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Subterranean water that saturates the Earth’s crust just below the surface

What is it?

How does water get underground?

• Cracks, crevices in rock

• Spaces between grains in the soil

• Rocks/beds that store and transmit groundwater in sufficient quantity to supply wells are called aquifers– 22% of all fresh water flows as groundwater

How does rock flow through soil?

• The only open spaces for water to flow are between individual grains, called pore spaces

• Porosity: percentage of the total volume that is taken up by pores

Porosity

• How loosely packed are the grains?

• Higher in sedimentary rocks

Permeability

• Are the pores connected?

• How big are they?

• How torturous is the path to be traveled?

• Some cave systems are so permeable that people can move through them

The (Ground) Water Table

Zone of saturation

“coke” table (aka water table)

Zone of aeration

The Effect of the Water Table

• If you drill a well below the water table, it will fill to the level of the water table– Straw in pop

• It forms a surface roughly parallel to the ground surface

• And it moves downhill under gravity and is exposed at rivers and lakes

Aquifers

• Rocks/beds that store and transmit groundwater in sufficient quantity to supply wells are called aquifers– Confined– Unconfined

Balancing Recharge and Discharge

• Normal “Problem”

Balancing Recharge and Discharge• Extreme

Problems– saltwater

intrusion

Land Subsidence

• Groundwater commonly flows through spaces in sediment (sand and gravel)

• Pumping groundwater can cause grains to rearrange and “compact”

• Irreversible

• Permanent

damage to

reservoirs

San Joaquin Valley

• CA 1st in agriculture– Central Valley = 25% of

nation’s food, 1% of land– “Groundwater mining”

• Valley is filled with river sediments and clays

– First noted here in 1935– >1/2 of San Joaquin Valley (5,200

mi2) irrigable land has sunk by 1’

Land Subsidence

Darn Kids…

Groundwater and Erosion

• Recall how water is a great dissolver– Rainwater contains carbon dioxide which

therefore is present in groundwater– Carbon dioxide acts to dissolve limestone– Carbonic acid (COKE) causes faster reactions

• Therefore the passage of groundwater through limestone can make HUGE caves

Just Below Water Table

• Water coming down from above can evaporate when it reaches opening– Stalactite

• Water that drips down to base can form– Stalagmite