Groundwater
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Groundwater. Importance of Groundwater. Supplies 40% of U.S. public drinking water. Almost all rural residents Agriculture can be dependent on groundwater for irrigation. Shapes the land surface (e.g. sinkholes, mudslides). Fig. 17.3. Occurrence of Groundwater.

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Groundwater

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Importance of groundwater
Importance of Groundwater

  • Supplies 40% of U.S. public drinking water.

    • Almost all rural residents

  • Agriculture can be dependent on groundwater for irrigation.

  • Shapes the land surface (e.g. sinkholes, mudslides).

Fig. 17.3


Occurrence of groundwater
Occurrence of Groundwater

  • Found everywhere below the surface, in all rock/sediment types.

  • Stored in and flows through pores spaces.

Fig. 17.5

Fig. 17.5


Occurrence of groundwater1
Occurrence of Groundwater

  • The watertable is the boundary between the unsaturated and saturated zones.

Unsat. – pores filled with air and water

Sat. – pores filled with only water

Fig. 17.6


Occurrence of groundwater2
Occurrence of Groundwater

  • Most surface water bodies (i.e. streams, lakes) occur where water table insects the land surface.

Quarries/mines must be dewatered while operating.

Fig. 17.4


Movement of groundwater
Movement of Groundwater

  • Precipitation infiltrates, flows through unsat. zone to water table.

  • Groundwater flows from higher to lower elevations (velocity – cm to m per day).

Fig. 17.17



Water table fluctuations
Water Table Fluctuations

  • The water table is continually rising and falling.

    • rises when precipitation infiltrates

    • falls as water discharges to streams

Fig. 17.13


Water table fluctuations1

The water table also drops when water is pumped from wells.

Water Table Fluctuations

  • Shallow wells go dry during dry periods.

  • water table drops below bottom of the well.



Effects of pumping groundwater1
Effects of Pumping Groundwater

  • Groundwater can be pumped out faster than it recharges.

    • water table drops, aquifer yields less water

Fig. 17.13


Effects of pumping groundwater2
Effects of Pumping Groundwater

Fig. 17.14

  • High Plains Aquifer:

    • Low recharge rate

    • High pumping rate (agriculture)

    • Max. water table drop is >50 m (160 ft.)

    • Half of aquifer has been drained


Effects of pumping groundwater3
Effects of Pumping Groundwater

Fig. 17.33

  • Land subsidence:

  • Water in pores supports grains, resists compaction.

  • Dewatering pores allows compaction.

  • Compaction lowers land surface elevation (subsidence).


Effects of pumping groundwater4
Effects of Pumping Groundwater

  • Land subsidence fissures in Arizona caused by groundwater pumping.

Fig. 17.15


Effects of pumping groundwater5
Effects of Pumping Groundwater

Fig. 17.33

  • Salt water intrusion:

  • Near coasts, groundwater at depth is salty.

  • Pumping fresh water may eventually draw salt water into a well.


Groundwater and landscapes
Groundwater and Landscapes

  • Limestone is easily dissolved by groundwater.

  • Sinkholes – where surface rocks collapse into the void left after limestone dissolves.

Fig. 17.36


Groundwater and landscapes1
Groundwater and Landscapes

  • Groundwater flows along joints, dissolves limestone.

  • Eventually, caves form and sinkholes form when cave roof collapses.

Fig. 17.36


Groundwater and landscapes2
Groundwater and Landscapes

  • Sinkhole damage in Florida.


Groundwater and landscapes3
Groundwater and Landscapes

  • Areas in the U.S. where rocks are easily dissolved.

Fig. 17.36


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