water supply comes from the surface  or  underground,  the  supply  must  suit  its intended use. Either source may produce water with too  high  a  concentration  of  mineral  salts,  color, suspended  matter,  incrusting  or  corrosive  agents,  or bacteria that prevents the use of water in its natural state for the purpose intended. If suitable water cannot be found, then other available sources must be used. The water must be treated to remove those elements that  make  its  direct  use  impossible.  For  human consumption, all harmful bacteria must be destroyed and the concentration of certain mineral salts and Figure 7-2.—Classification of underground water. 7-3