Transcript Aquifer Storage Properties (p.58ff)

Aquifer Storage Properties
CVEG 5243 Ground Water Hydrology
T. Soerens
Pump tests are used to determine aquifer parameters
(hydraulic conductivity and storage properties)
well
Q
drawdown curve
drawdown cone
cone of depression
h0 - h = drawdown
h0
h0
Darcy’s Law (K) describes the
shape of the drawdown curve
at equilibrium
h2
h1
We need more information to describe
the time-dependent (“transient”) behavior
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.
3
1
m3
Vdrained 0.10m
Sy 

 0.1
3
Vtotal
1m
• Specific Retention, Sr
– Volume of water retained ...
Sr 
Vretained
Vtotal
• Released + retained = total
– Sy + Sr = n
0.10 m3
• range of Sy = 0.01 clay to 0.30 gravel
• 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
1 m3
0.01 m3
 0.01m3 


3
1m 

1
Ss 
 0.01m
1m
• 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 = S s .b
well
(unitless)
– Ss = volume of water released per 1 m3 of
aquifer volume per 1m change in head
1 m2
• example: Ss = 0.01 m-1
upper confining layer
(0.01 m3 released per 1m3 box)
– Aquifer volume per 1 m2 of aquifer area =
aquifer thickness = b
b = 3m
lower confining layer
1 m3
1 m3
1 m3
• example: = 3m x 1m2 / 1m2 = 3m = b
Ss= 0.01 m-1
3
3

0
.
01
m
Vreleased
2
1
m
area
S  change in head 
 0.03  Ss  b
1m
3 boxes,
each 1 m3
•
range of S = 0.005 to 0.00005 (10-3 to 10-5)
• 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
Equilibrium Well Equations
Unconfined
1
h u( r )
2
h2
Q  r2
ln
r
 K
Confined
r2
Q
 ln
r
2 KB
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
h c( r  B )
h2
• 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