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-Las Vegas is the fastest growing metropolitan area in the U.S.
-It encompasses 1.2 million (2/3 of Nevada’s population)
-Visited by 30 million tourists each year
Las Vegas Subsidence
SubsidenceSubsidence
• Sudden sinking or gradual settling of the Earth’s surface due to the movement of subsurface earth material
• Causes:– Aquifer-system compaction– Underground mining– Thawing permafrost– Drainage of organic soils– Natural compaction
GeologyGeologyLas Vegas valley:
-fault-bounded, hydrologic basin
-highly compressible, fine-grained sediment
PrecipitationPrecipitation
• 1,564 square mile watershed
• 24 inches precipitation occur in Spring mountains to the west
• Valley receives 4 inches a year
• Measurable precipitation occurs 30 days out of the year
- The increase in population has caused an equal increase in the demand for water
- Groundwater makes up about 20% of water supply (~ 375,000 acre-feet/year)
- Since 1946 withdrawal > recharge
- Since 1968 efforts have been made to reduce withdrawal
- Water table has decreased by 90 m in some areas
• Since 1935, Las Vegas has subsided by six feet– Forms fissures– Reactivates faults
• Damages:– Cracks in curbs, roads, sidewalks, swimming
pools, sewage lines, well failures, ruptured gas and water lines, warped railroad tracks and cracked drainage canals
– Damages to homes in one subdivision are projected to be $14 million
Other Concerns…Other Concerns…
• Can create new flood-prone areas by altering natural water drainage patterns
• Fissures can connect contaminated surface water with aquifers
• Replacement costs of collapsed wells
InSARInSAR
• Interferometric Synthetic Aperture Radar• Remote sensing technique using radar satellite
images• Constantly shoots radar beams at Earth’s
surface and record them as they bounce back– Intensity: how much of the radar wave comes back– Phase: time it took the wave to get there and bounce
back
Artificial RechargeArtificial Recharge
• Began in 1988• Pump excess imported water from Lake Mead
into principle aquifers• Occurs during winter months• Temporarily raises groundwater levels to
postpone subsidence• Can contaminate aquifers
– Disinfecting byproducts (DBPs) specifically trihalomethanes (THMs)
• Despite recharge efforts, annual extraction still exceeds annual recharge
Possible alternativesPossible alternatives
• Reduce groundwater withdrawal to level of net annual recharge
• Designate “high-hazard” or “no build zones”
• Strict regulations in already developed areas built in these hazard zones
• Las Vegas Subsidence District
• Long-term monitoring
• More research
SourcesSources
• http://pubs.usgs.gov/circ/circ1182/pdf/08LasVegas.pdf• http://www.npagroup.com/insar/apps/pdf/
lasvegas_insar.pdf• http://water.usgs.gov/ogw/pubs/fs00165/• http://geochange.er.usgs.gov/sw/impacts/hydrology/
vegas_gw/• http://www.nbmg.unr.edu/dox/nl/nl16a.htm• http://www.reviewjournal.com/lvrj_home/2006/May-21-
Sun-2006/photos/3apocalypse.gif• http://quake.usgs.gov/research/deformation/modeling/
InSAR/whatisInSAR.html