Aquifer Storage Properties (p.58ff)

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Aquifer Storage Properties

• CVEG 5243 Ground Water Hydrology
• T. Soerens

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Pump tests are used to determine aquifer
parameters (hydraulic conductivity and storage
properties)
Aquifer Medium
drawdown curve drawdown cone cone of depression
h0 - h drawdown
Darcys Law (K) describes the shape of the
drawdown curve at equilibrium
h0
We need more information to describe the
time-dependent (transient) behavior
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Aquifer Storage Properties (p.58ff)

• Specific Yield, Sy for unconfined aquifers
• Volume of water released under gravity drainage
  from a volume of initially saturated material,
  divided by the total volume of the material.

1 m3

• Specific Retention, Sr
• Volume of water retained ...

• Released retained total
• Sy Sr n

0.10 m3

• range of Sy 0.01 clay to 0.30 gravel

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• Specific storage, Ss
• Volume of water released per unit volume from
  storage under a unit decline in hydraulic head.
• i.e., if head drops 1m while 0.01 m3 is released

0.01 m3
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• Storativity, (storage coefficient) for confined
  aquifers
• S volume of water released or taken into
  storage per unit of aquifer storage area per unit
  change in head.

• S Ss.b (unitless)
• Ss volume of water released per 1 m3 of aquifer
  volume per 1m change in head
• example Ss 0.01 m-1 (0.01 m3 released per
  1m3 box)
• Aquifer volume per 1 m2 of aquifer area aquifer
  thickness b
• example 3m x 1m2 / 1m2 3m b

Ss 0.01 m-1

• range of S 0.005 to 0.00005 (10-3 to 10-5)

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• Transmissivity, T
• T K.b
• Pump test
• assume confined
• calculate T, S
• if S 10-3 or lower ? confined
• if S gt 10-2 ? unconfined
• Water released from
• unconfined drainage S Sy
• confined
• water expansion - minimal S 10-6 m-1
• compression of aquifer

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Equilibrium Well Equations
Confined
Unconfined

• Well terms
• static water level (SWL)
• pumping water level (PWL)
• drawdown
• dif between initial and PWL
• s h - h0
• residual drawdown (during recovery)
• after pumping stops, water level comes up.
• well yield
• discharge from a well (gpm)
• specific capacity
• well yield per unit drawdown
• varies with time
• radius of influence
• horizontal distance from center of well to limit
  of cone of depression (no drawdown)
• gets larger with time
• larger in confined

• Assumptions
• uniform hydraulic conductivity
• aquifer not stratified
• saturated thickness constant
• unconfined - before pumping
• confined - constant
• well is 100 efficient
• fully penetrating
• no slope of water table or potentiometric surface
• laminar flow
• infinite extent