L10 - Continuation

Head decline in a confined aquifer

Doesn’t drain pores; aquifer stays saturated

How is water released in a confined aquifer?

Expansion of water & compression of aquifer matrix - both are small

Typical S for confined vs unconfined aquifer

Head decline in unconfined aquifer

Lowers water table

Pores above water table in unconfined aquifer - what happens if they aren’t saturated? What happens after?

No longer saturated = drain by gravity = releases orders of magnitudes more water than compressibility effects. Compressibility contribution is present but negligible relative to Sy

Unconfined aquifers release more or less water than confined aquifers? What’s the dominant method?

Orders of magnitude more water per unit head decline. Dominant mechanism is gravity drainage (not compressibility)

Specific yield (Sy)

Specific retention (Sr) & capillary forces

Storativity in unconfined aquifers

Confined vs unconfined storage: saturation during head decline

remains fully saturated vs pores drain above water table

Confined vs unconfined storage: primary storage mechanism

water expansion + matrix compression vs gravity drainage of pore space

Confined vs unconfined storage: storativity expression

Confined vs unconfined storage: water released per unit head decline

very small vs much larger (100-1000x)