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Groundwater

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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  1. Groundwater

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. The Water Table and Groundwater Flow

  8. Water Table Fluctuations • The water table is continually rising and falling. • rises when precipitation infiltrates • falls as water discharges to streams Fig. 17.13

  9. 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.

  10. Effects of Pumping Groundwater

  11. Effects of Pumping Groundwater • Groundwater can be pumped out faster than it recharges. • water table drops, aquifer yields less water Fig. 17.13

  12. 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

  13. 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).

  14. Effects of Pumping Groundwater • Land subsidence fissures in Arizona caused by groundwater pumping. Fig. 17.15

  15. 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.

  16. Groundwater and Landscapes • Limestone is easily dissolved by groundwater. • Sinkholes – where surface rocks collapse into the void left after limestone dissolves. Fig. 17.36

  17. Groundwater and Landscapes • Groundwater flows along joints, dissolves limestone. • Eventually, caves form and sinkholes form when cave roof collapses. Fig. 17.36

  18. Groundwater and Landscapes • Sinkhole damage in Florida.

  19. Groundwater and Landscapes • Areas in the U.S. where rocks are easily dissolved. Fig. 17.36

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