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 aspring.

Groundwater is an important source of water supply, especially in

Answer the following questions.

1) What is the main source of groundwater?

2) What formations are known as aquifers?

3) What does the amount of groundwater depend on?

4) What is measured by the porosity?

5) The soil pores contain both water and air in varying amounts don’t they?

6) Where does water move if the retention capacity of the soil in the zone of aeration is satisfied?

7) Is the water in the zone of saturation called the groundwater?

8) What is the specific yield?

9) Is the specific yield of fine grained materials less or more than that of coarse materials?

10) Give the examples of fine-grained and coarse materials.

11) What aquifers have a high specific yield?

12) Is Groundwater usually discharged only by evapotranspiration?

13) What is a spring?

14) Why is groundwater an important source of water supply, especially in areas where dry summer months or extended droughts cause streams to stop their flow?

15) What two sources of supply are interrelated?

Choose the contextual meaning of the words written in bold.

1) volume

2) move

3) well

4) deposits

5) spring

6) cause

Match the meanings of the terms with their definitions.

a) an aquifer        b) groundwater                         c) water table

d) soil moisture                  e) specific yield

1) The static level of water in wells extending up to the zone of saturation.

2) In hydrology, a rock layer or sequence that contains water and releases it in appreciable amounts.

3) Water in the zone of aeration.

4) Water that occurs below the surface of the Earth, where it occupies spaces in soils or geologic strata.

5) Volume of water draining freely from an aquifer.

Fill in the correct preposition, then choose any three items and make sentences.

Fill in the correct words from the list below.

Sometimes an aquifer is confined by strata of 1)______ permeability. Such an aquifer is called the 2)______ aquifer. They are analogous to pipelines. The static pressure at a point within the aquifer is equivalent to the elevation of the water table in the3)______ area. If the 4)______ is sufficient to raise water above the ground, the well is called 5)______. Artesian aquifers usually 6)______ relatively small recharge areas as 7)______ with other aquifers and yield less water. The econo­mic 8)______ of artesian aquifers lies in the fact that they transmit water substantial distances and deliver it above the level of the aquifer, thus minimizing pumping 9)______.

Look at the following scheme. Get ready to speak about how people in cities get fresh water. You may use the keys below.

Cities get fresh water from…

Keys: … by drilling wells and pumping up the water …

  … by drawing water …

  … is piped into …

  … flows to …

  … is pumped into …