Comment: 14:56 - 16:16 (01:20)
Source: Annenberg/CPB Resources - Earth Revealed - 21. Groundwater
Keywords: "James Sadd", groundwater, aquifer, "saturated zone", "hydrostatic pressure", weight, subsidence, infiltration, "groundwater withdrawal", Houston, Texas, well, "Crystal Cove", flood, Italy, "geologic hazard", "Leaning Tower of Pisa", Venice
Our transcription: Groundwater management requires as great deal of care, not only because the water itself is so vital, but because a diminished supply could cause the ground above to subside and this, in turn, could trigger a multitude of problems.
As water infiltrates into an aquifer, the permeable rock soaks up the water like a sponge until it's saturated.
The weight of this saturated zone generates an aquifer pressure that forces rock particles apart.
This pressure also helps to hold up the ground surface, even inflating it slightly.
When groundwater is withdrawn, the pressure decreases, and the particles pack more tightly together.
If the rate of water withdrawal exceeds the rate of infiltration, the overlying ground surface will often subside.
Ground subsidence can be dramatic and have disastrous consequences in areas of high groundwater withdrawal.
In Houston, Texas, for example, three meters of subsidence has occurred over 40 years of over pumping, resulting in tens of millions of dollars of property damage and the loss of 80 square kilometers of land.
One suburb, Crystal Cove, had to be abandoned completely because of flooding.
In Italy, groundwater withdrawal has caused the lean in the Tower of Pisa and contributes to continued flooding in the canal-city of Venice.
Geology School Keywords