Aquifer Storage Properties

CVEG 5243 Ground Water Hydrology

T. Soerens

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)

• 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)

• 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

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

• 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

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