Lecture 11
Storativity and Specific Yield
Fetter 4.8
Storativity - (Storage Coefficient) (S) - the volume of water that a permeable unit will
adsorb or expel from the storage per unit surface area unit change in head.
Below the water table or potentiometric surface in the saturated zone
| Hydraulic head creates pressure | ||||
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If pressure increases or decreases - mineral skeleton expands or contracts( respectively)
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Specific Storage (Ss)
Amount of water per unit volume of a saturated formation that is stored or expelled
from storage due to compressibility of the aquifer skeleton and pore water per unit
change in head.
| Aquifers and confining units | ||
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| r w = density of water (M/L3) | ||
| g = acceleration of gravity (L/T2) | ||
| a = compressibility of aquifer skeleton (1/(M/LT2) | ||
| n = porosity (L3/L3) | ||
| b = compressibility of water (1/(M/LT2) | ||
| Ss = 1/L | ||
Pump water from a confined aquifer (Diagram - nonpumping conditions versus pumping
| 1) Head declines | |
| 2) Hydraulic head remains above the aquifer unit. |
Hence, where does water come from?
Answer: Water is released from storage
Aquifer remains saturated
S = bSs |
S- storativity (confined) |
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= (ft) (1/L) = unitless |
Water release is accounted for by the compressibility of mineral skeleton
and the pore water.
S < 0.005 for confined aquifers
Cone of depression analysis diagram
Water yield in a confined and unconfined aquifer
In an unconfined Aquifer-
Where does the water come from?
| 1) Gravity drainage | |||
| 2) Water released from storage. | |||
| storativity unconfined | |||
| S = Sy +hSs L/L + L(1/L) | |||
| Sy = specific yield | |||
| h = thickness of saturated zone (L) | |||
| Ss = Specific storage (1/L) | |||
Sy is several orders of magnitude > h Ss
Sy = 0.02 to 0.30 |
Volume of water drained from an aquifer as head lowers
Vw = SA dh |
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ENV 302 - Lectures
