Text B Groundwater or Subsurface Water

The main source of groundwater is precipitation. Formations which contain and transmit groundwater are known as aquifers. The amount of groundwater which can be obtained in any area depends on the character of the aquifer. The capacity of an aquifer is measured by the porosity, or ratio of the pore volume to the total volume of the aquifer. Pores vary in size from submicroscopic ones in clay and shale to large caverns and tunnels in limestone and lava.

It is well known that the soil pores contain both water and air in varying amounts. After a rain, water may move downward through the zone of aeration. Water in the zone of aeration is known as soil moisture. If the retention capacity of the soil in the zone of aeration is satisfied, water moves downward into a region where the pores of the soil or rock are filled with water. The water in this zone of saturation is called the groundwater.

A high porosity does not necessarily indicate that an aquifer will yield large volumes of water to a well. The only water which can be obtained from the aquifer is that which will flow by gravity. The specific yield is the volume of water (expressed as a per cent of the total volume of the aquifer) which will drain freely from the aquifer. The specific yield of fine grained materials is much less than that of coarse materials. Clay (although having a high porosity) is so fine-grained that it ordinarily yields little water to wells. In contrast, a cavernous limestone or sandstone with low porosity may yield almost all the water they contain. The most important aquifers economically are deposits of sand and gravel, which have a fairly high specific yield.

Groundwater is usually discharged by evapotranspiration and surface discharge. When the capillary fringe reaches the root systems of vegetation, a route for direct transpiration to the atmosphere is provided. If the capillary fringe is near the ground surface, great quantities of water may be evaporated directly from the soil. If the water table or an artesian aquifer intersects the ground surface, water is discharged as surface flow. A large discharge from an aquifer concentrated in a small area is called a spring.

Groundwater is an important source of water supply, especially in areas where dry summer months or extended droughts cause streams to stop their flow. Many surface streams receive a major portion of their water from groundwater, and water from surface streams is the main source of recharge for the groundwater. These two sources of supply are interrelated. Therefore both surface and groundwater problems should be considered together in plans for water-resources development.