Lecture 19 Ground Water (2)

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Lecture 19 Ground Water (2)

• Ground water storage
• Porosity
• Water retention and yield
• Storage change

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Ground Water Storage

• Aquifers are reservoirs for water storage
• Water moves very slowly in aquifers acting as
  water pipelines
• The age of aquifers ranges from a few weeks to
  several tens of thousand years, depending on the
  depth and size.

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Porosity
Figure 6.10, WR
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Terms to Remember
Porosity percentage of the total volume of a
rock or soil which is occupied by interstices or
voids Original interstices pores which were
created at the time of origin of the
rock Secondary interstices pores results from
subsequent actions of geological, climatic and
biotic forces or factors upon the original rock,
such as faults and joints enlarged by weathering
and solution Effective porosity total porosity
less the portion of interstices which are not
interconnected with other and are hydrologically
inert.  
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Porosity vs. soil/rock type
Table 5.1, WR Ground Water
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Specific yield and specific retention
Figure 5.3, WR Ground Water
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Terms to Remember
Specific yield the volume of water than can
freely drain from a saturated rock or soil under
the influence of gravity, usually expressed in
percentage of the total aquifer volume Specific
retention the percentage of the total aquifer
volume which is retained by the surface tension
as films around the individual grains and in
capillary openings.  
Ground Water
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Ground Water Storage Change
??Qr-Qd
?? storage change per unit time Qr recharge
per unit time Qd discharge per unit time
Recharge

• Infiltration of precipitation
• Seepage from lakes, rivers, and even oceans
• Inflow from adjacent aquitards and aquifers
• Artificial recharge (irrigation, wells,
  pipelines, sewers, etc.)

• Evaporation
• Spring flow and seepage into surface water
  bodies
• Leakage to aquitards and other aquifers
• Artificial extraction

Discharge
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Self reading
Characteristics of change in unconfined
aquifers (WR Chapter 5.4.3) Characteristics of
change in confined aquifers (WR Chapter 5.4.3)