5.1 Water cycle and fresh water supply  (Page 4/47)

Groundwater is in constant motion due to interconnection between pore spaces. Porosity is the percentage of pore space in an earth material and it gives a measure of how much groundwater an earth material can hold. Permeability is a measure of the speed that groundwater can flow through an earth material, and it depends on the size and degree of interconnection among pores. An earth material that is capable of supplying groundwater from a well at a useful rate—i.e., it has relatively high permeability and medium to high porosity—is called an aquifer    . Examples of aquifers are earth materials with abundant, large, well-connected pore spaces such as sand, gravel, uncemented sandstone, and any highly fractured rock. An earth material with low hydraulic conductivity is an aquitard    . Examples of aquitards include clay, shale (sedimentary rock with abundant clay), and igneous and metamorphic rock, if they contain few fractures.

As discussed above, groundwater flows because most earth materials near the surface have finite (nonzero) porosity and permeability values. Another reason for groundwater movement is that the surface of the water table commonly is not completely flat but mimics the topography of the land surface, especially in humid climates. There is "topography" to the water table because groundwater moves slowly through rock and soil, so it builds up in higher elevation areas. In fact, when groundwater flows slowly through aquitards and deep underground, it can take many thousands of years to move relatively short distances. An unconfined aquifer    has no aquitard above it and, therefore, it is exposed to the atmosphere and surface waters through interconnected pores (See Figure Flowing Groundwater ). In an unconfined aquifer, groundwater flows because of gravity to lower water table levels, where it eventually may discharge or leave the groundwater flow system. Discharge areas include rivers, lakes, swamps, reservoirs, water wells, and springs (see Figure Fatzael Springs in Jordan Valley ). Springs are rivers that emerge from underground due to an abrupt intersection of the land surface and the water table caused by joints, caves, or faults that bring permeable earth materials to the surface. A confined aquifer    is bounded by aquitards below and above, which prevents recharge from the surface immediately above. Instead, the major recharge occurs where the confined aquifer intercepts the land surface, which may be a long distance from water wells and discharge areas (see Figure Schematic Cross Section of Aquifer Types ). Confined aquifers are commonly inclined away from recharge areas, so groundwater in a confined aquifer is under greater-than-atmospheric pressure due to the weight of water in the upslope direction. Similar to river discharge, groundwater discharge describes the volume of water moving through an aquifer over time. Total groundwater discharge depends on the permeability of the earth material, the pressure that drives groundwater flow, and the size of the aquifer. It is important to determine groundwater discharge to evaluate whether an aquifer can meet the water needs of an area.