Groundwater occurs in the natural openings in the soils and rocks of the Earth's crust. These spaces range from large caverns, which may have dimensions of the order of tens or hundreds of meters, to pore spaces which may be less than 1 μ m in size between in the grains or crystals of rocks. In the soil and near-surface rocks, it is usual for smaller pores to be filled or partly filled with water which is at a pressure less than that of the atmosphere, and for larger pores to be mainly filled with air; this region is called the unsaturated zone. At greater depths all the pores will be completely filled with water which is at a pressure greater than atmospheric pressure; this region is called the saturated zone, and the surface separating the two zones is the water table (
Figure G7) Some definitions of groundwater include all the water in pore spaces in rocks, but most restrict the term to water in the saturated zone, i.e. below the water table.
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