Environmental Problems, Their Causes, and Sustainability 1

Figure 15-16 Natural capital degradation: areas of greatestaquifer depletion from groundwater overdraft in the continentalUnited States. Aquifer depletion is also high in Hawaiiand Puerto Rico (not shown on map). It causes the landabove the aquifer to subside or sink in spots in most of theseareas. (U.S. Water Resources Council and U.S. GeologicalSurvey)GroundwaterOverdrafts:HighModerateMinor or nonecoastal areas of Florida, California, South Carolina, andTexas.Figure 15-18 lists ways to prevent or slow theproblem of groundwater depletion. Study this figurecarefully.Mexico City, built on a lakebed, has one of theworld’s worse subsidence problems because of an increasein groundwater overdrafts from rapid populationgrowth and urbanization. In recent years, someparts of the city have sunk as much as 8 meters (26 feet).Excessive withdrawal of groundwater can causethe roof of a cavern or underground conduit to collapsesuddenly and create a large crater. Such sinkholescan form suddenly without warning and swallowhouses, cars, and trees. Subsidence and the formationof sinkholes usually compress the rock particles inaquifers and thus lead to a permanent loss of theaquifer.Finally, groundwater overdrafts near coastal areascan contaminate groundwater supplies by causing intrusionof salt water into freshwater aquifers used tosupply water for irrigation and domestic purposes(Figure 15-17). This is an especially serious problem inCase Study: The Shrinking Ogallala Aquifer—Using Up Natural CapitalPumping water from the world’s largest aquifer hasgreatly increased food production, but overpumpingis a serious problem in some areas.PreventionWaste less waterSubsidize waterconservationSolutionsGroundwater DepletionControlRaise price of waterto discourage wasteMajor irrigationwellFreshgroundwateraquiferInterfaceSaltwaterIntrusionWell contaminatedwith salt waterWatertableInterfaceNormalInterfaceSea levelSaltwaterSeafloorBan new wells inaquifers nearsurface watersBuy and retiregroundwaterwithdrawal rightsin critical areasDo not growwater-intensivecrops in dry areasReduce birthratesTax water pumpedfrom wells nearsurface watersSet and enforceminimum streamflow levelsFigure 15-17 Natural capital degradation: saltwater intrusionalong a coastal region. When the water table is lowered, thenormal interface (dashed line) between fresh and salinegroundwater moves inland (solid line), making groundwatersupplies unusable for irrigation and domestic purposes.Figure 15-18 Solutions: ways to prevent or slow groundwaterdepletion. Which two of these solutions do you believe are themost important?320 CHAPTER 15 Water Resources