-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

Subsidence 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

Geology Las Vegas valley: -fault-bounded, hydrologic basin -highly compressible, fine-grained sediment

Precipitation 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… 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

InSAR 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 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 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

Sources pdfhttp:// pdf s_gw/ s_gw/ Sun-2006/photos/3apocalypse.gifhttp:// Sun-2006/photos/3apocalypse.gif SAR/whatisInSAR.html