单项选择题

As the American West enters its fifth year of drought--the longest stretch in 108 years--the region’s cities are instituting sweeping water-usage restrictions and conservation programs. In Aurora, Colo., where the reservoir system is at just 26% capacity and is expected to reach only half of normal levels by summer, planting new trees and shrubs is prohibited, and privately owned pools may not be filled.
In the thirsty, growing cities of Southern California, however, simple conservation simply won’t do the trick. This region imports more than 80% of its water from neighboring states. And even though it jealously guards those arrangements, they won’t be enough to compensate for the rapid growth that lies just ahead: San Diego County’s population alone is projected to rise about 29% by 2020, from 2.84 million to 3.67 million.
Drastic times call for drastic measures, so state water agencies are turning to desalination(脱盐作用), a technology that makes ocean and brackish(有盐味的) water drinkable by stripping it of salt and other minerals. California has plans in various stages to build 13 desalination plants along its coastline. The projects will cost billions, but planners say they’ll provide a far more reliable supply for California residents than waiting for Mother Nature to adjust her weather patterns.
Since just 3% of water on earth is fresh, this is a step that would have to be taken anyway as the global population grows. "Desalination will create a drought-proof supply of water," says Bob Yamada, the San Diego Water Authority’s seawater- desalination program manager. He adds that 20 years from now, 10% to 20% of the state’s water could come from the ocean. The American Water Works Assn., a Denver based nonprofit dedicated to improving drinking water quality and supply, predicts that the market for desalination plants and equipment, now just $2 billion, will grow to more than $70 billion over the next two decades.
Environmentalists embrace desalination. Studies show that pumping the cooling water and concentrate back into the ocean raises its salinity by less than 1%, which is equivalent to the natural rise and fall. Barry Nelson, a senior policy analyst with the Natural Resources Defense Council, says he became a proponent of desalination when a June, 1999, California report demonstrated that it was cheaper than building new dams, which often have a huge environmental impact.
Nelson still worries about energy consumption and coastal disruption. But he adds that "desalination is no longer on the h~natic fringe. It has entered the mainstream. That means we look at desalt projects on a case-by-case basis, as we would any other legitimate water policy."
As the technology continues to improve, experts say it’ll fast become a solution not only for municipalities but for hotels and resorts, corporations, and, someday, homeowners. Privately held water-treatment outfit Matrix Water, based in Fort Lauderdale, Fla., is installing a desalination plant that will process 800,000 gallons of water per day for the new Emerald Bay Four Seasons Resort in the Bahamas. And the new U.S. Homeland Security Dept. is investigating ways of using reverse osmosis to protect the nation’s water supply from bio-terrorism.
Barry Nelson became a supporter of desalination owing to its ______.