-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