aquifer storage properties
DESCRIPTION
Aquifer Storage Properties. CVEG 5243 Ground Water Hydrology T. Soerens. Q. well. Pump tests are used to determine aquifer parameters (hydraulic conductivity and storage properties). h 0. h 2. h 1. Aquifer Medium. drawdown curve drawdown cone cone of depression. h 0 - h = drawdown. - PowerPoint PPT PresentationTRANSCRIPT
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Aquifer Storage Properties
CVEG 5243 Ground Water Hydrology
T. Soerens
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h0 h2
h1
well
Darcy’s Law (K) describes theshape of the drawdown curve
at equilibrium
We need more information to describe the time-dependent (“transient”) behavior
Q
drawdown curvedrawdown cone
cone of depression
h0
h0 - h = drawdown
Pump tests are used to determine aquifer parameters(hydraulic conductivity and storage properties)
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• 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.
1.01
10.03
3
m
m
V
VS
total
drainedy
0.10 m3
1 m3
total
retainedr V
VS • Specific Retention, Sr
– Volume of water retained ...
• Released + retained = total – Sy + Sr = n
• range of Sy = 0.01 clay to 0.30 gravel
Aquifer Storage Properties (p.58ff)
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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
13
3
01.01
101.0
mm
mm
Ss
0.01 m3
1 m3
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upper confining layer
lower confining layer
b = 3m
• 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.
b S .mm
mVS s
released
03011
01.032
3
headin changearea
• 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
1 m2
well
1 m3
1 m3
1 m3
Ss= 0.01 m-1
• range of S = 0.005 to 0.00005 (10-3 to 10-5)
3 boxes, each 1 m3
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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 > 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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h u r( ) h 22 Q
Kln
r 2
r
1
2
• 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
• 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
h c r B( ) h 2Q
2 K Bln
r 2
r
Unconfined Confined
Equilibrium Well Equations