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C. An aquifer is an underground layer of water-bearing sand, gravel, permeable rock or rock fractures. The water from these layers is extracted by drilling wells to the depths of the layers. Overuse can drain an aquifer before it can be refilled by rain and surface water percolating down. Water in an aquifer also can become polluted.
D. Lake Mead and Lake Powell are the largest reservoirs on the Colorado River which supplies water for 40 million people. But as the climate grows hotter and drier in the West, water from major sources like the Colorado River and the aquifers is dwindling. That threatens both major agricultural production and record population growth.
B. Fed by water from Lake Mead, Las Vegas is the fastest growing area in the United States, but its metropolitan population is less than half of the 4.8 million people in the Phoenix region. The city of Phoenix is the nation’s fifth-most populous city and the most populous state capital in the United States.
C. If the 174,000-square-mile aquifer goes dry, more than $20 billion worth of food and fiber will vanish from the world's markets. Scientists say it would take natural processes 6,000 years to refill the reservoir.